The Hercules-Lyra Association revisited New age estimation and multiplicity study
T. Eisenbeiss, M. Ammler-von Eiff, T. Roell, M. Mugrauer, Ch. Adam, R. Neuhaeuser, T. O. B. Schmidt, A. Bedalov
aa r X i v : . [ a s t r o - ph . S R ] D ec Astronomy&Astrophysicsmanuscript no. HerLyrPaper˙revised˙3p0 c (cid:13)
ESO 2018September 11, 2018
The Hercules-Lyra Association revisited ⋆⋆⋆
New age estimation and multiplicity study
T. Eisenbeiss , M. Ammler-von Ei ff , , , T. Roell , M. Mugrauer , Ch. Adam , R. Neuh¨auser , T. O. B. Schmidt , andA. Bedalov , , Astrophysical Institute and University Observatory, Friedrich-Schiller Universit¨at, Schillerg¨asschen 2-3, 07745 Jena, Germany,e-mail: [email protected] Th¨uringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany Georg-August-Universit¨at, Institute for Astrophysics, Friedrich-Hund-Platz 1, 37077 G¨ottingen, Germany Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany Department of Physics and Astronomy K.U.Leuven, Celestijnenlaan 200D, B-3001 Leuven Faculty of Natural Sciences, University of Split, Teslina 12. 21000 Split, Croatia
ABSTRACT
Context.
The Hercules-Lyra association, a purported nearby young moving group, contains a few tens of zero age main sequencestars of spectral types F to M . The existence and the properties of the Her-Lyr association are controversial and discussed in theliterature.
Aims.
The present work reassesses the properties and the member list of the Her-Lyr association based on kinematics and age indica-tors. Many objects form multiple systems or have low-mass companions and so we need to properly account for multiplicity.
Methods.
We use our own new imaging observations and archival data to identify multiple systems. The colors and magnitudesof kinematic candidates are compared to isochrones. We derive further information on the age based on Li depletion, rotation, andcoronal and chromospheric activity. A set of canonical members is identified to infer mean properties. Membership criteria are derivedfrom the mean properties and used to discard non-members.
Results.
The candidates selected from the literature belong to 35 stellar systems, 42 . ±
46 Myr which is roughly in between the ages of the Pleiades and the Ursa Major group. The measures ofchromospheric and coronal activity support the young age.Four membership criteria are presented based on kinematics, lithium equivalent width, chromospheric activity, and gyrochronologicalage. In total, eleven stars are identified as certain members including co-moving objects plus additional 23 possible members while14 candidates are doubtful or can be rejected. A comparison to the mass function (MF), however, indicates the presence of a largenumber of additional low-mass members, which remain unidentified.
Conclusions.
Key words. solar neighborhood - open clusters and associations: individual: Her-Lyr - binaries: visual - stars: kinematics and dynam-ics - stars: individual: HH Leo
1. Introduction
The Hercules-Lyra (Her-Lyr) association is one of the closestyoung moving groups (MGs). The members of moving groups,also called stellar kinematic groups, are identified by similarspace motion. In many cases, common space motion is relatedto spatial clustering, for example in the well-known cases ofthe Hyades and Ursa Major (UMa, or Sirius) moving groupsand the eponymous open clusters. The discovery of additionaldistant and co-moving stars also led to the concept of super- ⋆ Based on observations made with ESO Telescopes at the ParanalObservatory under programs ID: 380.C-0248(A) (Service Mode, VLT-Yepun) and ID: 074.C-0084(B) (on 2005 Jan 06, VLT-Yepun). ⋆⋆ Based on observations collected at the Centro Astron´omicoHispano Alem´an (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut f¨ur Astronomie and the Instituto de Astrof´ısica deAndaluc´ıa (CSIC). clusters (Eggen 1994). The third large nearby group next to theHyades and UMa groups is the local association (LA) that is as-sociated with the Pleiades. Its members are young with a widerange of ages of 100 Myr and younger. In contrast to the Hyadesand UMa, the LA clearly lacks aspects of spatial coherence andembeds clusters and associations like α Per, IC 2602, and Sco-Cen (Montes et al. 2001; Zuckerman & Song 2004). In the pastdecade, many nearby young moving groups have been identifiedwith ages below 100 Myr (Zuckerman & Song 2004; Torres et al.2008). They are located at average distances of about 30 pc andbeyond (Torres et al. 2008).At even closer distances, young stars similar to membersof the LA and the Pleiades were noticed earlier (Young et al.1987; Soderblom & Clements 1987), while it was not beforeGaidos (1998) that these stars were considered a close movinggroup. This group was previously unnoticed because of the lack
1. Eisenbeiss et al.: The Hercules-Lyra Association revisited ° ° ° ° ° −90 ° − ° − ° ° ° ° ° V439 And HIP 1481 EP Eri G 112−35 HD 54371 DX Leo EE Leo HH Leo MN UMa LW Com PX Vir alpha Cir LTT 6256 V382 Ser HN Peg HD 207129 LTT 9081 V368 Cep G 248−16 G 270−82 LTT 10580 BC Ari V447 Lac 84 Cet EX Cet HD 25457 V538 Aur LHS 1775 HIP 63317 HIP 63322 NQ UMa NSV 6424 LTT 14623 LTT 14624 39 Tau HR 4758
Lyra Hercules
Fig. 1.
All Her-Lyr membership candidates and their locations on the sky. The arrows indicate the proper motion. The names of the stars areindicated. of bright stars unlike the UMa group, and because of its spatialscatter due to its proximity (Fuhrmann 2004).In more detail, Gaidos (1998) searched the Hipparcos cat-alog for young solar analogues within 25 pc of the Sun. Henoticed two distinct groups in kinematic space, one of whichwas identified with the UMa group. Another group of five stars,V439 And, DX Leo, MN UMa, HN Peg, and NQ UMa, displayskinematics similar, but not equal, to the LA with the radiant be-ing located in the constellation Hercules (Fig. 1).In his volume-complete study of northern G- and K-typestars in the solar neighborhood within 25 pc, Fuhrmann (2004)detected a concentration of stars with higher rotation rates typ-ical of stellar youth coinciding in UV space with the Herculesmoving group identified by Gaidos (1998). Depending on thedetails of the selection of up to 15 candidates, the radiant mayalso be located in the constellation Lyra, thus the name Her-Lyrassociation (Fuhrmann 2004). The equivalent widths of the H α and lithium lines of many of these stars resemble those of UMagroup members for which Fuhrmann (2004) gives an age of ap-proximately 200 Myr.While Fuhrmann (2004) founded the very existence of thegroup on the statistically enhanced concentration in kinematicalspace, L´opez-Santiago et al. (2006) gave additional quantitativecriteria for Her-Lyr membership. From the sample of Monteset al. (2001) of late-type members of young stellar kinematicgroups, they constructed an initial sample adding twelve starsto Fuhrmann’s sample. A velocity criterion was derived takingthe average UV velocity of Fuhrmann’s sample and adopting thedispersion of 6 km s − of the Castor moving group as the toler-ance limit. In addition, the lithium equivalent width and the po-sition in the color-magnitude diagram were required to be com-patible with a young age of 200 Myr.Maldonado et al. (2010) also investigate the properties ofyoung nearby stars and put the young nearby MGs into context,noticing that ”. . . the young MGs (8-50 Myr) are probably themost immediate dissipation products of the youngest associa- tions.” In their conclusion they find that ”. . . All previously pro-posed members of AB Dor or Her-Lyr fall into our classificationof probable LA members, . . . ”. The Her-Lyr group might there-fore be a sub-group of the LA or even indistinguishable fromit. The aim of this article, however, is not to discuss the veryexistence of the moving group. Its emergence in the UV -plane asa concentration of fast rotating young stars was well establishedby Fuhrmann (2004). In Fig. 1 all Her-Lyr candidates discussedin this paper are displayed along with their proper motion in aHammer-Aito ff projection. The radiant coincides with the con-stellations Hercules and Lyra. No kinematic candidates are lo-cated close to the radiant or the point of convergence. Any kine-matic candidates in these areas would have small, if not negligi-ble, proper motion and a value of radial velocity which is closeto the total space motion of the Her-Lyr association. Therefore,the lack of detections in these regions of the sky does not neces-sarily mean a real lack of Her-Lyr candidates but may instead re-flect the presence of an observational selection e ff ect. The Her-Lyr association in this area will have very small proper motions,moving mainly in the radial directions and may not be identifiedin the initial kinematic sample.In the present work we want to establish the mean propertiesof, as well as a refined member list of, the Her-Lyr association.In addition to the tools used by Fuhrmann (2004) and L´opez-Santiago et al. (2006) we investigate the multiplicity of the Her-Lyr candidates in Sect. 3, revealing the low-mass membershipcandidates, typically well-known sources that are visually re-solved because of the proximity of the parent stars. Finding late-type companions to the stars may indeed give better constraintson ages derived via isochrone-fitting (Sect. 4) as late-type starsarrive on the main sequence later.The assessment of an age has been unsure in the past as it isbased on a few stars only, assuming similarity to the UMa groupand adopting its age estimate (Fuhrmann 2004). In addition tothe study of the lithium equivalent width (Sect. 5.1) we use gy-
2. Eisenbeiss et al.: The Hercules-Lyra Association revisited rochronology (Barnes 2007, 2009; Mamajek 2009) in Sect. 5.2as a tool to investigate the age of the membership candidates. Inaddition we analyze the chromospheric activity index ( R ′ HK ) inSect. 5.3 and the coronal activity (X-ray luminosity) in Sect. 5.4.Another open question is the rather arbitrary adoption of theCastor velocity dispersion for the kinematic criterion given byL´opez-Santiago et al. (2006). Interestingly the dispersion of theirfinal sample is lower.To define the properties and the list of members a new ap-proach is followed in Sect. 6 in order to avoid any assumptionsor selection e ff ects, starting from the concentration of rapid ro-tators in the volume-complete work of Fuhrmann (2004). Theyouth indicators of these stars will be studied as a whole, i.e., thedistribution in lithium equivalent width and (gyrochronological)age. If a young Her-Lyr association exists as its own entity, agroup of young stars will stand out in all of the properties stud-ied, even if intrinsic variations might be present. It is assumedthat this group of stars is composed of the canonical memberswhich define the overall properties of the Her-Lyr association.In Sect. 7 individual interesting stars among the Her-Lyr candi-dates are addressed with emphasis given on multiplicity.Based on this information we present and discuss an updatedlist of Her-Lyr members and non-members in Sect. 8. We eval-uate the mass function of the membership candidates before weconclude in Sect. 9.
2. Definition of the Sample
We started with the Her-Lyr candidates defined by Fuhrmann(2004), with the four stars of the Hercules moving group (Gaidos1998) already included. We added new candidates introduced byL´opez-Santiago et al. (2006) and Fuhrmann (2008). We neverremoved a candidate beforehand, even if it was later rejected forsome reason. Also we did not add candidates, which had beenpreviously excluded within the same study in which they whereconsidered as candidates for the first time. In particular we didnot include objects, classified as members of the LA if they hadnever been taken into account as Her-Lyr candidates before. Ourmultiplicity study, which is described in detail in the followingsection, increased the number of candidates again, but did not in-crease the number of stellar systems. Most of these companionshad been discovered previously. An intense study of the litera-ture brought up additional candidates, including spectroscopicand sub-stellar companions.
3. Multiplicity
Our multiplicity study focuses on archival work and the searchfor common proper motion pairs. Using archived surveys alongwith state-of-the-art follow-up observations enables the combi-nation of di ff erent data sources (Caballero et al. 2010; Alonso-Floriano et al. 2011), o ff ers a way for accurate indirect deter-minations of quantities like parallax (Gould & Chanam´e 2004)or the evolutionary status (Makarov et al. 2008), and provides astarting point for extended surveys (like the Palomar / Keck sur-vey of Metchev & Hillenbrand 2009), as well as extreme findings(like the wide ultra-cool binary by Caballero 2007).Within our ongoing multiplicity study and based on theFuhrmann (2004) and L´opez-Santiago et al. (2006) sample weinitiated an archival search for faint companions. Given thatmost of the Her-Lyr candidates are within 25 pc of the Sunand hence bright, we can use archival data, for example, theTwo Micron All Sky Survey (2MASS) (Cutri et al. 2003). Furthermore, we used Schmidt plates from the Palomar and ESOsky surveys, scanned by the SuperCOSMOS machine (Hamblyet al. 1998), providing best available astrometric accuracy.We found co-moving objects for six nearby stars: V538 Aur,DX Leo, HH Leo, HIP 63317, LTT14623, and V382Ser. Eachone of the companions except for one were known already, so wepublished the new one in Eisenbeiss et al. (2007). This sectiongives an overview of the multiple systems among the Her-Lyrstars included in this ongoing study.The analysis follows the procedures outlined in Eisenbeisset al. (2007). Taking the scanned Schmidt plates as first epochand 2MASS as the last, we identify wide stellar binaries amongHer-Lyr members as common proper motion pairs. Calibrationand uncertainties are derived in a statistical way using the (non-moving) background stars in the stellar field observed. If theHer-Lyr star is saturated in the Schmidt plates, the di ff ractionspikes are used to determine an accurate position measurement.Because of the large epoch di ff erences, the proper motions de-rived have uncertainties of only ∼ / yr. Considering thelarge proper motion of the nearby Her-Lyr stars and possiblecompanions, this uncertainty corresponds to a relative error ofa few percentage points. Most of the companions we found werealready known before but were not taken into consideration byformer studies of the Her-Lyr association.For some stars we took additional data to look for closerand fainter companions. The search for common proper mo-tion pairs in the archives is supplemented by own new observa-tions. We observed with either VLT / NaCo (Lenzen et al. 2003;Rousset et al. 2003) or Calar Alto 3.5m / ALFA (or Omega-Cass,Lenzen et al. 1998; Kasper et al. 2000) isolated young nearbystars (with an age of up to some 100 Myr and a distance of up tosome 100 pc) in order to detect possible sub-stellar companions.Naos-Conica is the adaptive optics imager and spectrographinstalled at the Nasmyth B focus of UT4 (Yepun) at ESO’sParanal observatory in Chile. The adaptive optics Naos containsa tip-tilt mirror, as well as a deformable mirror with 185 actu-ators. The infrared Conica camera is equipped with an Aladdin3 InSb infrared detector (1024 × × µ m). The Naco data presented here (Table 1), were takenwith Naos’s visible wavefront sensor (VIS) using its 14 × / pixel).Omega-Cass is an infrared camera, which was operated atthe Cassegrain focus of the 3.5 m telescope at the Calar Altoobservatory in Spain. The camera is equipped with a RockwellHAWAII HgCdTe infrared detector (1024 × . × . µ m) operated in the non-destructive readoutmode. Omega-Cass could be used either as seeing limited in-frared imager or together (as done here) with the adaptive opticsAlfa for di ff raction limited imaging at the CAHA / = HD 37394), DX Leo ( = HD 82443), andV382 Ser ( = HD 141272) and their companions (indicated bythe letter B). The images were obtained using the Calar Alto3 . Ω -Cass (Lenzenet al. 1998). Two more objects are discussed in detail in the fol-lowing subsections. These data were reduced using ESO Eclipse
3. Eisenbeiss et al.: The Hercules-Lyra Association revisited(a) V538 Aur = HD 37394 and LHS 1775 (b) DX Leo = HD 82443 and GJ 354.1 B (c) V382 Ser = HD 141272 and HD 141272 B
Fig. 2.
Three Her-Lyr candidates (labeled ”A”) and their visual companions (labeled ”B”), observed with the Calar-Alto 3 . Ω -Cass. V382 Ser was already published in Eisenbeiss et al. (2007). Table 1.
Observations log
Target MJD Tel. / Inst. Filt. scale det. ori.[days] [mas / pix] [ ◦ ]V538 Aur 53800 CAHA 3.5 m /Ω -Cass H 192 ± .
43 358 . ± . /Ω -Cass H 192 ± .
43 358 . ± . /Ω -Cass H 192 ± .
43 358 . ± . / ALFA H 77 . ± .
05 18 . ± . / ALFA H 77 . ± .
05 18 . ± . / NaCo K s . ± .
02 359 . ± . / NaCo K s . ± .
05 359 . ± . Table 2.
Data of known binary pairs taken with Ω -Cass at the Calar–Alto 3 . Name Companion JD separation position angle[days] [”] [ ◦ ]V538 Aur LHS 1775 2453800 98.04 ± ± ± ± ± ± ± ± Notes.
Position angle measured from north over east. (Devillard 2001) and calibrated using background stars in thefield (not visible in Fig. 2 because of the cut levels and the smallsection of the images shown), listed in the 2MASS catalog. Inthe worst case, we had five reference stars in the field, still suf-ficient if field distortion, skew, and other non-linear e ff ects areneglected. The reader is referred to Eisenbeiss et al. (2007) forfurther details. We give the observations log in Table 1. In Table2 we give the separation and the position angle with respect tothe primaries.The binary status of V382 Ser was already published inEisenbeiss et al. (2007). Because of saturation e ff ects the binarywas barely resolved on Schmidt-plates. We succeeded using theSource extractor (Bertin & Arnouts 1996) and confirmed our re-sults with follow-up observations.Furthermore, a multiplicity study of nearby stars has beencarried out by Raghavan et al. (2010). Nevertheless, the Her-Lyr association was not addressed in particular. Combining theresults with the present work, the multiplicity census of the Her-Lyr candidates within 25 pc should be complete (see Raghavanet al. 2010, Sect. 2.2 and 5.2 for details) . Raghavan et al. (2010) combine a large variety of datasets, includ-ing data from high angular resolution imaging and high precision spec-troscopy. They detected all companions into the planetary regime at allrealistic separations.
NE10 arcsecA B
Fig. 3.
HN Peg A & B imaged at the 3 . Ω -Cass and ALFA. HN Peg B is almost outside theFoV of the detector, and so it was only imaged at some dither positions. The HN Peg system was observed on 2002 December 23 in theH-band at the 3.5 m telescope of the Calar Alto observatory inSpain using the adaptive optics system ALFA (Kasper et al.2000) in combination with the near-IR imager Ω -Cass (Table 1).The bright primary HN Peg A served as natural AO guide star.We used the shortest possible individual integration time of Ω -Cass (0.842 s) to suppress saturation e ff ects of the IR-detectordue to the bright primary star in the observed field of view.Forty-nine of these short exposures were averaged to one im-age and 24 of these frames were then taken at di ff erent telescopepositions in jitter-mode for proper sky-background subtraction,which yields 16.5 min of total integration time on target. For flat-fielding, skyflats were taken in the evening and morning twi-lights. All frames were background-subtracted, flatfielded, andaveraged to the image shown in Fig. 3, using the data reductionpackage Eclipse (Devillard 2001). The faint sub-stellar compan-ion HN Peg B is detected at the western border of our ALFAimage. The same object was detected by Luhman et al. (2007)in 2006 June, so our 2002 image is a 2nd pre-discovery after2MASS. Relative astrometry, based on the calibration publishedin Mugrauer et al. (2004), is summarized in Table 2.
4. Eisenbeiss et al.: The Hercules-Lyra Association revisited
Fig. 4.
The HH Leo ( = HD 96064) system from 2002 (Calar Alto Ω -Cass with ALFA, inset left), through 2005 (inset middle), to 2008 (insetright and main image, both VLT / NaCo). Orbital motion of the BC com-ponent can clearly be seen. For further explanations see text.
Table 3.
Separation and position angle of HH Leo B & C from 1989 to2008.
Epoch separation position angle Reference[”] [ ◦ ]1989 Nov 10 0.342 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Notes. *: original values corrected by ± ◦ for consistencyData are obtained via speckle interferometry (until 1999) and directimaging. We observed HH Leo (HD 96064) in 2002 December 23 with Ω -Cass and the adaptive optics system ALFA on Calar Alto. Weused the shortest possible DIT of 0.842 s and 49 integrationswere averaged to one frame per jitter position. We took imagesat 20 jitter positions, i.e., total integration times of 13.8 min. Thedata were reduced in a standard way and calibrated using thecalibration provided in Mugrauer et al. (2004). We resolved thebinary companion ∼ ′′ to the southwest of HH Leo (Fig. 4).Both, B and C, are listed in SIMBAD as objects LTT 4076 andBD-033040C, respectively. It is an equal mass binary orbitingHH Leo A. However, the objects are typically not resolved inmost photometric catalogs. Since the information found for thissystem is often part of a compilation of huge data sets, it is un-likely that the multiplicity of tight systems was treated correctly.In that sense, the magnitudes and colors given for both objectsin the literature have to be used with care.Within our direct imaging campaign of sub-stellar compan-ions, we observed HH Leo again with VLT / NaCo (Lenzen et al.2003; Rousset et al. 2003) in 2005 and 2008. The observed targetwas also used for the wavefront sensing. The observations wereperformed in the jitter mode with the shortest possible detec-tor integration time (DIT) of 0.347 s, and 200 such integrationswere always averaged to one frame per jitter position. In the 1st epoch 27 frames, and in the 2nd epoch 15 frames are taken atdi ff erent jitter positions, which results in total integration timesof 31.2 min and 17.4 min, respectively. We calibrated the NaCodata with the well-known binaries AB Dor and HIP 73357, re-spectively. Data reduction was done with ESO Eclipse . The re-sulting separation and position angle of the equal-mass binarysystem is shown in Table 3 and orbital motion is visible in Fig. 4.Realizing that we could determine the orbit more precisely, wesearched the literature and found six speckle interferometry mea-surements from 1989 to 1999, see Hartkopf et al. (1993, 2000),Horch et al. (2002), and Table 3. Together with our observations,this is almost a full orbit which is suspected to be ∼
23 years.Because of the similar brightness of both objects, the B and Ccomponents were not commonly defined in the literature. Wetherefore adopt a correction to the position angle of 180 ◦ wherenecessary.Using this dataset of nine observations (18 constraints: nineseparations and nine position angles), we fit the orbit of HH LeoB & C (see Fig. 5) by using an error weighted χ minimizationusing the SciPy package (Jones et al. 2001). The χ is calculatedas χ = X i (O i − C i ) σ , (1)where O i are the measured values, C i the model values, and σ i the measurement errors.The distance of the system was fixed to 26 .
27 pc, taken fromvan Leeuwen (2007). Although HH Leo is a hierarchical triplestar, the influence of the A component can be neglected andHH Leo B & C can be treated as a binary system. The achievedorbital parameters of the HH Leo B & C system are listed inTable 4 together with their 1 σ - and 3 σ -uncertainties which aredetermined by varying the fit parameters (see Press 2007, forfurther details).With seven orbital parameters to fit, there are ten degrees offreedom in total. The achieved reduced χ of the best orbital fit is22 .
63 which is quite large and also obvious in view of the largeresiduals of the separation and position angle measurements (seeFig. 5). Assuming that no systematic e ff ects a ff ected the mea-surements, there are two possible explanations for residuals thatlarge.First, the errors of the speckle interferometry measurementscould be underestimated because Hartkopf et al. (1993, 2000)published no more than the typical errors for each telescope ofthe observation program (see Hartkopf et al. 2000, Table 3), butnot the measurement errors for a specific target system itself.Horch et al. (2002) also published average errors for their ob-servation campaign. The second possibility is the presence ofan unseen astrometric companion in the HH Leo B & C system.However, attempts to fit a model of a binary plus an astrometriccompanion around one of the stellar components did not result insmaller residuals. In fact, the χ increases, especially becauseof the reduced degree of freedom. Hence, the existence of anunseen astrometric companion could neither be confirmed norruled out with the present data of nine observations. Additionalobservations are needed. The values and errors in Table 4 arebased on the assumption that the real system can be describedby a binary model.In the 2MASS catalog, K s magnitudes of 5 . ± .
021 magand 6 . ± .
016 mag are given for the A and the unresolved BCcomponent, respectively. We measured the instrumental magni-tude of the A component in our NaCo images to determine thezero point with an aperture radius of 27 pixels. Then, we mea-
5. Eisenbeiss et al.: The Hercules-Lyra Association revisited X app [as] (E ↔W) Y a pp [ a s ] ( S ↔ N ) P AΩ P A [ d e g ] Time [MJD] R e s i d u a l [ d e g ] S e p [ a s ] Time [MJD] R e s i d u a l [ m a s ] Fig. 5.
Orbital solution for the HH Leo B & C binary system obtained by a χ minimization. The separation (in arcsec) and position anglemeasurements as well as their residuals to the binary model are displayed on the right side. Table 4.
Parameters of the binary system HD 96064 B & C calculatedby a χ minimization.Parameter 1 σ − uncertainty 3 σ − uncertainty ε . + . − . . + . − . ω [ ◦ ] 292 . + . − . . + . − . M tot [ M ⊙ ] 1 . + . − . . + . − . P [d] 8410 . + . − . . + . − . T [MJD] 49843 . + . − . . + . − . d [pc] 26 . + . − . . + . − . Ω [ ◦ ] 71 . + . − . . + . − . i [ ◦ ] 62 . + . − . . + . − . Notes.
The listed uncertainties are determined according to Press(2007). sured the B and C component individually with a smaller aper-ture of 18 pixels. In addition, both B and C were measured withina single, larger aperture of 55 pixels.The apparent magnitudes of B, C, and the combined mag-nitude of BC are derived by aperture photometry relative to thewell-known A component. Using the parallax of A, we get theabsolute magnitude M K s . This measurement indicates that both objects are nearly similar in brightness and may also have a sim-ilar spectral type. Although this assumption cannot be provendirectly, we can now compare our dynamically determined massto evolutionary models. As in Sect. 4, we use the models byBara ff e et al. (1998) and Yi et al. (2001) for this comparison.Additional photometric information is taken from the 2MASScatalog. We utilized our measurement of the absolute K s magni-tude and the 2MASS J − K color and ran a Monte Carlo simu-lation of 1 million runs for each model to find the most probablemass and age. The result is shown in Fig. 6. For comparison, thetotal dynamical mass divided by two and the gyrochronologicalage (see Sect. 5.2) is shown as a data point. Figure 6 shows thatboth models estimate the mass of the object to be approximately0 . M ⊙ which is below but approximately in agreement with halfthe dynamical total mass of the system. The maximum probabil-ity isochronic age however is 40-50 Myr in disagreement withthe gyrochronological age of ≈
300 Myr. However, the age isnot very well constrained by the models.The photometric measurements are given along with themodel based spectral types and masses (Bara ff e et al. 1998) inTable 5. It turns out that the total mass of B + C is higher than themass of component A.
After collecting all candidates from the literature and perform-ing the multiplicity study, as many as 48 visually resolved Her-
6. Eisenbeiss et al.: The Hercules-Lyra Association revisited
Log(Age) [yr] M a ss [ M S un ] Model : BACH98AGE : 50.00 MyrMASS : 0.57 MsunPROP : 0.077
BACH98Orbit fit 0.010.020.030.040.050.060.077 7.5 8 8.5 9 9.5 100.10.20.30.40.50.60.70.8
Log(Age) [yr] M a ss [ M S un ] Model : Yi Y Fig. 6.
Surface plot of the results of a Monte-Carlo simulation of a com-parison of the photometric measurements ( M K s from NaCo and J − H from 2MASS) and evolutionary models by Bara ff e et al. (1998) and Yiet al. (2001). Large probabilities are shown in dark colors and low prob-abilities are shown in light colors. The color bar on the right show theencoded probability. For the highest probability, the age and the massare given in the corresponding subfigures. The datapoint shows the dy-namical mass of HH Leo B + C, divided by 2, and the gyrochronologicalage of HH Leo for comparison.
Table 5. K s band photometry of HH Leo A & BC as extracted out ofour VLT / NaCo images using relative aperture photometry.comp. K s M K s SpT Massmag mag M ⊙ A 5 . ± .
021 3 . ± .
193 G5 0.92BC 6 . ± . ≈ .
2B 7 . ± .
022 5 . ± .
194 K7 - M0 ≈ .
6C 7 . ± .
022 5 . ± .
194 K7 - M0 ≈ . Notes.
The 2MASS catalog serves as a calibration source. The massesare photometric (Bara ff e et al. 1998). Lyr candidates are taken into account in this analysis. In addi-tion, there are three brown dwarf companions, one for HN Peg(Luhman et al. 2007) and a brown dwarf binary companion toMN UMa (Bouy et al. 2003). These brown dwarfs are listed inTables 9 and 10 but are not counted separately. Furthermore,three membership candidates (BC Ari, HD 54371, and PX Vir)are known spectroscopic binaries. The complete census of Her-Lyr candidates amounts to 54 individual objects, distributed over35 stellar systems. This work is aimed at the 48 stellar objectsand does not discuss the spectroscopic binaries in detail. Formore information on individual objects, please refer to Sect. 7and Tables 9 and 10.
4. Photometry
Since most of the Her-Lyr members and even the late type mem-bers are su ffi ciently bright B − V and V − H data are availablefrom the HIPPARCOS (Perryman et al. 1997) and the 2MASScatalog (Cutri et al. 2003). These data can be compared withevolutionary models. The late-type companions may contribute important information to a photometric age estimation, based onevolutionary models since late-type stars arrive later on the mainsequence than early-type stars. We have chosen the models byBara ff e et al. (1998), Yi et al. (2001) (Fig. 7), and VandenBerget al. (2006) (Fig. 8). Giving a good representation of the mainsequence in the regime of solar-type stars, the models disagreefor late-type stars.Examining Fig. 7 (a) gives very limited insight into the age ofthe association. Most membership candidates are located on thezero age main sequence (ZAMS), the late end being at spectraltype K7 to M0. So from Fig. 7 (a) alone, it can be concluded thatthe Her-Lyr association is &
40 Myr old.Figure 7 (b) on the other hand shows the M dwarfs of thesample. Most of them are companions of brighter Her-Lyr stars.All stars except two down to M1 are located on or near theZAMS of the Bara ff e isochrones. The five mid- to late-M starsof the sample are only visible in Fig. 7 (b). Only the Bara ff e et al.(1998) isochrones are extended to their mass range, implying ayoung age for all five.Figure 8 shows the model grid of VandenBerg et al. (2006)intended to fit the ZAMS and the evolution of stars for a widerange of masses and ages. The isochrones of these models in-deed coincide with most of the Her-Lyr candidates. For this fig-ure, we calculated the bolometric magnitude using the correc-tions given in Kenyon & Hartmann (1995). The bolometric cor-rection is given in Table 10. Figure 8 indicates that most of theHer-Lyr candidates are coeval down to 0 . M ⊙ . They are locatedabove the isochrone of 0 . – It is not clear if any of the models predict correct colors forM stars. If they are correct, Fig. 7 (b) shows significant agespread among the M-type candidates. – EE Leo is a poorly known isolated M dwarf. No spectro-scopic measurements were found for this star to confirm ordisprove that this is indeed a young object. Gl 354.1 B isthe companion of DX Leo which was thought to be youngerthan the average of Her-Lyr stars because of its strong activ-ity (L´opez-Santiago et al. 2006). The stars NLTT 56532 andNLTT 56725 are companions to V368 Cep. There are manyindicators that this star (and its companions) is very young(12-50 Myr) and, therefore, a doubtful member of the Her-Lyr association. It cannot be excluded that the selection ofM-type objects for this study was composed almost entirelyof non-members. – As presented in Sect. 3, V538 Aur and LHS 1775 forma binary. Compared with the Yi et al. (2001) isochrones,LHS 1775 is located exactly on the 40 Myr isochrone.However, as seen from Table 9, the system seems to be olderthan the Her-Lyr association implying an inconsistency be-tween evolutionary models and other age indicators. – The A7 VpSrCrEu star α Cir, visible in Fig. 7 (a), is eithervery young ( <
10 Myr) or very old ( ∼ α Cir is located exactly on the1 Gyr isochrone suggesting that this object might be olderthan Her-Lyr but not a member. The absence of isochronesyounger than 0 . α Circannot be ruled out. If it is not a member of the Her-Lyr as-sociation, its companion LTT 5826 is also not a member.
7. Eisenbeiss et al.: The Hercules-Lyra Association revisited −0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4−2−1012345678 M V [ m ag ] B−V [mag]
Yi Isochrones for [Fe/H]= 0.0463, age in Gyr.
V538 AurDX Leoalpha Cir V368 Cep (a) Y isochrones for solar type stars M V [ m ag ] V−H [mag]
Baraffe isochrones, age in Gyr
LHS 1775 GJ 354.1 BEE LeoNLTT 56532 NLTT 56725 (b) Bara ff e isochrones for brown dwarfs Fig. 7. (a) The FGK-type Her-Lyr candidates compared with model isochrones by Yi et al. (2001), the Y isochrones for solar mixture ( Z = . Y = . OS = . l / H p = . / H] = . α/ Fe] = . M V is displayed vs. B − V which is a well-calibrated temperatureindicator for FGK-type stars. (b) The M-type Her-Lyr candidates compared with isochrones by Bara ff e et al. (1998), optimized for late-type starsand brown dwarfs. The V − H color index is used for the late-type stars.The photometry is taken from the HIPPARCOS catalog (Perryman et al. 1997), 2MASS (Cutri et al. 2003), and other catalogs if HIPPARCOS datawere not available (Ochsenbein et al. 2000). The full list of references is given below Table 9. According to Table 11, stars denote probable Her-Lyrmembers, circles denote uncertain members, and squares denote non-members. In the case of stars shown without error bars, uncertainties werenot given in the corresponding catalog. For conversion from apparent to absolute magnitude, we used the revised parallaxes from van Leeuwen(2007). For faint companions, we adopted the parallax of the primary. −0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6−4−20246810 Victoria Regina isochrones for [Fe/H] = 0.0, age in Gyr
B−V [mag] M bo l [ m ag ] alpha Cir Fig. 8.
Victoria Regina isochrones based on evolutionary models re-ported in VandenBerg et al. (2006). The models include α enhancementand core overshooting. Stellar mass ranges from 0 . . M ⊙ and stel-lar age from 0 . . In fact, evolutionary models cannot be used to rule out non-members or to estimate the association’s age. This is partlycaused by measurement inaccuracies and the density of theisochrones near the ZAMS, and partly by the fact that a validrepresentation of equally aged stars from late M-dwarfs to Astars is still di ffi cult to determine from stellar evolution theory.
5. Youth indicators
The most obvious evidence of a young Her-Lyr association isthe concentration of late-type fast rotators in kinematic space.Given the volume-completeness by Fuhrmann (2004) and the ab-sence of any pre-selection, this is an unbiased view. Therefore,this concentration in kinematic space provides a solid startingpoint to critically review Her-Lyr membership. In this work, all kinematic candidates (Gaidos 1998; Fuhrmann 2004; L´opez-Santiago et al. 2006; Fuhrmann 2008) are revisited without anypre-selection based on lithium, activity, or photometry. The strat-egy is to study the distribution of these properties of all kine-matic candidates. A population of young stars will stand outclearly in this distribution. In particular, this is expected for therotational velocities since these gave rise to the notion that theconcentration in kinematic space is a group of relatively youngstars (Fuhrmann 2004).
Figure 9 compares the Li equivalent widths of all candidatesto the distribution of the Pleiades, UMa, and the Hyades. Theupper envelope of the Pleiades is taken from Neuh¨auser et al.(1997) while the lower envelope is obtained from Soderblomet al. (1993). The UMa data are adopted from Ammler-von Ei ff & Guenther (2009) providing a homogeneous analysis. TheHyades region originates from Soderblom et al. (1990). Thecolors given by Soderblom et al. (1993) and Soderblom et al.(1990) are converted to e ff ective temperatures using the scalesof Bessell (1979) and Bessell (1991). The comparison is limitedto e ff ective temperatures above ≈ ff erent from the Pleiades and the UMa group. Theage of Her-Lyr without the Li-depleted candidates is thereforesimilar to UMa, or between Pleiades and UMa and younger thanthe Hyades.
8. Eisenbeiss et al.: The Hercules-Lyra Association revisited T eff [K]050100150200250300 L i I λ . Å e qu i v a l e n t w i d t h [ m Å ] period < 6.0 d 6.0 < period < 10.0 dperiod > 10.0 d Fig. 9.
Lithium equivalent widths of Her-Lyr candidates compiled inthis work (circles) compared to the areas populated by the Pleiades(gray, Neuh¨auser et al. 1997; Soderblom et al. 1993) and Hyades (darkgray, Soderblom et al. 1990). UMa group members (Ammler-von Ei ff & Guenther 2009) are indicated by squares. E ff ective temperatures andlithium equivalent widths of Her-Lyr candidates are taken from Table 9.Error bars have been omitted for clarity . The symbol sizes of Her-Lyrstars scale with rotational period as is indicated in the figure. Symbolsdenote final membership according to Table 11. Filled circles identifymembers ( + ) while doubtful (o) or possible members (?) are highlightedby open circles. Crossed circles denote rejected (-) candidates. Table 6.
Parameters for Eq. 2 as estimated by various authors.Barnes (2007) Barnes (2009) Mamajek (2009) n . ± .
007 0 . ± .
007 0 . ± . a . ± .
011 0 . ± . . ± . b . ± .
024 0 . ± .
052 0 . ± . c . . ± .
027 0 . ± , B − V ) > c > c . . . . . Notes.
The last row gives the applicable range of ( B − V ). It is well known that stellar rotation slows down as a star be-comes older. However, this e ff ect varies with stellar mass andradius as well as with starting conditions, accretion history, anddisk life time. Therefore, it is possible to roughly estimate theage of a single main sequence star from its rotational period bytaking into account its spectral type or its B − V color.An empirical relation was found by Barnes (2007) and wasimproved by Mamajek & Hillenbrand (2008) and Mamajek(2009) to belog t = n (cid:0) log P − log a − b log( B − V − c ) (cid:1) (2)with t being the age in millions of years and P the rotationalperiod in days.The sets of parameters for Eq. 2 are summarized in Table 6.In the analysis described in the following, we use either the morerecent values of Mamajek (2009), if possible, or the values ofBarnes (2009) if B − V is outside the applicable range but stilllarger than c . If ( B − V ) < .
472 but still larger than 0.4, we useBarnes (2007). Uncertainties in all cases are calculated followingBarnes (2009). It should also be mentioned that this method is
Age [Myr] N u m be r o f s t a r s log(Age [Myr]) N u m be r o f s t a r s Fig. 10.
Age distribution of Her-Lyr candidates derived from the rota-tional period (black). In the gray data, rotational periods assessed from v sin i measurements are included. From these measurements, a lowerlimit of the rotational speed is derived, leading to an upper limit of thegyrochronological age. In the top panel, the age is displayed on a lin-ear scale, emphasizing the existence of a young concentration of 11stars (18 including upper limits). In the lower panel, the age is plottedlogarithmically to illustrate that the concentration of young stars con-solidates with six (eight) stars with similar age. The binning of the his-tograms employs the Freedman-Diaconis rule (Freedman & Diaconis1981). not applicable to mid- and late-M dwarfs as these form a fullyconvective envelope leading to a di ff erent rotational behavior.In principle, the rotational period can be derived in two ways.The direct way is to observe a photometric light curve. If peri-odic brightness variations caused by star spots are observed, thephotometric periodicity is a direct measure of the rotational pe-riod. A possible complication might be the observation of mul-tiples of the rotational period if several spots are located at unfa-vorable stellar longitudes.On the other hand, the broadening of su ffi ciently resolvedspectral lines gives a measure of the rotational velocity, but mul-tiplied by the sine of the inclination. The rotational period then is P / sin i = π R / v sin i with the stellar radius R . This introduces anadditional uncertainty. We use the photometric period, if avail-able, or the spectroscopic period. In cases where both are avail-able, we confirmed that the di ff erences are generally small.Photometric rotational periods are available for 16 Her-Lyr candidates with spectral types F, G, and K (see Table 9).Figure 10 shows the distribution of the ages derived for these 16stars in linear and logarithmic scale. It can be noticed that there isa pronounced peak of stars at ∼
250 Myr which we identify withthe concentration of rapidly rotating young stars in the UV dia-gram noticed by Fuhrmann (2004). This peak is significant at the2 σ -level in the linear plot and significant in the logarithmic plot(Fig. 10 and Table 7). While older intruders can be clearly sepa-rated from the Her-Lyr association, there are four to six youngerstars whose membership cannot be conclusively determined.Measurements of v sin i were found for 13 additional stars.These measurements give an upper limit to the gyrochronolog-ical age. The full sample of 29 stars with gyrochronologicalage estimates is shown in gray in Fig. 10. Including these ad-ditional members, the Her-Lyr association becomes even morepronounced.
9. Eisenbeiss et al.: The Hercules-Lyra Association revisited
Table 7.
Histogram statistics.number 1 σ σ confidence max.of stars per bin at 0.05 level bin linear only phot. 3 - 7 1 - 9 2 - 8 11all 3 - 6 1 - 9 2 - 8 18 logarithmic only phot. 2 - 5 1 - 7 1 - 6 6all 3 -6 1 - 9 2 - 8 8 Notes.
The table corresponds to Fig. 10. The number of stars per bin,predicted for a uniform distribution, is given for the upper gray data(linear age, including upper limit data from v sin i measurements), theupper black data (without data obtained from v sin i ), lower gray data(same as upper gray data for logarithmic age), and lower black data(without v sin i data). The 0 . Most of the measurements indicate an association with anage of approximately250 Myr while it is not clear whether thelarge spread is an intrinsic property of the association or whetherit reflects the uncertainty of the gyrochronology. In addition,the sample is contaminated by old intruders. There are five starsthat clearly show longer rotational periods and that are signifi-cantly older. From Table 9 it can be determined that these are thestars that also have very small lithium equivalent widths or areeven lithium-depleted. While V538 Aur and MN UMa show Liabundance and rotation speed similar to the Hyades, V382 Ser,39 Tau, and LW Com might be ordinary field stars, sharing thekinematics of Her-Lyr by chance. The individual ages of stars inTable 9 should be considered with care.
Since the late-type Her-Lyr members are young and ro-tate rapidly, measurable chromospheric activity is expected.Chromospheric activity is commonly assessed from emission inthe cores of the Ca II H&K lines. The R ′ HK index is corrected forphotospheric flux contributions and allows one to compare thelevel of activity independent of spectral type.Figure 11 displays R ′ HK measurements of Her-Lyr candidatestaken from the literature (Table 9) and compares to the activitylevels of the Pleiades, the UMa group, and the Hyades (Mamajek& Hillenbrand 2008).The distinction of the activity levels of the Pleiades, UMa,and the Hyades is not very clear as the distributions of R ′ HK strongly overlap. Therefore, linear fits were obtained to get themean level of activity. The standard deviation of the fits qualita-tively indicates the relative width of the distributions. Individualdata points are only shown for the Her-Lyr stars which can becompared to the average level of the Her-Lyr members accord-ing to Table 11 (gray filled area). The di ff erence of the averagerelations is less than the total widths of the distributions. Fig. 11shows that for each association, the average levels of activity in-crease with color and are higher with younger average age ofthe respective association. Furthermore, the width of the distri-butions decrease with the average age of the respective associa-tion.The Her-Lyr relation is in between the Pleiades and the UMarelations regarding both the average activity level and the width (B−V) −5.5−5.0−4.5−4.0−3.5 R ’ HK period < 6.0 d 6.0d < period < 10.0 dperiod > 10.0 d Fig. 11.
The activity level measured for Her-Lyr candidates is shownvs. color. The symbols exhibit individual measurements taken from theliterature (Table 9). The symbols have the same meaning as in Fig. 9.The filled areas represent mean levels derived from the distributions ofthe Pleiades (horizontally hashed), Her-Lyr (gray), UMa (hashed), andthe Hyades (vertically hashed). The widths are due to the uncertainty ofthe coe ffi cients of the linear fits. supporting a young age of the Her-Lyr association between theage of Pleiades and UMa.Among the kinematical Her-Lyr candidates, stars are identi-fied above which display high levels of Li absorption and shortrotation periods (Fig. 9) unlike another group of purportedlyolder stars. The distinction is not as clear in terms of chromo-spheric activity. The older and younger objects widely overlap.This finding does not weaken the evidence of the two di ff erentgroups identified since the activity distributions of the Pleiades,the Hyades, and the UMa group strongly overlap as well.We note, however, that the least active Her-Lyr candidates(HD 4915 and HD 207129) show activity levels similar to theless active Hyades members and are not among the rapid Li-richrotators. these rotators are located above the mean Hyades rela-tion, except for HD 166 which also displays the least Li equiva-lent width among the gyrochronologicalally young and Li-richstars.Similar to the gyrochronology, the chromospheric activityindex can be used in a logarithmic fit to derive an age – activityrelation for main sequence stars. Mamajek & Hillenbrand (2008)derivedlog t = − . − .
912 log R ′ HK − . R ′ HK ) , (3)where t is the age in years and R ′ HK is the chromospheric activityindex. The equation is appropriate between − . > R ′ HK > − . Young, late-type stars often show X-ray emission connected tocoronal activity, making the X-ray luminosity function a tool forage investigation. We convert count rate R and hardness ratiofrom the ROSAT All-Sky Bright Source Catalog (Voges et al.
10. Eisenbeiss et al.: The Hercules-Lyra Association revisited
Age [Myr] N u m be r o f s t a r s log(Age [Myr]) N u m be r o f s t a r s Fig. 12.
The histogram of age derived from chromospheric activity isshown in linear and logarithmic scale as in to Fig. 10. The distributionshows that the majority of Her-Lyr candidates with measured activitylevel is younger than 500 Myr with the logarithmic plot showing a con-centration around 250 Myr. The two older stars identified in the linearplot are HD 207129 and G 270-82. The youngest star in the logarithmicplot is DX Leo.
Second ROSAT PSPC Catalog (Rosat Consortium2000), if available, to X-ray fluxes for the Her-Lyr members andcandidates and for the Ursa Major stars. The energy conversionfactor of ROSAT is calculated byECF = (5 . · HR1 + . · − ergcm · counts , (4)(Schmitt et al. 1995) with the hardness ratio HR1. We obtainedupper limits for undetected Her-Lyr and UMa stars from the ROSAT All Sky Survey (RASS) data using
XIMAGE . Upper lim-its on R are converted to upper limits on luminosity L X by amedian ECF of 6 × − erg · cm − · counts − . The X-ray fluxeswere transformed into luminosities via the stellar distances takenfrom van Leeuwen (2007). For many stars in our sample, thiswas already done by other authors (e.g. Pizzolato et al. 2003;L´opez-Santiago et al. 2009). We repeated the analysis for con-sistency. In addition, we took X-ray luminosities and upper lim-its for the Pleiades and Hyades from the WEBDA database(Mermilliod 1996). We calculated the Kaplan-Meier estimator(Meier 1958) for right-censored data following the procedureoutlined in Feigelson & Nelson (1985) to create an empiricalcumulative distribution function. The result of this calculation isshown in Fig. 13.From Fig. 13 we can conclude that the Her-Lyr associationis younger than the Hyades. Apparently, the X-ray luminosityis comparable to the Pleiades. The Ursa Major association in-cludes a few very active stars, statistically comparable to theHyades. On the low activity end, the Ursa Major associationappears more Pleiades like. The Ursa Major group is thereforeyounger than the Hyades and older than the Pleiades and (as fol-lows from the comparison of X-ray luminosity alone) also olderthan Her-Lyr.Then, we also estimated the L X over L bol luminosity ratio forthose of our objects for which distance and X-ray luminosity areknown. We estimated the bolometric luminosity as usual from V-band magnitude, bolometric correction (from spectral types fol- XIMAGE is provided by the HEASARC X-ray archive.
28 28.5 29 29.5 30 30.5 3100.10.20.30.40.50.60.70.80.91 log L X [erg/s] K ap l an − M e i e r E s t i m a t o r PleiadesHyadesHer−LyrUrsa Major
Fig. 13.
X-ray luminosity function of the Pleiades, Hyades, Her-Lyr(asterisks), and Ursa Major (squares). The logarithm of the luminos-ity measured by ROSAT is displayed with respect to the Kaplan-Meierestimator (Meier 1958; Feigelson & Nelson 1985) as an empirical cu-mulative distribution function of the X-ray luminosity. The X-ray dataof the Pleiades and the Hyades was taken from the WEBDA database(Mermilliod 1996). For the Ursa Major and Her-Lyr stars, data fromthe
ROSAT All-Sky Bright Source Catalog (Voges et al. 1999) and the
Second ROSAT PSPC Catalog (Rosat Consortium 2000) was used. UrsaMajor members were taken from the rather comprehensive study ofKing et al. (2003).
Table 8.
New X-ray upper limits for Her-Lyr and UMa stars.
Name log R log L X Name log R log L X [cts / s] [erg / s] [cts / s] [erg / s]Her-Lyr HD 87696 < − . < . < − . < .
338 HD 89025 < − . < . < − . < .
259 HD 95418 < − . < . < − . < .
511 HD 102070 < − . < . < − . < .
745 HD 106591 < − . < . < − . < .
478 HD 109799 < − . < . α Cir < − . < .
094 HD 111397 < − . < . < − . < . < − . < .
625 HD 112185 < − . < . < − . < .
324 GJ 519 < − . < . < − . < .
276 HD 129246 < − . < . < − . < .
662 HD 129798 < − . < . < − . < .
981 HD 141003 < − . < . < − . < .
937 HD 173950 < − . < . < − . < .
443 HD 216627 < − . < . Notes. σ upper limits on count rate and luminosity are given. lowing Kenyon & Hartmann 1995), and distance. We then plot-ted the ratio of L X to L bol vs. the B − V color in Fig. 14. Wecompare our objects with the Hyades and Pleiades as plotted inMaldonado et al. (2010, Fig. 8). Our Her-Lyr objects are locatedin between the Pleiades and Hyades and are quite similar to theLA objects studied in Maldonado et al. (2010). Hence, the ageof Her-Lyr is in between the ages of the Pleiades and the Hyadesin agreement with our other approaches presented here.
11. Eisenbeiss et al.: The Hercules-Lyra Association revisited ≤ Period < 10 dPeriod ≥
10 d PleiadesHyades
B−V [mag] l og ( L X / L bo l ) Fig. 14. B − V color index vs. the logarithmic ratio between X-ray andbolometric luminosity. Her-Lyr objects are plotted in the same wayas in Fig. 9. Then, envelopes to the distributions of the Pleiades andHyades are displayed according to Maldonado et al. (2010, Fig. 8).The figure shows that the age of Her-Lyr is in between the ages of thePleiades and the Hyades and similar to the LA. It can be concluded that the age indicators give results which arein line with earlier estimates (Fuhrmann 2004; L´opez-Santiagoet al. 2006).The seven stars (V439 And, EX Cet, EP Eri, DX Leo,HH Leo, PX Vir, HN Peg) showing similar rotational periods,similar lithium equivalent width, and comparable space velocitycan be used to define the properties of the Her-Lyr association.They are identified with the stars which make up the concen-tration of fast rotators in UV -space found by Fuhrmann (2004)and studied by L´opez-Santiago et al. (2006). They comprisethree of the five stars which initially defined the Hercules mov-ing group (Gaidos 1998): V439 And, DX Leo, and HN Peg. Theother two stars, MN UMa and NQ UMa, share the same spacemotion but have been discarded because they seem older thanthe other stars as is indicated by lithium equivalent width or gy-rochronological age. Some objects that were adopted canonicalmembers are doubted by Fuhrmann (2004): EP Eri, DX Leo,and PX Vir. However, the evidence collected in this section thatthese three objects belong to the canonical members which forma physically associated group of stars is very convincing.
6. Kinematics
Using the canonical list of seven stars we can now redefine theaverage
UVW velocity of the association. Space velocities aretaken from Montes et al. (2001). Figure 15 displays the VU planeof kinematic space. In addition to the Her-Lyr candidates, otherassociations are displayed. We restrict this evaluation to starswithin 25 pc to reach volume completeness. The space veloc-ity of Her-Lyr is similar to the LA so that there is controversyabout whether the Her-Lyr association does exist as an individ-ual entity or, rather if it is a part of the LA (Fuhrmann 2004;Maldonado et al. 2010). Taking the mean and the standard de-viation of the seven canonical members, the space velocity ofHer-Lyr is U = ( − . ± . − (5) −40 −30 −20 −10 0 10 20−40−35−30−25−20−15−10−50510 Her−Lyr Castor HS IC 2391 LA UMa
U [km/s] V [ k m / s ] Fig. 15.
The UV diagram shows the distribution of nearby (within25 pc) late-type stars in UV -space. The Her-Lyr candidates are shownas black circles as in Fig. 9. The data for Her-Lyr is taken from L´opez-Santiago et al. (2006). Downward triangles are Ursa Major (UMa) stars.The ellipses show the 3 σ boundary of the displayed clusters as cal-culated from the space velocities of individual members. UV data aretaken from Montes et al. (2001). In addition, the large black pluses showthe mean velocities of some moving groups. The location and the ellipseof the Her-Lyr association is calculated from the canonical members. V = ( − . ± .
59) km s − W = ( − . ± .
80) km s − . Based on the Fuhrmann (2004) members, L´opez-Santiagoet al. (2006) derive ( U , V ) = ( − . , − .
4) km s − . With theirrefined member list, they get ( U , V ) = ( − . , − .
64) km s − with a standard deviation of ( σ U , σ V ) = (2 . , .
61) km s − . Asthey note, this deviation is smaller than that of other coeval MGslike Ursa Major. The dispersion in W is σ W ≈ . − andcompares well to our value.Except for 39 Tau, all rejected candidates concentrate in aspecific region in the UV -diagram of Fig. 15. This could be atrace of an older stellar stream. The galactic V velocity of thisstream would then be in between that of Her-Lyr and IC 2391while the U velocity is similar to Her-Lyr.
7. Notes on individual stars
84 Cet = HD 16765:
The binary status of 84 Cet was discov-ered photographically by The (1975). The separation of 4 . ′′ ◦ have both been measured byHIPPARCOS. However, Raghavan et al. (2010) published an up-dated value of separation of 3 . ′′
3, yielding a projected distanceof 74 . BC Ari = HD 17382:
This is a spectroscopic binary with an or-bital solution giving an orbital period of 16 . ± . M A = . M ⊙ and M B = . M ⊙ . At an angular separation of 20 ′′ , there is an additionalM-dwarf companion (NLTT 8996) with a spectral type of M7and a mass estimate of ≈ . M ⊙ (L´epine & Bongiorno 2007).Including this wide companion, BC Ari is a triple system.
12. Eisenbeiss et al.: The Hercules-Lyra Association revisited
Table 9.
Spectroscopy and gyrochronology of all Her-Lyr candidates. Canonical members are written in bold face.Spectroscopy Gyrochronology ReferencesStar SpT M tot T e ff EW (Li) R ′ HK t R ′ HK P t gyro error
Name HD / HIP M ⊙ K mÅ Myr days Myr %
1. V439 And
HD 166 K0 V 0.99 5471 75.5 -4.64 1440 5.69 231 12 2,3,4,5,9,10,13,23,25,30,322. HIP 1481 HD 1466 F9 V 0.98 6149 125 -4.36 220 ≤ ≤
161 31 1,2,5,13,24,283. G 270-82 HD 4915 G1.5 0.91 5630 ∼ -4.86 4180 ≤ ¶ ≤
4. EX Cet
HD 10008 G5 V 0.93 5170 103 -4.29 126 7.15 315 12 2,3,4,5,7,24,25,275. LTT 10580 HD 10086 G5 IV 1.04 5685 16 -4.60 1430 ≤ ≤ ≤ ≤
148 77 3,9,11,13,24,337. 84 Cet B HD 16765 B K2 V 0.70 . . . . . . . . . . . . . . . . . . . . . 98. BC Ari (SB) HD 17382 K1 V 0.90 5234 < ≤ ≤
901 13 2,3,7,8,9,10,12,20,24,259. NLTT 8996 HD 17382 C M 7 0.13 . . . . . . . . . . . . . . . . . . . . . 7,9
10. EP Eri
HD 17925 K1 V 0.89 5150 212 -4.33 170 6.725 242 11 1,3,4,5,6,8,9,10,11,16,23,25,30,31,3311. 1E 0318-19.4 . . . K7 V 0.65 . . . 63 . . . . . . . . . . . . . . . 3,5,11,2412. G 112-35 HIP 37288 M0 V 0.6 4156 . . . . . . . . . . . . . . . . . . 3,5,24,2513. HD 25457 HD 25457 F5 V 1.17 6246 118 -4.417 340 ≤ ≤
381 59 3,4,5,8,9.13,15,20,24,25,27,29,3314. G 248-16 HD 25665 K2.5 V 0.66 4950 . . . . . . . . . ≤ ≤
719 12 3,7,8,10,20,23,24,25,27,30,3215. 39 Tau HD 25680 G5 V 1.05 5807 61 -4.30 140 12.00 1303 17 2,7,8,9,23,24,25,30,3216. V538 Aur HD 37394 K1 V 0.91 5200 2.2 -4.55 840 10.00 527 12 2,3.4,5,7,8,16,23,24,25,30,3217. LHS 1775 HD 233153 M0.5 V 0.45 3740 16 . . . . . . . . . . . . . . . 2,3,5,7,8,20,2618. HD 54371 (SB) HD 54371 G8 V 1.03 5503 . . . . . . . . . . . . . . . . . . 2,4,13,24,25,27,3219. HD 70573 HD 70573 G1 / ∗
14 3,5,7,11,16,22,24
20. DX Leo
HD 82443 K1 V 0.84 5292 176.3 -4.08 20 5.38 197 11 2,3,4,5,6,7,8,9,16,24,25,32
21. GJ 354.1 B . . . M5.5 0.1 . . . . . . . . . . . . . . . . . . . . . 2,922. EE Leo HIP 53020 M4 0.15 . . . . . . -5.33 . . . . . . . . . . . . 3,5,26
23. HH Leo
HD 96064 G5 V 0.92 5410 114 -4.34 190 6.9 312 12 2,3,4,5,9,24,25,33
24. LTT4076 . . . M0 0.6 . . . . . . . . . . . . . . . . . . . . . 9
25. BD-03 3040C . . . M0 0.6 . . . . . . . . . . . . . . . . . . . . . 926. MN UMa HD 97334 G0 V 1.08 5898 10 -4.27 110 7.60 †
643 17 2,3,4,5,9,13,23,24,25,30,3226a. Gl 471 Ba . . . L4.5 0.035 . . . . . . . . . . . . . . . . . . . . . 926b. Gl 471 Bb . . . L6 . . . . . . . . . . . . . . . . . . . . . . . . 927. HR 4758 HD 108799 G1 / ≤ ≤ § / ≤ ≤
32. PX Vir (SB) HD 113449 G5 V 0.83 5187 142 . . . . . . 6.47 231 11 2,3,4,5,6,7,9,24,2533. NQ UMa HD 116956 G9 V 0.94 5352 40 -4.22 70 4.27 124 10 2,3,4,5,13,24,25,3234. NSV 6424 HIP 67092 K5 0.65 4162 . . . -3.96 . . . . . . . . . . . . 3,5,8,2435. α Cir HD 128898 A7 V 1.81 7452 . . . . . . . . . 4.47 . . . . . . 13,15,21,2436. LTT 5826 . . . K5 V 0.72 4346 . . . . . . . . . . . . . . . . . . 3,537. LTT 6256 HD 139664 F4 V 1.36 7111 < . ≤ ≤
223 94 3,13,14,18,24,29,3338. LTT 14623 HD 139777 G0 1.03 5746 151 -4.41 320 ≤ ≤
767 15 4,6,13,24,25,3239. LTT 14624 HD 139813 G5 0.89 5343 142 -4.40 300 ≤ ≤
408 12 2,4,20,23,24,25,30,3240. V382 Ser HD 141272 G8 V 0.88 5270 6 -4.30 140 14.045 1029 13 2,3,4,6,9,20,24,25,3241. HD 141272 B . . . M3 0.3 . . . . . . . . . . . . . . . . . . . . . 6,9
42. HN Peg
HD 206860 G0 V 0.90 5930 124.8 -4.48 530 4.70 296 18 1,2,3,4,5,7,8,9,13,15,16,23,24,25,27,30,32 . . . T2.5 0.021 . . . . . . . . . . . . . . . . . . . . . 943. HD 207129 HD 207129 G0 V 1.00 6025 44 -4.80 3200 . . . . . . . . . 1,3,5,13,14,19,23,24,28,3044. V447 Lac HD 211472 K1 V 0.88 5258 . . . . . . . . . 4.43 132 10 2,3,7,9,2445. LTT 9081 HD 213845 F5 V 1.33 7037 . . . -4.512 660 ≤ ≤
261 29 3,5,8,13,15,24,25,29,3346. V368 Cep HD 220140 G9 V 0.9 5144 205.3 -3.622 . . . 2.74 49 9 3,8,9,16,24,3247. NLTT 56532 . . . early M 0.2 . . . . . . . . . . . . . . . . . . . . . 948. NLTT 56725 . . . M5.0 0.04 . . . . . . . . . . . . . . . . . . . . . 9
References. (1) Henry et al. (1996); (2) Gaidos (1998); (3) Montes et al. (2001); (4) Fuhrmann (2004); (5) L´opez-Santiago et al. (2006); (6) Eisenbeiss et al. (2007); (7) Fuhrmann(2008); (8) L´opez-Santiago et al. (2010); (9) Raghavan et al. (2010); (10) Tokovinin (1992); (11) Favata et al. (1995); (12) Favata et al. (1996); (13) Allende Prieto & Lambert (1999);(14) Lachaume et al. (1999); (15) Reiners & Schmitt (2003); (16) Messina et al. (2003); (17) Cutispoto et al. (2003); (18) Mallik et al. (2003); (19) Israelian et al. (2004); (20) Wright et al.(2004); (21) Bychkov et al. (2005); (22) Pojmanski (2002); Reiners & Schmitt (2003); Pojmanski & Maciejewski (2004, 2005); Pojmanski et al. (2005); (23) Valenti & Fischer (2005);(24) Ammons et al. (2006); (25) Maldonado et al. (2010); (26) Browning et al. (2010); (27) Masana et al. (2006); (28) Torres et al. (2006); (29) Reiners (2006); (30) Takeda et al. (2007);(31) White et al. (2007); (32) Mishenina et al. (2008); (33) Schr¨oder et al. (2009); (34) Mart´ınez-Arn´aiz et al. (2010)
Notes.
The names in the first column represent the SIMBAD name, in the second column the HD or HIP name is given. Spectral type, mass, e ff ective temperature, lithium equivalentwidth, calcium HK index R ′ HK , and age derived from chromospheric activity are presented as used in Sect. 5. The ages are calculated using the chromospheric activity index andgyrochronology (this work). All other values are taken from the literature. Brown dwarf companions are further denoted by the primary star’s number, followed by a small letter, startingwith ’a’. Spectroscopic binaries have a ”(SB)” after their name in the first column. All companions are indented. ¶ : based on v sin i = .
97 km s − (Mart´ınez-Arn´aiz et al. 2010), this object might be very young ( .
46 Myr); ∗ : the double period is published as well, leading to an age of 425 Myr; † : asecond period of 8.25 days is published, leading to an age of 740 Myr; § : Possibly the result of a stellar merger (Fuhrmann 2008); : Conversion from Li abundance to equivalent width(Soderblom et al. 1993) reveals very small values, which are consistent with zero
39 Tau = HD 25665:
A single epoch observation of a compan-ion at 0 . ′′
216 is reported (McAlister et al. 1993), but this is con-trasted with a constant radial velocity.
V538 Aur = HD 37394:
V538 Aur is a K1 star at 12 . . ′′ ′′ anda position angle of 71 ◦ (Woolf & Wallerstein 2006), the M0.5star LHS 1775 ( = HD 233152) moves with the same proper mo- tion (Fig. 2a and Eggen 1956). It is surprising, however, thatL´opez-Santiago et al. (2006) classified LHS 1775 as a Her-Lyrmember while V538 Aur was excluded. Our analysis shows thataccording to lithium and rotation, this system is too old to bea Her-Lyr member. Adopting approximate photometric masses M A = . M ⊙ and M B = . M ⊙ , assuming circular orbits, andneglecting projection e ff ects, the system has a semi-major axisof a ≈ T ≈
37 000 yr.
13. Eisenbeiss et al.: The Hercules-Lyra Association revisited
Table 10.
Photometric and astrometric properties of all Her-Lyr candidates. Photometry Space velocity ReferencesStar d p B − V V V − H B.C.
U V W
Name HD / HIP pc phot. mag km s −
1. V439 And
HD 166 13.67 0.752 6.06 1.441 -0.20 -15.0 -21.6 -10.0 2,3,4,5,8,9,10,13,30,342. HIP 1481 HD 1466 41.55 0.537 7.76 1.212 -0.17 -8.8 -20.0 -1.2 3,5,13,24,28,343. G 270-82 HD 4915 21.52 0.663 6.98 1.565 -0.21 -15.6 -25.2 -1.1 7,24,30,34
4. EX Cet
HD 10008 23.95 0.797 7.66 1.761 -0.28 -13.2 -18.1 -11.1 2,3,4,5,7,24,345. LTT 10580 HD 10086 21.37 0.690 6.60 1.578 -0.22 -19.7 -17.1 -19.4 7,13,24,30,346. 84 Cet HD 16765 22.59 0.511 5.72 1.078 -0.16 -12.7 -24.1 -2.7 3,9,13,24,347. 84 Cet B HD 16765 B 21.62 . . . 9.67 . . . . . . 3 . ′′ , 74 . ′′ , 493 AU 7,9,34
10. EP Eri
HD 17925 10.35 0.862 6.04 1.820 -0.37 -15.0 -21.8 -8.7 3,4,5,6,9,10,24,30,3411. 1E 0318-19.4 . . . 27.03 1.139 10.24 2.605 -0.55 -12.7 -17.3 -11.8 35,24,3412. G 112-35 HIP 37288 14.58 1.379 9.65 3.538 -0.92 -11.0 -21.5 -13.1 3,5,24,3413. HD 25457 HD 25457 18.83 0.516 5.38 1.038 -0.16 -6.0 -28.3 -10.5 3,4,5,8,9,13,24,31,3414. G 248-16 HD 25665 18.75 0.952 7.71 2.172 -0.48 -7.4 -23.8 -17.0 3,7,8,10,24,30,3415. 39 Tau HD 25680 16.90 0.620 5.90 1.402 -0.20 -27.0 -13.2 -7.1 2,7,8,9,24,30,3416. V538 Aur HD 37394 12.28 0.840 6.20 2.219 -0.37 -12.9 -23.3 -14.5 2,3,4,5,7,8,24,30,3417. LHS 1775 HD 233153 12.44 1.473 9.75 3.817 -1.43 -14.4 -22.9 -14.3 2,3,5,7,8,3418. HD 54371 (SB) HD 54371 25.17 0.700 7.08 1.640 -0.23 -21.1 -17.5 -15.7 2,4,7,13,24,3419. HD 70573 HD 70573 40.36 0.635 8.68 1.464 -0.22 -14.7 -18.8 -6.7 3,5,24,31,34
20. DX Leo
HD 82443 17.79 0.779 7.06 1.808 -0.28 -9.9 -22.8 -5.6 2,3,4,5,6,7,8,9,24,31,34
21. GJ 354.1 B . . . . . . . . . 14.7 6.172 3.29 65 ′′ , 1160 AU 2,922. EE Leo HIP 53020 6.76 1.679 11.64 4.933 -2.56 -7.9 -22.5 -19.1 3,5,34
23. HH Leo
HD 96064 26.27 0.770 7.60 1.737 -0.28 -14.2 -26.7 -0.6 2,3,4,5,9,24,34
24. LTT 4076 . . . . . . . . . . . . . . . . . . 11 ′′ , 244 AU 9
25. BD-03 3040C . . . . . . . . . . . . . . . . . . 11 ′′ , 244 AU 926. MN UMa HD 97334 21.93 0.600 6.40 1.389 -0.19 -15.8 -23.2 -11.2 2,3,4,5,9,13,24,30,3426a. Gl 471 Ba . . . . . . . . . . . . . . . . . . 90 ′′ , 1974 AU 926b. Gl 471 Bb . . . . . . . . . . . . . . . . . . 90 ′′ , 1974 AU 927. HR 4758 HD 108799 24.65 0.591 6.37 1.438 -23.4 -19.6 -6.4 7,13,24,31,3428. GJ 469.2 B . . . 24.65 1.310 9.58 . . . -0.20 2”, 49 AU29. LW Com HD 111395 16.93 0.703 6.29 1.585 -0.82 -18.4 -21.6 -9.2 3,4,5,13,24,30,3430. HIP 63317 HD 112733 44.21 0.738 8.65 1.720 -0.22 -17.6 -23.3 -0.8 5,8,13,24,3431. HIP 63322 HIP 63322 40.57 0.852 9.23 2.284 -0.26 -13.9 -20.3 -4.3 24,31,34
32. PX Vir (SB) HD 113449 21.69 0.874 7.70 2.016 -0.37 -5.0 -28.8 -9.8 2,3,4,5,6,7,9,24,31,3433. NQ UMa HD 116956 21.59 0.804 7.28 1.809 -0.31 -15.9 -18.8 -8.8 2,3,4,5,13,24,3434. NSV 6424 HIP 67092 25.10 1.344 10.52 3.212 -0.92 -8.0 -22.4 -1.8 3,5,8,24,3435. α Cir HD 128898 16.57 0.256 3.17 0.709 -0.10 -10.9 -19.2 -10.8 13,24,3436. LTT 5826 . . . . . . . . . 8.47 2.780 -0.77 16 ′′ , 262 AU 3,537. LTT 6256 HD 139664 17.44 0.413 4.64 0.908 -0.14 -15.1 -19.8 -9.7 3,5,13,14,24,3438. LTT 14623 HD 139777 21.85 0.665 6.57 1.415 -0.20 -14.7 -26.6 -2.2 4,5,6,13,24,31,3439. LTT 14624 HD 139813 21.51 0.803 7.29 1.741 -0.29 -14.7 -26.6 -2.2 2,4,5,24,30,31,3440. V382 Ser HD 141272 21.29 0.801 7.44 1.830 -0.31 -19.2 -27.6 -14.0 2,3,4,5,6,9,24,3441. HD 141272 B . . . . . . . . . . . . . . . . . . 18 ′′ , 383 AU 6,9
42. HN Peg
HD 206860 17.89 0.587 5.95 1.362 -0.18 -14.6 -21.4 -11.0 2,3,4,5,7,8,9,13,24,30,34 . . . . . . . . . . . . . . . . . . 18 ′′ , 383 AU 943. HD 207129 HD 207129 15.99 0.601 5.57 1.264 -0.19 -13.3 -22.2 -0.3 3,5,13,14,24,28,30,3444. V447 Lac HD 211472 21.54 0.810 7.50 1.859 -0.31 -19.6 -13.0 -5.9 2,3,7,9,24,3445. LTT 9081 HD 213845 22.68 0.446 5.21 0.944 -0.14 -15.1 -20.6 -12.9 3,5,8,13,24,3446. V368 Cep HD 220140 19.20 0.893 7.53 2.018 -0.42 -10.2 -23.5 -5.5 3,8,9,24,3447. NLTT 56532 . . . . . . 1.51 12.24 4.850 -1.64 10 ′′ , 192 AU 948. NLTT 56725 . . . 19.38 1.83 16.16 6.318 -3.29 16 ′′ , 304 AU 9,34 References. (2) Gaidos (1998); (3) Montes et al. (2001); (4) Fuhrmann (2004); (5) L´opez-Santiago et al. (2006); (6) Eisenbeiss et al. (2007); (7) Fuhrmann (2008); (8) L´opez-Santiagoet al. (2010); (9) Raghavan et al. (2010); (10) Tokovinin (1992); (13) Allende Prieto & Lambert (1999); (14) Lachaume et al. (1999); (24) Ammons et al. (2006); (28) Torres et al. (2006);(30) Takeda et al. (2007); (34) van Leeuwen (2007)
Notes.
Object names follow the same conventions as in Table 9. Distance is given in the subsequent column, followed by the BVH photometry used in Sect. 4. Hipparcos distancesare available for most of the objects and have been adopted from van Leeuwen (2007). The
UVW velocity data occupies the next three columns. All values are taken from the literature.Separation and position angle of the companions are given in the
UVW columns. The canonical members are written in bold face.
HD 54371:
The photometric and kinematic analysis ofHD 54371 su ff ers because this object is a spectroscopic binarywith a period of 32 .
81 days (Beavers & Salzer 1985; Raghavanet al. 2010). The companion is a K dwarf with a magnitude dif-ference of ∆ m = HD 70573:
Setiawan et al. (2007) reported a planet candidatewith a mass of m sin i = . ± . M Jup at a semi-major axis of1 . ± .
05 AU and ∼
850 days orbital period using the radialvelocity technique. The planet has a comparably large eccentric- ity of e = . ± .
1. It is one of the few radial velocity planetsdiscovered in orbit around an apparently young star.
DX Leo = HD 82443:
The K0 star DX Leo at 17 . Ω -Cass images we confirmed the companionGJ 354.1 B astrometrically and photometrically at a separationof 65 ′′ and a position angle of 67 ◦ (Fig. 2b). Using the photomet-ric masses M A = . M ⊙ and M B = . M ⊙ this translates intoa linear separation of a ≈ T ≈
38 400 yr( e =
14. Eisenbeiss et al.: The Hercules-Lyra Association revisited
HH Leo = HD 96064:
The multiplicity of HH Leo was investi-gated by Heintz (1980). The object, located at a separation of11” ( a ≈
244 AU, T ≈ MN UMa = HD 97334:
At a separation of 90 ′′ at 245 ◦ ,Kirkpatrick et al. (2001) discovered a L4.5 companion(GJ 417 B). Common proper motion was proven using the2MASS catalog. This object is too red and too faint to be vis-ible on Schmidt plates so we missed it in our analysis. Later,Bouy et al. (2003) resolved this brown dwarf companion into abinary using HST WFPC2. They report a separation of 70 masor 1 . ff erence of ∼ HR 4758 = HD 108799:
This object might be an example ofa former stellar merger (Fuhrmann 2008). This idea arises fromthe fact that this star has all indicators of a young object (rotation,lithium, H α , etc.) but looks like an old object on evolutionarytracks. From its kinematics, however, a Her-Lyr membershipcannot be excluded. It has a K-dwarf companion with 0 . M ⊙ ata separation of ∼ ′′ and the orbital period is estimated to be P =
151 yr (Fuhrmann 2008).
HIP 63317 = HD 112733:
The G5 star HD 112733 forms acommon proper motion pair with its neighboring twin, the G6star HIP 63322. Halbwachs (1986) discovered this pair with aprojected separation of a =
807 AU giving a period of T =
17 124 yr if the projected separation is close to the semi-majoraxis. Given the di ff erences in parallax and proper motion whichare slightly larger than the corresponding uncertainty, it is notclear however whether these two stars are bound. PX Vir = HD 113449:
This is a stellar binary. Various pho-tometric and spectroscopic measurements were taken (Moore& Paddock 1950; Gaidos et al. 2000; Latham et al. 2010). Aphotometric period of 231 days and radial velocity changes of20 km s − were observed. The companion was resolved in 2007with a brightness di ff erence of ∆ H ∼ . . ± . . ◦ ± . ◦ (Raghavanet al. 2010). Astrometric and spectroscopic measurements areconsistent. α Cir = HD 128898: α Cir is the brightest star in the Circiniusconstellation. Being a nearby late A-type star, it is too bright foranalysis via Schmidt plates. In addition, it shows astroseismicvariations as studied in detail by Bruntt et al. (2009). The K5star LTT 5826 is located at the same distance, thus it is proba-bly a wide companion of α Cir. This companion candidate wasconsidered a Her-Lyr candidate by L´opez-Santiago et al. (2006)while α Cir remains unattended in their work.
LTT 14623 = HD 139777 and LTT 14624 = HD 139813:
Thewell-known common proper motion pair was first mentioned byStephenson (1960). With spectral types of G0 and G5, both aresolar analogs. The upper limits on the age derived via v sin i mea-surements are, however, insu ffi cient to confirm a probable Her-Lyr membership. Currently, their linear separation is ∼
680 AU corresponding to an orbital period of T ≈
13 000 yr in case of acircular orbit.
V382 Ser = HD 141272:
Eisenbeiss et al. (2007) confirmed thatthe object ≈ ′′ north of V382 Ser is a co-moving companionusing Schmidt plates, Ω -Cass images (Fig. 2c), the 2MASS cata-log as well as EMMI / NTT spectroscopy. Spectroscopy indicatesa spectral type of M3 and a mass of ∼ . M ⊙ (Eisenbeiss et al.2007) for the companion (see also Raghavan et al. 2010). HN Peg = HD 206860:
Luhman et al. (2007) discovered a T2.5dwarf companion using Spitzer IRAC and confirmed the com-mon proper motion using the 2MASS catalog. In the archivalSchmidt plates, the object is not detected. However, we imagedthe companion with ALFA (see Fig. 3). At a mass of 0 . M ⊙ (Luhman et al. 2007), this is the least massive directly detectedmember of the Her-Lyr association (see also Raghavan et al.2010). V368 Cep = HD 220140:
This G9V star is probably theyoungest Her-Lyr candidate in our list. Since it was not inthe initial sample of Fuhrmann (2004), it was also not consid-ered by L´opez-Santiago et al. (2006). The kinematics make it agood candidate for the Her-Lyr association although new lithiumequivalent widths (L´opez-Santiago et al. 2010) and photome-try of the star and its companions suggest a somewhat youngerage (12-20 Myr, see Raghavan et al. 2010 and Makarov et al.2007). The gyrochronology suggests ∼
50 Myr. The celestiallocation co-incides with the Cepheus-Cassiopeia complex anda group of four co-moving isolated T Tauri stars identified byGuillout et al. (2010). They argue, however, that the proper mo-tion and the distances rule out a common origin with V368 Cepand its companions. The brighter companion, NLTT 5632, at aseparation of 10 ′′ , shows common proper motion with the pri-mary and its photometry yields a spectral type of early M at theprimary’s distance (Raghavan et al. 2010). The second compan-ion, NLTT 56725, at 16 ′′ separation is a late-M dwarf which wasdiscovered by L´epine & Shara (2005) and later confirmed bymeasuring the parallax (Makarov et al. 2007). This star is over-luminous in the K s band, again indicating a young age of thesystem.All directly detected companions outside 10 AU are listedseparately in Tables 9, 10, and 11.
8. Discussion
Tables 9 and 10 summarize all properties of Her-Lyr candidatesused in this analysis. In the following, our conclusions regard-ing age, membership, and multiplicity statistics are presented.Quantitative membership criteria are developed as far as possi-ble.L´opez-Santiago et al. (2006) already noted that isochrone fit-ting fails to give tight constraints on the age of the association.The stars are simply too old and most of them have reached themain-sequence. We have searched for late-type companions toHer-Lyr candidates to obtain some improvement on that issue.Late-M stars may still be contracting and provide a possibility ofderiving an association age via theoretical isochrones. However,our analysis shows that either the models themselves or the cal-ibration of observables (color and magnitude) lacks precision atthe late-type end. We suspect that the problems occur becauseM dwarfs become fully convective around spectral type M3-M4.
15. Eisenbeiss et al.: The Hercules-Lyra Association revisited
The isochrone method fails to derive an age estimate for the Her-Lyr association. Alternatively, the late-M stars in question (espe-cially the companions to V368 Cep) may indeed be very young.From the list of candidates, selected via space velocity,we were able to identify seven canonical members using gy-rochronology and lithium equivalent width. These members de-fine the properties of the association hence the average gy-rochronological age of the Her-Lyr association is (with 1 σ errorbars)age = ±
46 Myr . (6)The youth of the Her-Lyr association is confirmed by theX-ray luminosity function, the L X / L bol ratio, the analysis oflithium equivalent width, the R ′ HK index, and the age derivedfrom chromospheric activity. The activity–age relation usedhere ignores spectral type or color hence should be used withcare when looking at individual ages. The average overall candi-dates ( ∼
285 Myr), however, coincides with the gyrochronolog-ical age. The Her-Lyr association is at least as old as the Pleiadesand younger than the Hyades. There are some hints that an agesimilar to Ursa Major can be assumed.The canonical member list is identified in Sect. 5.5and is composed of the young lithium-enriched stars inFig. 9, i.e., V 439 And (HD 166), EX Cet (HD 10008), EP Eri(HD 17925), DX Leo (HD 82443), HH Leo (HD 96064), PX Vir(HD 113449), and HN Peg (HD 206860). Several additional stars(G 112-35, HD 25457, HIP 63317 & HIP 63322, NSV 6424, α Cir A&B, LTT 6256, LTT 14623 & LTT 14624, and LTT 9081)show good agreement, but cannot be used for the canonical listof members since some information is missing (typically lithiummeasurements or rotational periods).Based on the average properties derived from the canonicallist of members, three membership criteria are defined in orderto establish membership of additional candidates:1. The U and V velocities must not deviate by more than 2 σ from the mean U and V velocities of the association (Eq. 5).2. The lithium equivalent width of a Her-Lyr candidate shouldbe as least as high as for an UMa member of the same e ff ec-tive temperature (applicable above 5000K).3. Chromospheric activity should not be significantly below themean levels of the Hyades and UMa in Fig. 11. As the cal-cium R ′ HK index is a very uncertain criterion, we use it as asupporting argument.4. The gyrochronological age should be within 2 σ of the meangyrochronological age of the association (Eq. 6).These criteria are applied to each candidate. The results aregiven in Table 11. Given our definition of Her-Lyr like kinemat-ics (Eq. 5) and age (Eq. 6), we give the deviation (in σ ) for eachcandidate in Table 11. We consider a star a doubtful candidate ifit has a deviation of more than two σ in one or more propertiesand a non-member in the case of a deviation of more than three σ . The lithium equivalent width and the chromospheric activityindex R ′ HK are used as an additional qualitative argument.The member list also includes all co-moving companionsof members which are GJ 354.1 B, LTT 4076, BD-03 3040 C,PX Vir B, and HN Peg B (see Sect. 7); HIP 63317 (HD 112733)with its companion HIP 63322 are possible members as well as1E 0318-19.4, G 248-16, and HD 54371. The criteria on Li and R ′ HK only apply to late-type stars. For this reason, LTT 6256 hasbeen kept as candidate even though no significant Li absorptionhas been measured.Interestingly, all doubtful candidates have a similar age tooyoung to include them in our current list of Her-Lyr members. Table 11.
Membership of Her-Lyr candidates.Primary σ UV σ W EW(Li) R ′ HK σ age Mem. F04 L06
1. V439 And (cid:8) (cid:8) + + +
2. HIP 1481 1 1.9 (cid:8) (cid:8) & (cid:18) .
4. EX Cet (cid:8) (cid:8) + + +
5. LTT 10580 2.0 3.0 (cid:18) (cid:18) . (cid:8) (cid:8) & (cid:18) (cid:8) .
10. EP Eri (cid:8) (cid:8) + o o11. 1E 0318-19.4 1.6 1 (cid:8) ? ? ? o12. G 112-35 1 1.4 ? ? ? ? +
13. HD 25457 1.8 1 (cid:8) (cid:8) . . (cid:8) (cid:8) (cid:18) (cid:8) + o17. LHS 1775 1 1.7 (cid:18) ? ? - +
18. HD 54371 2.4 2.0 ? ? ? ? o19. HD 70573 1.2 1 (cid:8) ? 3.0 o +
20. DX Leo (cid:8) (cid:8) + o o
21. GJ 354.1 B +
22. EE Leo 1.3 2.9 ? o - - +
23. HH Leo (cid:8) (cid:8) + + o
24. LTT 4076 +
25. BD 033040 C +
26. MN UMa 1 1 (cid:18) (cid:8) + o27. HR 4758 3.0 1 (cid:8) (cid:8) & GJ 469.2 B (cid:18) (cid:8) (cid:8) (cid:8) . (cid:8) ? ? ? o
32. PX Vir (cid:8) ? 1 + o -33. NQ UMa 1.2 1 o (cid:8) + o34. NSV 6424 1.2 1.7 ? (cid:8) ? ? o35. α Cir 1.1 1 ? ? ? ?36. LTT 5826 ? ? ? ? ? ? o37. LTT 6256 1 1 ? (cid:8) & + +
38. LTT 14623 1 1.6 (cid:8) (cid:8) . + o39. LTT 14624 1 1.6 (cid:8) (cid:8) . + o40. V382 Ser 1.9 1.6 (cid:18) (cid:8) + o41. HD 141272 B 1.9 1.6 ? ? ? -
42. HN Peg (cid:8) (cid:8) + + +
43. HD 207129 1 2.1 (cid:8) (cid:18) ? - o44. V447 Lac 2.8 1 ? ? 2.8 o45. LTT 9081 1 1.3 ? (cid:8) & +
46. V368 Cep 1 1 (cid:8) (cid:18)
Notes.
The numbers in Cols. 2 and 3 give the deviation of the space velocity of each starfrom the mean space velocity of Her-Lyr in Sect. 6 in units of σ . The criteria defined in Sect.6 are applied to each single candidate. A discrepancy of more than two σ in UV velocityand age disqualifies a star as a probable member. For lithium, a more qualitative argument(EW(Li) UMa < EW(Li)
Her − Lyr . EW(Li)
Pleiades ) is given. A Her-Lyr candidate is assigneda (cid:8) if the equivalent width fits that of the canonical members, a (cid:18) if the star is lithium-depleted, and a ? if no information was found or if the criterion is not applicable to thisparticular object. Similarly, a (cid:8) is given if a candidate displays significant chromosphericactivity R ′ HK , otherwise (cid:18) or ? if unknown. The & and . signs in Col. 6 indicate that theage was derived based on an upper limit of the rotational period, i.e., v sin i . Based on thesecriteria, the membership probability is qualitatively indicated by a + for a high membershipprobability and a o for doubtful membership. Improbable members do not fulfill at least oneof the criteria and are indicated by -. In case of unavailable information or upper limits, thecandidate is kept as a possible member and marked by ? ( Col. 7). For comparison, resultsby Fuhrmann (2004) and L´opez-Santiago et al. (2006) are given in the last two columns. Incases of resolved multiple systems where common proper motion is confirmed (see Sects. 3and 7), similar UVW velocity is assumed and the membership is assessed as for the primary.Canonical members are written in bold face.
The question arises whether this is another group of stars or if theage distribution is bimodal indicating that star formation did notoccur as a single event in the Her-Lyr association. At present, thenumber of stars and the data available are insu ffi cient to firmlyanswer this question.A critical review of earlier assessments must neccesar-ily be done before reconsidering the member list. Therefore,the member lists of Fuhrmann (2004) and L´opez-Santiagoet al. (2006) are given for comparison in the last two
16. Eisenbeiss et al.: The Hercules-Lyra Association revisited columns of Table 11. Fuhrmann (2004) identified a largeinitial list of candidates. L´opez-Santiago et al. (2006) dis-carded HD 25457 since the kinematics of that star matchthe B4 subgroup of the Pleiades. The systems HH Leo( = HD 96064), NSV 6424 ( = HIP 67092), HD 207129, and the bi-naries HIP 63317 ( = HD 112773) & HIP 63322, and LTT 14623( = HD 139777) & LTT 14624 ( = HD 139813) were doubted be-cause of their W velocity. This is not a strong argument, however,because the Her-Lyr candidates have already completed ∼ W velocity can be significantly changedthanks to the galactic potential (see e.g., Fuhrmann 2004, Fig. 23and discussion).The PX Vir system is discarded by L´opez-Santiago et al.(2006) because it was earlier classified as an AB Dor member(Zuckerman et al. 2004). It is not clear, however, to what extentthe Her-Lyr association di ff ers from the AB Dor moving group.In contrast to L´opez-Santiago et al. (2006), we exclude stars de-pleted of lithium and showing excessively slow rotational be-havior. Seven stars (HIP 1481, 84 Cet, HD 70573, HR 4758,NQ UMa, V447 Lac, and HD 220140) are younger than the av-erage age of the Her-Lyr association.The present work adds new membership criteria to the kine-matic criteria in a coherent way, based on a well-defined list ofcanonical members and consolidates the properties and memberlist of the Her-Lyr association.The multiplicity study revealed that our sample consistsof 35 stellar systems, counting just the primaries, includ-ing the spectroscopic binaries BC Ari, HD 54371, and PX Vir( = HD 113449). Fithteen (42 . . . are considered, aswell as non-solar like stars. Setiawan et al. (2007) detected aplanet candidate around HD 70573. To our knowledge no furtherplanetary mass companion was found for any Her-Lyr memberor candidate.The mass function of the Her-Lyr association, when com-pared to the initial mass function, su ff ers from various selectione ff ects and should therefore be used with care (Fig. 16). The lowmass content of the Her-Lyr association is dramatically underes-timated in the MF since the census of K, M, and brown dwarfs isnot at all complete. Thus, the low mass members are only repre-sented through the companions, found close to various Her-Lyrmembers. Observations predict that the number of stars is givenby N = µ M − α , with the proportionality factor µ and the index α = .
35 for M > . M ⊙ (Salpeter 1955)2 . . < M ≤ . M ⊙ (Kroupa 2002)1 . . < M ≤ . M ⊙ (Kroupa 2002)0 . M ≤ . M ⊙ (Kroupa 2002) . (7)In Fig. 16, the MF is binned the same way as the histogramand the free parameter µ is adjusted to the star count above With the re-reduction of the
Hipparcos catalog by van Leeuwen(2007), some of the stars, included in the 25 pc sample of Fuhrmann(2004), are now located outside the 25 pc sphere (e.g., HH Leo). Thisnecessarily leads to inconsistencies regarding the 25 pc sample.
Mass [M
Sun ], µ = 4.822 O n l y m e m be r s and c and i da t e s Fig. 16.
Mass function of the Her-Lyr association compared to the initialmass function (IMF). Only good candidates (membership criterion + and ? in Table 11) are included. The number of stars is N = µ M − α withthe stellar mass M in M ⊙ , the proportionality factor µ , and the index α (see text). Only stars with M ≥ . M ⊙ are used to fit µ . See text forfurther explanations. . M ⊙ , to use only bright stars for the fit which are countedproperly. Figure 16 suggests that the stellar content of Her-Lyris complete down to ∼ . M ⊙ . The general trend of the MF iswell reproduced above ∼ . M ⊙ .
9. Outlook
In the past, the very existence of the Her-Lyr association wasdoubted by various authors (e.g., Maldonado et al. 2010). At thevery least, following Fuhrmann (2004), the Her-Lyr associationis an individual entity in kinematic space. Applying our selectioncriteria, the often-discussed heterogeneity in age and evolution-ary state vanished to some extent but, a large age spread is stillpresent. It is not clear if this is due to the uncertainties of themethod these ages were derived with, or if this is an intrinsicproperty of the stars considered as members. However, the exis-tence of the Her-Lyr association, as its own entity, or at least asa sub-moving-group of the LA is becoming clearer.The Her-Lyr association displays a substantial spread in gy-rochronological age and lithium absorption. If one assumes thatthis spread is real and intrinsic to Her-Lyr, there are impor-tant implications for the nature of this association. Although,a spread in lithium is well-known for the Pleiades which havesimilar age, there could also be a sub-structure in Her-Lyr whichhas not been resolved yet because of the small number of knownmembers.The Her-Lyr association could be part of the LA which is amixture of Pleiades-like stars in the solar neighborhood (Eggen1956). The unclear composition of the LA is reflected by theuncertainty of further sub-structure in Her-Lyr. The X-ray lu-minosity function, which is similar to that of the Pleiades, sup-ports this idea. Future work clearly needs to address the rela-tionship of Her-Lyr with other local moving groups (e.g., ABDor, Zuckerman et al. 2004, SACY, Torres et al. 2006, etc.) andto show in detail whether the local moving groups are di ff erentfrom Her-Lyr.Recently, a self-consistent study of the UMa and Hyadesclusters revealed that the UMa association could be only about100 Myr younger than the Hyades (King & Schuler 2005, but
17. Eisenbeiss et al.: The Hercules-Lyra Association revisited also see Fuhrmann 2004), i.e., significantly older than the Her-Lyr association. The Hercules-Lyra association could thereforebe an intermediate case in between the Pleiades (which are 100to 110 Myr old, see Meynet et al. 1993; Terndrup et al. 2000,respectively) and UMa. This impression is strengthened by thecomparison of lithium equivalent width (Fig. 9) and by the av-erage gyrochronological age of about 250 Myr derived for Her-Lyr. However, the spread in lithium of Her-Lyr is generally com-parable to that of the Pleiades and seems to be larger than inUMa. In the future, lithium abundance needs to be studied in-stead of lithium equivalent widths. Stellar parameters, as wellas iron and lithium abundance, have to be studied in a self-consistent way (cf. Ammler-von Ei ff & Guenther 2009).For many candidates subject to the present work, spectro-scopic information or rotational periods are missing. Additionalobservations are needed to obtain the missing data for these starsand to update the member list and criteria.The distribution of non-members in the UV -space (seeFig. 15) possibly corresponds to a ∼ ≈ . M ⊙ . This is further evidence that there is a large number ofunidentified low mass stars in the solar neighborhood, as waspointed out by Fuhrmann (2004) and other studies before.Hercules-Lyra is an interesting laboratory for astrophysics.Similar to the Hyades, UMa group, and other local movinggroups, the members are bright so that even late-type mem-bers can be studied in detail. The Her-Lyr association providesan important additional snapshot in the rotational and Li evo-lution of young stars. The stars are close so that companionscan be resolved and common proper motion can be assessedquickly. Low-mass companions are particularly interesting sincethey possibly have not yet reached the main-sequence or mighteven be brown dwarfs at a very early cooling stage. The multi-plicity study presented in this work contributes to the total stellarcensus of young moving groups. Acknowledgements. The authors want to thank the sta ff of ESO Paranal andof the-ppl Calar Alto observatory, for helping to carry out observations used inthis work. TE would like to thank the Deutsche Forschungsgemeinschaft (DFG),projects SFB / TR-7 and NE 515 / / Deutes Zentrum f¨ur Luft- und Raumfahrt (DLR)under the projects 50OO1007 and 50OW0204. T.O.B.S. and R.N. like to thankthe DFG under project NE 515 / / / / / GSFC and the High Energy Astrophysics Division ofthe Smithsonian Astrophysical Observatory. Last but not least we would like tothank the anonymous referee, who made numerous suggestions to improve thearticle and even suggested additional analysis methods to secure our results.
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