Detection of the lithium depletion boundary in the young open cluster IC 4665
aa r X i v : . [ a s t r o - ph ] D ec Astronomy&Astrophysicsmanuscript no. 8226 c (cid:13)
ESO 201826th October 2018
Detection of the lithium depletion boundary in the young opencluster IC 4665 ⋆ S. Manzi , S.Randich , W.J. de Wit , , and F. Palla INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy Laboratoire d’Astrophysique, Observatoire de Grenoble, BP 53, F-38041 Grenoble, C´edex 9, France School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UKReceived Date: Accepted Date
Abstract
Context.
The so-called lithium depletion boundary (LDB) provides a secure and independent tool for deriving the ages of young open clusters.
Aims.
In this context, our goal is to determine membership for a sample of 147 photometrically selected candidates of the young open clusterIC 4665 and to use confirmed members to establish an age based on the LDB.
Methods.
Employing the FLAMES multi-object spectrograph on VLT / UT2, we have obtained intermediate-resolution spectra of the clustercandidates. The spectra were used to measure radial velocities and to infer the presence of the Li i α emission. Results.
We have identified 39 bona fide cluster members based on radial velocity, H α emission, and Li absorption. The mean radial velocityof IC 4665 is found to be V rad = − . ± .
13 km / s. Confirmed cluster members display a sharp transition in magnitude between stars with andwithout lithium, both in the I m vs. I m − z and in the K s vs. I C − K s diagrams. From this boundary, we deduce a cluster age of 27 . + . − . ± . ± Conclusions.
IC 4665 is the fifth cluster for which an LDB age has been determined, and it is the youngest cluster among these five. Thus,the LDB is established from relatively bright stars still in the contracting pre-main sequence phase. The mass of the boundary is M ∗ = . ± . M ⊙ . The LDB age agrees well with the ages derived from isochrone fitting of both low and high mass, turn–o ff stars, a result similarto what is found in the slightly older NGC 2547. Key words. open clusters and associations: individual: IC 4665 – stars: low-mass – stars: pre–main-sequence – stars: abundances
1. Introduction
Pre-main sequence (PMS) clusters with an age of 5-50 Myrrepresent an ideal tool for investigating several aspects relatedto star formation and the early phases of (sub-)stellar evolu-tion. These clusters provide the youngest samples of PMS starsoutside a star forming environment. Indeed, unlike star form-ing regions, PMS clusters show the complete and final productof the formation process, immediately after the active phaseof star birth. Moreover, the low-mass members (both stars andbrown dwarfs) are still bright and readily detectable, and not af-fected by severe extinction since most of the circumstellar andinterstellar material has been accreted and dispersed. Finally,the age interval of PMS clusters is critical with respect to theearly evolution of protoplanetary disks, stellar rotation, andactivity. Despite these benefits, at present only three systemsare confirmed PMS clusters: NGC 2547 ( ∼
35 Myr; Je ff ries& Oliveira 2005), NGC 2169 ( ∼
10 Myr; Je ff ries et al. 2007),IC 2391 ( ∼
50 Myr; Barrado y Navascu´es et al. 2004); a fewother candidates exist. The identification of additional PMS
Send o ff print requests to : S. Randich, email:[email protected] ⋆ Based on observations collected at ESO-VLT, ParanalObservatory, Chile. Program number 073.D-0587(A). cluster candidates and confirmation of their nature through asecure age determination would therefore represent a signifi-cant improvement from the phenomenological and statisticalpoint of view (see discussion in Je ff ries et al. 2007).Ages of stellar clusters can be obtained from the location ofthe main sequence turn-o ff (t nucl ) or from the isochronal distri-bution of the PMS population in the HR diagram. These classi-cal methods are widely used, but su ff er from large uncertain-ties of up to a factor of two in age (e.g., Mermilliod 2000;Je ff ries & Oliveira 2005). On the contrary, the method basedon the lithium depletion boundary (LDB) has proven to be ro-bust and less model dependent since it relies on well knownphysics (Bildsten et al. 1997; Ushomirsky et al. 1998). Duringthe PMS phase, stars undergo a gradual gravitational contrac-tion that causes a progressive, mass-dependent rise of the cen-tral temperature. Li burning starts when the core reaches a tem-perature ≃ × K (depending on density); hence, in fullyconvective low-mass stars (M < ∼ . ⊙ ) Li is depleted fromthe initial interstellar abundance on a time scale that is a sensi-tive function of mass. In a young stellar cluster three regimesof Li depletion are present: i) relatively massive stars (with ra-diative interiors) that su ff er only a little amount of Li deple-tion; ii) stars in the so-called Li chasm (Basri 1997) that have S. Manzi: LDB in IC 4665 fully depleted their initial Li supply; iii) low mass stars thathave preserved the initial Li content. The transition betweenlow-mass stars with and without Li is very sharp and the lu-minosity of the faintest star that has depleted 99 % of its ini-tial Li identifies the boundary (LDB) and the age (t
LDB ) of thecluster (e.g., Basri et al. 1996; Basri 1997; Stau ff er 2000). Theolder the cluster is, the fainter the stars at the boundary are. Sofar, the LDB has been detected in four clusters: the Pleiades(Stau ff er et al. 1998), α Persei (Stau ff er et al. 1999), IC 2391(Barrado y Navascu´es et al. 2004), and NGC 2547 (Je ff ries &Oliveira 2005). Remarkably, the LDB ages determined for thePleiades, α Per, and IC 2391 exceed the nuclear ages by a fac-tor ∼ .
5; these particular nuclear ages were derived from fit-ting the cluster turn-o ff (TO) with evolutionary models withoutovershooting. On the other hand, the two dating methods yieldsimilar ages for the younger cluster NGC 2547, although thecluster’s nuclear age is still rather uncertain.In this paper, we report on the determination of the LDBin the open cluster IC 4665 whose properties have been exten-sively discussed by de Wit et al. (2006). IC 4665 is an inter-esting candidate PMS cluster, located relatively far from theGalactic plane at b ∼ + ◦ . Its nuclear age is about t nucl =
36 Myr (Mermilliod 1981), but other properties might suggestan age as high as 100 Myr (Prosser 1993). The Hipparcos dis-tance is 385 ±
40 pc (Hoogerwerf et al. 2001), while a lowervalue of 320 pc has been derived by Crawford & Barnes (1972).IC 4665 was targeted for a wide and deep survey in I m (Mould)and z filters at the Canada-France-Hawaii telescope (CFHT).This deep photometric survey led to the detection of 786 newlow-mass stellar and brown dwarf candidate members (14 . < I m < jup (de Wit et al. 2006). Giventhe nuclear age of IC 4665, the new low-mass candidate mem-bers provide a sample suitable for the detection of the LDB andfor the derivation of an accurate and independent age estimate.The importance of such determination was already stressed byMart´ın & Montes (1997) who were the first to obtain Li abun-dances in a small sample of cluster stars (mainly of G and early-K spectral-type). Although a spread in Li was found, the obser-vations did not reach the low luminosity population of IC 4665where the LDB is expected to occur.Our paper is structured as follows: Section 2 describes thesample selection, observations, and data analysis. The resultson membership and the lithium boundary are given in Section3. The age of IC 4665 and the comparison with other youngPMS clusters is discussed in Section 4. The conclusions closethe paper.
2. Sample selection, observations and dataanalysis
Target stars were selected from di ff erent sources:Prosser (1993), Giampapa et al. (1998), de Wit et al. (2006),and a literature search on SIMBAD. The sample includes 96stars from Prosser (1993 –of these, 88 were covered by theCFHT survey, but not included in the catalog of de Wit etal.), one star from Giampapa et al. (1998), 15 stars from deWit et al. (2006), 24 stars included both in Prosser and de Witet al. catalogs, and eight new candidate members. The latter Figure 1. I m vs. I m − z color-magnitude diagram of all the 137candidate members observed with Gira ff e and with availablephotometry from the CFHT survey. Target stars have been re-trieved from i. Prosser (1993) and Giampapa et al. (1998): opencircles, 89 stars; ii. de Wit et al. (2006), open squares andcrosses – 15 and eight stars, respectively; iii.
Luyten (1961):triangles, two stars. Filled squares instead represent candi-dates common to Prosser and de Wit et al. (24 stars). Crossesare the eight objects brighter than the minimum magnitudeselection limit in de Wit et al. and not reported in Prosser.Three isochrones for 30, 50, and 100 Myr are shown from theNextgen model of Bara ff e et al. (1998) with a mixing lengthparameter α = m = ff limit used in that paper. Finally, we also observedtwo stars from Luyten (1961) and one IRAS source (IRAS17447 + m and z photometry available from the CFHT survey. Fig. 1shows their distribution in the color-magnitude diagram. Starswith I m magnitudes between ∼
16 and 18 bracket the expectedapparent brightness of the LDB boundary at the distance ofIC 4665 corresponding to a relatively young ( ∼
20 Myr) andold ( ∼
50 Myr) age. The 147 target stars are listed in Table 1that gives a running number (Col. 1); coordinates (Cols. 2-3);data source (Col. 4: 1- only in Prosser or Giampapa et al.; 2-from Prosser and de Wit et al.; 3- only in de Wit et al.; 4- notin Prosser nor in de Wit et al. because brighter than the I m limit of that paper; 9- from IRAS or Luyten); name (Col. 5);I m and z magnitudes (Col. 6-7); radial velocity and associatederror (Col. 8-9; a “-” means that we were not able to measurethe velocity); presence (Y) or lack (N) of the Li line (Col. . Manzi: LDB in IC 4665 3 Figure 2.
Spectra of 13 stars ordered by increasing magnitude representative of the Li range: above the chasm ( ∼
672 nm are not visiblein the spectra with very low S / N due to poor sky subtraction.10: “?” means that the S / N did not allow to confirm or rejectthe presence of Li); presence of the H α line in absorptionor emission (Col. 11); and membership flag (Col. 12 –seeSect. 3.1). The I m and z magnitudes listed in the table are fromthe CFHT measurements. Note that for a large fraction of theseobjects VI photometry in the Kron system is also availablefrom Prosser (1993) and that near-infrared photometry for allthe stars was retrieved from the Two Micron All Sky Survey(2MASS) catalogue. In the following, we will use our runningnumbers given in Col. 1.The observations were carried out in Service Mode dur-ing May, June, July, and September 2004 using the FLAMESinstrument (Pasquini et al. 2002) on VLT / UT2. The spectrawere obtained with the GIRAFFE spectrograph in conjunc-tion with the MEDUSA fibre system and a 600 lines / mm grat-ing (L6). The resolving power is R ∼ / N ratio ∼
20 for the faintest tar-gets. The spectra cover the wavelength range between 643.8 nmand 718.4 nm, which includes the H α line, besides the Li i ff erent fields. The con-figurations were centered at RA(2000) =
17h 46m 46.27s andDEC(2000) =+
05d 38m 17.7s, and RA(2000) =
17h 45m 26.91sand DEC(2000) =+
05d 56m 53.7s, respectively. 94 and 53 starswere allocated in configurations A and B. We obtained six45 min exposures for configuration A and four 45 min expo-sures for configuration B. Data reduction was performed using Gira ff e BLDRS , fol-lowing the standard procedure and steps (Blecha & Simond2004). The sky contribution was subtracted separately; namely,for each configuration, we considered 15 sky spectra, subdi-vided them in three groups of five spectra, and derived the me-dian sky from each group. Then, we formed a “master” sky bytaking the average of the three median sky spectra. Due to thefact that the sky on the CCD was rather inhomogeneous and tothe presence of scattered light from the fibers allocated to verybright objects, for the faint stars an appropriate sky subtractionwas impossible to perform. For this reason, while both the Liand H α lines when present are in most cases clearly visible inthe spectrum, we prefer not to give any quantitative measure-ment of their equivalent widths.Data handling and analysis has been carried out both withMIDAS and IRAF software packages. In most cases multiplesky-subtracted spectra of the same target have been combined,after adjusting them for Doppler shift due to the motion of theearth after sky subtraction. For a few stars we excluded one ormore exposures, due to bad quality. Final S / N ratios per reso-lution element are in the range ∼ −
15. In Fig. 2 we presentsome representative spectra spanning the magnitude range cor- version 1.12 – http: // girbldrs.sourceforge.net / IRAF is distributed by the National Optical AstronomicalObservatories, which are operated by the Association of Universitiesfor Research in Astronomy, under contract with the National ScienceFoundation. S. Manzi: LDB in IC 4665 responding to the three regimes mentioned in Sect. 1: solar-type stars with a strong Li line; stars lacking Li that fall in theLi chasm; stars below the LDB, showing again the Li feature.Radial velocities (V rad ) have been measured from the aver-age shift of the spectral lines in the co-added spectra. For somecritical spectra (e.g. low S / N, suspected binary) we have de-termined V rad from the individual exposures. Measurements ofV rad were carried out using IRAF and the
RVIDLINES proce-dure. We typically used 10-20 lines per star, depending on S / N.For the faintest stars, V rad was determined using a couple oflines only. Resulting heliocentric radial velocities have errorsbetween 0.5 and 6 km / sec and are listed in Cols. 8 and 9 ofTable 1.
3. Results
In order to confirm or reject membership of the cluster candi-dates, we applied the usual radial velocity criterion, togetherwith the requirement on the presence of Li (for bright stars)and / or H α . We have first estimated the cluster average V rad andits standard deviation from the observed V rad distribution ofthe sample. In Fig. 3 we show the distribution of measuredradial velocities (stars with variable V rad are obviously notincluded in the figure): there is a clear peak at V rad ≈ − / s which indicates the presence of the cluster. The aver-age velocity was derived by fitting the observed distributionswith two gaussian functions, one for the cluster and one forfield stars; the best fit was then determined using a maximumlikelihood algorithm. For IC 4665 we find V rad (IC 4665) = − .
95 km / s and σ (IC 4665) = / s, while for the fieldwe obtain V rad (field) = − / s and σ (field) = / s.Interestingly, the field and cluster velocities are very similar,although, as expected, the distribution of the field stars is muchbroader. The cluster average velocity is compatible with pre-vious estimates, notably that from the high-mass members viz. V rad = − . / s and σ = . / s (Crampton et al. 1976). Onthe other hand, Prosser & Giampapa (1994) found the slightlyhigher value of V rad = −
13 km / s. We have considered as clus-ter members those stars with V rad within ± σ from the aver-age value. In addition, we have also included three stars ( rad slightly outside this limit, but withlarge errors on V rad and with other indicators consistent withmembership (see below). With this criterion 42 radial velocitymembers were found, with an expected statistical contamina-tion (i.e., non members with V rad consistent with the cluster) offive stars, as estimated from the fitting procedure. Furthermore,14 possible members with variable radial velocity and / or ev-idence for a double line system were considered as possiblemembers. Thus, we have a total of 56 possible candidates.The presence of H α emission and / or Li absorption providesadditional membership criteria for the V rad members and forstars without a radial velocity measurement. In Fig. 4 we showthe same color-magnitude (CM) diagram of Fig. 1, but with theadditional information on V rad and H α of each individual star.The figure clearly shows that stars with I m > ∼
14 must have H α emission in order to be members. Therefore, we considered as Figure 3.
Radial velocity distribution of IC 4665 candidatemembers. The two gaussians (solid curves) indicate the bestfits for the cluster and field, respectively.
Figure 4.
CM diagram of the observed targets with availableIz photometry including information on V rad and presence ofH α emission. Symbols are as follows: open circles - V rad mem-bers without H α emission; filled circles - V rad members withH α emission, including the three marginal V rad members; opensquares - V rad variables without H α emission; filled squares -V rad variables with H α emission or ”abs / em”; crosses - all otherstars.non members all stars fainter than this magnitude limit withoutH α emission: we found 10 such stars ( rad consistentwith membership, and six with variable V rad . We also discardedthe star α . Interestingly, the number of stars with consistent V rad ,but not satisfying the H α / Li criteria (seven) is comparable to . Manzi: LDB in IC 4665 5
Table 2.
Stars confirmed as members. The values for I c are converted from I m , as explained in the text. m I m − z I c − K s EW(Li) m I m − z I c − K s EW(Li) m I m − z I c − K s EW(Li)(mag) (mag) (mag) (mÅ) (mag) (mag) (mag) (mÅ) (mag) (mag) (mag) (mÅ)20 12.440 0.270 1.639 30 ± ± ± ± ± ±
10 145 15.373 0.584 2.591 — 85 16.706 0.637 2.661 Y62 13.074 0.233 1.567 236 ± ± ± ± that of the expected number of contaminants. Finally, we notethe presence of four stars with uncertain membership status,because we could not retrieve enough / secure information fromtheir spectra. One of them, star α emission and uncertain Li status; given its position in the CMdiagram (below the sequence), it is likely a non member. A fi-nal membership flag is given in Col. 12 of Table 1. Specifically,we assigned a “Y” status to stars with secure membership andconsistency between all indicators; we marked “Y?’ stars withtwo indicators (out of three) consistent with membership andthe three stars with V rad slightly outside the permitted range,while we gave uncertain status (“?”) to the four stars with vari-able radial velocity, H α emission, and uncertain Li line. Finally,stars that turned out to be non members are marked with “N”.In summary, of the 147 candidates, 39 stars are members (Y orY? status), 104 non members, and 4 with uncertain member-ship. The 39 confirmed members are listed in Table 2, wherewe also give the Li equivalent widths for the brighter objectsand the presence of the Li line for the faintest ones.Our analysis indicates that 27% of the sample stars arelikely cluster members, in reasonable agreement with the es-timate of de Wit et al. (2006). A detailed discussion of mem-bership and contamination in di ff erent mass bins cannot be pre-sented here, since not only our spectroscopic sample is incom-plete as a whole, but stars in di ff erent mass bins are charac-terized by varying degrees of completeness. This discussion isdeferred to a forthcoming paper (de Wit et al. in preparation)where the analysis of a much larger sample of very low-masscluster stars and brown dwarfs will be presented, based on lowresolution optical and near-IR spectra. In Fig. 5 we show the I m vs. I m − z and K s vs. I C − K s color-magnitude diagrams of the 38 likely cluster members listed inTable 2 with available photometry.A chasm and boundary are clearly present in both diagrams.We determine the observed LDB of IC 4665 from the bright- est star with secure Li detection (“Y” status in Table 2) on thefaint side of the chasm. In the I m vs. I m − z diagram this cor-responds to the star m = .
68. Note that, whereasthe Li line might be present in the spectra of stars / N of these twospectra makes the detection of Li less secure than in the caseof star m = .
47 magFor most of faint members in this paper, standard VI C pho-tometry is not available. In order to convert Mould to CousinI magnitudes, we used new photometry of IC 4665 that willbe presented in James et al. (2008). More specifically, Jameset al. have performed a shallow survey of IC 4665 in BVI C ,allowing us to derive relationships both between I m − z andV − I C colors and between I C and I m magnitudes. The re-lation between I m − z and V − I C was obtained for objectspresent in both James et al. and de Wit et al. (2006) with0 . < I m − z < .
35 or, correspondingly, 0.7 < V–I C < − I C = + × (I m − z ) + × (I m − z ) . This was thenextrapolated up to I m − z = .
75. Using the 1 σ errors on thefit and extrapolating the 1 σ upper and lower limit of the fit,delivers a range in values for V–I C of 0.3 mag. As to magni-tude conversion, we directly compared I m and I C magnitudes ofstars in common in the two surveys and found median valuesI C − I m = m − z lower and greater than 0.2 mag,respectively. The typical scatter is 0.05 mag.As a check, we estimated the expected I C − I m di ff erencefor stars of di ff erent temperatures based on the filter transmis-sion curves and found that it increases from ∼ .
03 mag at6000 K to ∼ .
06 mag at 3500 K; we derived V − I C colorsfrom V − I K colors published by Prosser (1993), employing thetransformation of Bessel (1979); from these V − I C colors and Vmagnitudes given in Prosser, we also estimated I C magnitudes.We found a good agreement between magnitudes and colorsestimated in this way and those obtained extrapolating from S. Manzi: LDB in IC 4665
James photometry: namely, ∆ (V − I C ) mean = . ± . ∆ I Cmean = . ± .
28 mag.As mentioned in Sect. 1, the distance to IC 4665 is notaccurately known with values ranging from 320 pc (Crawford& Barnes 1972) to 385 + =
425 pc (Hoogerwerf et al. 2001).Without convincing evidence for a short or a long distance,we adopt a compromise between the two extremes, i.e. a dis-tance of 370 ±
50 pc. We have also estimated our own distanceto IC 4665 by comparing the photometry (V, B–V) of high massstars in IC 4665 to the Pleiades and determining a vertical o ff -set. By assuming a distance to the Pleiades of 133 pc and E(B–V) = C ) = − V) = I c = .
37 and 8.57, respec-tively. Considering the average of the two values, we find thatthe LDB occurs at M I c = . + . − . ± .
10 mag, where the firstcontribution to the error is due to uncertainty in distance andthe second one reflects the uncertainty in the LDB determina-tion. Clearly the error is dominated by the distance uncertainty.Similarly, the brightest / faintest stars with / without Li inthe K s vs. I C − K s diagram are s = s = K = .
06 mag, this yields an LDBat K s = . + . − . ± . The age of IC 4665 can now be determined using model calcu-lations. Fig. 6 displays the time variation of the absolute I c andK s magnitudes of a star which has depleted 99 % of the initialLi abundance, according to the models of Chabrier & Bara ff e(1997). The LDB age that we derive from the two diagrams forIC 4665 is very similar, namely t LDB (M(I c )) = + . − . Myr andt
LDB (M(K s )) = + . − . Myr. Also shown in the figure are theLDBs for NGC 2547 and IC 2391. In Table 3 we summarizeLDB magnitudes, ages, and masses for the three PMS clus-ters, and using di ff erent magnitudes. Ages and masses weredetermined using the models of Chabrier & Bara ff e (1997);LDB magnitudes for IC 2391 and NGC 2547 were taken fromJe ff ries & Oliveira (2005). M bol (LDB) for IC 4665 was com-puted starting from I C and K s absolute magnitudes using thebolometric corrections (BCs) of Leggett et al. (1996) as a func-tion of V − I and I C − K s colors. Those corrections were used byJe ff ries & Oliveira (2005) and we assumed them for consis-tency; we found M bol (I C ) = .
65 and M bol (K s ) = .
84. In theprevious section and in Table 3 we considered only the uncer-tainty due to distance and LDB determination. However othersources of error are present: i. Uncertainty in the I m to I C con-version ( ∼ .
05 mag), that also reflects into I C − K s colors andthus slightly a ff ects BCs for K s magnitudes ( ∼ .
01 mag); ii. uncertainty in the I m − z to V − I C conversion ( ∼ .
3) mag, thata ff ects BCs for I C magnitudes (by ∼ . iii. uncertaintyin reddening, that should not be larger than 0.05 mag; iv. finally,the choice of the bolometric correction vs. color calibration. Byusing the calibrations of Bessel (1991) we would have obtained ∼ .
07 brighter M bol values. Each of these errors is at most ofthe same order of the uncertainty in the LDB determination andresults in an error of about 1 Myr in the LDB age. Remarkably,LDB ages listed in Table 3 are all within 2.4 Myr. In particu-lar, the LDB age from I C magnitude is very similar to that fromK s magnitude, that is not a ff ected by the error due to magnitudeconversion. The average of the four values listed in Table 3 is27.7 ± . ff erent evolutionary models, which is of the order of2 Myr, as shown by Je ff ries & Oliveira (2005). Summarizing,our LDB age for IC 4665 is 27 . + . − . ± . ± ff e (1997) modelsyield a value of M(LDB) = .
24 M ⊙ , independent of the choiceof the absolute magnitude. As expected, this mass is above thatof NGC 2547 (M(LDB) = .
17 M ⊙ ) and IC 2391 (M(LDB) = .
12 M ⊙ ).
4. Discussion
As mentioned in Sect. 1, prior to our estimate, Mermilliod(1981) was the first to determine a nuclear age for IC 4665 andincluded it in the age group of 36 Myr, along with IC 2391.Later studies have found that the cluster could be almost asold as the Pleiades (Prosser 1993; Prosser & Giampapa 1994),although Prosser (1993) noted that the sequence of clustercandidates in the I K vs. I C − K s diagram was suggestive of arather young age. Our analysis confirms the young age, makingIC 4665 the youngest cluster for which the LDB has been de-tected, and, equally important, allows us to firmly establish itsPMS status. In addition, the LDB age matches the nuclear age.This is similar to the case of NGC 2547 (Je ff ries & Oliveira2005), but at variance with IC 2391, α Per, and the Pleiadeswhere the age estimates di ff er by a factor ∼ ff erence is usuallyinterpreted as evidence for the occurrence of some convec-tive core overshooting in high-mass stars that could lengthenthe duration of the main sequence life time. The question thenarises why a large di ff erence between TO and LDB ages is in-stead not found for the two youngest clusters NGC 2547 andIC 4665. One possibility is indeed that the amount of over-shooting in models of massive stars is a step function of TOmass, with more overshooting needed for lower TO masses ofthe older clusters. Still, it is puzzling that IC 2391 and IC 4665were originally included in the same age group by Mermilliod(1981), based on the CM and color-color diagrams for highmass stars. We suggest that a careful re-analysis and compar-ison of the two cluster upper main sequence photometry andCM diagrams should be performed, taking into account thepossible e ff ects of binaries and rotation. This, along with use ofupdated stellar evolution models including overshooting, mightprovide insights on this issue. . Manzi: LDB in IC 4665 7 Figure 5. I m vs. I m − z (left panel) and K s vs. I C − K s (right panel) diagrams of the 38 likely cluster members with availablephotometry listed in Table 2. Filled and open symbols indicate stars with detected / undetected Li line, respectively. The twocrossed circles denote stars with uncertain Li detection. The theoretical position of the boundary from Chabrier & Bara ff e (1997)models for ages of 20, 30, and 40 Myr is also shown (see text), along with the 30 Myr isochrone. Note that, by definition, theLDB is actually just a point on the isochrone. What we show here, following Je ff ries & Oliveira (2005), are constant luminositycurves, with the luminosity corresponding to the LDB luminosity at a given age. These curves are not flat since, for a given M bol ,the bolometric correction changes with color (T e ff ). To construct the curves, we estimated bolometric corrections for I m and K s asfunction of color from Bara ff e et al. (1998) models. Table 3.
Properties of the LDB in IC 4665, NGC 2547, and IC 2391. The upper part lists the absolute magnitudes of the LDB.The bottom part gives the age and mass of the LDB derived from the three values of the absolute magnitudes listed in the upperpart, using the Chabrier & Bara ff e (1997) models with α =
1. Errors are the quadratic sum of the uncertainty in distance and theuncertainty in the exact magnitude of the LDB.
IC 4665 NGC 2547 IC 2391 M I c + . − . ± ± K s + . − . ± ± bol (I C ) = M bol1 + . − . ± ± bol (K s ) = M bol2 + . − . LDB(M I c ): t (Myr); M (M ⊙ ) 28.4 + . − . ; 0.24 ± .
04 35.4 ± ± ± ± K s ): t (Myr); M (M ⊙ ) 28.0 + . − . ; 0.24 ± ± ± ± ± bol1 ): t (Myr); M (M ⊙ ) 26.1 + . − . ; 0.24 ± ± ± ± ± bol2 ): t (Myr); M (M ⊙ ) 28.3 + . − . ; 0.24 ± In Fig. 7 we compare the distribution of confirmed members ofIC 4665, IC 2391, and NGC 2547 in the absolute K s magnitudevs. I C − K s diagram. The 20, 30, and 50 Myr isochrones and thepredicted location of the LDB for di ff erent ages from Bara ff eet al. (2002) are also shown, along with the ZAMS (solid line).Comparison of isochrones and datapoints up to I C − K s ∼ . ± C − K s > ∼ ff ries & Oliveira (2005)for NGC 2547, who found that the K s vs. I C − K s diagram givesan isochronal age smaller than optical diagrams. As a possi-ble explanation, these authors note that model atmospheres ofcool stars do not take into account the e ff ect of spots, plages,and magnetic activity that could result in a significant amountof I − K excess (see also Stau ff er et al. 2003). Interestingly,the e ff ect of magnetic activity would explain not only the o ff -set, but also the observed dispersion in the CM diagram. Inthis respect, we also mention the recent theoretical study byChabrier et al. (2007), where an analysis of the e ff ects of rota-tion and magnetic fields on the evolution of M dwarfs is pre-sented. They show that rapid rotation and / or magnetic field in- S. Manzi: LDB in IC 4665
Figure 6.
Location of currently known LDB of PMS clusters in absolute I c (left) and K s (right) magnitudes with uncertainties(dotted lines). Clusters are ordered from young to old: IC 4665, NGC 2547, IC 2391. The solid curve shows the predictions of theLDB as a function of age from the evolutionary models of Chabrier & Bara ff e (1997) with α = ff ective temperatures than for normalstars. As a consequence, the stars would appear younger in acolor-magnitude diagram.As to IC 2391, its sequence generally lies below thoseof IC 4665 and NGC 2547, yielding an age ∼
50 Myr ingood agreement with the LDB age. Thus, we conclude that theoverall distribution of cluster members shown in Fig. 7 indi-cates a smooth progression of increasing ages from IC 4665 toIC 2391.
5. Conclusions
We have obtained intermediate-resolution GIRAFFE spectra of147 cluster candidate low-mass members of IC 4665. The spec-tra have been used to measure radial velocities and to establishthe presence of the Li i α emission.Using these features as membership diagnostics, we have iden-tified a subsample of 39 bona-fide cluster members with a meanradial velocity of − ± − . From the distribution ofthese stars in the I m vs. I m − z and K s vs. I C − K s color-magnitudediagrams, a clear separation of stars with and without Li isfound. From this boundary, an age of 27 . + . − . ± . ± M ∗ = . ± .
04 M ⊙ . Comparison ofthe LDB age with the standard TO age from Mermilliod (1981)and that inferred from isochrone fitting of the cluster low-masssequence shows an excellent agreement, a result similar tothat found in NGC 2547 by Je ff ries & Oliveira (2005). Thisis at variance with the trend observed in older clusters (e.g.,IC 2391, α Per, Pleiades) where the LDB age exceeds the TOnuclear age by a factor of ∼ ff ort to find and characterize PMS clusters in the agerange 5–50 Myr is being actively pursued by several groups andthe first results are encouraging. In addition to the two similarclusters IC 4665 and NGC 2547 with age 25–35 Myr, two veryyoung clusters have been found with ages ∼
10 Myr, namelyNGC 7160 (Sicilia-Aguilar et al. 2005) and NGC 2169 (Je ff ries Figure 7.
The distribution of confirmed members of IC 4665,NGC 2547, and IC 2391 in the K s vs. I C − K s diagram. Filledand empty symbols represent stars of each clusters with andwithout lithium, respectively. The hatched squares are the lociof the predicted LDB at di ff erent ages, according to the modelsof Bara ff e et al. (2002). Also shown are the 20 (dot-dash), 30(dash), and 50 Myr (dotted) isochrones from the same models,along with the ZAMS (solid line). The light diagonal curvesmark the position of the LDB in each cluster, as labeled.et al. 2007), as well as several young moving groups in the solarneighborhood (e.g., TW Hya, η Cha, Cha-Near; see Zuckerman& Song 2004). Now, the next observational challenge is to fillin the age gap with clusters between ∼
10 and 30 Myr to extendour knowledge on fundamental processes related to the early . Manzi: LDB in IC 4665 9 evolution of stars and circumstellar disks, as well as on starforming process and its duration.
Acknowledgements.
We thank the ESO Paranal sta ff for performingthe service mode observations. We thank the referee, Dr. J. Stau ff er,for the very useful suggestions. We are grateful to Germano Sacco forproviding help with the maximum likelihood analysis of radial veloc-ities. This work has made extensive use of the services of WEBDA,ADS, CDS etc. WJDW is grateful for the warm hospitality and supportof the Osservatorio di Arcetri. The research of F. Palla and S. Randichhas been supported by an INAF grant on Young clusters as probes ofstar formation and early stellar evolution . References
Bara ff e, I., Chabrier, G., Allard, F., Hauschildt, P. H. 1998, A&A, 337,403Bara ff e, I., Chabrier, G., Allard, F., Hauschildt, P. H. 2002, A&A, 382,563Barrado y Navascu´es, D., Stau ff er, J. R., Jayawardhana, R. 2004, ApJ,614, 386Basri, G., Marcy, G.W., Graham, J.R. 1996, ApJ, 458, 600Basri, G. 1997, Mem. SAIt, 68, 917Bessell, M. S. 1979, PASP, 91, 589Bessell, M. S. 1991, AJ, 101, 662Bildsten, L., Brown, E. F., Matzner, C. D., Ushomirsky, G. 1997, ApJ,482, 442Blecha, A., & Simond, G. 2004, Technical report, GIRAFFE BLDRSoftware - Reference Manual Version 1.12, Observatoire deGeneveChabrier, G., Bara ff e, I. 1997, A&A, 327, 1039Chabrier, G., Gallardo, J., Bara ff e, I. 1997, A&A, 327, 1039Crampton, D., Hill, G., Fisher, W.A. 1976, ApJ, 204, 502Crawford, D.L., Barnes, J.V. 1972, AJ, 77, 862Dean, J. F., Warren, P. R., Cousins, A. W. J. 1978, MNRAS, 183, 569de Wit, W. J., Bouvier, J., Palla, F., et al. 2006, A&A, 448, 189Giampapa, M., Prosser, C.F., Fleming, T.A. 1998, ApJ, 501, 624Hogg, A.R., Kron, G.E. 1955, AJ, 60, 365Hoogerwerf, R., de Bruijne, J. H. J., de Zeeuw, P. T. 2001, A&A, 365,49James, D. et al. 2008, Proceedings of the XIV Cambridge Workshopon Cool Stars, Stellar Systems, and the Sun, G. van Belle (ed), inpressJe ff ries, R. D. & Oliveira, J. M. 2005, MNRAS, 358, 13Je ff ries, R. D., Oliveira, J. M., Naylor, T., Mayne, N.J., Littlefair, S.P.2007, MNRAS, 376, 580Leggett, S.K., Allard, F., Berriman, G., Dahn, C.C., Hauschildt, P.H.1996, ApJS, 104, 117Luyten, W. J. LB 1961, C24, 1LMart´ın, E.L., Montes, D. 1997, A&A, 318, 805Mermilliod, J. C. 1981, A&A, 97, 235MMermilliod, J. C. 2000, ASP Conf. Series, 198, 105Pasquini, L., Avila, G., Blecha, A., et al. 2002, Messenger, 110, 1Prosser, C. F. 1993, AJ, 105, 1441PProsser, C. F., Giampapa, M. S. 1994, AJ, 108, 964PSicilia-Aguilar, Hartmann, L.W., Hern´andez, J., Brice˜no, C., Calvet,N. 2005, AJ, 130, 188Stau ff er, J. R., Schultz, G., Kirkpatrick, J. D. 1998, ApJ, 499, L199Stau ff er, J. R., Barrado y Navascu´es, D., Bouvier, J., et al. 1999, ApJ,527, 219Stau ff er, J.R. 2000, ASP Conf. Ser., 198, p. 255Stau ff er, J.R., Jones, B.F., Bouvier, J., et al. 2003, AJ, 126, 833Ushomirsky, G., Matzner, C.D., Brown, E.F., et al. 1998, ApJ, 497,253Zuckerman, B., Song, I. 2004, ARA&A, 42, 685. Manzi: LDB in IC 4665 , Online Material p 1
Online Material . Manzi: LDB in IC 4665 , Online Material p 2
Table 1.
The 147 candidate members of IC 4665 observed with FLAMES. α δ Ref. Name I m z V rad δ V rad Li H α Mem.(mag) (mag) (km / s) (km / s)1 17 44 45.015 +
06 02 11.45 4 D.08.2.901 14.691 14.228 − +
06 01 52.78 1 P176 — — var — N abs N3 17 44 48.120 +
06 02 01.22 3 D.08.2.877 15.350 14.731 − +
05 55 13.39 3 B.05.30.194 17.461 16.764 var — ? em ?5 17 44 51.438 +
05 49 53.39 1 P188 — — − +
05 48 35.43 1 P059 12.614 12.346 − +
05 49 40.43 1 P195 15.763 15.395 − +
05 47 20.14 3 A.00.30.2558 16.390 15.74 − +
05 51 32.93 1 P060 12.623 12.295 − +
05 56 22.32 1 P202 15.529 15.116 − +
05 51 55.58 1 P204 16.537 16.102 SB2 — ? em ?12 17 45 02.388 +
06 07 32.45 4 D.09.2.1536 12.841 12.526 − +
05 54 33.05 1 P206 13.674 13.319 13.4 1.0 N abs N14 17 45 07.463 +
05 50 59.39 1 P064 13.035 12.766 − +
05 49 21.50 1 P214 14.124 13.863 − +
05 46 17.38 1 P215 13.413 13.154 − +
05 47 57.40 1 P216 16.074 15.614 − +
05 56 06.81 1 P217 15.757 15.246 − +
05 45 25.30 1 P220 17.183 16.743 — — N abs N20 17 45 12.935 +
05 49 50.54 1 P065 12.440 12.17 var — Y abs Y?21 17 45 13.301 +
05 55 34.93 1 P222 13.651 13.312 − +
05 49 42.24 1 P067 13.912 13.66 57.2 1.5 N abs N23 17 45 15.190 +
06 07 44.77 3 D.09.30.3076 16.975 16.316 − +
05 53 56.41 1 P227 15.439 15.012 − +
05 44 58.84 1 GPF98-R05 17.265 16.639 − +
05 47 40.13 1 P071 12.696 12.493 − +
05 46 29.53 1 P232 14.147 13.804 − +
05 53 20.74 1 P233 16.074 15.501 − +
05 54 27.68 4 A.00.2.270 12.761 12.483 − +
05 58 25.67 4 D.09.2.377 12.823 12.528 28.3 1.5 N abs N31 17 45 23.174 +
05 57 06.51 1 P238 16.928 16.314 − +
06 00 07.91 3 D.09.30.1207 16.486 15.841 − +
05 51 38.75 1 P075 12.697 12.492 − +
05 45 09.24 3 A.01.30.3322 17.716 16.964 — — N abs N35 17 45 30.051 +
05 48 49.04 2 P242 15.139 14.719 25.6 1.0 N abs N36 17 45 30.139 +
05 47 07.46 3 A.01.2.1244 14.913 14.405 − +
05 58 22.28 4 D.10.2.409 12.883 12.595 22.0 1.8 N abs N38 17 45 33.091 +
05 46 24.35 1 P155 12.488 12.111 − +
05 49 04.33 1 P250 13.513 13.25 − +
05 53 53.40 1 P155 12.830 12.515 71.7 1.0 N abs N41 17 45 37.837 +
05 45 33.41 1 P251 15.201 14.861 var — N abs N42 17 45 38.302 +
05 44 44.30 1 P253 16.226 15.812 − +
05 45 16.25 1 P258 — — — — N abs N44 17 45 41.023 +
05 54 23.47 1 P260 15.378 15.005 − +
05 55 41.13 4 D.10.2.38 13.177 12.837 − +
05 55 56.46 1 P262 17.152 16.624 — — N abs N47 17 45 44.987 +
05 49 51.57 9 LB-3885 17.059 16.943 70.0 4.0 N abs N48 17 45 56.008 +
05 52 45.17 1 P272 16.570 16.069 SB2 — N abs N49 17 45 56.492 +
05 48 44.64 1 K057 — — var — N abs N50 17 45 56.733 +
05 52 24.04 1 P273 14.505 14.135 6.1 1.0 N abs N51 17 45 59.539 +
05 50 45.52 1 P276 13.654 13.395 16.0 1.5 Y abs N52 17 45 59.912 +
05 36 18.05 1 P277 16.098 15.649 − +
05 37 11.58 1 P278 15.458 15.101 10.0 1.0 N abs N54 17 46 01.604 +
05 36 52.79 2 P279 15.155 14.695 20.0 1.0 N abs N55 17 46 03.252 +
05 33 12.58 1 P283 15.474 14.987 − +
05 50 57.51 1 P284 16.435 15.935 − +
05 49 42.63 2 P285 15.813 15.335 − +
05 41 54.62 2 P286 15.442 15.008 − +
05 40 58.13 1 P290 13.443 13.134 var / SB2 — Y abs / em Y?. Manzi: LDB in IC 4665 , Online Material p 3
Table 1. continued. α δ R Name I m z V rad δ V rad Li H α Mem.(mag) (mag) (km / s) (km / s)60 17 46 10.312 +
05 30 56.34 3 A.08.30.655 16.940 16.241 − +
05 42 21.47 1 P292 — — 20.0 10.0 N abs N62 17 46 11.975 +
05 41 25.85 1 P100 13.074 12.841 − +
05 38 54.72 1 P296 13.940 13.642 − +
05 30 21.42 2 P298 16.203 15.681 − +
05 36 17.15 1 P300 16.916 16.365 − +
05 29 25.29 1 P101 12.761 12.359 − +
05 33 51.83 1 P303 15.542 15.110 − +
05 45 08.95 1 P306 13.344 13.068 28.8 0.5 N abs N69 17 46 18.999 +
05 46 20.37 1 P309 — — − +
05 45 00.19 1 P311 16.226 15.747 −
170 10 N abs N71 17 46 21.270 +
05 29 14.80 2 P313 16.736 15.995 var — Y em Y?72 17 46 23.313 +
05 37 17.87 2 P315 14.876 14.398 var — N em Y?73 17 46 23.892 +
05 47 26.94 1 P317 14.151 13.804 6.4 0.7 N abs N74 17 46 24.778 +
05 35 38.13 1 P108 12.718 12.542 − +
05 29 28.57 2 P320 16.096 15.559 20.0 5.0 N em N76 17 46 28.403 +
05 40 18.02 1 P322 13.858 13.598 35.5 0.6 N abs N77 17 46 28.923 +
05 33 45.03 1 P323 15.650 15.290 − +
05 31 19.95 1 P113 13.421 13.174 − +
05 28 45.69 1 P326 13.672 13.303 − +
05 30 32.08 1 P328 14.353 14.044 − +
05 29 13.98 1 P329 15.589 15.162 SB2 — N abs N82 17 46 31.736 +
05 28 35.03 3 A.09.30.47 15.940 15.291 − +
05 40 54.06 1 P331 13.705 13.449 − +
05 48 53.10 1 P332 13.231 12.979 10.1 1.2 N abs N85 17 46 34.731 +
05 33 33.56 2 P333 16.706 16.069 − +
05 26 28.85 1 P334 — — − +
05 36 10.62 2 P335 15.902 15.318 − +
05 31 07.58 2 P336 14.910 14.443 − +
05 40 06.38 1 P337 16.568 16.105 5.0 1.0 N abs N90 17 46 38.379 +
05 35 48.63 2 P338 16.685 16.100 − +
05 28 54.13 1 P339 13.822 13.455 52.6 0.6 N abs N92 17 46 40.649 +
05 40 19.82 1 P341 16.819 16.329 − +
05 28 21.36 3 A.09.30.14 16.840 16.118 − +
05 49 02.67 1 P344 16.650 16.047 − +
05 44 18.42 1 P343 15.228 14.778 − +
05 33 41.98 1 P346 13.988 13.715 − +
05 35 14.01 1 P347 14.768 14.407 27.0 1.0 N abs N98 17 46 43.770 +
05 30 07.65 1 P348 15.741 15.177 − +
05 26 58.24 1 P349 13.306 13.043 119.8 0.7 Y abs N100 17 46 45.359 +
05 45 06.06 2 P350 16.475 15.854 − . +
05 37 09.58 9 LB3900 17.270 17.018 − +
05 35 06.85 1 P352 13.325 13.108 43.2 0.5 N abs N103 17 46 46.736 +
05 49 09.35 1 P119 12.807 12.526 − +
05 47 05.15 1 P120 13.251 12.965 10.4 0.6 N abs N105 17 46 47.622 +
05 31 38.05 2 P354 15.278 14.800 − +
05 47 58.74 1 P121 — — 84.0 2.6 N abs N107 17 46 48.743 +
05 43 00.62 1 P122 13.803 13.504 − +
05 41 59.01 2 P359 16.276 15.757 − +
05 48 54.98 1 P360 15.853 15.451 − +
05 36 41.06 1 P362 15.785 15.41 − +
05 31 27.73 1 P364 16.027 15.614 − +
05 32 36.92 1 P123 14.059 13.780 49.9 0.4 N abs N113 17 46 53.013 +
05 32 47.02 1 P366 14.083 13.799 − +
05 31 26.18 1 P367 17.898 17.292 − +
05 41 59.74 3 A.10.30.3980 17.854 17.108 var – ? em ?116 17 46 54.990 +
05 37 45.17 1 P126 13.465 13.149 101 0.8 N abs N117 17 46 55.159 +
05 41 12.70 2 P369 16.322 15.715 − +
05 30 29.42 2 P370 15.412 14.962 6.0 4.0 N abs N. Manzi: LDB in IC 4665 , Online Material p 4
Table 1. continued. α δ R Name I m z V rad δ V rad Li H α Mem.(mag) (mag) (km / s) (km / s)119 17 46 56.016 +
05 38 34.99 2 P372 16.608 16.028 − +
05 47 44.62 1 P374 14.554 14.155 − +
05 36 07.30 2 P373 16.169 15.537 − +
05 47 26.03 1 P375 13.377 13.111 − +
05 45 44.77 3 A.04.2.1503 14.966 14.422 − +
05 30 10.30 1 P129 12.509 12.149 − +
05 30 29.04 1 P377 13.680 13.403 − +
05 45 22.94 1 P379 16.281 15.799 − +
05 30 45.41 2 P378 17.119 16.568 — — N abs N128 17 47 03.098 +
05 34 46.35 2 P380 16.116 15.544 − +
05 43 31.34 1 P132 12.882 12.612 − +
05 28 04.14 1 P382 13.829 13.587 − +
05 36 14.70 1 P383 14.467 14.221 25.0 1.0 N abs N132 17 47 08.518 +
05 37 37.65 4 A.10.2.1392 13.025 12.744 − +
05 28 13.37 1 P385 17.043 16.116 — — N abs N134 17 47 09.961 +
05 43 14.06 2 P387 15.674 15.239 − +
05 29 06.42 1 P386 14.712 14.175 − +
05 39 55.98 1 P390 13.596 13.324 61.1 0.6 N abs N137 17 47 11.660 +
05 33 09.98 3 A.10.30.1418 17.250 16.598 var — ? em ?138 17 47 11.865 +
05 29 24.78 3 A.10.30.316 16.236 15.566 − +
05 43 00.54 9 IRAS17447 +
054 — — SB2? — N abs N140 17 47 12.480 +
05 36 33.85 1 P137 13.058 12.713 67.8 0.9 N abs N141 17 47 12.532 +
05 42 14.95 1 P394 — — − +
05 29 49.50 1 P396 13.844 13.457 − +
05 31 24.08 4 A.11.2.358 12.509 12.204 33.7 0.5 N abs N144 17 47 19.490 +
05 44 47.38 3 A.05.30.3622 17.248 16.547 − +
05 30 41.39 2 P398 15.373 14.789 − +
05 43 40.74 1 P399 16.290 15.745 − +
05 46 53.71 2 P400 15.269 14.815 −−