Discovery of "isolated" comoving T Tauri stars in Cepheus
aa r X i v : . [ a s t r o - ph . S R ] S e p Astronomy&Astrophysicsmanuscript no. Cepheus˙MV˙Published c (cid:13)
ESO 2018October 15, 2018
Discovery of “isolated” comoving T Tauri stars in Cepheus ⋆,⋆⋆ (Research Note)
P. Guillout , A. Frasca , A. Klutsch , , E. Marilli , and D. Montes Observatoire Astronomique, Universit´e de Strasbourg & CNRS, UMR 7550, 11 rue de l’Universit´e, 67 000 Strasbourg, France INAF - Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy Departamento de Astrof´ısica y Ciencias de la Atm´osfera, Universidad Complutense de Madrid, E-28040 Madrid, SpainReceived 15 April 2010 / Accepted 19 July 2010
ABSTRACT
Context.
During the course of a large spectroscopic survey of X-ray active late-type stars in the solar neighbourhood, we discoveredfour lithium-rich stars packed within just a few degrees on the sky. Although located in a sky area rich in CO molecular regions anddark clouds, the Cepheus-Cassiopeia complex, these very young stars are projected several degrees away from clouds in front of anarea void of interstellar matter. As such, they are very good “isolated” T Tauri star candidates.
Aims.
We present optical observations of these stars conducted with 1–2 meter class telescopes. We acquired high-resolution opticalspectra as well as photometric data allowing us to investigate in detail their nature and physical parameters with the aim of testing the“runaway” and “in-situ” formation scenarios. Their kinematical properties are also analyzed to investigate their possible connectionto already known stellar kinematic groups.
Methods.
We use the cross-correlation technique and other tools developed by us to derive accurate radial and rotational velocitiesand perform an automatic spectral classification. The spectral subtraction technique is used to infer chromospheric activity level inthe H α line core and clean the spectra of photospheric lines before measuring the equivalent width of the lithium absorption line. Results.
Both physical (lithium content, chromospheric, and coronal activities) and kinematical indicators show that all stars are veryyoung, with ages probably in the range 10–30 Myr. In particular, the spectral energy distribution of TYC 4496-780-1 displays a strongnear- and far-infrared excess, typical of T Tauri stars still surrounded by an accretion disc. They also share the same Galactic motion,proving that they form a homogeneous moving group of stars with the same origin.
Conclusions.
The most plausible explanation of how these “isolated” T Tauri stars formed is the “in-situ” model, although accuratedistances are needed to clarify their connection with the Cepheus-Cassiopeia complex. The discovery of this loose association of“isolated” T Tauri stars can help to shed light on atypical formation processes of stars and planets in low-mass clouds.
Key words. stars: fundamental parameters – stars: pre-main sequence – stars: formation – stars: kinematics – X-rays: stars
1. Introduction
Although most studies of very young low-mass stars have con-centrated on star-forming regions (SFRs) or young clusters, ithas become increasingly evident that a considerable number ofvery young stars unrelated to prominent SFRs is present in thesolar neighbourhood. Their existence far from interstellar ma-terial is di ffi cult to reconcile with the standard picture of starformation. Di ff erent scenarios, such as “runaway” T Tauri stars(Sterzik & Durisen 1995, 1998) or formation in small turbulentclouds (Feigelson 1996), have been proposed as possible expla-nations. Most of the comoving associations of nearly coeval stars(i.e., moving groups (MG)) discovered so far are located in thesouthern hemisphere and may be tied to the Scorpius-Centaurusassociation (see, e.g., Zuckerman & Song 2004 and Torres et al.2008).Single active stars in the field, selected on the basis of theirhigh coronal emission, are mostly young stars with an age of Send o ff print requests to : P. Guilloute-mail: [email protected] ⋆ Based on observations collected at the
Observatoire de HauteProvence (OHP, France) and the
Catania Astrophysical Observatory (OAC, Italy). ⋆⋆ Figure 5 is only available in electronic form at a few 100 Myr, i.e. in the zero-age main sequence (ZAMS), oreven younger: post-T Tauri stars (PTTS) or T Tauri stars (TTS).Guillout et al. (2009) conducted a large ground-based observingprogram designed to perform the physical characterization of ≈ RasTyc stellar X-ray sources (Guillout et al. 1999) byanalysing their high-resolution optical spectra. Although thesky density of the youngest stars (identified thanks to theirvery high-lithium content) is more or less uniform (Klutsch2008), we discovered an unusual group of four lithium-richstars towards the Cepheus-Cassiopeia (Cep-Cas) complex (see,e.g., Kun et al. 2008, and references therein). However, theirspace distribution in a wide region devoid of dense CO cloudsand their stellar properties make the relation between these starsand the Cep-Cas complex uncertain. Young stars in the skyregion around the Cep-Cas complex are indeed not necessarilyassociated with these SFRs. A good example is representedby the nearby young visual binary HIP 115147 (V368 Cep)and its reported comoving companion (Makarov et al. 2007),which are also projected towards the same region of the sky.HIP 115147 is currently classified as a very young “naked”PTTS (Nations et al. 1990; Chugainov et al. 1991, 1993), 20to 50 Myr old, located at 20 pc whose origin remains disputedalthough it is certainly unrelated to the Cep-Cas complex(Montes et al. 2001a; L´opez-Santiago et al. 2010).
P. Guillout et al.: Discovery of “isolated” comoving T Tauri stars in Cepheus (RN)
Table 1.
Main data of our sources from the literature and dates of observations.
Name RasTyc source α (2000) δ (2000) V T ( B − V ) T PSPC Li obs. H α obs.h m s ◦ ′ ′′ (ct / s)BD +
78 853 RasTyc0000 + +
79 40 36.9 10.35 0.79 0.10 2001 / /
10 2001 / / + +
77 02 10.9 9.83 0.70 0.10 2001 / /
12 2001 / / + +
79 03 28.5 10.37 1.04 0.11 2003 / /
14 2009 / / +
78 19 RasTyc0039 + +
79 05 30.8 9.67 0.77 0.05 2002 / /
02 2002 / / Table 2.
Johnson-Cousins and Str¨omgren photometric data.
Name
V R C I C B − V U − B b − y m c BD +
78 853 10.29 (0.05) 9.77 a b a c a (0.03) 9.96 d (0.03) 9.60 d (0.02) 0.87 (0.03) 0.40 (0.02) 0.572 0.243 0.400BD +
78 19 9.57 (0.03) 9.11 (0.03) 8.89 (0.02) 0.72 (0.03) 0.09 (0.02) 0.457 0.218 0.260 a Zacharias et al. (2004) b Monet et al. (2003) c Droege et al. (2006) d Corrected from the contribution of the nearby companion.
In the present paper, we concentrate on the four stars that wepreviously discovered and study their properties, evolutionarystatus, and kinematics. The observations and data reduction areoutlined in Sect. 2. The determination of their stellar parametersas well as the level of chromospheric activity, lithium content,and age are briefly discussed in Sect. 3. We discuss the prop-erties of each star under scrutiny in Sect. 4. We conclude andsummarize the results from the ongoing study of these sourcesin Sect. 5.
2. Observations and data reduction
Spectroscopic observations were conducted in 2001, 2002, and2003 (see Table 1) at the OHP 1.52-m telescope using the
Aurelie spectrograph to acquire spectra in both the H α ( λλ ≈ − λλ ≈ − R ≈
38 000. An echelle spectrum at R ≈
21 000of TYC 4500-1478-1 was also taken with the FRESCO spectro-graph of the 0.91-m telescope of the OAC in 2009. For detailsabout the reduction of
Aurelie and FRESCO spectra, the readeris referred to Guillout et al. (2009).In 2008, complementary photometric observations were per-formed in the standard
U BV system and Str¨omgren uvby filterswith the 91-cm telescope of the OAC and a photon-counting re-frigerated photometer equipped with an EMI 9893QA /
350 pho-tomultiplier, cooled to − ◦ C. For details about the reduction ofphotometric data, the reader is referred to Frasca et al. (2006).CCD images in Johnson-Cousins V , R C , and I C filters werealso acquired for two stars using the OAC focal-reducer imag-ing camera. The reduction of these data was performed follow-ing standard methods and the VR C I C magnitudes were extractedfrom the corrected images by means of aperture photometry.Photometric data are summarized in Table 2.
3. Astrophysical parameters, chromosphericactivity, and lithium abundance
High-resolution spectroscopic observations allow us to deriveradial ( RV ) and projected rotational velocities ( v sin i ), spec-tral type, luminosity class, and metallicity. They also enable usto identify spectroscopic binaries. Cross-correlation functions(CCFs) were computed to derive radial velocities (Table 4) andwe used the ROTFIT code (Frasca et al. 2003, 2006) to evalu-ate e ff ective the temperature ( T e ff ), gravity (log g ), metallicity ([Fe / H]), and v sin i of our targets (Table 3). We were also able toinfer both the level of chromospheric activity (from the emissionin the H α line core) and the age (from the lithium abundance).The equivalent width of the lithium line ( W Li ) and the net equiv-alent width of the H α line ( W em H α ) were measured in the spectrumobtained after subtracting the non-active template by integrat-ing the residual H α and lithium profile. We refer to Frasca et al.(2006) and Guillout et al. (2009) for a more detailed presenta-tion of the methods we used to derive lithium abundances, alongwith chromospheric and coronal luminosities (Table 3).As an independent estimate of the main astrophysical param-eters, we evaluated the e ff ective temperature, radius, and metal-licity of these stars from our Str¨omgren photometry using the uvby β algorithm (Moon 1985), which also provides an estimateof the color excess E ( b − y ). As seen in Table 3, the values ofe ff ective temperature derived from spectroscopy and Str¨omgrenphotometry agree with each other to within 200 K. The metallic-ity index δ m is in substantial agreement with the spectroscopicvalue [Fe / H] ≈ g valuesexcluded the possibility of lithium-rich giant stars.
4. Discussion
The high resolution spectra and CCFs of our stars are shownin Figs. 1 and 5 for both H α and Li i spectral regions. Thecharacteristics of the stars, derived from the analysis of thespectra acquired in the H α and lithium spectral regions, are nowsummarized below: BD +
78 853 : Although both the H α and lithium spectradisplay broad absorption lines, which are sometimes asymmet-ric, the automatic procedure failed to detect double peaks inany of the CCF. The H α profile is seen in absorption, but theanalysis found that the line is partly filled. The equivalent widthof the lithium line is W Li = N (Li) = +
78 853 isyoung. The rotational velocity derived from the line broadeningdecreases by a factor two or so from the H α ( v sin i ≈
50 km s − )to lithium spectra ( v sin i ≈
28 km s − ). This could be indicativeof a spectroscopic binary, but may also be the result of a di ff erent Available in electronic form only.. Guillout et al.: Discovery of “isolated” comoving T Tauri stars in Cepheus (RN) Table 3.
Astrophysical parameters from spectroscopy and Str¨omgren photometry, lithium abundance, and chromospheric / coronalluminosities. Name T ff log g [Fe / H] T ff R δ m E ( b − y ) W Li / log N (Li) W em H α / log L H α log L X (K) (K) (R ⊙ ) (mag) (Å) / (Å) / (erg s − ) (erg s − )BD +
78 853 5738 4.19 − .
03 5750 0.98 0.032 0.068 0.18 / / − .
04 5650 0.99 0.012 0.024 0.18 / / − .
06 5330 0.88 0.052 0.099 0.30 / / +
78 19 5444 4.14 − .
11 5630 0.96 0.003 0.040 0.22 / / Derived from spectra Derived from Str¨omgren photometry
CCF shape in the two spectral domains. The radial velocitiesderived from H α and Li i spectral regions are compatible within0.2 km s − at RV = − . − . Our present data suggest thatBD +
78 853 is a single star that is rotating relatively quickly, butthey do not exclude a spectroscopic binary system. More dataare needed to settle this point.
TYC 4496-780-1 : The H α line exhibits an absorption featureroughly centered on a strong asymmetric H α emission profile.The Li i λ W Li = N (Li) = +
78 853. In both spectra, the lines are very broad andthe code detected two Doppler-shifted dips (at − . − ) in the H α CCF with very di ff erent depths, sug-gesting a binary in which the two stars of the system havesignificantly di ff erent luminosities. In the lithium CCF, thefootprint of a secondary component is clearly visible (althoughnot detected by the automatic analysis) as an asymmetryon the red side of the main CCF dip, which is centered onabout − − . We derived an RV ≈
25 km s − for thesecondary dip, which would imply an RV variation of + −
20 km s − for the primary and secondary component, respec-tively, of this SB2 candidate. The projected rotational velocityof TYC 4496-780-1 is estimted to be v sin i = + / −
10 km s − . TYC 4500-1478-1 : The FRESCO spectrum displays an H α profile markedly filled-in by emission (see Fig. 1). The lithiumregion shows very sharp absorption lines and exhibits a verystrong Li i λ W Li = N (Li) = RV = − . − ) andprojected rotational velocity ( v sin i ≈ − ). Unless we haveobserved a binary system at a conjunction, we consider TYC4500-1478-1 as a single star. BD +
78 19 : The H α spectrum is typical of a mid-G type starwithout any remarkable characteristics. Both RV = − . − and v sin i ≈
12 km s − were derived from the Gaussian fit of theCCF. Apart from the observed Li i λ W Li = N (Li) = +
78 19 may bea binary system of mass ratio ≈ Aurelie close toconjunction at both epochs.The chromospheric and coronal luminosities (see Table 3)were computed assuming that all stars are 15 Myr old (seeTable 4 for the adopted distances d and Sect. 4.3 for its justification). We found X-ray luminosities L X ≃ . erg s − for all sources (within 0.2 dex), which is typical of weak-line T Tauri stars (WTTS) in Taurus-Auriga-Perseus SFRs. Forboth BD +
78 853 and BD +
78 19, we found L H α = . erg s − ,a value slightly higher than that found by Guillout et al. (2009)for their PTTS candidates and about an order of magnitude lowerthan that in old binary systems, in which the coupling of spinand orbital motions by tidal actions can maintain high chromo-spheric activity for a very long time (Frasca et al. 2006). The H α luminosity of TYC 4496-780-1 is an order of magnitude higherthan that of BD +
78 853 and BD +
78 19, but both the shape of theH α profile and the IR excess (see Sect. 4.2) infer that accretionis the primary cause of the observed emission. The standard
U BVR C I C photometry (Table 2), complementedwith JHK s magnitudes from the 2MASS catalogue (Cutri et al.2003), allowed us to reconstruct the spectral energy distribution(SED) from the optical to the near infrared (IR) domain for allsources.We used the grid of NextGen low-resolution synthetic spec-tra, with log g = . ff ective temperature( T e ff ) was kept fixed to the value derived with ROTFIT (Table 3)and we allowed the interstellar extinction ( A V ), which a ff ectsthe SED shape particularly at the shortest wavelengths, andthe angular diameter, which scales the model surface fluxto the stellar flux at Earth, free to vary. The Cardelli et al.(1989) extinction law with R V = . U BVRI J data, which aredominated by the photospheric flux of the star and are normallynot appreciably a ff ected by infrared excesses. The A V valuesare in the range 0.06–0.2 mag, in agreement with the colorexcesses E ( b − y ), which were inferred from the Str¨omgrenphotometry. The rather low A V values are typical of sourcesat ≈ φ ), all on the order of 0.1 mas, are reported in Table 4.Figure 2 displays the results of our fitting procedure. Asis clearly evident, the SEDs are reproduced well by the syn-thetic spectrum blueward of the K s band, with the exception ofTYC 4496-780-1, which displays a strong near- and far-IR ex-cess (IRAS fluxes). The lack of significant IR excess in threeof our targets implies that these stars are likely to be eitherWTTS or PTTS. The slope of the SED between K s and 24 µ m, α ≃ − .
1, allows us to tentatively classify TYC 4496-780-1 asa Class II young infrared source, i.e. a T Tauri star surroundedby an accretion disc, according to the definition of Lada (1987).
P. Guillout et al.: Discovery of “isolated” comoving T Tauri stars in Cepheus (RN)
Fig. 1.
Spectra (black line) in the H α ( upper panels ) and lithium ( lower panels ) region of the Cepheus comoving stars, and thetemplate spectra (red lines) of non-active lithium-poor reference stars broadened to the v sin i of the targets and Doppler-shiftedaccording to their RV . Fig. 2.
Spectral energy distributions of the four stars (dots). The best-fit NextGen spectrum is overplotted with continuous lines ineach box.
Table 4.
Parameters deduced from the SEDs and kinematics. The proper motions are from the TYCHO-2 catalogue.
Name φ d ZAMS d µ α cos δ µ δ RV U
ZAMS V ZAMS W ZAMS U V W (mas) (pc) (pc) (mas yr − ) (km s − ) (km s − ) (km s − )BD +
78 853 0.088 105 175 23 . − . − . . − . − . − . − . . . − . . − . − . − . − . . − . − . . − . − . − . − . +
78 19 0.092 90 155 23 . − . − . . − . − . − . − . This classification is consistent with its H α emission profile.Similar H α line profiles are often observed in accreting TTS(Appenzeller et al. 2005). According to the concept of the mag-netospheric accretion model, the large line width (several hun-dred km s − ) is indicative of large-scale gas flows, while the in-verse P Cygni profile traces the gas infall (Muzerolle et al. 2001;Kurosawa et al. 2006). The spatial distribution of BD +
78 853, TYC 4496-780-1, TYC4500-1478-1 and, BD +
78 19 is shown in Fig. 3 overplotted onthe extinction map (Av) published by Dobashi et al. (2005), ina similar way to Fig. 2 of Tachihara et al. (2005). The posi-tion of the nearby young visual binary HIP 115147 and its co-moving companion, discovered by Makarov et al. (2007), is alsoplotted together with the “isolated” young stars discovered byTachihara et al. (2005). The closest prominent star-forming re-gion is the Cep-Cas complex, which surrounds our stars. They are located in particular, at Galactic latitudes 14 ◦ < b < ◦ ,on the west side of the Cepheus flare , a region of significant ex-tinction outside the Galactic plane first recognized by Hubble(1934). Across the
Cepheus flare region, Kun (1998) determinedthe distances to several dark clouds and concluded that the inter-stellar matter is concentrated at 200, 300, and 450 pc (althoughthe whole Cep-Cas complex extends to 800 pc or so) separatedroughly at regular intervals along Galactic latitudes. Olano et al.(2006) suggested that the
Cepheus flare forms part of a largeshell, 50 pc in radius, expanding at 4 km s − , that encloses anold supernovae remnant (Grenier et al. 1989), whose center canbe located at l = ◦ , b = ◦ . The Cepheus flare is a region ofactive star formation as demonstrated by Kun et al. (2009), whodiscovered about 70 new PMS stars in the star-forming clouds.All the four stars investigated by ourselves are projected infront of the void located between the dark clouds LDN 1333(west), LDN 1241-1251 (east), and north of LDN 1259-1262.A group of seven stars, including TYC 4500-1478-1, locatedin this void and displaying typical T Tauri characteristics, wasidentified by Tachihara et al. (2005). For a few stars, including . Guillout et al.: Discovery of “isolated” comoving T Tauri stars in Cepheus (RN) Fig. 3.
Spatial distribution of our “isolated” comoving TTS (bigdiamonds) overplotted on a map of dark clouds enhancing in-terstellar material. The “isolated” young stars previously discov-ered in the CO void region are shown with square symbols, whilethe young “naked” PTTS HIP 115147 and its comoving com-panion are indicated by small asterisks. The major clouds in thevicinity are also labelled.TYC 4500-1478-1, they found a lithium equivalent width closeto that often measured at the Pleiades upper envelope, implyinga PTTS or ZAMS evolutionary status for them. Although theyconsider these stars to be associated with the Cep-Cas complex,only on the basis of their proximity to this region, and adopt adistance of 200 pc for them, their relation to the cloud complexremains uncertain.The distance of our stars is crucial in determining theirorigin and a possible association with the Cep-Cas complex.Unfortunately, the Tycho parallaxes are useless and one mustrely on photometric distance estimates. To cover all the possi-bilities, we estimated for each star two distances that we as-sume to be a lower and upper limit. The lower limit was cal-culated for the hypothesis that the star is on the ZAMS. Thestar radius of a ZAMS star with the same T e ff was adopted andthe distance was deduced from the angular diameter φ reportedin Table 4. The upper limit was estimated assuming a mini-mum age and the corresponding radius was computed from theSiess et al. (2000) evolutionary tracks. An age of 5 Myr wouldplace stars at distances mostly exceeding 250 pc, i.e. fartherthan the closest boundary of the Cepheus flare shell in thatdirection. This would contradict our photometric observationsthat no stars experience significant interstellar extinction. Anage older than ≈ d ZAMS and d , respectively) are reported in Table 4.As seen in this table, the ZAMS assumption would placeour stars roughly within 100 pc of the Sun ruling out theirassociation with the Cep-Cas complex whose CO clouds arefarther away by nearly a factor of two. Needless to say thatthis assumption is also hardly compatible with the detection ofthe IR excess from an accretion disc around TYC 4496-780-1.On the other hand, the
15 Myr assumption would place themroughly at the distance of LDN 1333, LDN 1261, and LDN 1228
Fig. 4. U − V kinematic diagram for our comoving stars (labelledas in Table 5). Blue circles and red triangles refer to the
ZAMS and
15 Myr assumptions , respectively. The average velocitycomponents (dots) of some young SKG and those of some late-type stars members of these young SKG are also plotted (square,triangle, circle, upside -down triangle, and U symbols for theIC 2391 supercluster, Pleiades, Castor, UMa moving groups, andHyades supercluster, respectively). The loci of the young-disc(YD) and old-disc (OD) populations are also marked. The posi-tion of HIP 115147 is plotted as a filled square.(see Fig.5 of Kun et al. 2008) and can thus partly reconciletheir possible association with at least the nearest clouds of the
Cepheus Flare .Additional crucial information to elucidate their origin ten-tatively can be derived from kinematics. Because errors in thedistances are the major uncertainties (proper motions and radialvelocity errors are smaller than or equal to 2 mas and 1.5 km s − respectively), we computed the heliocentric space velocity com-ponents, UVW , in a left-handed coordinate system, both withdistances corresponding to ZAMS radii ( U ZAMS V ZAMS W ZAMS )and adopting the d distances ( U V W ), i.e. as-suming that all stars are located in proximity to the closest cloudsof the Cep-Cas complex. The RV values measured on our spec-tra and Tycho-2 proper motions (Høg et al. 2000) have beenused. The positions of our four stars and HIP 115147 in the U - V kinematical diagram are shown in Fig. 4, together with themean position of the major stellar kinematics groups (SKG) dis-cussed in Montes et al. (2001b), namely the IC 2391 superclus-ter ( ∼
50 Myr), the Pleiades SKG ( ∼
100 Myr), the Castor SKG( ∼
200 Myr), the Ursa Major (UMa) group ( ∼
300 Myr), and theHyades supercluster ( ∼
600 Myr). Figure 4 readily shows that,whatever the distance is, all our stars share the same kinematics(within a few km s − ) proving that they form a homogeneousMG with the same origin. We also note that both distances andkinematics seem to rule out an origin in common origin withHIP 115147.To assess their membership to the aforementioned SKGs onmore objective grounds, we used the probabilistic approach de-scribed in Klutsch (2008), i.e. we computed the probability P that a star with heliocentric velocities U , V , and W has a Galacticmotion compatible with a given SKG. Independently of the dis-tance assumption, the quantitative analysis excludes any possi-ble association with the oldest UMa and Hyades MGs ( P ≈ P. Guillout et al.: Discovery of “isolated” comoving T Tauri stars in Cepheus (RN)
Table 5.
Membership probability P (%) of our four stars to ma-jor MGs computed with the ZAMS (left) and
15 Myr assumption (right). +
78 853 0.0 / / / / / / / / / +
78 19 0.0 / / / our stars. Table 5 suggests a possible link with the Castor MG,which is, however, again not supported by the very high-lithiumabundance measurements, hardly compatible with a 200 Myr oldMG. With a less than 5 % probability, the IC 2391 MG ori-gin is highly improbable. We are left with the Pleiades SKGfor which the probabilities rise from ≈
15 % for the
ZAMS as-sumption to ≈
40 % for the
15 Myr assumption . However, as re-called by Makarov et al. (2007), this stream includes isolatedstars, groups, and associations of diverse ages in the range from1 Myr to about 200 Myr, which by itself, does not shed light onthe origin of our stars.We finally consider a possible link with the Cep-Cascomplex. Projected on the sky, the closest dark clouds to ourstars are LDN 1251 and LDN 1241 at both 5–8 ◦ east of themand about 300 pc from the Sun (Kun et al. 2008). The averageproper motions of LDN 1251, as estimated thanks to its cloudmembers discovered by Kun et al. (2009), are µ α = . ± . µ δ = − . ± . − . The angular distance of 5–8 ◦ corresponds to 25–40 pc at the cloud distance. With a propermotion of about 21 mas yr − relative to LDN 1251, nearlyaligned in the east-west direction and corresponding to avelocity of about 30 km s − at the cloud distance, they shouldhave travelled for 0.8–1.3 Myr from their parental cloud. Theclosest cloud on the approaching side of the expanding shell(namely LDN 1228, d ≃
180 pc, Kun et al. 2008) is about 10 ◦ east of our stars ( ≈
30 pc at the cloud distance). By completingthe same calculations for LDN 1251 / v esc ) from LDN 1228 of about 22 km s − and a traveltime of about 1.3 Myr. These estimates do not allow us toexclude that these stars are “runaway” objects that originated inLDN 1251, LDN 1241, or LDN 1228. Nevertheless, although aclose encounter with massive stars can result in high velocity“runaway” stars, Sterzik & Durisen (1998) found that 60 %of the runaways have speeds larger than 3 km s − but thatthis fraction steeply decreases, becoming smaller than 5 %for v esc >
10 km s − . Both the high escape velocites and thecommon space motions cast doubt on the “runaway” hypothesisbeing applicable.The more plausible explanation of how these “isolated”TTS formed is given by the “in-situ” model (Feigelson 1996).This scenario would support the conclusion of Tachihara et al.(2005) although the lack of accurate distances does not al-low us to draw firm conclusions about the origin of this groupof stars. The advent of GAIA mission, with its unprecedentedastrometric precision, will certainly shed light on this openissue. Similar conclusions have been reached in the Taurus(Magazzu et al. 1997), Chamaeleon-Musca (Mizuno et al. 1998)and Lupus (Tachihara et al. 2001) SFRs although no accretingTTS have been found outside these SFR’s cores.
5. Conclusion
We have reported the discovery of four comoving very youngstars located in a region devoid of dark matter and molecularclouds. On the basis of its H α profile and infrared excess, one ofthe stars investigated has been tentatively classified as a Class IIyoung infrared source, i.e. a TTS surrounded by an accretiondisc. Owing to the distance uncertainty, we cannot assert thatthese stars are completely unrelated to the Cep-Cas star-formingregion. However, their kinematics prove that they form a homo-geneous comoving group of stars with the same origin. Theiro ff -cloud positions imply that they are very good “isolated” TTScandidates. The “runaway” hypothesis is highly improbable be-cause of their kinematical properties. Our conclusions raise aquestion about the applicability of the “in-situ” star-formationscenario to very low-mass cloud environments. TYC 4496-780-1 could be a TW Hya analog, although a little bit older, and itscomoving companions may represent the peak of the iceberg ofa young loose association. Acknowledgements.
We are grateful to the OHP night assistant sta ff in con-ducting our Key Program, and those of the OAC observatories for their sup-port and help with the observations. We thank Luigia Santagati of INAF-Cataniafor the English revision of the text. This research made use of SIMBAD andVIZIER databases, operated at the CDS, Strasbourg, France. This publica-tion uses ROSAT data. Part of this work was supported by the UniversidadComplutense de Madrid (UCM), the Spanish MICINN, Ministerio de Cienciae Innovaci´on under grant AyA2008-00695 and the
Comunidad Aut´onoma deMadrid , under PRICIT project S-2009 / ESP-1496 (AstroMadrid).
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Fig. 5.
Cross-correlation functions (cross symbols) of the Cepheus moving groups TTS candidates BD +
78 853(RasTyc 0000 + + + +
78 19(RasTyc 0039 + α ( upper panels ) and lithium ( lower panelslower panels