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III. Ages and Li Abundances�, A&A 508, 833–839 (2009) Astronomy DOI: 10.1051/0004-6361/200911736 & c ESO 2009 Astrophysics Search for associations containing young stars (SACY) III. Ages and Li abundances, L. da Silva1,C.A.O.Torres2, R. de la Reza1,G.R.Quast2,C.H.F.Melo3, and M. F. Sterzik3 1 Observatório Nacional-MCT, Rio de Janeiro, Brazil e-mail: [email protected] 2 Laboratório Nacional de Astrofísica-MCT, Itajubá, Brazil 3 European Southern Observatory, Alonso de Cordova 3107, Casilla 19, Santiago, Chile Received 27 January 2009 / Accepted 28 August 2009 ABSTRACT Context. Our study is a follow-up of the SACY project, an extended high spectral resolution survey of more than two thousand optical counterparts to X-ray sources in the southern hemisphere targeted to search for young nearby association. Nine associations have either been newly identified, or have had their member list revised. Groups belonging to the Sco-Cen-Oph complex are not considered in the present study. Aims. These nine associations, with ages of between about 6 Myr and 70 Myr, form an excellent sample to study the Li depletion in the pre-main sequence (PMS) evolution. In the present paper, we investigate the use of Li abundances as an independent clock to constrain the PMS evolution. Methods. Using our measurements of the equivalent widths of the Li resonance line and assuming fixed metallicities and microturbu- lence, we calculated the LTE Li abundances for 376 members of various young associations. In addition, we considered the effects of their projected stellar rotation. Results. We present the Li depletion as a function of age in the first hundred million years for the first time for the most extended sample of Li abundances in young stellar associations. Conclusions. A clear Li depletion can be measured in the temperature range from 5000 K to 3500 K for the age span covered by the nine associations studied in this paper. The age sequence based on the Li-clock agrees well with the isochronal ages, the Cha as- sociation being the only possible exception. The lithium depletion patterns for the associations presented here resemble those of the young open clusters with similar ages, strengthening the notion that the members proposed for these loose young associations have indeed a common physical origin. The observed scatter in the Li abundances hampers the use of Li in determining reliable ages for individual stars. For velocities above 20 km s−1, rotation seems to play an important role in inhibiting the Li depletion. Key words. stars: abundances – stars: pre-main sequence – stars: late-type – star: evolution 1. Introduction Li abundances have been studied over more than one decade (see Pallavicini et al. 2000), the results of Paper I was the first analy- In Torres et al. (2006) (hereafter Paper I) we report the results of sis of this kind for a young association. a high-resolution optical spectroscopic survey to search for as- Using the method described in Paper I, Torres et al. sociations containing young stars (SACY) among optical coun- (2008, hereafter TQMS08) defined nine new young associations, terparts of ROSAT All-Sky X-ray sources in the southern hemi- namely, Chamaleontis (ChA), TW Hydrae (TWA), β Pictoris, sphere. There we present the catalog resulting from the survey. Octants (OctA), Tucana-Horologium (THA), Columba (ColA), We describe the convergence method developed to search for Carina (CarA), Argus (ArgA), and AB Doradus (ABDA). The members of an association and a corresponding membership present work continues this study and aims to derive the distri- probability model. A membership to an association is defined bution of Li abundances for each of the nine associations result- in the hexa-dimensional space formed by the (UVW) velocity ing from a consolidated list of members. Since these associa- space and the (XYZ) spatial coordinates distribution. We also tions are young, covering ages from about 5 Myr up to that of take into account the position in the HR diagram, eliminating the Pleiades, they form an interesting “laboratory” for studying very discrepant stars. Finally, we check each member proposed the Li depletion with age, as achieved for some open clusters by comparing its Li content with the Li distribution of the as- (Randich et al. 2001; Jeffries 2006). sociation. The β Pictoris association (βPA) is presented as an example of the method outlined in Paper I. In Paper I, we also present the Li abundance analysis of the 2. Sample βPA to confirm its youth. In contrast to open clusters where In Table 1, we present some properties of the young associations Based on observations collected at the ESO – La Silla and at the studied in this paper derived in TQMS08. The number of mem- LNA-OPD. bers (N), the number of Li eliminated stars (“intruders”, n), the Tables 3–11 are only available in electronic form at average distance (in parsecs), and the age (in Myr) taken from http://www.aanda.org TQMS08 are given in Table 1. Li abundances were measured Article published by EDP Sciences 834 L. da Silva et al.: Age and Li in young associations Table 1. Properties of the associations studied in this paper. Table 2. Variation in Li abundance as a function of EW and model parameters. Association Nndistance Age Chamaleontis (ChA) 23 0 93–123 6 Parameter change ΔA(Li)1 ΔA(Li)2 TW Hydrae (TWA) 22 0 28–73 8 +0.18 +0.08 T ff[K] ±100 β Pictoris (βPA) 48 2 10–80 10 e −0.19 −0.07 log g ±0.5 +0.16 +0.12 Octans (OctA) 15 0 82–175 20? −0.11 −0.06 −1 ± +0.05 +0.02 Tucana-Horologium (THA) 45 1 37–78 30 microturbulence [km s ] 0.5 −0.05 −0.02 ± +0.04 +0.05 Columba (ColA) 44 0 35–150 30 EW 10% −0.05 −0.05 ± −0.03 −0.01 Carina (CarA) 23 1 45–160 30 [Fe/H] 0.1 +0.04 +0.02 Argus (ArgA) 64 1 29–164 40 AB Doradus (ABDA) 92 4 7–143 70 λ 6707.754,λ6707.766,λ6707.904,λ6707.917; and for log g f : −0.430, −0.209, −0.733, −0.510, respectively. for 376 stars. Although the data are mainly from Paper I, a few Effective temperatures were obtained from the photomet- additional data obtained since then are included and will be pub- ric and spectroscopic data available. The calibrations used were lished in forthcoming papers. mainly those of Kenyon & Hartmann (1995)andSchmidt-Kaler These new observations allowed us to refine the definitions (1982). Some additional information was included from Bessell of some of the associations. For the ColA, we are able to obtain (1979) and from Ducatietal.(2001). If a reliable Cousins a similar but more consistent solution with a few changes to the (V − I)c color index was available, either from our observa- member list with respect to those of TQMS08 (three stars now tions, from Hipparcos, or from other sources in the literature, being rejected and six new ones included). this was used to derive Teff. In the absence of (V − I)c,we For the ABDA, three new members have been proposed, one used the Johnson (B − V), mainly derived from TYCHO-2 but of them, HD 82879, previously proposed to be a member of the also obtained from various sources in the literature. We consid- ChA. Proposed by Zuckerman & Song (2004) as a member of ered the (B − V) colors from TYCHO-2 reliable only for stars the THA, HD 53842 was previously rejected in TQMS08 as a brighter than magnitude 10. Finally, if no reliable photometry member of THA because of a compilation error. Its reintroduc- was available, we used the spectral type to obtain the effective tion now as a member proposed for the THA has no other con- temperature. sequences for the mean values of this association1. The other model parameters were kept fixed: metallicity at As explained in TQMS08, IC 2391 members were incorpo- [Fe/H] = 0.1 (see Castilho et al. 2005); and surface gravity log g rated into the ArgA member list. Similarly, members of the open was fixed at 4.5 for the dwarfs, and at 4.0 for the subgiants, ac- cluster η Cha were combined with the ChA members forming cording to the spectral classification of Paper I. The microturbu- a unique group. The link between young loose associations and lence velocity was fixed at 1.5 km s−1 for all stars. some open clusters will be discussed in forthcoming papers. 3.1. Error analysis 3. Li abundance determinations Table 2 summarizes the internal errors expected in the Li abun- The observations were carried out using the FEROS spectro- dances as a function of the variations in the model parame- Δ Δ graph (Kaufer et al. 1999) at la Silla, ESO, and the Coudé spec- ters and in the equivalent width. A(Li)1 and A(Li)2 are the trograph of the 1.62 m telescope of the Observatório do Pico Li abundances variations for a star with an effective temperature dos Dias, LNA, Brazil (see Paper I for details). The Li abun- of 4000 K and 6100 K, respectively. Our main error source is the ff / dances (ALi) of the stars, in dex, in the system log A(H) = 12, e ective temperature. The unknown parameters, i.e., [Fe H] and where A(H) is the H abundance (ALi = logN(Li)/N(H) + 12), the microturbulence velocity, are less important, and the values were determined using the programs of M.
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