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تسجيل مستخدم جديدSearch for Associations Containing Young stars (SACY): I. Sample & Searching Method
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We report results from a high-resolution optical spectroscopic survey aimed to search for nearby young associations and young stars among optical counterparts of ROSAT All-Sky Survey X-ray sources in the Southern Hemisphere. We selected 1953 late-type (B-V >= 0.6), potentially young, optical counterparts out of a total of 9574 1RXS sources for follow-up observations. At least one high-resolution spectrum was obtained for each of 1511 targets. This paper is the first in a series presenting the results of the SACY survey. Here we describe our sample and our observations. We describe a convergence method in the (UVW) velocity space to find associations. As an example, we discuss the validity of this method in the framework of the BetaPic Association.
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We seek to update the spectroscopy binary fraction of the SACY (Search for Associations Containing Young stars) sample taking in consideration all possible biases in our identification of binary candidates, such as activity and rotation. Using high-r
esolution spectroscopic observations we have produced $sim$1300 cross-correlation functions (CCFs) to disentangle the previously mentioned sources of contamination. The radial velocity values obtained were cross-matched with the literature and were used to revise and update the spectroscopic binary (SB) fraction in each of the SACY association. In order to better describe the CCF profile, we calculated a set of high-order cross-correlation features to determine the origin of the variations in radial velocities. We identified 68 SB candidates from our sample of 410 objects. Our results hint that the youngest associations have a higher SB fraction. Specifically, we found sensitivity-corrected SB fractions of $22 substack{+15 -11} %$ for $epsilon$~Cha , $31 substack{+16 -14} %$ for TW Hya and $32 substack{+9 -8} %$ for $beta$~Pictoris, in contrast with the five oldest ($sim 35-125$ Myr) associations we have sampled which are $sim 10%$ or lower. This result seems independent of the methodology used to asses membership to the associations. The new CCF analysis, radial velocity estimates and SB candidates are particularly relevant for membership revision of targets in young stellar associations. These targets would be ideal candidates for follow-up campaigns using high-resolution techniques in order to confirm binarity, resolve the orbits, and ideally calculate dynamical masses. Additionally, if the results on SB fraction in the youngest associations are confirmed, it could hint of non-universal multiplicity among SACY associations.
Our study is a follow-up of the SACY project, an extended survey in the Southern Hemisphere targeted to search for young nearby associations. Nine associations have either been newly identified, or had their member list better defined. These associat
ions, with ages between about 6 Myr and 70 Myr, form an excellent sample to study the Li depletion in the pre-main sequence (PMS) evolution. We investigate the use of Li abundances as an independent clock to constrain the PMS evolution. We have calculated the LTE Li abundances for 376 members of different young associations. In addition we considered the effects of their projected stellar rotation. We present the Li depletion as 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. A clear Li depletion can be measured in the temperature range from 5000K to 3500K for the age span covered by these nine associations. The age sequence based on the Li-clock agrees well with the isochronal ages, $epsilon$Cha association being the only possible exception. The lithium depletion patterns for those associations resemble those of the young open clusters, 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 to determine reliable ages for individual stars. Rotation velocities above 20 km s$^{-1}$ seem to inhibit the Li depletion.
Context. Young loose nearby associations are unique samples of close (<150 pc), young (approx 5-100 Myr) pre-main sequence (PMS) stars. A significant number of members of these associations have been identified in the SACY collaboration. We can use t
he proximity and youth of these members to investigate key ingredients in star formation processes, such as multiplicity. Aims. We present the statistics of identified multiple systems from 113 confirmed SACY members. We derive multiplicity frequencies, mass-ratio, and physical separation distributions in a consistent parameter space, and compare our results to other PMS populations and the field. Methods. We have obtained adaptive-optics assisted near-infrared observations with NACO (ESO/VLT) and IRCAL (Lick Observatory) for at least one epoch of all 113 SACY members. We have identified multiple systems using co-moving proper-motion analysis and using contamination estimates. We have explored ranges in projected separation and mass-ratio of a [3-1000 au], and q [0.1-1], respectively. Results. We have identified 31 multiple systems (28 binaries and 3 triples). We derive a multiplicity frequency (MF) of MF_(3-1000au)=28.4 +4.7, -3.9% and a triple frequency (TF) of TF_(3-1000au)=2.8 +2.5, -0.8% in the separation range of 3-1000 au. We do not find any evidence for an increase in the MF with primary mass. The estimated mass-ratio of our statistical sample (with power-law index gamma=-0.04 +/- 0.14) is consistent with a flat distribution (gamma = 0). Conclusions. We show further similarities (but also hints of discrepancies) between SACY and the Taurus region: flat mass-ratio distributions and statistically similar MF and TF values. We also compared the SACY sample to the field (in the separation range of 19-100 au), finding that the two distributions are indistinguishable, suggesting a similar formation mechanism.
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