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Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| Search for associations containing young stars (SACY). II. Chemical abundances of stars in 11 young associations in the solar neighborhood
The recently discovered coeval, moving groups of young stellar objectsin the solar neighborhood represent invaluable laboratories for studyingrecent star formation and searching for high metallicity stars that canbe included in future exo-planet surveys.In this study, we derived through an uniform and homogeneous methodstellar atmospheric parameters and abundances for iron, silicium, andnickel in 63 post-T Tauri stars from 11 nearby young associations. Wefurther compare the results with two different pre-main sequence (PMS)and main sequence (MS) star populations. The stellar atmosphericparameters and the abundances presented here were derivedusing the equivalent width of individual lines in the stellar spectra byassuming the excitation/ionization equilibrium of iron.Moreover, we compared the observed Balmer lines with synthetic profilescalculated for model atmospheres with a different line-formationcode. We found that the synthetic profiles agree reasonably well withthe observed profiles, although the Balmer lines of many stars aresubstantially filled-in, probably by chromospheric emission. Solarmetallicity is found to be a common trend in all thenearby young associations studied.The low abundance dispersion within each association strengthens theidea that the origin of these nearby young associations is related tothe nearby star-forming regions (SFR).Abundances of elements other than iron are consistent with previousresults for Main Sequence stars in the solar neighborhood. The chemicalcharacterization of the members of the newly found nearby youngassociations, performed in this study and intended to proceed insubsequent works, is essential to understanding and testing the contextof local star formation and the evolutionary history of the galaxy.Based on observations collected with the UVES spectrograph at theVLT/UT2 8.2-m Kueyen Telescope (ESO run ID. 079.C-0556(A)) at theParanal Observatory, Chile. Tables 1, 2 and 5 are only available inelectronic form at http://www.aanda.org
| On the common origin of the AB Doradus moving group and the Pleiades cluster
AB Doradus (AB Dor) is the nearest identified moving group. As withother such groups, the age is important for understanding of several keyquestions. It is important, for example, in establishing the origin ofthe group and also in comparative studies of the properties of planetarysystems, eventually surrounding some of the AB Dor group members, withthose existing in other groups. For AB Dor two rather differentestimates for its age have been proposed: the first one, of the order of50 Myr, by Zuckerman and coworkers from a comparison with theTucana/Horologium moving group and a second one of about 100-125 Myr byLuhman and coworkers from colour-magnitude diagrams. Using this lastvalue and the closeness in velocity space of AB Dor and the Pleiadesgalactic cluster, Luhman and coworkers suggested coevality for thesesystems. Because strictly speaking such a closeness does not stillguarantee coevality, here we address this problem by computing andcomparing the full 3D orbits of AB Dor, Pleiades, α Persei and IC2602. The latter two open clusters have estimated ages of about 85-90and 50 Myr. The resulting age 119 +/- 20 Myr is consistent with AB Dorand Pleiades being coeval. Our solution and the scenario of open clusterformation proposed by Kroupa and collaborators suggest that the AB Dormoving group may be identified with the expanding subpopulation (GroupI) present in this scenario. We also discuss other related aspects asiron and lithium abundances, eventual stellar mass segregation duringthe formation of the systems and possible fraction of debris discs inthe AB Dor group.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
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| Sað Açýklýk: | 21h48m48.51s |
| Yükselim: | -39°29'09.5" |
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