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Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association

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López Santiago, J. and Micela, G. and Montes Gutiérrez, David (2009) Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association. Astronomy & Astrophysics, 499 (1). pp. 129-135. ISSN 0004-6361

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Official URL: http://dx.doi.org/10.1051/0004-6361/200811590


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Abstract

Context. The associations and moving groups of young stars are excellent laboratories for investigating stellar formation in the solar neighborhood. Previous results have confirmed that a non-negligible fraction of old main-sequence stars is present in the lists of possible members of young stellar kinematic groups. A detailed study of the properties of these samples is needed to separate the young stars from old main-sequence stars with similar space motion, and identify the origin of these structures. Aims. Our intention is to characterize members of the young moving groups, determine their age distribution, and quantify the contamination by old main-sequence stars, in particular, for the Local Association. Methods. We used stars possible members of the young (~10-650 Myr) moving groups from the literature. To determine the age of the stars, we used several suitable age indicators for young main sequence stars, i.e., X-ray fluxes from the Rosat All-sky Survey database, photometric data from the Tycho-2, Hipparcos, and 2MASS database. We also used spectroscopic data, in particular the equivalent width of the lithium line Li I λ6707.8 Å and H_α, to constrain the range of ages of the stars. Results. By combining photometric and spectroscopic data, we were able to separate the young stars (10-650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that the Local Association is contaminated by old field stars at the level of ~30%. This value must be considered as the contamination for our particular sample, and not of the entire Local Association. For other young moving groups, it is more difficult to estimate the fraction of old stars among possible members. However, the level of X-ray emission can, at least, help to separate two age populations: stars with <200 Myr and stars older than this. Conclusions. Among the candidate members of the classical moving groups, there is a non-negligible fraction of old field stars that should be taken into account when studying the stellar birthrate in the solar neighborhood. Our results are consistent with a scenario in which the moving groups contain both groups of young stars formed in a recent star-formation episode and old field stars with similar space motion. Only by combining X-ray and optical spectroscopic data is it possible to distinguish between these two age populations.


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© ESO 2009. J. López Santiago acknowledges financial support by the Marie Curie Fellowship contract No. MTKD-CT-2004-002769 and financial contribution by MERG-CT-2007-046535. The Madrid group acknowledges financial contribution by the Universidad Complutense de Madrid and the Programa Nacional de Astronomía y Astrofísica of the Spanish Ministerio de Educación y Ciencia (MEC), under grants AYA2005–02750 and AYA2008- 000695 and to the PRICIT project S-0505/ESP-0237 (ASTROCAM) of the Comunidad de Madrid. G. Micela acknowledges financial contribution from contract ASI-INAF I/088/06/0. We thanks the referee for useful comments that allowed to improve our manuscript. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research made use of the SIMBAD database, operated at the CDS, Strasbourg, France.

Uncontrolled Keywords:X-ray sources; Stellar kinematic groups; T tauri stars; Solar neighborhood; Mass stars; Evolution; Lithium; Sample; Age; Spectroscopy
Subjects:Sciences > Physics > Astrophysics
Sciences > Physics > Astronomy
ID Code:37836
Deposited On:24 May 2016 15:20
Last Modified:10 Dec 2018 15:05

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