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

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.

A high-resolution spectroscopic survey of late-type stars: chromospheric activity, rotation, kinematics, and age

“…West et al 2008), adding the condition of being member of a young moving group places a greater restriction on a star's age. Nevertheless, since the Montes et al (2001a) sample also contains some old stars (López-Santiago et al 2009), several stars of our present work could be older than ∼1 Gyr. The remaining 39 stars of our sample were selected because they showed a high level of magnetic activity, rotational rate, and/or lithium abundance.…”

“…In a recent work, López-Santiago et al (2009) demonstrated that the sample of possible members of the young stellar kinematic groups of Montes et al (2001a) is partially contaminated by active field stars that do not belong to the moving groups. In that work, the stars in the X-ray saturation regime also showed high Hα fluxes.…”

“…In particular, the level of magnetic activity is a powerful tool for this purpose (see López-Santiago et al 2009). The activity level of late-type stars is inversely correlated with age due to the decrease of stellar rotation with increasing age (e.g.…”

“…Through age indicators (mainly chromospheric activity and lithium abundance), we showed that the Local Association is indeed a mixture of subgroups of stars with different ages. In López-Santiago et al (2009), we used the information on the chromospheric activity of the stars provided by the spectroscopic survey, together with X-ray data from the ROSAT All Sky Survey (RASS) to quantify the contamination by old main-sequence stars of the sample of possible members of the Local Association in Montes et al (2001a).…”

“…In this new era of Virtual Observatory and extensive photometric databases and catalogs, the compilation of spectroscopic data is important for the purpose of constraining the properties of different astronomical objects, in particular, of stars (e.g. Sciortino et al 1995;Takeda et al 2007;Micela et al 2007;López-Santiago et al 2007;Klutsch et al 2008;López-Santiago et al 2009). The utility of substantial spectroscopic compilations has been demonstrated in the past by various groups.…”