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Description
The associations and moving groups of young stars are excellent laboratories to investigate the stellar formation in the solar neighborhood. Past results confirmed that a non-negligible fraction of old main-sequence stars is present in the lists of members of the young stellar kinematic groups. A detailed study on the properties of these samples is needed to separate the young stars from old main-sequence stars with similar space motion, in order to identify the origin of these structures. Our intention is to characterize the sample of stars members of the young moving groups and determine their age distribution to quantify the contamination by old main-sequence stars, in particular, in the Local Association. For our study, we used the sample of stars possible members of the young (~10-650Myr) moving groups from the literature. In order to determine the age of the stars, we used several age indicators well suited for young main sequence stars: 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 LiI lambda6707.8{AA} and Halpha, to constrain the range of ages of the stars in the sample. The combination of photometric and spectroscopic data permitted to separate the young stars (10-650Myr) from the old (>1Gyr) field ones. In particular, the member list of the Local Association results contaminated by ~30% of old field stars. This value must be considered as the contamination of our particular sample, not to the entire Local Association. In other young moving groups it is more difficult to estimate the fraction of old stars in the samples of possible members. However, the X-ray emission level can help to separate, at least, two age populations: stars with 200Myr and stars older than this. The samples of candidates of the classical moving groups contain a non-negligible amount of old field stars that should be considered in studies on the stellar birthrate in the solar neighborhood. Our results agree with the scenario of the moving groups being composed by groups of young stars formed in a recent star formation episode and old field stars with similar space motion. Only combining both X-ray and optical spectroscopic data it is possible to distinguish between the two age populations.
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