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J/A+A/499/129    Old MS stars in young moving groups     (Lopez-Santiago+, 2009)

Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association. Lopez-Santiago J., Micela G., Montes D. <Astron. Astrophys. 499, 129 (2009)> =2009A&A...499..129L
ADC_Keywords: Stars, nearby ; Stars, dwarfs ; X-ray sources ; Photometry, infrared Keywords: Galaxy: stellar content - Galaxy: solar neighbourhood - stars: kinematics - stars: activity - stars: coronae Abstract: 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 lambda6707.8Å 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. Description: Rosat All Sky Survey (RASS) and 2MASS counterparts of 556 stars possible members of the young moving groups: Local Association, Ursa Major Moving Group, Hyades Supercluster, Castor Moving Group, and IC 2391 Supercluster and other young stars from Montes et al. (2001MNRAS.328...45M). For each star equatorial coordinates, parallax, visual magnitude, V-I colour, 2MASS id., J, H, and K magnitudes, RASS id., X-ray count-rate in the ROSAT energy band, the two hardness-ratio in the RASS, and X-ray luminosity in the ROSAT energy band are given. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file catalog.dat 145 556 Data of the cross-correlation with the RASS and the 2MASS of the stars in the sample
See also: II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003) IX/10 : ROSAT All-Sky Bright Source Catalogue (1RXS) (Voges+ 1999) Byte-by-byte Description of file: catalog.dat
Bytes Format Units Label Explanations
1- 24 A24 --- Name Star name 26- 27 I2 h RAh Right ascension (J2000.0) 29- 30 I2 min RAm Right ascension (J2000.0) 32- 35 F4.1 s RAs Right ascension (J2000.0) 37 A1 --- DE- Declination sign (J2000.0) 38- 39 I2 deg DEd Declination (J2000.0) 41- 42 I2 arcmin DEm Declination (J2000.0) 44- 47 F4.1 arcsec DEs Declination (J2000.0) 49- 54 F6.2 mas Plx Paralax 56- 60 F5.2 mag Vmag ? V magnitude 62- 65 F4.2 mag V-Ic ? V-Ic colour index 67- 82 A16 --- 2MASS 2MASS Identification 84- 88 F5.2 mag Jmag ? 2MASS J magnitude 90- 94 F5.2 mag Hmag ? 2MASS H magnitude 96-100 F5.2 mag Kmag ? 2MASS K magnitude 102-121 A20 --- RASS RASS Identification 123-127 F5.3 ct/s CR ? Count-rate in the ROSAT energy band 129-133 F5.2 --- HR1 ? Hardness-ratio 1 of the RASS catalogue 135-139 F5.2 --- HR2 ? Hardness-ratio 2 of the RASS catalogue 141-145 F5.2 [10-7W] logLX ? Logarithm of the X-ray luminosity in the ROSAT energy band
Acknowledgements: Javier Lopez-Santiago, jls(at)
(End) Patricia Vannier [CDS] 17-Mar-2009
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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