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 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 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:
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FileName Lrecl Records Explanations
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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
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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
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Bytes Format Units Label Explanations
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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
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Acknowledgements:
Javier Lopez-Santiago, jls(at)astrax.fis.ucm.es
(End) Patricia Vannier [CDS] 17-Mar-2009