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Proper motion and X-ray selected search for new members of the young TW Hya association
We have searched for new members of the TW Hya association (TWA) amongunidentified ROSAT X-ray sources by identifying them in proper motioncatalogues and selecting those that would be consistent with kinematicalmembership to the TWA. Spectroscopic follow-up observations of 19 membercandidates revealed the detection of moderate lithium absorption linesfor the following three stars: GSC 7206 845, TYC 7216-55, and TYC7247-12. The isochronal ages of the latter TYC stars are estimated to be 20 Myr while the other one has 100 Myr age based on akinematic distance estimate that assumes TWA membership. However, themoderately Li-rich stars are not likely to be new pre-main sequencemembers of TWA partly because of the discrepant radial velocities.Infrared follow-up imaging in the H-band for the 3 stars shows companioncandidates near two of them. While one system (TYC 7216-55) is probablya near-equal-magnitude stellar binary, our follow-up H-band spectrum ofthe faint companion candidate near GSC 7206 845 shows that it is insteada background K-type star rather than a companion.Based on observations obtained in ESO programs 66.C-0540, 66.C-0138B,and 68.C-0009 on La Silla, Chile, as well as with ROSAT and theUniversity of Hawaii Institute for Astronomy 2.2m telescope on MaunaKea.
| F, G and K stars in the ROSAT all-sky survey. I. Photometry
We present accurate BV(RI)_c photometry for a sample of F, G and K starsdetected in selected areas of the ROSAT all-sky survey (RASS). We haveused the photometry, in addition to low-resolution spectroscopy, toestimate spectral classifications, distances and X-ray luminosities. Thelog (L_X / L_V) in the sample lies below -2. Although the samplecontains also nearby, inactive stars, it is dominated by active objects.The median X-ray luminosity in our sample is < L_X > = 29.88 andthe mean value of the hardness ratios = 0.13 +/- 0.35. Wecompare the derived X-ray luminosity function with similar functionsobtained from the serendipitous samples of the Einstein Observatorymedium sensitivity survey (EMSS) and EXOSAT. Our sample is completelyconsistent with the EMSS sample of solar type stars, indicating thatboth our sources and the EMSS sources are representative of the highgalactic latitude X-ray stellar population. We do not find extremelyactive stars (log (L_X) >= 32), as are found in the EMSS sample, andwe argue that these objects are rare. Table 3 is also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
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