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Astron. Astrophys. 337, 113-124 (1998)

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2. The RASS-Tycho/Hipparcos sample

The primary data used for our study consist in the so-called RASS - Tycho (hereafter RasTyc) sample which resulted from the cross-correlation of the RASS and Tycho catalogs. Among the 13 875 RasTyc stars, 6 200 (the so-called RASS - Hipparcos sample (hereafter RasHip)) are also contained in the Hipparcos catalog and therefore for almost half (45%) of our primary sample even more accurate Hipparcos data (magnitudes, positions, parallax, proper motions) are available. We refer to Guillout et al. (1998b) for a detailled description of the RasTyc and RasHip samples.

Since the RASS and Tycho catalogs are X-ray and optical flux limited, the RasTyc (and also the RasHip) sample is strongly biased toward young (because of the X-ray selection) and relatively nearby (because of the Tycho completeness level) stars. Moreover, since in zeroth order, X-ray emission is color-independent, young M-type stars are basically detected in the RASS up to distances at which young F-type stars can be detected, too. However, the limited Tycho sensitivity excludes such optically faint stars located at large distances, implying that the RasTyc sample is incomplete for K and M stars. For exemple at the Tycho completness threshold ([FORMULA] 10.5 mag) a K5 ZAMS star can only be detected within 40 pc from the Sun while an F0 ZAMS star is detectable up to 600 pc at the faintest Tycho limit ([FORMULA] 11.5 mag). This bias towards young F - G stars does not pose a problem for this study because it is sky position independent. However, unlike the Tycho sensitivity, the RASS detection threshold varies significantly over the sky because of the different exposure times for different parts of the sky. In order to avoid this sky position dependent bias we adopted, accordingly to Guillout et al. (1998b), S = 0.03 cts s- 1 as our deepest PSPC count-rate threshold. Finally, we focus on low galactic latitudes ([FORMULA] [FORMULA] [FORMULA]), thereby reducing the total number of stars to [FORMULA] 6 348 in the primary RasTyc sample.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998