4. Number of stars in excess
We quantify in Table 1 the number of RasTyc stars detected at Sthr = 0.03 cts s- 1 outside (Nout) and inside (Nin) GB regions towards the four quadrants previously defined. N computed as Nout(Sin/Sout) (where Sin and Sout are Galactic plane and GB areas respectively) represent the number of stars expected in GB regions for a pure Galactic plane population. Table 1 demonstrates a statistically significant (10 deviation) excess of stars (N+) in GB and GB. This quantitative analysis also reveals 66 stars in excess (3.7 ) towards GB where the Gould Disk was expected to be marginally detected due to the Tycho threshold (see Sect. 3.1.1). Again, this additional information would be unexplained in case of a belt scenario. 153 excess stars also show up towards GB, the far side of GB. Of these, 120 sources are located in a wide square centered on the Hyades cluster, among which 8 are expected to be contribution of the galactic plane. If we remove those sources we reduce the number of excess stars to 41. Moreover, 15 - 20 of those are probably Pleiades members, thus leaving an unsignificant (1 deviation) excess population towards GB as expected in both scenarios.
Table 1. Summary of the number of detected RasTyc stars and X-ray bright RasTyc stars (LX erg s-1, in brackets) at Sthr = 0.03 cts s- 1 outside (Nout) and inside (Nin) GB regions towards the four quadrants previously defined. N is the area corrected number of galactic plane stars in GB regions ; N+ = Nin - N is the observed number in excess with respect to N. Also quoted are the success rate Srate = N+/N100 and Dev = N+/ the deviation from the expected number.
According to Sect. 3.3.4 most RasTyc GB member should have X-ray luminosities above LX = erg s-1. We can use this constraint as a powerful selection method to enhance the fraction of GB members. The second set of numbers (in brackets) in Table 1 was obtained by restricting the sample to X-ray bright stars (LX erg s-1). On one hand this selection method enhances the fraction of excess stars towards GB, GB (10.0 to 12.5 deviation) and GB (3.7 to 4.0 deviation), but on the other hand reduces it towards GB (7.6 to 4.5 ) thus confirming the Hyades cluster as responsible for the excess. The X-ray bright RasTyc sample thus appears as a powerful method for picking genuine late type GB members with an expected success rate 40 to 45% towards GB regions located between l = and l = .
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998