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

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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[FORMULA] computed as Nout[FORMULA](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 [FORMULA] deviation) excess of stars (N+) in GB[FORMULA] and GB[FORMULA]. This quantitative analysis also reveals 66 stars in excess (3.7 [FORMULA]) towards GB[FORMULA] 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[FORMULA], the far side of GB. Of these, 120 sources are located in a [FORMULA] 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[FORMULA] deviation) excess population towards GB[FORMULA] as expected in both scenarios.


[TABLE]

Table 1. Summary of the number of detected RasTyc stars and X-ray bright RasTyc stars (LX [FORMULA] [FORMULA]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[FORMULA] is the area corrected number of galactic plane stars in GB regions ; N+ = Nin - N[FORMULA] is the observed number in excess with respect to N[FORMULA]. Also quoted are the success rate Srate = N+/N[FORMULA]100 and Dev = N+/[FORMULA] the deviation from the expected number.


According to Sect. 3.3.4 most RasTyc GB member should have X-ray luminosities above LX = [FORMULA]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 [FORMULA] [FORMULA]erg s-1). On one hand this selection method enhances the fraction of excess stars towards GB[FORMULA], GB[FORMULA] (10.0 to 12.5 [FORMULA] deviation) and GB[FORMULA] (3.7 to 4.0 [FORMULA] deviation), but on the other hand reduces it towards GB[FORMULA] (7.6 to 4.5 [FORMULA]) 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 [FORMULA] 40 to 45% towards GB regions located between l = [FORMULA] and l = [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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