6. Conclusion and perspectives
We analyzed the RASS - Tycho sample in the galactic plane with the aim to test the Gould Belt as an explanation for the stellar density enhancement observed between l and l . From the analysis of the distance and X-ray luminosity distributions of the X-ray emitting stars inside and outside the proposed GB regions we concluded that these stars are not located at the edge of a disk (i.e. in a "belt"). We are rather led to a slightly modified scenario, a Gould Disk , in which GB members are distributed in a disk disrupted near the Sun, but with the same dimension and orientation of the traditionally assumed picture. We then show that the Gould Disk scenario is indeed able to account for the observed density enhancements both at low and high sensitivity in all four quadrants of the galactic plane.
Specifically, the analysis of the distance distribution shows that the excess stars are distributed in between the solar vicinity and 150 - 300 pc from the Sun depending on the direction of observation. These stars have X-ray luminosities LX erg s-1, typical of very young, active late type stars. The 3D-distribution of the stars (which can be calculated on the basis of good parallax information for many of the stars) projected on a plane perpendicular to the nodal line of the GB shows a striking bar feature inclined by for stars with X-ray luminosities LX erg s-1. Such X-ray luminosities favor young and bona fide members of the GB system, while the bar feature is absent for the less X-ray luminous stars which are distributed uniformly on a sphere as predicted by stellar X-ray population models.
We interpret these observations as the first detection of the GB late type stars population born within the last 30 - 80 Myr. Our results agree with the spatial arrangement of the GB as derived from star forming regions and OB associations located along the outer edge of the belt. The RASS - Tycho sample, which contains stars with known distances and known X-ray luminosities, appears to be very powerful for finding genuine GB members with an expected success rate 40 to 45%. We demonstrated that the discovery of many young late-type stars in the RASS based on optical spectroscopy is consistent with the proposed, revised GB picture. We interpret a fraction of the so-called "distributed" population of young stars present in the RASS as members of the Gould Disk .
Our analysis favors the interpretation of the GB as a physical entity but we still cannot draw unambiguous conclusions. Because of the insufficient sensitivity of the Tycho and RASS catalogs, the horizon of the RasTyc sample only reaches the closest parts of the GB. This makes it impossible to infer a coherent and continuous structure. The detection of similar enhancements of young stars towards the far side of the GB in various directions would be needed but is not feasible with the RasTyc. Such detections would also put strong constraints on the distance of the inner-ring which is only loosely defined.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998