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

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1. Introduction

More than one century ago Gould (1879) noticed an asymmetry in the distribution of the brightest stars on the sky with respect to the galactic equator and concluded that these bright stars were aligned along a great circle crossing the Milky-Way at an angle close to [FORMULA]. Since then many astronomers have worked on this structure known as the Gould Belt (GB hereafter). Numerous studies of the sky distributions of bright stars, OB associations and young galactic clusters confirmed its existence. Furthermore, the GB appears to be tightly correlated with many of the bright and dark nebulae as well as with large masses of neutral interstellar gas from which these young stars were probably born. The modern picture of the GB consists of a stellar component (population I stars with ages up to 8[FORMULA]107 yrs, OB associations and young clusters) and associated interstellar matter. It has an ellipsoidal shape with a semi-major axis of about 500 pc and a semi-minor axis of about 340 pc, the Sun being located 150 to 250 pc off center. The formation of the GB is still a puzzle. Some authors suggest collisions of high velocity clouds with the galactic disk (e.g. Comerón & Torra 1992, 1994), while others propose supernova explosions, or induced star formation by strong stellar winds originating in the central Cas-Tau association (e. g. Blaauw 1991). Others dispute the spatial coherence of the GB structure (and therefore an unified formation picture) at all, pointing out that even a random spatial distribution of a few prominent OB associations (which are usually selected to trace the GB) would mimic the impression of an "inclined belt" in the sky. We note that a combined, or even alternative, scenario cannot be excluded since none of the individual models is able to account for all observations. An extensive review of the present knowledge on the GB system and the interstellar medium can be found in Pöppel (1997) with references therein.

If an expanding ring is indeed responsible for the GB (whatever the origin is : an explosive event or a high velocity cloud collision), it can be expected to have triggered a large scale propagating star formation process, and galactic shear would then cause the orientation of the outer ring (Olano 1982). However, one of the major problems in all studies of the GB stellar component is to find criteria that allow a reliable selection of GB members. The structure crosses the galactic plane with a small inclination angle and therefore GB members are immersed in the ambient galactic plane population along most lines of sight. In particular, the identification of late-type (F-M) GB stars is an especially arduous task. Due to these difficulties only the tip of the GB stellar iceberg has been recognized in star forming regions (SFR) and OB associations. So far, only Fresneau et al. (1996) have been successful in selecting GB low mass star candidates in the field on the basis of kinematic anomalies albeit on a small sample and only with [FORMULA] 20% success rate.

In the X-ray domain the situation is completely different. Because of the sharp decrease of X-ray activity with age for late-type stars (Rosner et al. 1985, Micela et al. 1988, 1990, Randich et al. 1995, Caillault 1996), X-ray selected stellar samples greatly enhance young stellar structures often unnoticed at optical wavelengths due to the lack of contrast. A large number of young zero-age main sequence (ZAMS) and pre-main sequence (PMS) stars were therefore expected to be present in the ROSAT All-Sky Survey (RASS, Trümper 1983, Voges 1992). Systematic identification of RASS sources in test regions indeed reveals the major contribution of ZAMS stars to soft X-ray survey (Motch et al. 1997, Zickgraf et al. 1997) while a large numbers of new PMS stars could be identified towards nearby SFRs like Orion (Alcalá et al. 1996), Taurus (Wichmann et al. 1996), Lupus (Krautter et al. 1997) and Chamaeleon (Alcalá et al. 1995, Covino et al. 1997). However, the somewhat unexpected finding of apparently rather young low-mass stars widely distributed around these star forming regions led to a controversial debate about the true age and the origin of this population (Sterzik et al. 1995, Feigelson 1996, Briceno et al. 1997, see also Neuhäuser 1997 and Caillault 1998 for summaries). There is evidence that the RASS young stellar population is at least not everywhere compatible with expectations from standard galactic modeling : Wichmann et al. (1997) report an excess of late-type, Lithium-rich RASS counterparts spatially coincident with the location of the GB above of the galactic plane in a region adjacent to the Lupus SFR, while Sterzik et al. (1998) report a similar finding near the Orion SFR, where the enhancement of Lithium-rich stars is below the galactic plane, again more consistent with the location of the GB in that particular direction in the sky. Recently, Guillout et al. (1998a) reported first results on the whole sky distribution of young X-ray active stars resulting from the cross-correlation of RASS with the Tycho catalog. Modeling the observed sky distribution of a few thousand stellar X-ray sources with a uniform "background" plus an exponential disk, they found the best fit for the disk position with an inclination angle of [FORMULA] with respect to the galactic equator and an ascending node [FORMULA] = [FORMULA], i.e., values quite consistent with those previously derived for the GB.

In this paper we extend the analysis of the stellar density enhancement discovered by Guillout et al. (1998a) within [FORMULA] from the galactic plane and located towards the third and fourth galactic quadrant by taking into account the distance information of our sample stars. Our major finding is that the excesses can be understood as a part of a coherent structure filling a large fraction within the solar vicinity which is geometrically in agreement with a standard (but somewhat modified) GB picture.

The paper is organized as follow : In Sect. 2 we briefly present the RASS - Tycho and RASS - Hipparcos samples on which this analysis is based on. In Sect. 3 we compare in detail our data with two different GB scenarios, taking into account different X-ray flux limits. The interpretation of distance and X-ray property distributions leads us to propose a favourable geometrical model of a modified Gould Belt namely a Gould Disk . Sect. 4 presents the actual number of stars responsible for the excess. In Sect. 5 we discuss more general implications of a young late-type population filling the GB plane in a disk-like manner. We draw conclusions of our major findings in Sect. 6.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998