SIMBAD references

The recent star formation history in the solar vicinity is not yet well constrained, and the real nature of the so-called Gould Belt is still unclear. We present a study of the large-scale spatial distribution of 6482 ROSAT All-Sky Survey (RASS) X-ray sources in approximately 5000deg² in the general direction of Orion. We examine the astrophysical properties of a sub-sample of ∼100 optical counterparts, using optical spectroscopy. This sub-sample is then used to investigate the space density of the RASS young star candidates by comparing X-ray number counts with Galactic model predictions. The young star candidates were selected from the RASS using X-ray criteria. We characterize the observed sub-sample in terms of spectral type, lithium content, radial and rotational velocities, and iron abundance. A population synthesis model is then applied to analyze the stellar content of the RASS in the studied area. We find that stars associated with the Orion star-forming region, as expected, do show a high lithium content. As in previous RASS studies, a population of late-type stars with lithium equivalent widths larger than that of the Pleiades stars of the same spectral type (hence younger than ∼70-100Myr) is found widely spread over the studied area. Two new young stellar aggregates, namely ``X-ray Clump 0534+22'' (age∼2-10Myr) and ``X-ray Clump 0430-08'' (age∼2-20Myr), are also identified. The spectroscopic follow-up and comparison with Galactic model predictions reveal that the X-ray selected stellar population in the general direction of Orion is characterized by three distinct components, namely the clustered, the young dispersed, and the widespread field populations. The clustered population is mainly associated with regions of recent or ongoing star formation and correlates spatially with molecular clouds. The dispersed young population follows a broad lane that apparently coincides spatially with the Gould Belt, while the widespread population consists primarily of active field stars older than 100 Myr. We expect the still ``bi-dimensional'' picture emerging from this study to grow in depth as soon as the distance and the kinematics of the studied sources will become available from the future Gaia mission.