Volume 542, June 2012
|Number of page(s)||16|
|Section||Galactic structure, stellar clusters and populations|
|Published online||18 June 2012|
1 INAF – Osservatorio Astronomico di Capodimonte, via Moiariello, 16, 80131 Napoli, Italy
2 ESO – European Southern Observatory, Casilla 19001, Santiago 19, Chile
3 Observatoire Astronomique de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
4 Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, Ensenada, B. C., 22800, Mexico
5 INAF – Osservatorio Astrofisico di Catania, via S. Sofia, 78, 95123 Catania, Italy
6 Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
Received: 2 November 2011
Accepted: 16 April 2012
Context. 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.
Aims. We present a study of the large-scale spatial distribution of 6482 ROSAT All-Sky Survey (RASS) X-ray sources in approximately 5000 deg2 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.
Methods. 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.
Results. 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 ~70−100 Myr) is found widely spread over the studied area. Two new young stellar aggregates, namely “X-ray Clump 0534+22” (age ~ 2−10 Myr) and “X-ray Clump 0430−08” (age ~ 2−20 Myr), are also identified.
Conclusions. 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.
Key words: stars: late-type / stars: fundamental parameters / stars: individual: Gould Belt / techniques: miscellaneous / surveys / stars: individual: Orion
Based on ROSAT All-Sky Survey data, low-resolution spectroscopic observations performed at the European Southern Observatory (Chile; Program 05.E-0566) and at the Observatorio Astronómico Nacional de San Pedro Mártir (Mexico), and high-resolution spectroscopic observations carried out at the Calar Alto Astronomical Observatory (Spain).
Table 4 and Appendix A are only available in electronic form at http://www.aanda.org
© ESO, 2012
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