INAF – Osservatorio Astronomico di Palermo Giuseppe S. Vaiana, Piazza Parlamento 1, 90134 Palermo, Italy e-mail: email@example.com
2 Astrophysics Missions Division, Research and Science Support Department of ESA/ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
3 INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy
4 X-ray Astrophysics Laboratory, Code 662, NASA/Goodard Space Flight Center, Greenbelt, MD 20771, USA
5 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
Accepted: 2 October 2006
Context.The comparison of observed counts in a given sky direction with predictions by Galactic models yields constraints on the spatial distribution and the stellar birthrate of young stellar populations. The XMM-Newton Bright Serendipitous Survey (XBSS) is an unbiased survey that includes a total of 58 stellar sources selected in the 0.5–4.5 keV energy band, having a limiting sensitivity of 10-2 cnt s-1 and covering an area of 28.10 sq deg.
Aims.We present the results of analysing the stellar content of the XBSS so as to understand the recent star formation history of the Galaxy in the vicinity of the Sun.
Methods.We compared the observations with the predictions obtained with XCOUNT, a model of the stellar X-ray content of the Galaxy. The model predicts the number and properties of the stars to be observed in terms of magnitude, colour, population and ratio distributions of the coronal sources detected with a given instrument and sensitivity in a specific sky direction.
Results.As in other shallow surveys, we observe an excess of stars not predicted by our Galaxy model. Comparing the colours of the identified infrared counterparts with the model predictions, we observe that this excess is produced by yellow (G+K) stars. The study of the X-ray spectrum of each source reveals a main population of stars with the coronal temperature stratification typical of intermediate-age stars. As no assumptions have been made for the selection of the sample, our results must be representative of the entire solar neighbourhood. Some stars show infrared excess due to circumstellar absorption, which is indicative of youth.
Key words: Galaxy: stellar content / stars: activity / stars: coronae / stars: formation / stars: magnetic fields / X-rays: stars
The XMM-Newton Bright Serendipitous Survey was conceived by the XMM-Newton Survey Science Center (SSC), a consortium of 10 institutions appointed by ESA to help the SOC in distinct technical aspects, including the exploitation of the XMM-Newton serendipitous detections (see http://xmmssc-www.star.le.ac.uk/).
© ESO, 2007