X-Ray Observations of Stars: First Results from Rosat

Extract

At the time of writing (i.e., September 1991), the ROSAT all-sky survey has been completed and almost the entire sky has been scanned with an imaging X-ray telescope down to a limiting flux of approximately 2 10⁻¹³erg/s/cm² in the pass band 0.1 — 2.0 keV; in regions of deeper exposures near the poles of the ecliptic considerably fainter flux limits have been achieved. While the processing and analysis of this huge body of data is still in progress and hence final results on the number of detected sources and their distribution in flux are not yet available, the total number of detected X-ray sources will be around 60 000. Preliminary results from optical identifications of selected fields show that about one quarter of the X-ray sources discovered at high galactic latitudes come from by comparison nearby stellar sources (Fleming 1991), while at lower galactic latitudes up to one half of the detected X-ray sources are of stellar origin; in areas occupied by star forming regions (for example, Orion) or open clusters (for example, Hyades or Pleiades) a large number of the detected X-ray sources can be identified with young stars, yielding up to 80 percent of the total source count as galactic stars. For the whole of the ROSAT all-sky survey we may therefore expect about one third of the total sources to be of stellar origin. The vast majority of these stellar X-ray sources is of coronal origin (i.e., late-type low mass stars). Only a relatively small number of stellar X-ray sources will be associated with early-type massive stars where the X-ray emission is thought to arise from instabilities in their radiatively driven winds or metal-poor degenerate stars where the X-ray emission comes from portions of the atmosphere considerably hotter than the optically visible photosphere. From the preliminary analyses performed so far it is already clear now that supersoft sources such as white dwarfs do not constitute a major fraction of the X-ray source population found in the ROSAT all-sky survey and the number of newly X-ray discovered white dwarfs will certainly be considerably less than one thousand. The X-ray emitting late-type stars are commonly referred to as ”active” stars, and the ROSAT all-sky survey catalog will comprise the most extensive list of such objects.