ROSAT Galactic Plane Survey

These results together with those of similar optical campaigns demonstrate that the soft X-ray population of the Milky Way is largely dominated by active stars. We show that the density and distribution in flux and spectral type of the active coronae detected in X-rays are consistent with the picture drawn from current stellar population models and age dependent X-ray luminosity functions. The modelling of this population suggests that most of the stars detected by ROSAT in this direction are younger than 1Gyr.

This opens the possibility to extract in a novel way large samples of young stars from the ROSAT all-sky survey. The small number of unidentified sources at low X-ray flux put rather strong constraints on the hypothetical X-ray emission from old neutron stars accreting from the interstellar medium. Our observations clearly rule out models which assume no dynamical heating for this population and a total number of N_ns_=10^9^ neutron stars in the Galaxy.

If accretion on polar caps is the dominant mode then our upper limit may imply N_ns_=~10^8^. Among the non coronal identifications are three white dwarfs, a Seyfert 1 active nucleus, two early type stars and one cataclysmic variable. We also report the discovery of a Me+WD close binary system with P_orb_=~12 .