STUDIES OF HOT B SUBDWARFS. XL A REDETERMINATION OF THE DENSITY OF HOT, HYDROGEN-RICH SUBDWARFS IN THE GALACTIC PLANE

摘要

We have reanalyzed the statistical properties of the sample of hydrogen-rich hot subdwarfs identified in the Kitt Peak-Downes survey in order to derive an improved value of the density D_0 of these stars in the plane of the Galaxy. Our analysis is based on photoelectric Stroemgren photometry for 22 of these stars, all brighter than V ～ 15.0. The value of the average absolute magnitude of the KPD objects we redetermine is at least 1 mag brighter than that derived in the initial investigation. Three methods are then considered to derive D_0, under the assumption of an exponential distribution of stars in the direction perpendicular to the Galactic plane: fits to the number of stars as a function of apparent brightness and as a function of vertical distance above the plane, as well as the 1/V′_m method. We find that the distribution of hot, hydrogen-rich subdwarf stars can be satisfactorily reproducee with a midplane space density D_0 = 3.8 ± 1.7 x 10~(-7) pc~(-3), a factor of ～5 lower than initial estimates, but in agreement with the recent determination of Villeneuve et al. based on the Palomar-Green sample at high galactic latitudes.