Studies of hot B subdwarfs. X. The distribution and space density of hot, hydrogen-rich subdwarfs determined from the palomar-green survey

We have computed the space density, as well as the characteristic scale height of the distribution, of hot, hydrogen-rich subdwarfs in the Galaxy through the use of photoelectric Strömgren photometry of a sample of 209 of these stars culled from the Palomar-Green Survey. Homogeneous, exponential, and isothermal-sheet model distributions in the direction perpendicular to the Galactic plane are considered in turn, together with various subsamples of objects reaching down to different limiting b magnitudes. Taking into account the results of the V/Vm test on the brightest subsamples, which suggest that the PG survey might not be complete at bright magnitudes, we find that the distribution of hot, hydrogen-rich subdwarf stars can be satisfactorily reproduced with an isothermal scale height of 600 ± 150 pc, a value characteristic of the disk population of spiral galaxies or, alternatively, with an exponential scale height of 450 ± 150 pc. The resulting values of the space density and birthrate in both cases are (3 ± 1) ×10-7 pc-3 and ∼(1-3) × 10-15 pc-3 yr-1, a factor of ∼5-10 lower than previous estimates. Detailed comparisons with previous determinations are provided, and the implications of the large exponential scale height, and low space density and birthrate, for this particular channel of pre-white dwarf evolution are considered.