Non-LTE analysis of the Palomar Green subdwarf O stars

Abstract

Using NETE model spectra, the sdO subdwarfs found in the Palomar-Green (PG) survey, are analyzed in order to deterniine their relative contribution to the white dwarfcooling sequence. We can confirm that only a small fraction of the WD's, and thus also of the DB WD's, can be related to the sdO cliannel. i Goal and Method Which role do the heliurn-rich sdO stars play in late stages of stellar evolution? In particular, do they form an important link to the helium-rich white dwarfs. By fitting optical and UV-spectra to H and He-line blanketed NLTE models we find Teif, log(g) and composition. Then Wc calculate distances and find population descriptors such as scale height, density in plane and rate of evolution, and finally compare to WD's. Our model grid covers from 35,000K to 65,000K, log(g)=4.0 to 6.5 and from 50 to 99% helium. The observed spectra are assumcd to form a random selection of objects from the statistically complete PO survey. There are about 230 hot subdwarfs, showing at least traces of helium, in the PO survey of blue high-galactic latitude objects. They are labeled as sdO, sdOA, sdOB, sdOC or sdOD in the survey and will in this work be referred to as "sdO's". We have about 130 iow to high quality medium dispersion sdO spectra. We attempted to fit 70 of these and were able to fit 35. Most spectra that we failed to fit were more rich iii hydrogen than the maximum of 50% that limits the present grid. We calculated the distances above planetary nebulae Birth Rate