Hidden SD/WD Stars Among the Faust UV Sources Toward Ophiuchus

Abstract

No white dwarf or hot subdwarf stars are found as optical counterparts of the 228 UV sources detected in a UV image toward Ophiuchus (Formiggini et al. 2002). The image was obtained at 1600 Å, with the FAUST instrument on board of the space shuttle Atlantis. Hot subluminous stars are numerous among blue objects, and dominate the population of blue stars down to B=16.5. To search for such stars, we cross-identified the UV sources positions with objects in the Main Part of the Tycho catalog, finding a significant parallax value for 46 entries. A significant number of objects have My values as expected for WD or subdwarf stars (Fig. la), while their spectral classification spans types from B to early F. For all these stars the parallax errors are very large, hence their classification as subluminous stars on the basis of the parallax data is rather unreliable.