Infrared flux excesses from hot subdwarfs.

Abstract

Observations of 27 hot subdwarf stars and white dwarfs have been performed with an IR photometer in a search for excess fluxes in the JHK bands. 22 of the objects are observed for the first time in the infrared. In 11 of the stars a significant excess flux was found, 2 of which are new discoveries: GD 274 and GD 299. We confirm the suspicion of Moehler et al. (1990) that PG0232+095 is a composite system, and we reinforce the suggestion of Thejll et al. (1991, 1994a) that Feige 34 is a composite system. The excesses are most likely due to companion stars, mainly of spectral types A - K. We estimate the subdwarf gravities, assuming zero-age main sequence companions, and thus find upper limits on log(g). For the hot sdO Feige 34 we estimate from the published trigonometric parallaxes an absolute magnitude M_v_^F34^=5.6^+0.9^_-1.6_ and surface gravity log(g_pi_)=6.8^+0.3^_-0.7_.