Improved effective temperatures for hydrogen-deficient binary stars.

Abstract

Effective temperatures are obtained for all four known hydrogen-deficient binary stars, using published and archival photometry and new line-blanketed model atmospheres. These model atmospheres largely resolve the well-known mismatch between the theoretical and observed flux distribution for Upsilon Sgr, although a discrepancy persists at short wavelengths (less than 1500 A). The effective temperature obtained is in excellent agreement with that derived from the ionization equilibrium in the photosphere. The effective temperatures of all four stars are substantially revised from previous measurements, and are all within 500 K of 12,000 K. With a reported radius of 60 solar radii for v Sgr, its luminosity becomes 63,000 solar luminosities. Assuming this radius for all objects, their distances are estimated, and lie between 1.4 and 9.6 kpc. All four objects show IR excesses. These are modeled assuming a single-temperature blackbody continuum; the results are broadly consistent with previous analyses. Three objects exhibit a 1000-K dust shell of some 10 to 100 times the stellar radius, while a 3500-K excess in LSS 1922 is probably due to a nearby K or M star.