Interferometric Astrometry of the Detached White Dwarf-M Dwarf Binary Feige 24 Using HST Fine Guidance Sensor 3: White Dwarf Radius and Component Mass Estimates

Abstract

With Hubble Space Telescope Fine Guidance Sensor 3 we have determined a parallax for the white dwarf-M dwarf interacting binary, Feige 24. The white dwarf (DA) component has an effective temperature T_eff~56,000 K. A weighted average with past parallax determinations (π_abs=14.6+/-0.4 mas) narrows the range of possible radius values, compared with past estimates. We obtain R_DA=0.0185+/-0.0008 R_solar with uncertainty in the temperature and bolometric correction the dominant contributors to the error. Fine Guidance Sensor 3 photometry provides a light curve entirely consistent with reflection effects. A recently refined model mass-luminosity relation for low-mass stars provides a mass estimate for the M dwarf companion, M_dM=0.37+/-0.20 M_solar, where the mass range is due to metallicity and age uncertainties. Radial velocities from Vennes and Thorstensen provide a mass ratio from which we obtain M_DA=0.49^+0.19_-0.05 M_solar. Independently, our radius and recent logg determinations yield 0.44 M_solar<M_DA<0.47 M_solar. In each case, the minimum DA mass is that derived by Vennes & Thorstensen from their radial velocities and Keplerian circular orbits with i<=90°. Locating Feige 24 on an (M,R)-plane suggests a carbon core. Our radius and these mass estimates yield a value of γ_grav inconsistent with that derived by Vennes & Thorstensen. We speculate on the nature of a third component whose existence would resolve the discrepancy. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.