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

With Hubble Space Telescope Fine Guidance Sensor 3 we have determined a par allax for the white dwarf-M dwarf interacting binary, Feige 24. The white d warf (DA) component has an effective temperature T-eff similar to 56,000 K. A weighted average with past parallax determinations (pi(abs) = 14.6 +/- 0 .4 mas) narrows the range of possible radius values, compared with past est imates. We obtain R-DA = 0.0185 +/- 0.0008 R. with uncertainty in the tempe rature and bolometric correction the dominant contributors to the error. Fi ne 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., where the mass range is due to metallicity and age unc ertainties. Radial velocities from Vennes and Thorstensen provide a mass ra tio from which we obtain M-DA = 0.49(-0.05)(+0.19) M.. Independently, our r adius and recent log g determinations yield 0.44 M. < M-DA < 0.47 M.. In ea ch case, the minimum DA mass is that derived by Vennes & Thorstensen from t heir radial velocities and Keplerian circular orbits with i less than or eq ual to 90 degrees. Locating Feige 24 on an (M, R)plane suggests a carbon co re. Our radius and these mass estimates yield a value of gamma(gray) incons istent with that derived by Vennes & Thorstensen. We speculate on the natur e of a third component whose existence would resolve the discrepancy.