Hubble Space Telescope Imaging Spectrograph Observations of the Hot White Dwarf in the Close Binary Feige 24

Abstract

We obtained and analyzed two Hubble Space Telescope Imaging Spectrograph spectra of the white dwarf in the DA plus dMe binary Feige 24. The spectra, obtained at orbital quadratures, provide new estimates of the white dwarf motion and gravitational redshift resulting in revised white dwarf parameters. An analysis of interstellar absorption lines reveals the presence of two clouds ( +3.1 km s-1, +17.6 km s-1) in the line of sight toward Feige 24; one of these clouds (+17.6 km s-1) is identified with the "local cloud." A study of the Lyα H I and D I interstellar medium lines shows that the deuterium-to-hydrogen abundance ratio (D/H = 1.3 × 10-5) is consistent with other measurements supporting a relative constancy of this ratio throughout the local interstellar medium. The total hydrogen column density measured with Lyα (log nH = 2.95 × 1018 cm-2) is in agreement with EUV Lyman continuum flux measurements. Finally, we present a complete abundance pattern for the white dwarf, demonstrating the predominance of iron and nickel over lighter elements. Residual ionization imbalance in the case of several elements, most notably in the case of O IV/O V, which cannot be explained by temperature or surface gravity variations, may indicate the presence of other atmospheric constituents, inhomogeneous stratification of oxygen in the photosphere, and/or remaining inaccuracies in the treatment of model atoms. The abundance patterns in Feige 24 and in the hot DA white dwarf G191-B2B are remarkably similar, indicating that the same processes are operating equally in both stars.