Interferometric Astrometry of the Detached White Dwarf–M Dwarf Binary Feige 24 Using [ITAL]HST[/ITAL] Fine Guidance Sensor 3: White Dwarf Radius and Component Mass Estimates

Abstract

With Hubble Space T elescope Fine Guidance Sensor 3 we have determined a parallax for the white dwarfÈM dwarf interacting binary, Feige 24. The white dwarf (DA) component has an e †ective temperature K. A weighted average with past parallax determinations mas) T eff D 56,000 (n abs \ 14.6^0.4 narrows the range of possible radius values, compared with past estimates. We obtain R DA \ 0.0185 with uncertainty in the temperature and bolometric correction the dominant contributorŝ 0.0008 R _ to the error. Fine Guidance Sensor 3 photometry provides a light curve entirely consistent with reÑec-tion e †ects. A recently reÐned model mass-luminosity relation for low-mass stars provides a mass estimate for the M dwarf companion, where the mass range is due to metallicity M dM \ 0.37^0.20 M _ , and age uncertainties. Radial velocities from Vennes and Thorstensen provide a mass ratio from which we obtain Independently, our radius and recent log g determinations yield 0.44 M DA \ 0.49~0 .05 0.19 M _ . In each case, the minimum DA mass is that derived by Vennes & Thorstensen M _ \ M DA \ 0.47 M _ . from their radial velocities and Keplerian circular orbits with i ¹ 90¡. Locating Feige 24 on an (M, R)-plane suggests a carbon core. Our radius and these mass estimates yield a value of inconsistent with c grav that derived by Vennes & Thorstensen. We speculate on the nature of a third component whose existence would resolve the discrepancy.

Equilibrium abundances in hot DA white dwarfs as derived from self-consistent diffusion models

“…One independent result, however, might be worth considering in this context: detailed parallax and gravitational redshift measurements with HST of the binary system Feige 24 (Benedict et al 2000;Vennes et al 2000) also yielded a higher surface gravity for the WD component than several optical analyses (log g = 7.2−7.5). The consistency of our result (log g = 7.7) with Vennes et al's (log g = 7.5−7.7) might be interpreted as a hint that our current results do not suffer from an inaccurate g rad determination in the models after all.…”

Astrometry withHubble Space TelescopeFine Guidance Sensors—A Review

“…See Harrison (2014) for a recent overview. Related in a sense to this group is Feige 24 (55, Benedict et al 2000), a WD-M dwarf binary described as the prototypical post common-envelope detached system with a low probability of becoming a cataclysmic variable (CV) within a Hubble time. This object was selected for the STAT original Guaranteed Time Observing parallax program because a directly measured distance could reduce the uncertainty of the radius of one of the hottest white dwarfs.…”

Direct distance determination using parallax: Techniques, promises and limitations

“…Benedict et al 1999) yields an accuracy of approximately 1 mas per observation for the magnitude range V = 3 to 17. The HST parallax programme includes a number of Cepheids and RR Lyrae stars for the calibration of their period-luminosity relations, selected Hyades and Pleiades stars, and some fainter stars (e.g., Benedict et al 2000Benedict et al , 2002Benedict et al , 2007Benedict et al , 2011Soderblom et al 2005;McArthur et al 2011). The typical parallax accuracy is 0.1-0.2 mas.…”