SPECTROSCOPIC, ORBITAL, AND PHYSICAL-PROPERTIES OF THE BINARY FEIGE-24 AND DETECTION OF TRANSIENT HE-II ABSORPTION IN THE SYSTEM

We have obtained new high-dispersion optical spectroscopy at Kitt Peak National Observatory (KPNO) and new International Ultraviolet Explorer (IUE) spectroscopy of the white dwarf+red dwarf binary system Feige 24 (P=4.(d)23160). The optical range shows a composite DA+dM spectrum, together with H I Balmer and He I emission. The orbital phase dependence of the emission shows that it results from extreme ultraviolet (EUV) light reprocessing in the red dwarf upper atmosphere. The systems close enough and hot enough to show this reprocessing signature only recently emerged from common-envelope evolution. The ultraviolet spectrum exclusively emanates from the white dwarf and shows numerous heavy element absorption lines (C, N, O, Si, S, Fe). We measured accurate (1 km s(-1)) radial velocities of the red dwarf component motion, traced by both optical absorption and emission lines, and new radial velocities of the white dwarf, traced by ultraviolet Fe V lines. Combining these measurements, we refined the orbital parameters presented by Vennes et al. [ApJ, 372, L37 (1991)], and we confirmed that the white dwarf gravitational redshift is exceptionally small with 9+/-2 km s(-1). From this we deduced that the interior is either pure helium or carbon with a thick hydrogen layer (approximate to 10(-4) M(circle dot)), and we derived, for the combined interior compositions, a white dwarf mass and radius of M(WD)=0.44-0.50 M(circle dot) and R(WD)=0.028-0.036 Ro. We suggest that Feige 24 could be a typical case of close binary evolution leading to the formation of a low-mass helium white dwarf. The mass of the red dwarf and the inclination of the system naturally follow: M(dM)=0.26-0.33 M(circle dot) , i greater than or equal to 75 degrees. High-dispersion H alpha line profiles are asymmetrical, strongly enhanced toward the blue, suggesting a moving atmosphere possibly linked to a mass loss rate of 10(-10) M(circle dot) yr(-1). The IUE spectra taken when the system is near inferior conjunction show strong He II 1640 Angstrom absorption. The profile is highly variable in width (maximum FWHM=90 km s(-1)) and intensity. Because it is correlated with the passage of the white dwarf at inferior conjunction, the absorption may occur in some foreground plasma emanated by the red dwarf and accumulating near a Lagrangian point or, alternatively, it may originate in an accretion spot on the white dwarf surface coaligned with the major orbital axis. Either way, the He II detection may imply substantial mass loss from the red dwarf with a corollary reclassification of Feige 24 as a mixed He/H DAO white dwarf resulting from accretion of secondary mass-loss material. Feige 24 is the prototype of a class of young, EW-emitting, binary systems comprising a late main sequence secondary and a hot H-rich white dwarf; the class is characterized by optical (lambda 4686) and ultraviolet (lambda 1640) photospheric He II absorption, circumstellar C IV lambda 1550 absorption, and by the presence of EUV-induced, phase-dependent Balmer fluorescence. These young systems present the best opportunity to constrain theory of common-envelope evolution.