Spectroscopic and Orbital Properties of the Binary Feige 24 and Discovery of External Plasma at Inferior Conjunction
Session 54 -- Interacting Binaries: Hot Stars
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [54.07] Spectroscopic and Orbital Properties of the Binary Feige 24 and Discovery of External Plasma at Inferior Conjunction

S. Vennes (CEA/UCB), J.R. Thorstensen (Dartmouth College)

We have obtained new high-dispersion optical (KPNO) and ultraviolet spectroscopy (IUE ) of the close white dwarf + red dwarf binary system Feige 24 (P = 4.2316 d). The optical range shows a composite DA+dM spectrum, together with H~{\sc i} Balmer and He~{\sc i} emission. The orbital phase dependence of the emission shows that it results from extreme ultraviolet (EUV) light reprocessing in the red dwarf photosphere. The systems close enough and hot enough to show this reprocessing signature must arise from common-envelope evolution. The ultraviolet spectrum is dominated by the white dwarf. It shows numerous Fe~{\sc v} absorption lines together with C~{\sc iv}, N~{\sc v}, and Si~{\sc iv} resonance doublets and few excited lines from the most abundant elements (N~{\sc iv}, O~{\sc iv}, S~{\sc v}). We measured accurate ($1~\mbox{km~s}^{-1}$) 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 UV Fe~{\sc v} lines. Combining these measurements, we refine the orbital parameters presented by Vennes et al. (1991, ApJ, 372, L37), and we confirm that the white dwarf gravitational redshift is exceptionally small ($8 \pm 2\mbox{~km~s}^{-1}$). Using theoretical radii for thin hydrogen layers we can uniquely constrain its mass and radius to $M_{WD} = 0.40 \pm 0.04 M_{\odot}$ and $R_{WD} = 0.024-0.032 R_{\odot}$. The mass of the red dwarf and the inclination of the system naturally follow: $M_{dM} = 0.27 \pm 0.03 M_{\odot}$, $i = 65 \deg$. The IUE spectra taken when the system is near inferior conjunction show strong He~{\sc ii} 1640 absorption. The profile is highly variable in width and intensity and appears correlated with the passage of the white dwarf in the background of plasma associated with the red dwarf, almost $4 R_{\odot}$ above the orbital plane. At maximum, the line absorption is broad ($130\mbox{~km~s}^{-1}$) and blueshifted ($-20\mbox{~km~s}^{-1}$) relative to the systemic velocity. The plasma probably consists of coronal material and/or wind material. Additional UV spectroscopy will help determine the nature, dynamics, and temperature of this external plasma. This work is supported by NASA contract NAS5-30180 and grant NAG5-1805.