Discovery of Strong EUV-induced Balmer Emission in the New WD+dM Binary EUVE J2013+40.0 (RE 2013+400)
Session 54 -- Interacting Binaries: Hot Stars
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [54.06] Discovery of Strong EUV-induced Balmer Emission in the New WD+dM Binary EUVE J2013+40.0 (RE 2013+400)

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

The binary system EUVE J2013+40.0 (= RE 2013+400) was discovered in the EUV-selected sample of white dwarfs identified in the course of the ROSAT Wide Field Camera (WFC) all-sky survey (Pounds et al.~1993, MNRAS, 260, 77). The intense extreme ultraviolet (EUV) emission from the hot white dwarf (DAO type) was also detected in the course of the Extreme Ultraviolet Explorer (EUVE) all-sky survey (Bowyer et al.~1993, ApJ, submitted), and the subsequent optical identification campaign suggested the association of EUVE J2013+40.0 with the Feige 24 class of binary systems (see Vennes \& Thorstensen, these proceedings). Such systems consist of a hot H-rich white dwarf (DA/DAO) and a red dwarf companion (dM) and are characterized by strong, narrow, variable Balmer emission. We obtained spectroscopy with 4 \AA\ resolution at the Michigan-Dartmouth-MIT Hiltner 2.4~m, covering the H${\alpha}$ and H${\beta}$ range. The H$\alpha$ emission line velocity and equivalent widths varied with a period of $0.708 \pm 0.003$ d; the velocity semiamplitude is $89 \pm 3$ km s$^{-1}$. The emission equivalent width reaches maximum strength $0.251 \pm 0.007$ cycle after maximum emission-line velocity, that is, when the emission source reaches superior conjunction. This is just as expected if the emission arises from reprocessing of the EUV radiation incident upon the face of the dM star facing the white dwarf, as proposed for Feige 24 by Thorstensen et al.~(1978, ApJ, 223, 260). EUVE J2013+40.0 is one of a handful of WD+dM binary systems in which the illumination effect is observed with unambiguous clarity. By comparing Feige 24 and EUVE J2013+40.0, and modelling the white dwarf EUV emission and red dwarf Balmer emission, we constrain the orbital inclinations. Additional spectroscopy of EUVE J2013+40.0 is being scheduled to determine the component masses. These are important input data for the study of the close binary systems which arise from common envelope evolution. This work is supported by a forthcoming NASA Guest Observer grant.