Cool companions to hot white dwarfs

Low-mass companions to high-mass stars are difficult to detect, which is partly why the binary fraction for high-mass stars is still poorly constrained. Low-mass companions can be detected more easily however, once high-mass stars turn into white dwarfs. These systems are also interesting as the progenitors of a variety of intensely studied interacting binary systems, like novae, CVs, symbiotics, Ba and CH giants, Feige 24-type systems, and dwarf carbon stars. We describe a near-IR photometric search for cool red dwarf companions to hot white dwarfs (WDs). IR photometry o ers a sensitive test for low-mass main-sequence (MS) companions. Our sample of EUV-detected WDs o ers several advantages over previous (largely proper motionÈselected) WD samples: (1) the high WD temperatures (24,000\Teff\ 70,000 K) insure excellent IR Ñux contrast with cool dwarfs; (2) the range of evolutionary parameter space occupied by the WDs is considerably narrowed; and (3) the random e ects of the intervening ISM provide a complete but reasonably sized sample. While some composite systems have been found optically among WDs detected in recent EUV All-Sky Surveys, we develop an IR technique that probes farther down the main sequence, detecting yet more companions. We use detailed DA model atmosphere Ðts to optical spectra to predict K magnitudes and distances, against which we contrast our near-IR observations. Our photometric survey reveals 10 DAs with a signiÐcant excess in both J and K. Half are newly discovered and are most likely previously unrecognized binary systems. Neither the frequency of infrared excess nor the mass estimate of the red dwarf companion correlate with white dwarf mass, as might be expected if either the EUV detectability or mass of the white dwarfs were signiÐcantly a ected by a companion. Infrared spectra of these systems should help to determine the mass and spectral type of the cool companions presumably causing the IR excess, leading to better estimates of the mass ratio distribution in binaries. Counting previously known binaries, and resolved pairs, we Ðnd the total binary fraction of the sample is at least a third. Since most WD progenitors had initial masses º2 M we thus _, provide a photometric measure of the binary fraction of high-mass stars that would be difficult to perform in high-mass main-sequence stars. We estimate that 90% of the companions are of type K or later.