Searching for Close Double Degenerates

Abstract

Despite extensive efforts (Robinson & Shafter 1987; Bragaglia et al. 1990; Foss, Wade & Green 1991; Bragaglia, Greggio & Renzini 1994), not a single “double degenerate” (DD) system with the right properties to be a Supernova Ia precursor has yet been found. We need to increase the number of White Dwarfs examined and to better discriminate in some dubious cases of possible binaries for a decisive observational test. We have now reliable data on more than 80 WDs (2/3 DAs, 1/3 DBs); several of them show marginal detection of radial velocity variations between spectra, a few have a mass so low (Bragaglia, Renzini & Bergeron 1995) to be suggestive of evolution in a close binary system, and three are confirmed binaries (one not-too-close DD, and two WD + red dwarf pairs, Bragaglia et al. 1990). During 1993 we reobserved 6 field DA White Dwarfs, at the ESO 1.5m telescope. From several sky lines and bands we derive a precision of our wavelength scale of ~ 30 kms⁻¹. None of the WDs shows incontrovertible evidence of binarity. We note that all confirmed or suspect binary in our sample show Δv _r ’s of the order of 100 kms⁻¹, at odds with the expectation (Iben 1990; Yungelson et al. 1994) of a nearly equal number of DDs with the semiamplitude of the radial velocity curve K ≤ 100 kms⁻¹and with 100 ≤ K ≤ 500 kms⁻¹. Further study of all objects showing possible Δv _r ’s is foreseen.