White Dwarf–Red Dwarf Systems Resolved with theHubble Space Telescope. I. First Results

Abstract

First results are presented for a Hubble Space Telescope (HST ) Advanced Camera for Surveys snapshot study of white dwarfs with likely red dwarf companions. Of 48 targets observed and analyzed so far, 27 are totally or partially resolved into two or more components, while an additional 15 systems are almost certainly unresolved binaries. These results provide the first direct empirical evidence for a bimodal distribution of orbital separations among binary systems containing at least one white dwarf.

Ultracool Companions to White Dwarfs

“…Consideration of the post-main sequence evolution of these systems and observations by Farihi, Hoard & Wachter (2006b) suggests that there may be a bimodal distribution of orbital separations among binaries containing at least one white dwarf: wide pairs with orbits > 10 AU and very close systems (< few solar radii) in which the companion was dragged in during the common envelope phase. The failure to resolve the system by Farihi et al (2005) might be indicating that PG 1234 is a close binary, but it should be cautioned that the pair also may be aligned by chance.…”

“…Many white dwarfs are known to have low mass M dwarf companions, and analysis of their population statistics allows the investigation of binary formation and evolution. In particular, consideration of stellar evolution suggests the possible existence of two distinct populations: close systems (Orbital distance < 0.1AU) in which the secondary has survived a phase of CEE and which may eventually lead to the formation of a CV, and wide pairs (Orbital separation > few AU) where the secondary has migrated outwards in response to mass-loss from the white dwarf's progenitor (Farihi, Hoard & Wachter 2006a). An object originally orbiting a white dwarf progenitor at a > 5 AU will have its orbit expanded by a maximum factor equal to the ratio of the main-sequence mass to the white dwarf mass (M ms /M wd ) (Jeans 1924, Zuckerman & Becklin 1987b, Burleigh et al 2002, a value typically around 3.…”

“…Shortly afterwards the system was resolved with the Hubble Space Telescope by Farihi, Hoard & Wachter (2006b) who then estimated the spectral type of the companion, again from photometry, as an M3.5 orbiting at a projected distance of 21 AU. Figure 3.4 shows the observed J and K and modelled spectra of GD 984 and composite WD+M3, WD+M3.5 and WD+M4 models.…”

“…PG 1412−049 was identified as having a companion by Wachter et al (2003) and later resolved with the Hubble Space telescope by Farihi, Hoard & Wachter (2006b) who, using photometry, assigned a spectral type for the companion as an M0 star at a projected orbital separation of 1200 AU. Figure B.10 shows the observed and modelled spectra of WD 1412−049 and composite WD+M0, WD+M1 and WD+M2 models.…”

White dwarfs in the WTS: Eclipsing binaries

“…Two distinct populations are thought to be the outcome of stellar evolution; wide pairs where the secondary has migrated outwards due to the mass loss of the WDs progenitor (Farihi et al 2006, Nordhaus et al 2010, and the close systems in which the secondary has survived a stage of common envelope evolution and may eventually lead to the formation of a cataclysmic variable (CV). In these close binaries, the BD is expected to be irradiated by the WD's high UV flux, leading to substantial differences in the "day" and "night" side hemispheres.…”