40 research outputs found
<i>Gaia</i> reveals evidence for merged white dwarfs
We use Gaia Data Release 2 to identify 13,928 white dwarfs within 100 pc of
the Sun. The exquisite astrometry from Gaia reveals for the first time a
bifurcation in the observed white dwarf sequence in both Gaia and the Sloan
Digital Sky Survey (SDSS) passbands. The latter is easily explained by a helium
atmosphere white dwarf fraction of 36%. However, the bifurcation in the Gaia
colour-magnitude diagram depends on both the atmospheric composition and the
mass distribution. We simulate theoretical colour-magnitude diagrams for single
and binary white dwarfs using a population synthesis approach and demonstrate
that there is a significant contribution from relatively massive white dwarfs
that likely formed through mergers. These include white dwarf remnants of
main-sequence (blue stragglers) and post-main sequence mergers. The mass
distribution of the SDSS subsample, including the spectroscopically confirmed
white dwarfs, also shows this massive bump. This is the first direct detection
of such a population in a volume-limited sample.Comment: MNRAS, in pres
The composition of a disrupted extrasolar planetesimal at SDSS J0845+2257 (Ton 345)
We present a detailed study of the metal-polluted DB white dwarf SDSS J0845+2257 (Ton 345). Using high-resolution Hubble Space Telescope/Cosmic Origins Spectrograph and Very Large Telescope spectroscopy, we have detected hydrogen and 11 metals in the atmosphere of the white dwarf. The origin of these metals is almost certainly the circumstellar disc of dusty and gaseous debris from a tidally disrupted planetesimal, accreting at a rate of 1.6 Ă 1010âgâsâ1. Studying the chemical abundances of the accreted material demonstrates that the planetesimal had a composition similar to the Earth, dominated by rocky silicates and metallic iron, with a low water content. The mass of metals within the convection zone of the white dwarf corresponds to an asteroid of at least âź130â170 km in diameter, although the presence of ongoing accretion from the debris disc implies that the planetesimal was probably larger than this. While a previous abundance study of the accreted material has shown an anomalously high mass fraction of carbon (15 per cent) compared to the bulk Earth, our independent analysis results in a carbon abundance of just 2.5 per cent. Enhanced abundances of core material (Fe, Ni) suggest that the accreted object may have lost a portion of its mantle, possibly due to stellar wind stripping in the asymptotic giant branch. Time series spectroscopy reveals variable emission from the orbiting gaseous disc, demonstrating that the evolved planetary system at SDSS J0845+2257 is dynamically active