August 11-15, 2014


The frequency of metal-contamination in young DA white dwarfs

Boris Gaensicke (University of Warwick)

B. Gaensicke (University of Warwick), D. Koester (University of Kiel), J. Farihi (UCL)

We have obtained high-resolution (R~20000) ultraviolet spectroscopy of 85 DA white dwarfs (WDs) using COS onboard the Hubble Space Telescope, with the aim to determine the frequency of metal pollution at young cooling ages (~20-200Myr). We did not apply any prior knowledge in the target selection, apart from a cut in temperature (17000K<Teff<25000K)and in the predicted UV flux. We find that 56% of the DAs observed with HST display photospheric Si absorption lines, 16 stars also have photospheric C, and 7 exhibit additional elements. Our COS survey identified one of the most heavily contaminated DAZ, WD0843+516, with nine elements detected in its UV spectrum - which went unnoticed in previous ground-based studies. We discuss our results in the context of accretion vs. radiative levitation, and we review the implications of the abundance ratios determined for the stars that are undoubtedly in accretion-diffusion equilibrium for the nature of their circumstellar debris.

Mode of presentation: oral