Journal Article

A trio of metal-rich dust and gas discs found orbiting candidate white dwarfs with <i>K</i>-band excess

J. Farihi, B. T. Gänsicke, P. R. Steele, J. Girven, M. R. Burleigh, E. Breedt and D. Koester

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 421, issue 2, pages 1635-1643
Published in print April 2012 | ISSN: 0035-8711
Published online March 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20421.x
A trio of metal-rich dust and gas discs found orbiting candidate white dwarfs with K-band excess

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This paper reports follow-up photometric and spectroscopic observations, including warm Spitzer IRAC photometry of seven white dwarfs from the SDSS with apparent excess flux in UKIDSS K-band observations. Six of the science targets were selected from 16 785 DA star candidates identified either spectroscopically or photometrically within SDSS DR7, spatially cross-correlated with HK detections in UKIDSS DR8. Thus, the selection criteria are completely independent of stellar mass, effective temperature above 8000 K and the presence (or absence) of atmospheric metals. The infrared fluxes of one target are compatible with a spatially unresolved late M or early L-type companion, while three stars exhibit excess emissions consistent with warm circumstellar dust. These latter targets have spectral energy distributions similar to known dusty white dwarfs with high fractional infrared luminosities (thus the K-band excesses). Optical spectroscopy reveals the stars with disc-like excesses are polluted with heavy elements, denoting the ongoing accretion of circumstellar material. One of the discs exhibits a gaseous component – the fourth reported to date – and orbits a relatively cool star, indicating the gas is produced via collisions as opposed to sublimation, supporting the picture of a recent event. The resulting statistics yield a lower limit of 0.8 per cent for the fraction dust discs at DA-type white dwarfs with cooling ages less than 1 Gyr. Two overall results are noteworthy: (i) all stars whose excess infrared emission is consistent with dust are metal rich and (ii) no stars warmer than 25 000 K are found to have this type of excess, despite sufficient sensitivity.

Keywords: stars: abundances; brown dwarfs; circumstellar matter; stars: low-mass; planetary systems; white dwarfs

Journal Article.  6129 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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