Infrared observations of white dwarfs and the implications for the accretion of dusty planetary material


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Authors
Bonsor, A 
Farihi, J 
Wyatt, MC 
van Lieshout, R 
Abstract

Infrared excesses around metal-polluted white dwarfs have been associated with the accretion of dusty planetary material. This work analyses the available infrared data for an unbiased sample of white dwarfs and demonstrates that no more than 3.3 per cent can have a wide, flat, opaque dust disc, extending to the Roche radius, with a temperature at the disc inner edge of Tin = 1400 K, the standard model for the observed excesses. This is in stark contrast to the incidence of pollution of about 30 per cent. We present four potential reasons for the absence of an infrared excess in polluted white dwarfs, depending on their stellar properties and inferred accretion rates: (i) their dust discs are opaque, but narrow, thus evading detection if more than 85 per cent of polluted white dwarfs have dust discs narrower than δr < 0.04r, (ii) their dust discs have been fully consumed, which only works for the oldest white dwarfs with sinking time-scales longer than hundreds of years, (iii) their dust is optically thin, which can supply low accretion rates of <107 gs−1 if dominated by (Poynting–Robertson) PR-drag, and higher accretion rates, if inwards transport of material is enhanced, e.g. due to the presence of gas, (iv) their accretion is supplied by a pure gas disc, which could result from the sublimation of optically thin dust for T > 20 000 K. Future observations sensitive to faint infrared excesses or the presence of gas can test the scenarios presented here, thereby better constraining the nature of the material fuelling accretion in polluted white dwarfs.

Description
Keywords
planets and satellites: general, circumstellar matter, planetary systems, white dwarfs
Journal Title
Monthly Notices of the Royal Astronomical Society
Conference Name
Journal ISSN
0035-8711
1365-2966
Volume Title
468
Publisher
Oxford University Press
Sponsorship
Science and Technology Facilities Council (ST/J003344/2)
Science and Technology Facilities Council (ST/K003453/1)
Science and Technology Facilities Council (ST/N000927/1)
European Research Council (279973)
AB, MCW and RL acknowledge the support of the European Union through ERC grant number 279973. JF acknowledges support from the United Kingdom Science and Technology Facilities Council in the form of an Ernest Rutherford Fellowship (ST/J003344/1). This publication makes use of data products from WISE, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.