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

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% can have a wide, flat, opaque dust disc, extending to the Roche radius, with a temperature at the disc inner edge of T in = 1, 400K, the standard model for the observed excesses. This is in stark contrast to the incidence of pollution of about 30%. 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% 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 timescales longer than hundreds of years, iii) their dust is optically thin, which can supply low accretion rates of < 10 7 gs −1 if dominated by PR-drag, and higher accretion rates, if inwards transport of material is enhanced, for example 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, 000K. 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.

Magnetism, X-rays and accretion rates in WD 1145+017 and other polluted white dwarf systems

“…In the case of 0322-019 with T eff ≈ 5300 K, this star sits among a large class of metal-lined white dwarfs older than 0.5 Gyr, where infrared excesses are rarely observed (Xu & Jura 2012;Bergfors et al 2014), and in which the heavy element sinking timescales are sufficiently long that accretion may have ended. For 2105-820, it has a steady-state accretion rate of 3 × 10 7 g s −1 based on its calcium abundance, and likely below the ability of space-and ground-based detection of its disk (Rocchetto et al 2015;Bonsor et al 2017).…”

Compositions of Planetary Debris around Dusty White Dwarfs

“…Note-These are average values for the WDs presented in Fig. 3. natures; alternatively, the dust disk can be completely accreted but the heavy elements have not fully settled (Jura 2008;Bonsor et al 2017). As shown in Fig.…”

Numerical Simulations of Collisional Cascades at the Roche Limits of White Dwarf Stars

“…Stars in the DZ class have strong metallic lines with little or no H or He (e.g., Sion et al 1990;Koester et al 2011;Sion et al 2014;Kepler et al 2015Kepler et al , 2016. A few per cent of these stars have near-IR excess emission from warm dust which reprocesses roughly 1% of the stellar luminosity (e.g., Kilic et al 2005;Reach et al 2005;Hansen et al 2006;Kilic et al 2006;Tremblay & Bergeron 2007;von Hippel et al 2007; Kilic et al 2008;Farihi et al 2009;Girven et al 2011;Debes et al 2011;Chu et al 2011;Girven et al 2012;Barber et al 2012;Hoard et al 2013;Bergfors et al 2014;Rocchetto et al 2015;Barber et al 2016;Bonsor et al 2017). A few systems also have metallic emission features which sometimes display the characteristic double-peaked profile of a circumstellar disk (e.g., Gänsicke et al 2006Gänsicke et al , 2007Gänsicke et al , 2008Melis et al 2010;Farihi et al 2012;Melis et al 2012;Debes et al 2012a;Wilson et al 2014).…”