Post-main-sequence evolution of A star debris discs

Abstract

While the population of main‐sequence debris discs is well constrained, little is known about debris discs around evolved stars. This paper provides a theoretical framework considering the effects of stellar evolution on debris discs, particularly the production and loss of dust within them. Here, we repeat a steady‐state model fit to disc evolution statistics for main‐sequence A stars, this time using realistic grain optical properties, then evolve that population to consider its detectability at later epochs. Our model predicts that debris discs around giant stars are harder to detect than on the main sequence because radiation pressure is more effective at removing small dust around higher luminosity stars. Just 12 per cent of the first ascent giants within 100 pc are predicted to have discs detectable with Herschel at 160 μm. However, this is subject to the uncertain effect of sublimation on the disc, which we propose can thus be constrained with such observations. Our model also finds that the rapid decline in stellar luminosity results in only very young white dwarfs having luminous discs. As such systems are on average at larger distances they are hard to detect, but we predict that the stellar parameters most likely to yield a disc detection are a white dwarf at 200 pc with cooling age of 0.1 Myr, in line with observations of the helix nebula. Our model does not predict close‐in (<0.01 au) dust, as observed for some white dwarfs; however we find that stellar wind drag leaves significant mass (∼10−2 M⊕), in bodies up to ∼10 m in diameter, inside the disc at the end of the asymptotic giant branch (AGB) phase which may replenish these discs.

Circumstellar debris and pollution at white dwarf stars

“…While extrasolar asteroid belts of the necessary size and mass are consistent with observational limits, and hence not ruled out empirically (Wyatt, 2008), owing to collisional evolution, massive disks within 10 AU are not expected to survive beyond a few Myr, and should be depleted by orders of magnitude by 100 Myr, well before the post-main sequence (Wyatt et al, 2007;Bonsor & Wyatt, 2010). There is currently no proposed solution to this puzzle: chemistry favors terrestrial-like parent bodies formed interior to a snow line, and planetesimal belt evolution favors the icy, outer reaches.…”

A DEBRIS disk around the planet hosting M-star GJ 581 spatially resolved withHerschel

“…Grains smaller than 1 μm are not important because they emit so inefficiently that their flux density is negligible (Bonsor & Wyatt 2010). The total cross-sectional area A can be converted into mass assuming a size distribution and a mass density ρ for the material.…”

ALMA and Herschel observations of the prototype dusty and polluted white dwarf G29-38

“…While the ALMA observations were best suited to outer regions analogous to evolved Kuiper belt analogs, the Herschel data were uniquely sensitive to an evolved, asteroid-like belt at G29-38 and in general to dust at temperatures and orbital regions intermediate to the relatively warm dust seen at polluted white dwarfs and the cooler dust often detected at main sequence stars. The non-detection at G29-38 is not wholly unexpected, as disk evolution models supported by observations of main sequence stars predict that the available mass in both dust and parent bodies decreases significantly over timescales of several hundred Myr (Wyatt et al 2007), and this depletion is likely enhanced during the post-main sequence (Bonsor & Wyatt 2010).…”

“…The collision rate of the largest surviving particles can be long, but the smallest grains are repopulated from the integrated collisions of all large bodies, resulting in a replenishment timescale equal to their depletion timescale in a steady state . Still, detailed models for the post-main sequence evolution of Kuiper belt analogs (Bonsor et al 2011;Bonsor & Wyatt 2010) suggest that detecting these disks in cool white dwarf systems like G29-38 is challenging, primarily due to collisional depletion of the disc material. Such collisional depletion would occur on even shorter timescales for asteroid belt analogs favored by the volatile poor abundance patterns seen via atmospheric pollution , including G29-38 in particular .…”

“…Following Bonsor & Wyatt (2010), considering the realistic emission properties of small grains illuminated by the faint white dwarf, only particles larger than 0.1 µm contribute significantly to the emission, and thus the change in fractional luminosity will be…”