Abstract
Debris discs are commonly detected orbiting main-sequence stars, yet little is known regarding their fate as the star evolves to become a giant. Recent observations of radial velocity detected planets orbiting giant stars highlight this population and its importance for probing, for example, the population of planetary systems orbiting intermediate mass stars. Our Herschel survey observed a subset of the Johnson et al program subgiants, finding that 4/36 exhibit excess emission thought to indicate debris, of which 3/19 are planet-hosting stars and 1/17 are stars with no current planet detections. Given the small numbers involved, there is no evidence that the disc detection rate around stars with planets is different to that around stars without planets. Our detections provide a clear indication that large quantities of dusty material can survive the stars' main-sequence lifetime and be detected on the subgiant branch, with important implications for the evolution of planetary systems and observations of polluted or dusty white dwarfs. Our detection rates also provide an important constraint that can be included in models of debris disc evolution.
Highlights
Belts of rocks and dust, known as debris discs are commonly detected around main-sequence stars
These data were used to find the best fitting stellar model, using the PHOENIX Gaia grid (Brott & Hauschildt 2005), via a χ2 minimisation, as in Kennedy et al (2012a,b); Wyatt et al (2012). This method uses synthetic photometry over known bandpasses and has been validated against high signal to noise ratio (S/N) MIPS 24μm data for DEBRIS targets, showing that the photospheric fluxes are accurate to a few percent for main-sequence, AFG-type, stars. Most stars in this sample are faint and have predicted photospheric fluxes lower than the PACS detection limit, any detected emission is likely to result from a debris disc, at 160μm
There is clear evidence that some previous Herschel observations of debris discs have been contaminated by emission from background objects (e.g. Donaldson et al 2012), at 160μm, this is only found to be important for 1 or 2 of our sample
Summary
Belts of rocks and dust, known as debris discs are commonly detected around main-sequence stars. There are a growing number of detections of planets around ‘retired’ A stars, on the subgiant or giant branch (e.g. Johnson et al 2006, 2007; Bowler et al 2010; Sato et al 2010), some controversy does exist regarding the exact evolutionary paths of these stars (Lloyd 2011, 2013; Schlaufman & Winn 2013) These observations provide some key insights into the potential differences between the planetary population around sun-like and intermediate mass stars, that otherwise can only be probed by direct imaging of planets around main-sequence A stars The interest in the fate of debris discs has grown with the growing evidence for planetary systems orbiting white dwarfs
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