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Confusion limited surveys: using WISE to quantify the rarity of warm dust around Kepler stars




We describe a search for infrared excess emission from dusty circumstellar material around 180 000 stars observed by the Kepler and Wide-field Infrared Survey Explorer missions. This study is motivated by (i) the potential to find bright warm discs around planet host stars, (ii) a need to characterize the distribution of rare warm discs and (iii) the possible identification of candidates for discovering transiting dust concentrations. We find about 8000 stars that have excess emission, mostly at 12 μm. The positions of these stars correlate with the 100 μm background level, so most of the flux measurements associated with these excesses are spurious. We identify 271 stars with plausible excesses by making a 5 MJy sr−1 cut in the IRAS 100 μm emission. The number counts of these excesses, at both 12 and 22 μm, have the same distribution as extra-Galactic number counts. Thus, although some excesses may be circumstellar, most can be explained as chance alignments with background galaxies. The one exception is a 22 μm excess associated with a relatively nearby A-type star that we were able to confirm because the disc occurrence rate is independent of stellar distance. This detection implies a disc occurrence rate consistent with that found for nearby A-stars. Despite our low detection rate, these results place valuable upper limits on the distribution of large mid-infrared excesses: e.g. fewer than 1:1000 stars have 12 μm excesses (Fobs/F) larger than a factor of 5. In contrast to previous studies, we find no evidence for discs around 1790 stars with candidate planets (we attribute one significant 12 μm excess to a background galaxy), and no evidence that the disc distribution around planet hosts is different from the distribution around the bulk population. Higher resolution imaging of stars with excesses is the best way to rule out galaxy confusion and identify more reliable disc candidates among Kepler stars. A similar survey to ours that focuses on nearby stars would be well suited to find the distribution of rare warm discs.