This paper discusses an algorithm for detecting single transits in
photometric time-series data. Specifically, we aim to identify asymmetric
transits with ingress that is more rapid than egress, as expected for cometary
bodies with a significant tail. The algorithm is automated, so can be applied
to large samples and only a relatively small number of events need to be
manually vetted. We applied this algorithm to all long cadence light curves
from the Kepler mission, finding 16 candidate transits with significant
asymmetry, 11 of which were found to be artefacts or symmetric transits after
manual inspection. Of the 5 remaining events, four are the 0.1% depth events
previously identified for KIC 3542116 and 11084727. We identify HD 182952 (KIC
8027456) as a third system showing a potential comet transit. All three stars
showing these events have H-R diagram locations consistent with
∼100Myr-old open cluster stars, as might be expected given that cometary
source regions deplete with age, and giving credence to the comet hypothesis.
If these events are part of the same population of events as seen for KIC
8462852, the small increase in detections at 0.1% depth compared to 10% depth
suggests that future work should consider whether the distribution is naturally
flat, or if comets with symmetric transits in this depth range remain
undiscovered. Future searches relying on asymmetry should be more successful if
they focus on larger samples and young stars, rather than digging further into
the noise.GMK is supported by the Royal Society as a Royal Society University Research Fellow. STH acknowledges support from an STFC Consolidated Grant
