Quantifying spectroscopic Ca <scp>ii</scp> exocomet transit occurrence in two decades of HARPS data

ABSTRACT The field of exocomets has been built around the unmatched number of detections made in the circumstellar disc of the archetypal star Beta Pictoris. An exocomet detection in spectroscopy is identified by variable atomic absorption features in a stellar spectrum, associated with transiting gas in and trailing an exocomet coma. This paper presents the largest spectroscopic search for exocomet transits to date, which overcomes the limitations of biased samples of stars with debris discs, and instead looks through the ${\approx} 7500$ stars in the High Accuracy Radial velocity Planet Searcher (HARPS) archive for signs of exocomets in the Ca ii doublet (H: 396.847 nm and K: 393.366 nm). The search resulted in 155 candidate stars, which after filtering for false positives (e.g. binaries, stellar activity, etc.) were cut down to 22 stars. These 22 stars are classified into Tier 1, Tier 2, and Tier 3 exocomet candidates, reflecting the confidence level of their exocomet detection. Our two best candidates (Tier 1: Beta Pictoris and HD 172555) and four lower confidence candidates (Tier 2: Gl 1, HIP 5158, HD 94771, and HR 1996) are discussed, yielding a detection rate of 0.03 per cent (Tier 1 only) and 0.1 per cent (Tier 1 and Tier 2) in the HARPS sample. Both Tier 1 stars are known exocomet host stars. These two young A-type stars correspond to 0.4 per cent of all A-types in the sample, suggesting that detecting signs of exocomet transits using Ca ii is more likely around young A-type stars. Reanalysing a past HARPS study, we found no evidence to support the previously claimed four exocomet detections, indicating either that those detections are not robust or that we are only sensitive to the strongest signals.