A Search for Transiting Exocomets in TESS Sectors 1-26
We present a search for single photometric exocomet transits using a magnitude-limited sample of stars observed by the TESS primary mission. These events are asymmetric, with a sharp ingress and more gradual egress expected because the comet tail trails behind the coma. Our goals are to estimate the occurrence rate of exocomet transits, and given sufficient numbers comment on whether the host stars are biased towards being A/F spectral types, as suggested by a previous survey with Kepler data. We recovered the previously identified exocomet transit with TESS around $\beta$ Pic (TIC 270577175) and identified three additional main sequence systems with exocomet-like transits (TIC 280832588, TIC 73149665, and TIC 143152957). We also identified one exocomet candidate around a giant star (TIC 229790952) and one around a probable supergiant (TIC 110969638). We find a total occurrence rate of $2.64\times10^{-4}$ star$^{-1}$ year$^{-1}$, much higher than Kepler's rate of $6.7 \times 10^{-6}$ star$^{-1}$ year$^{-1}$. Some of this difference may be because our rate includes a correction for detection efficiency, where the Kepler search did not. However, with only a handful of detections in each survey, the rates are also very uncertain. In contrast to the Kepler search, we find two candidate hosts that may be G types, but the spectral types would be better supported with spectroscopic follow-up. Primarily, we conclude that exocomet-like transits are very rare at 0.1%-1% transit depth levels, and that higher precision photometry to detect and characterise shallower transits effectively is the most likely path to more detections and stronger statistical conclusions.