TESS Discovers a Second System of Transiting Exocomets in the Extreme Debris Disk of RZ Psc

Abstract

We present the TESS discovery of only the second system of transiting exocomets with a sufficient number of events to measure the size distribution in the RZ Psc system, enabling comparisons with the $\beta$ Pictoris and Solar System size distributions. Twenty-four transits with absorption depths (AD) of 1--20\% were observed across three TESS sectors of the 20-50 Myr K0V star, detected as part of our TESS survey of extreme debris disks identified by their IR excess. We discover that the ADs (and hence exocomet radii) follow a broken power-law cumulative frequency distribution not previously seen in extrasolar contexts but similar to that observed in Solar System Kuiper Belt Object sizes, with power-law slopes above and below the break of $\gamma_\mathrm{AD>break}$=2.32$\pm$0.12 and $\gamma_\mathrm{AD<break}$=0.11$\pm$0.04, respectively. We derive size distributions of 1--7~km from two independent lines of evidence. We use the RZ Psc exocomet rate to predict exocomet yields for the Early eVolution Explorer (EVE) NASA astrophysics Small Explorer (SMEX) mission concept to obtain simultaneous photometry of 10$^4$ young stars in NUV, optical, and NIR bands. Assuming occurrence rates scaled from RZ Psc, EVE would detect 590 exocomets from $\approx$70 young systems in the optical band, with $\approx$120 simultaneous 5$\sigma$ detections in all three bands. These data would enable grain sizes of 200--700~nm and graphite--olivine compositions of dozens of events to be distinguished at 2.5--3$\sigma$, as well as a 4$\sigma$ determination of the accuracy of the Herschel-derived M-debris disk fraction.