Wolf 327b: A new member of the pack of ultra-short-period super-Earths around M dwarfs

Abstract

Planets with orbital periods shorter than 1 day are rare and have formation histories that are not completely understood. Small ($R_{\mathrm{p}}<2R_{\oplus }$) ultra-short-period (USP) planets are highly irradiated, probably have rocky compositions with high bulk densities, and are often found in multi-planet systems. Additionally, USP planets found around small stars are excellent candidates for characterization using present-day instrumentation. Of the current full sample of approximately 5500 confirmed exoplanets, only 130 are USP planets and around 40 have mass and radius measurements. Wolf 327 (TOI-5747) is an M dwarf ($R_{\star }=0.406\pm 0.015R_{\odot }$, $M_{\star }=0.405\pm 0.019M_{\odot }$, $T_{\mathrm{eff}}=3542\pm 70$ K, and $V=13$ mag) located at a distance $d=28.5$ pc. NASA's planet hunter satellite, TESS, detected transits in this star with a period of 0.573 d (13.7 h) and with a transit depth of 818 ppm. Ground-based follow-up photometry, high resolution imaging, and radial velocity (RV) measurements taken with the CARMENES spectrograph confirm the presence of this new USP planet. Wolf 327b is a super-Earth with a radius of $R_{\mathrm{p}}=1.24\pm 0.06R_{\oplus }$ and a mass of $M_{\mathrm{p}}=2.53\pm 0.46M_{\oplus }$, yielding a bulk density of $7.24\pm 1.66$\,g cm${}^{-3}$ and thus suggesting a rocky composition. Owing to its close proximity to its host star ($a=0.01$ au), Wolf 327b has an equilibrium temperature of $996\pm 22$ K. This planet has a mass and radius similar to K2-229b, a planet with an inferred Mercury-like internal composition. Planet interior models suggest that Wolf 327b has a large iron core, a small rocky mantle, and a negligible (if any) H/He atmosphere.

26 pages, 19 figures. Accepted for publication in A&A