Spot-Crossing Variations Confirm a Misaligned Orbit for a Planet Transiting an M Dwarf

Abstract

TOI-3884~b is an unusual 6.4~R${}_{\oplus }$ planet orbiting an M4 host, whose transits display large and persistent spot-crossing events. We used the \textit{Tierras} Observatory to monitor both the long-term photometric variability of TOI-3884 and changes in the spot-crossing events across multiple transits of the planet. We show that the star rotates with a period of $11.020\pm 0.015$~days. We simultaneously model the rotational modulation of the star and variations in transit shapes that arise due to rotation of the spot, allowing us to determine the true stellar obliquity, ${\psi }_{\star }$. The data are best described by a planet on a misaligned orbit around a highly inclined star (${\psi }_{\star }={77.4^{\circ }}_{-2.5^{\circ }}^{+2.3^{\circ }}$; $i_{\star }={22.3^{\circ }}_{-1.6^{\circ }}^{+1.8^{\circ }}$) that hosts a large polar starspot ($r_{\mathrm{spot}}={31.2^{\circ }}_{-1.9^{\circ }}^{+2.4^{\circ }}$; ${\lambda }_{\mathrm{spot}}=80.5^{\circ }\pm 1.2^{\circ }$). Archival photometry from the Zwicky Transient Facility suggests that this polar spot has persisted on TOI-3884 for at least seven years. The TOI-3884 system provides a benchmark for studying the evolution of a polar spot on an M dwarf.

17 pages, 9 figures, accepted to AJ