[P-EM11] Exoplanet

[PEM11-P09] Discovery of an Ultra-short Period, Ultra-cool Dwarf around an M dwarf

*Akihiko Fukui¹, Teruyuki Hirano², Norio Narita¹, Takeru K. Suzuki¹, Takato Tokuno¹, Masahiro Ikoma³, The MuSCAT team, The IRD Intensive team, The IRD instrument team (1.The University of Tokyo, 2.Astrobiology Center, 3.National Astronomical Observatory of Japan)

Keywords:transit, M dwarf, TESS, magnetic braking, tidal interaction

The rotational periods of aged (Gyr-old) Sun-like stars have a unique trend with stellar mass, which can be well explained by a simple magnetic braking law. However, the rotational periods of similar-aged M dwarfs have a bimodal distribution, whose cause has not yet been well understood. On the other hand, any stars that host a close-in companion (planet or stellar/substellar object) tidally interact with their companions, which can also affect the evolution of stellar spin. Here we report on the discovery of an ultra-cool dwarf that transits a mid-M dwarf with a period of 0.435 days (10.4 hours). This object (TOI-XXXX) was first identified from the TESS photometric survey and later characterized by ground-based follow-up observations including multiband photometry with MuSCATs and radial velocity measurements with Subaru/IRD. From these observations, we find that the orbit of the companion is likely decaying at a rate of about 30 ms per year. We also find that the stellar rotation is syncronized with the companion's orbit, meaning that the star is also likely spinning down at the same rate. This spin-down rate is consistent with the prediction by the well-known Skumanich law that well explains the evolution of rotational periods of Sun-like stars. In our presentation, we will show the result of our observations of this interesting sytem and also discuss the orbital evolution of hot Jupiters around M dwarfs due to tidal interaction.