Exoplanets in the Mid-to-Late M-Dwarf Context: Insights on the Spindown of Fully Convective Stars and the Rarity of Their Jupiter Analogs

Abstract

Low-mass M dwarfs (0.1-0.3M⊙) comprise over half of stars in the solar neighborhood. These tiny stars are ideal for exoplanet characterization, providing the highest-precision measurements of planetary masses and radii that enable envelope-pushing explorations of planetary atmospheres and bulk compositions. My work explores how these stars differ from our Sun and the consequences for attendant planets. The rotation periods of low-mass field M dwarfs are bimodal, suggesting an abrupt transition between fast and slow rotation rates. However, a detailed understanding of the spindown process has been stymied by difficulties in measuring M-dwarf ages. By studying M dwarfs in wide binaries with stars of known age, I observe gradual spindown within the rapidly rotating mode for a few billion years before the abrupt transition. There is, however, a large dispersion and some stars make the jump by 600Myr. I establish that G 68-34, the one star in my sample that did not follow this trend, is in fact an eclipsing binary. The pair has masses of 0.3280±0.0034M⊙ and 0.3207±0.0036M⊙, radii of 0.345±0.014R⊙ and 0.342±0.014R⊙, an orbital period of 0.655 days, and has maintained rapid rotation for >5Gyr due to tidal interactions. I also identify a strong mass dependence to the spindown process: 0.1M⊙ M dwarfs spin down to a quiescent state billions of years later on average than their 0.3M⊙ counterparts, at an average age of 4.4Gyr for 0.1M⊙, 2.8Gyr for 0.2M⊙, and 1.3Gyr for 0.3M⊙. By measuring the Hα activity of stars in M-M binaries (which presumably share a common age, metallicity, and high-energy birth environment), I find ≤0.5Gyr dispersion in the mass-dependent relation when these variables are controlled. The long active lifetimes of low-mass M dwarfs may imply that their terrestrial planets are unable to retain atmospheres. I also present a multi-epoch radial-velocity survey of the volume-complete sample of 0.1-0.3M⊙ M dwarfs within 15pc, which enables several studies: 1) For the 123 active, single stars, I measure rotation periods, Hα equivalent widths, inclinations, and radial velocities. I find that 92±3% of active, low-mass M dwarfs have rotation periods shorter than 20 days, with 74±4% having periods shorter than 2 days. Stars in the gap between the rapidly and slowly rotating modes have enhanced magnetic activity, perhaps reflecting a change in magnetic field structure during this phase of abrupt spindown; 2) I search the 200 inactive, single stars for giant planets. My null detection implies .7% of low-mass M dwarfs have a Jupiter-analog companion beyond the snow line, with 95% confidence. Terrestrial planets are abundant around low-mass M dwarfs, implying those worlds evolved in a very different dynamical environment to the rocky worlds of our solar system, with implications for their sizes, compositions, and habitability; 3) I calibrate a novel spectroscopic metallicity relation for mid-to-late M dwarfs, which I will use to estimate metallicities for the volume-complete sample. This relation leverages the metallicity sensitivity of the near-IR Ca II triplet, a strong feature deeply embedded in the metal lines of the pseudocontinuum; and 4) For one star in the survey, LTT 1445A, I analyze new HST photometry, finding that the transit of planet c is nongrazing with 97% confidence and thus the planet has a radius of 1.07-0.07+0.10R⊕ and a terrestrial composition. This star is the nearest M dwarf with transiting planets, making LTT 1445Ac one of the best opportunities to characterize a terrestrial exoplanetary atmosphere.