paper

TOI-1235 b: a keystone super-Earth for testing radius valley emergence models around early M dwarfs

arXiv:2004.06682 · doi:10.3847/1538-3881/ab9534

Abstract

Small planets on close-in orbits tend to exhibit envelope mass fractions of either effectively zero or up to a few percent depending on their size and orbital period. Models of thermally-driven atmospheric mass loss and of terrestrial planet formation in a gas-poor environment make distinct predictions regarding the location of this rocky/non-rocky transition in period-radius space. Here we present the confirmation of TOI-1235 b ( $P = 3.44$ days, $r_{p} = 1.73 8_{- 0.076}^{+ 0.087}$ R $_{\oplus}$ ), a planet whose size and period are intermediate between the competing model predictions thus making the system an important test case for emergence models of the rocky/non-rocky transition around early M dwarfs ( $R_{s} = 0.630 \pm 0.015$ R $_{⊙}$ , $M_{s} = 0.640 \pm 0.016$ M $_{⊙}$ ). We confirm the TESS planet discovery using reconnaissance spectroscopy, ground-based photometry, high-resolution imaging, and a set of 38 precise radial-velocities from HARPS-N and HIRES. We measure a planet mass of $6.9 1_{- 0.85}^{+ 0.75}$ M $_{\oplus}$ , which implies an iron core mass fraction of $2 0_{- 12}^{+ 15}$ % in the absence of a gaseous envelope. The bulk composition of TOI-1235 b is therefore consistent with being Earth-like and we constrain a H/He envelope mass fraction to be $< 0.5$ % at 90% confidence. Our results are consistent with model predictions from thermally-driven atmospheric mass loss but not with gas-poor formation, suggesting that the former class of processes remain efficient at sculpting close-in planets around early M dwarfs. Our RV analysis also reveals a strong periodicity close to the first harmonic of the photometrically-determined stellar rotation period that we treat as stellar activity, despite other lines of evidence favoring a planetary origin ( $P = 21. 8_{- 0.8}^{+ 0.9}$ days, $m_{p} sin i = 13. 0_{- 5.3}^{+ 3.8}$ M $_{\oplus}$ ) that cannot be firmly ruled out by our data.

Accepted to The Astronomical Journal. 8 figures & 5 tables. Table 2 is provided in the arXiv source code