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

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, $r_p=1.738_{-0.076}^{+0.087}$ 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${}_{\odot }$, $M_s=0.640\pm 0.016$ M${}_{\odot }$). 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 $6.91_{-0.85}^{+0.75}$ M${}_{\oplus }$, which implies an iron core mass fraction of $20_{-12}^{+15}$% 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

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