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

Kavli Affiliate: Christopher Burke

| First 5 Authors: Ryan Cloutier, Joseph E. Rodriguez, Jonathan Irwin, David Charbonneau, Keivan G. Stassun

| Summary:

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, $r_p=1.738^{+0.087}_{-0.076}$ 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 ($R_s=0.630pm 0.015$ R$_{odot}$,
$M_s=0.640pm 0.016$ 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 $6.91^{+0.75}_{-0.85}$ M$_{oplus}$, which implies an iron core
mass fraction of $20^{+15}_{-12}$% 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 ($P=21.8^{+0.9}_{-0.8}$ days, $m_psin{i}=13.0^{+3.8}_{-5.3}$
M$_{oplus}$) that cannot be firmly ruled out by our data.

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