Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions

Kavli Affiliate: Roland K. Vanderspek

| First 5 Authors: Teruyuki Hirano, John H. Livingston, Akihiko Fukui, Norio Narita, Hiroki Harakawa

| Summary:

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685:
$J=9.5-9.6$) hosting ultra-short period (USP) planets, identified by the TESS
mission. The two stars are similar in temperature, mass, and radius
($T_mathrm{eff},approx,3500$ K, $M_star,approx,0.45-0.46,M_odot$, and
$R_starapprox 0.45-0.46,R_odot$), and the planets are both
super-Earth-sized ($1.25,R_oplus<R_p<2.0,R_oplus$). For both systems, light
curves from the ground-based photometry exhibit planetary transits, whose
depths are consistent with those by the TESS photometry. We also refine the
transit ephemerides based on the ground-based photometry, finding the orbital
periods of $P=0.9893436pm0.0000020$ day and $P=0.6691416pm0.0000019$ day for
TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV)
observations using IRD on the Subaru 8.2m telescope, we confirm the planetary
nature of the TOIs, and measure their masses: $10.14pm0.95,M_oplus$ and
$3.43pm0.93,M_oplus$ for TOI-1634b and TOI-1685b, respectively, when the
observed RVs are fitted with a single-planet circular-orbit model. Combining
those with the planet radii of $R_p=1.749pm 0.079,R_oplus$ (TOI-1634b) and
$1.459pm0.065,R_oplus$ (TOI-1685b), we find that both USP planets have mean
densities consistent with an Earth-like internal composition, which is typical
for small USP planets. TOI-1634b is currently the most massive USP planet in
this category, and it resides near the radius valley, which makes it a
benchmark planet in the context of discussing the size limit of rocky planet
cores as well as testing the formation scenarios for USP planets. Excess
scatter in the RV residuals for TOI-1685 suggests the presence of a possible
secondary planet or unknown activity/instrumental noise in the RV data, but
further observations are required to check those possibilities.

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