Hot planets around cool stars -- two short-period mini-Neptunes transiting the late K-dwarf TOI-1260

TL;DR

This paper reports the discovery and detailed characterization of two mini-Neptunes orbiting the low-metallicity K-dwarf star TOI-1260, including mass, radius, and activity analysis, highlighting the importance of Gaussian process regression in disentangling stellar activity from planetary signals.

Contribution

The study combines TESS photometry, ground-based follow-up, and Gaussian process modeling to accurately characterize the masses and radii of two sub-Neptunes and a tentative outer planet around TOI-1260, demonstrating advanced analysis techniques.

Findings

Two sub-Neptunes with radii 2.33 and 2.82 R⊕ and periods of 3.13 and 7.49 days.

Masses of the planets are approximately 8.6 and 11.8 Earth masses.

The planets are in the 2-3 R⊕ range with ambiguous structure and composition.

Abstract

We present the discovery and characterization of two sub-Neptunes in close orbits, as well as a tentative outer planet of a similar size, orbiting TOI-1260 - a low metallicity K6V dwarf star. Photometry from TESS yields radii of $R_{\rm b} = 2.33 \pm 0.10$ $R_{\oplus}$ and $R_{\rm c} = 2.82 \pm 0.15$ $R_{\oplus}$, and periods of 3.13 and 7.49 days for TOI-1260b and TOI-1260c, respectively. We combined the TESS data with a series of ground-based follow-up observations to characterize the planetary system. From HARPS-N high-precision radial velocities we obtain $M_{\rm b} = 8.61_{ - 1.46 } ^ { + 1.36 }$ $M_{\oplus}$ and $M_{\rm c} = 11.84_{ - 3.23 } ^ { + 3.38 }$ $M_{\oplus}$. The star is moderately active with a complex activity pattern, which necessitated the use of Gaussian process regression for both the light curve detrending and the radial velocity modelling, in the latter case guided by suitable activity indicators. We successfully disentangle the stellar-induced signal from the planetary signals, underlining the importance and usefulness of the Gaussian Process approach. We test the system's stability against atmospheric photoevaporation and find that the TOI-1260 planets are classic examples of the structure and composition ambiguity typical for the $2-3$ $R_{\oplus}$ range.