TOI-1759 b: a transiting sub-Neptune around a low mass star characterized with SPIRou and TESS

TL;DR

This paper reports the discovery and detailed characterization of the transiting sub-Neptune planet TOI-1759 b orbiting a low-mass star, using combined photometric and spectropolarimetric data to analyze its properties and stellar magnetic activity.

Contribution

It provides the first detailed characterization of TOI-1759 b, including mass, radius, and magnetic environment of its host star, using innovative near-infrared spectropolarimetry and Bayesian analysis.

Findings

TOI-1759 b has a radius of 3.06 R$_ ext{Earth}$ and a mass of 6.8 M$_ ext{Earth}$.

The host star exhibits a magnetic field with a mean strength of 18 G and a rotation period of 35.65 days.

The planet receives high stellar flux and has an equilibrium temperature of 433 K.

Abstract

We report the detection and characterization of the transiting sub-Neptune TOI-1759 b, using photometric time-series from TESS and near infrared spectropolarimetric data from SPIRou on the CFHT. TOI-1759 b orbits a moderately active M0V star with an orbital period of $18.849975\pm0.000006$ d, and we measure a planetary radius and mass of $3.06\pm0.22$ R$_\oplus$ and $6.8\pm2.0$ M$_\oplus$. Radial velocities were extracted from the SPIRou spectra using both the CCF and the LBL methods, optimizing the velocity measurements in the near infrared domain. We analyzed the broadband SED of the star and the high-resolution SPIRou spectra to constrain the stellar parameters and thus improve the accuracy of the derived planet parameters. A LSD analysis of the SPIRou Stokes $V$ polarized spectra detects Zeeman signatures in TOI-1759. We model the rotational modulation of the magnetic stellar activity using a GP regression with a quasi-periodic covariance function, and find a rotation period of $35.65^{+0.17}_{-0.15}$ d. We reconstruct the large-scale surface magnetic field of the star using ZDI, which gives a predominantly poloidal field with a mean strength of $18\pm4$ G. Finally, we perform a joint Bayesian MCMC analysis of the TESS photometry and SPIRou RVs to optimally constrain the system parameters. At $0.1176\pm0.0013$ au from the star, the planet receives $6.4$ times the bolometric flux incident on Earth, and its equilibrium temperature is estimated at $433\pm14$ K. TOI-1759 b is a likely gas-dominated sub-Neptune with an expected high rate of photoevaporation. Therefore, it is an interesting target to search for neutral hydrogen escape, which may provide important constraints on the planetary formation mechanisms responsible for the observed sub-Neptune radius desert.