Characterisation of 92 Southern TESS Candidate Planet Hosts and a New Photometric [Fe/H] Relation for Cool Dwarfs

TL;DR

This study analyzes 92 cool TESS candidate planet hosts through optical spectroscopy, develops a new photometric [Fe/H] calibration, and models exoplanet radii, revealing the planet-radius gap among cool dwarfs.

Contribution

It introduces an updated photometric [Fe/H] relation for cool dwarfs and provides precise stellar and planetary parameters for a large sample of TESS candidates.

Findings

Median stellar parameter precisions: 0.8% in $T_{eff}$, 1.7% in $R_*$.

Exoplanet radii determined with 3.5% accuracy.

Evidence of the planet-radius gap in cool dwarf hosts.

Abstract

We present the results of a medium resolution optical spectroscopic survey of 92 cool ($3,000 \lesssim T_{\rm eff} \lesssim 4,500\,$K) southern TESS candidate planet hosts, and describe our spectral fitting methodology used to recover stellar parameters. We quantify model deficiencies at predicting optical fluxes, and while our technique works well for $T_{\rm eff}$, further improvements are needed for [Fe/H]. To this end, we developed an updated photometric [Fe/H] calibration for isolated main sequence stars built upon a calibration sample of 69 cool dwarfs in binary systems, precise to $\pm0.19\,$dex, from super-solar to metal poor, over $1.51 < {\rm Gaia}~(B_P-R_P) < 3.3$. Our fitted $T_{\rm eff}$ and $R_\star$ have median precisions of 0.8% and 1.7%, respectively and are consistent with our sample of standard stars. We use these to model the transit light curves and determine exoplanet radii for 100 candidate planets to 3.5% precision and see evidence that the planet-radius gap is also present for cool dwarfs. Our results are consistent with the sample of confirmed TESS planets, with this survey representing one of the largest uniform analyses of cool TESS candidate planet hosts to date.