TESS Hunt for Young and Maturing Exoplanets (THYME) VII : Membership, rotation, and lithium in the young cluster Group-X and a new young exoplanet

TL;DR

This study confirms the age and membership of the young stellar association Group-X, identifies a new nearby group MELANGE-2, and validates a young exoplanet, TOI 2048 b, orbiting a 300-million-year-old star.

Contribution

It provides new membership and age estimates for Group-X, discovers MELANGE-2, and confirms the exoplanet TOI 2048 b as a young, small-radius planet.

Findings

Group-X is approximately 300 million years old.

A new nearby association MELANGE-2 was identified.

TOI 2048 b is a validated young exoplanet with a 13.8-day orbit.

Abstract

The public, all-sky surveys Gaia and TESS provide the ability to identify new young associations and determine their ages. These associations enable study of planetary evolution by providing new opportunities to discover young exoplanets. A young association was recently identified by Tang et al. and F{\"u}rnkranz et al. using astrometry from Gaia (called "Group-X" by the former). In this work, we investigate the age and membership of this association; and we validate the exoplanet TOI 2048 b, which was identified to transit a young, late G dwarf in Group-X using photometry from TESS. We first identified new candidate members of Group-X using Gaia EDR3 data. To infer the age of the association, we measured rotation periods for candidate members using TESS data. The clear color--period sequence indicates that the association is the same age as the $300\pm50$ Myr-old NGC 3532. We obtained optical spectra for candidate members that show lithium absorption consistent with this young age. Further, we serendipitously identify a new, small association nearby Group-X, which we call MELANGE-2. Lastly, we statistically validate TOI 2048 b, which is $2.6\pm0.2$ \rearth\ radius planet on a 13.8-day orbit around its 300 Myr-old host star.