TESS Hunt for Young and Maturing Exoplanets (THYME). IX. A 27 Myr Extended Population of Lower Centaurus Crux with a Transiting Two-planet System

Abstract

We report the discovery and characterization of a nearby (∼85 pc), older (27 ± 3 Myr), distributed stellar population near Lower Centaurus Crux (LCC), initially identified by searching for stars comoving with a candidate transiting planet from TESS (HD 109833; TOI 1097). We determine the association membership using Gaia kinematics, color–magnitude information, and rotation periods of candidate members. We measure its age using isochrones, gyrochronology, and Li depletion. While the association is near known populations of LCC, we find that it is older than any previously found LCC subgroup (10–16 Myr), and distinct in both position and velocity. In addition to the candidate planets around HD 109833, the association contains four directly imaged planetary-mass companions around three stars, YSES-1, YSES-2, and HD 95086, all of which were previously assigned membership in the younger LCC. Using the Notch pipeline, we identify a second candidate transiting planet around HD 109833. We use a suite of ground-based follow-up observations to validate the two transit signals as planetary in nature. HD 109833 b and c join the small but growing population of <100 Myr transiting planets from TESS. HD 109833 has a rotation period and Li abundance indicative of a young age (≲100 Myr), but a position and velocity on the outskirts of the new population, lower Li levels than similar members, and a color–magnitude diagram position below model predictions for 27 Myr. So, we cannot reject the possibility that HD 109833 is a young field star coincidentally nearby the population.

Using the Gaia Excess Uncertainty as a Proxy for Stellar Variability and Age

“…Binaries. High renormalised unit weight error (RUWE; Lindegren et al 2018) values (1.2) are often used to signify binary systems (Pearce et al 2019;Ziegler et al 2020;Wood et al 2023). More restrictive RUWE cuts will not remove all binaries, but should remove enough of them to see if binaries have a significant impact on the result.…”

“…We are able to get quick age estimates using available Gaia DR3 data and without the need for collecting additional rotation period and lithium measurements. For example, the age for MELANGE-4 is based on lithium absorption, which requires multiple nights of observations (Wood et al 2023). While not as precise, we calculated a similar age from just Gaia data alone (26 5 8 -+ Myr compared to 27 ± 3 Myr).…”

“…Outside the Sun, stars with the most precise and reliable ages are usually in coeval associations (Soderblom et al 2014). Ages can then be estimated using the bulk properties of the cluster, such as the lithium abundances (e.g., Burke et al 2004;Wood et al 2023) or main-sequence turnoff (Conroy & Gunn 2010), or from a subset of stars with more easily determined properties (e.g., asteroseismic pulsators; Grunblatt et al 2021;Bedding et al 2023).…”

“…Precision astrometry from the Gaia mission (Gaia Collaboration et al 2016) has been invaluable for finding new stellar associations (e.g., Meingast et al 2019;Moranta et al 2022), subpopulations of known associations (e.g., Wood et al 2023), and additional members of known populations (e.g., Gagné & Faherty 2018;Röser & Schilbach 2020). Identifying and finding members of sparse groups is still challenging.…”

A Lithium Depletion Age for the Carina Association

“…BANYAN Σ uses kinematic models of nearby young associations to calculate the probability of a given star's membership within each association or the field. We use the kinematic models defined by Gagné et al (2018) for most of the associations, but due to the proximity of Carina to Lower Centaurus Crux (LCC), and MELANGE-4 (Wood et al 2023), we use an updated model for those groups. The new model of LCC, incorporating its subpopulations, and the model of MELANGE-4 are described in Mann et al (2022) and Wood et al (2023), respectively.…”