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  1. Abstract

    Stars are known to be more active when they are young, resulting in a strong correlation between age and photometric variability. The amplitude variation between stars of a given age is large, but the age–variability relation becomes strong over large groups of stars. We explore this relation using the excess photometric uncertainty in Gaia photometry (VarG,VarBP, andVarRP) as a proxy for variability. The metrics follow a Skumanich-like relation, scaling as ≃t−0.4. By calibrating against a set of associations with known ages, we show how theVar of population members can predict group ages within 10%–20% for associations younger than ≃2.5 Gyr. In practice, age uncertainties are larger, primarily due to the finite group size. The index is most useful at the youngest ages (<100 Myr), where the uncertainties are comparable to or better than those derived from a color–magnitude diagram (CMD). The index is also widely available, easy to calculate, and can be used at intermediate ages where there are few or no pre- or post-main-sequence stars. We further show howVar can be used to find new associations and test if a group of comoving stars is a real coeval population. We apply our methods to Theia groups within 350 pc and find ≳90% are inconsistent with drawing stars from the field and ≃80% have variability ages consistent with those derived from the CMD. Our findings suggest the great majority of these groups contain real populations.

     
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  2. Abstract

    The dispersed remnants of stellar nurseries, stellar associations, provide unparalleled samples of coeval stars critical for studies of stellar and planetary formation and evolution. The Carina Stellar Association is one of the closest stellar associations to Earth, and yet measurements of its age have varied from 13 to 45 Myr. We aim to update the age of Carina using the lithium depletion boundary (LDB) method. We obtain new measurements of the Li 6708 Å absorption feature in likely members using optical spectra from the Goodman High Throughput Spectrograph on SOAR and NRES on LCO. We detect the depletion boundary atMK≃ 6.8 (M5). This age is consistent within uncertainties across six different models, including those that account for magnetic fields and spots. We also estimate the age through analysis of the group’s overall variability, and by comparing the association members’ color–magnitude diagram to stellar evolutionary models using a Gaussian Mixture Model, recovering ages consistent with the LDB. Combining these age measures we obtain an age for the Carina association of415+3Myr. The resulting age agrees with the older end of previous age measurements and is consistent with the lithium depletion age for the neighboring Tucana-Horologium moving group.

     
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  3. Abstract

    Young planets provide a window into the early stages and evolution of planetary systems. Ideal planets for such research are in coeval associations, where the parent population can precisely determine their ages. We describe a young association (MELANGE-3) in the Kepler field, which harbors two transiting planetary systems (KOI-3876 and Kepler-970). We identify MELANGE-3 by searching for kinematic and spatial overdensities around Kepler planet hosts with high levels of lithium. To determine the age and membership of MELANGE-3, we combine new high-resolution spectra with archival light curves, velocities, and astrometry of stars near KOI-3876 spatially and kinematically. We use the resulting rotation sequence, lithium levels, and color–magnitude diagram of candidate members to confirm the presence of a coeval 105 ± 10 Myr population. MELANGE-3 may be part of the recently identified Theia 316 stream. For the two exoplanet systems, we revise the stellar and planetary parameters, taking into account the newly determined age. Fitting the 4.5 yr Kepler light curves, we find that KOI-3876b is a 2.0 ± 0.1Rplanet on a 19.58 day orbit, while Kepler-970 b is a 2.8 ± 0.2Rplanet on a 16.73 day orbit. KOI-3876 was previously flagged as an eclipsing binary, which we rule out using radial velocities from APOGEE and statistically validate the signal as planetary in origin. Given its overlap with the Kepler field, MELANGE-3 is valuable for studies of spot evolution on year timescales, and both planets contribute to the growing work on transiting planets in young stellar associations.

     
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  4. 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.

     
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  5. Abstract

    Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15Rplanets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05RJ(9.5R) planet transiting a very-low-mass star (0.170 ± 0.015M) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the color–magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5MJ. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5Rplanets.

     
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