The Gaia-ESO survey: Constraining evolutionary models and ages for young low mass stars with measurements of lithium depletion and rotation

Abstract

A growing disquiet has emerged in recent years that standard stellar models are at odds with observations of the colour-magnitude diagrams (CMDs) and lithium depletion patterns of pre main sequence (PMS) stars in clusters. In this work we select 1,246 high probability K/M-type constituent members of 5 young open clusters (5–125 Myr) in the Gaia-ESO Survey to test a series of models that use standard input physics and others that incorporate surface magnetic fields or cool starspots. We find that: standard models provide systematically under-luminous isochrones for low-mass stars in the CMD and fail to predict Li-depletion of the right strength at the right colour; magnetic models provide better CMD fits with isochrones that are ∼1.5 − 2 times older, and provide better matches to Li depletion patterns. We investigate how rotation periods, most of which are determined here for the first time from Transiting Exoplanet Survey Satellite data, correlate with CMD position and Li. Among the K-stars in the older clusters we find the brightest and least Li-depleted are the fastest rotators, demonstrating the classic ‘Li-rotation connection’ for the first time at ∼35 Myr in NGC 2547, and finding some evidence that it exists in the early M-stars of NGC 2264 at <10 Myr. However, the wide dispersion in Li depletion observed in fully-convective M-dwarfs in the γ Vel cluster at ∼20 Myr appears not to be correlated with rotation and is challenging to explain without a very large (>10 Myr) age spread.

Star-spots and magnetism: testing the activity paradigm in the Pleiades and M67

“…Observations of these huge spot coverage fractions have been made (Gully-Santiago et al 2017), and they appear to satisfactorily explain age skew and radius inflation in isochrones (Somers et al 2020;Somers & Stassun 2017;Jackson et al 2016;Feiden 2016). Models incorporating starspots and magnetic effects result in more consistent ages inferred with magnetic models compared to nonmagnetic ones (Cao et al 2022;David et al 2019;Simon et al 2019); spotted models also allow lithium abundance variations to be explained as an "activity spread" in starspots rather than an unphysically large age spread (Binks et al 2022). An observed spread in lithium can also be produced with a distribution of starspots, as was the insight of Soderblom et al (1993b).…”

Kepler and the Behemoth: Three Mini-Neptunes in a 40 Million Year Old Association

“…If so, one would expect the IC 2602 and Tuc-Hor K-dwarfs to show a larger intrinsic lithium dispersion. A recent analysis of the ≈40 Myr NGC 2547 by Binks et al (2022) suggests that this may be the case, though that study only had ≈10 stars in the relevant effective temperature range. An alternative explanation could be that the overall metallicity of Cep-Her is different from Tuc-Hor and IC 2602, but this seems unlikely given the near-solar metallicities we have measured for the KOIs.…”

The origin of free-floating planets

“…Young stars are photometrically variable (due to activity and accretion processes) and they are usually affected by interstellar extinction. All this makes difficult the age determination of young stars by isochrone fitting (Jeffries et al 2014(Jeffries et al , 2021Binks et al 2022).…”