The Gaia-ESO survey: A lithium depletion boundary age for NGC 2232

TL;DR

This paper uses Gaia and Gaia-ESO data to determine the lithium depletion boundary in NGC 2232, deriving a more accurate cluster age that aligns better with models including magnetic activity and starspots.

Contribution

It introduces a specialized spectral analysis for lithium detection and compares LDB and isochrone ages, highlighting the importance of magnetic effects in stellar models.

Findings

LDB age of 38±3 Myr for NGC 2232

Standard models underestimate cluster age by more than double

Magnetic models with starspots yield ages of 45-50 Myr

Abstract

Astrometry and photometry from {\it Gaia} and spectroscopic data from the {\it Gaia}-ESO Survey (GES) are used to identify the lithium depletion boundary (LDB) in the young cluster NGC 2232. A specialised spectral line analysis procedure was used to recover the signature of undepleted lithium in very low luminosity cluster members. An age of $38\pm 3$ Myr is inferred by comparing the LDB location in absolute colour-magnitude diagrams (CMDs) with the predictions of standard models. This is more than twice the age derived from fitting isochrones to low-mass stars in the CMD with the same models. Much closer agreement between LDB and CMD ages is obtained from models that incorporate magnetically suppressed convection or flux-blocking by dark, magnetic starspots. The best agreement is found at ages of $45-50$\,Myr for models with high levels of magnetic activity and starspot coverage fractions $>50$ per cent, although a uniformly high spot coverage does not match the CMD well across the full luminosity range considered.