Abstract
ABSTRACT Astrometry and photometry from Gaia and spectroscopic data from the Gaia-ESO Survey (GES) are used to identify the lithium depletion boundary (LDB) in the young cluster NGC 2232. A specialized spectral line analysis procedure was used to recover the signature of undepleted lithium in very low luminosity cluster members. An age of 38 ± 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.
Highlights
Estimating the ages of stars and star clusters is of great importance in astrophysics, but age is something which cannot directly be measured (Soderblom 2010)
A fundamental test of stellar models that can be applied very effectively in star clusters is that ages derived from multiple methods, that are sensitive to different aspects of stellar physics or that sample different parts of the stellar mass spectrum, should agree
In this paper we report a new lithium depletion boundary (LDB) age determination for the young cluster NGC 2232, using spectroscopy obtained as part of the Gaia-ESO spectroscopic survey (Gilmore et al 2012; Randich, Gilmore & Gaia-ESO Consortium 2013, hereafter Gaia-ESO Survey (GES))
Summary
Estimating the ages of stars and star clusters is of great importance in astrophysics, but age is something which cannot directly be measured (Soderblom 2010). There are direct and indirect indications that magnetically active stars, whether they are fast-rotating and young or members of close, tidally locked binary systems, have larger radii than predicted by the most commonly used stellar models (Morales et al 2009; Torres 2013; Malo et al 2014b; Kraus et al 2015, 2017; Rizzuto et al 2020) This has led to suggestions that rotation, magnetic fields, and high surface coverage of starspots may significantly alter the evolutionary tracks and isochrones in young clusters (Feiden & Chaboyer 2013; Jackson & Jeffries 2014a; Somers & Pinsonneault 2015; MacDonald & Mullan 2017).
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