The radii of M-dwarfs in the young open cluster NGC 2516

TL;DR

This study measures the radii of fast-rotating M-dwarfs in NGC 2516 using a novel method, revealing they are larger than theoretical models predict, especially in rapidly rotating stars with significant starspot coverage.

Contribution

It introduces a new technique combining rotation periods and spectroscopic data to determine M-dwarf radii in a young cluster, highlighting discrepancies with models.

Findings

Measured radii exceed model predictions by up to 50%.

Rapid rotators show high starspot coverage (~50%).

Results suggest magnetic activity impacts stellar radius estimates.

Abstract

Using a novel technique, which combines previously determined rotation periods with new spectroscopic determinations of projected rotation velocity, we have determined radii for fast rotating, low-mass (0.2-0.7 Msun) M-dwarfs in the young, solar-metallicity open cluster, NGC 2516. The mean radii are larger than model predictions at a given absolute I or K magnitude and also larger than the measured radii of magnetically inactive M-dwarfs; the difference increases from a few per cent, to 50 per cent for the lowest luminosity stars in our sample. We show that a simple two-temperature starspot model is broadly capable of explaining these observations, but requires spot coverage fractions of about 50 per cent in rapidly rotating M-dwarfs.