The mass-to-light ratio of rich star clusters

TL;DR

This paper demonstrates that the mass-to-light ratio of star clusters evolves significantly over time, affecting mass estimates and the inferred distribution functions, with implications for cluster modeling.

Contribution

It introduces gas-dynamical models coupled with stellar evolution to quantify the time evolution of the mass-to-light ratio in star clusters.

Findings

The mass-to-light ratio eta can increase by up to a factor of 3 over 50 million years.

Cluster mass estimates are significantly affected by the evolving eta.

Fitting light profiles can overestimate the concentration parameter c.

Abstract

We point out a strong time-evolution of the mass-to-light conversion factor eta commonly used to estimate masses of unresolved star clusters from observed cluster spectro-photometric measures. We present a series of gas-dynamical models coupled with the Cambridge stellar evolution tracks to compute line-of-sight velocity dispersions and half-light radii weighted by the luminosity. We explore a range of initial conditions, varying in turn the cluster mass and/or density, and the stellar population's IMF. We find that eta, and hence the estimated cluster mass, may increase by factors as large as 3 over time-scales of 50 million years. We apply these results to an hypothetic cluster mass distribution function (d.f.) and show that the d.f. shape may be strongly affected at the low-mass end by this effect. Fitting truncated isothermal (Michie-King) models to the projected light profile leads to over-estimates of the concentration parameter c of delta c ~ 0.3 compared to the same functional fit applied to the projected mass density.