The M/L ratio of massive young clusters

TL;DR

This paper demonstrates that the mass-to-light ratio of young dense star clusters evolves significantly within 20 million years, affecting mass estimates and model fits.

Contribution

It introduces a gas-dynamical model coupled with stellar evolution tracks to quantify the time evolution of the mass-to-light conversion factor in young clusters.

Findings

Mass-to-light ratio increases by up to 3 times over 20 million years.

Higher upper mass limits cause faster increases in ta within 10 million years.

Fitting isothermal models overestimates the concentration parameter c by about 0.3.

Abstract

We point out a strong time-evolution of the mass-to-light conversion factor \eta commonly used to estimate masses of dense star clusters from observed cluster radii and stellar velocity dispersions. We use a gas-dynamical model coupled with the Cambridge stellar evolution tracks to compute line-of-sight velocity dispersions and half-light radii weighted by the luminosity. Stars at birth are assumed to follow the Salpeter mass function in the range [0.15--17 M_\sun]. We find that $\eta$, and hence the estimated cluster mass, increases by factors as large as 3 over time-scales of 20 million years. Increasing the upper mass limit to $50 M_\sun$ leads to a sharp rise of similar amplitude but in as little as 10 million years. Fitting truncated isothermal (Michie-King) models to the projected light profile leads to over-estimates of the concentration par ameter c of $\delta c\approx 0.3$ compared to the same functional fit applied to the proj ected mass density.