2005ApJ...620L..27B

We point out a strong time evolution of the mass-to-light conversion factor η commonly used to estimate masses of dense star clusters from observed cluster radii and stellar velocity dispersions. We use a gasdynamical 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 initial mass function in the range of 0.15-17 M_☉. We find that η, and hence the estimated cluster mass, increases by factors as large as 3 over timescales of 20 million years. Increasing the upper mass limit to 50 M_☉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 overestimates of the concentration parameter c of δc~0.3 compared to the same functional fit applied to the projected mass density.