  The majority of observed mass-to-light ratios of globular clusters are too
low to be explained by `canonical' cluster models, in which dynamical effects
are not accounted for. Moreover, these models do not reproduce a recently
reported trend of increasing M/L with cluster mass, but instead predict
mass-to-light ratios that are independent of cluster mass for a fixed age and
metallicity. This study aims to explain the M/L of globular clusters in four
galaxies by including stellar evolution, stellar remnants, and the preferential
loss of low-mass stars due to energy equipartition. Analytical cluster models
are applied that account for stellar evolution and dynamical cluster
dissolution to samples of globular clusters in Cen A, the Milky Way, M31 and
the LMC. The models include stellar remnants and cover metallicities in the
range Z=0.0004-0.05. Both the low observed mass-to-light ratios and the trend
of increasing M/L with cluster mass can be reproduced by including the
preferential loss of low-mass stars, assuming reasonable values for the
dissolution timescale. This leads to a mass-dependent M/L evolution and
increases the explained percentage of the observations from 39% to 92%. This
study shows that the hitherto unexplained discrepancy between observations and
models of the mass-to-light ratios of globular clusters can be explained by
dynamical effects, provided that the globular clusters exhibiting low M/L have
dissolution timescales within the ranges assumed in this Letter. Furthermore,
it substantiates that M/L cannot be assumed to be constant with mass at fixed
age and metallicity.
