2009A&A...500..785K

The observed dynamical mass-to-light (M/L) ratios of globular clusters (GCs) are systematically lower than the value expected from ``canonical'' simple stellar population models, which do not account for dynamical effects such as the preferential loss of low-mass stars due to energy equipartition. It has recently been shown that low-mass star depletion can qualitatively explain this discrepancy for globular clusters in several galaxies. To verify whether low-mass star depletion is indeed the driving mechanism behind the M/L decrease, we aim to predict the M/L_V ratios of individual GCs for which orbital parameters and dynamical V-band mass-to-light ratios M/L_V are known. There is a sample of 24 Galactic GCs for which this is possible. We used the SPACE cluster models, which include dynamical dissolution, low-mass star depletion, stellar evolution, stellar remnants, and various metallicities. We derived the dissolution timescales due to two-body relaxation and disc shocking from the orbital parameters of our GC sample and used these to predict the M/L_V ratios of the individual GCs. To verify our findings, we also predicted the slopes of their low-mass stellar mass functions. The computed dissolution timescales agree well with earlier empirical studies. The predicted M/L_V are in 1σ agreement with the observations for 12 out of 24 GCs. The discrepancy for the other GCs probably arises because our predictions give global M/L ratios, while the observations represent extrapolated central values that are different from global ones in the case of mass segregation and a long dissolution timescale. The GCs in our sample that likely have dissimilar global and central M/L ratios can be excluded by imposing limits on the dissolution timescale and King parameter. For the remaining GCs, the observed and predicted average M/L_V are 78⁺⁹_–11% and 78±2% of the canonically expected values, while the values are 74⁺⁶_–7% and 85±1% for the entire sample. The predicted correlation between the slope of the low-mass stellar mass function and M/L_V drop is found to be qualitatively consistent with observed mass function slopes. The dissolution timescales of Galactic GCs are such that the ∼20% gap between canonically expected and observed M/L_V ratios is bridged by accounting for the preferential loss of low-mass stars, also when considering individual clusters. It is concluded that the variation in M/L ratio due to dissolution and low-mass star depletion is a plausible explanation for the discrepancy between the observed and canonically expected M/L ratios of GCs.