Dissolution is the solution: on the reduced mass-to-light ratios of Galactic globular clusters

¹ Astronomical Institute, Utrecht University, PO Box 80000, 3508TA Utrecht, The Netherlands e-mail: kruijssen@astro.uu.nl
² Sterrewacht Leiden, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: smieske@eso.org

Received: 1 December 2008
Accepted: 27 March 2009

Abstract

Context. The observed dynamical mass-to-light () 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.

Aims. To verify whether low-mass star depletion is indeed the driving mechanism behind the decrease, we aim to predict the  ratios of individual GCs for which orbital parameters and dynamical V-band mass-to-light ratios  are known. There is a sample of 24 Galactic GCs for which this is possible.

Methods. 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  ratios of the individual GCs. To verify our findings, we also predicted the slopes of their low-mass stellar mass functions.

Results. The computed dissolution timescales agree well with earlier empirical studies. The predicted are in  agreement with the observations for 12 out of 24 GCs. The discrepancy for the other GCs probably arises because our predictions give global  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  ratios can be excluded by imposing limits on the dissolution timescale and King parameter. For the remaining GCs, the observed and predicted average  are 78% and 78 ± 2% of the canonically expected values, while the values are 74% and 85 ± 1% for the entire sample. The predicted correlation between the slope of the low-mass stellar mass function and  drop is found to be qualitatively consistent with observed mass function slopes.

Conclusions. The dissolution timescales of Galactic GCs are such that the % gap between canonically expected and observed  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  ratio due to dissolution and low-mass star depletion is a plausible explanation for the discrepancy between the observed and canonically expected  ratios of GCs.