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

Abstract

Context. 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. Aims. 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. 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 M/L V 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 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. Conclusions. 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.

MONDian dynamical modelling of NGC 288 with β ≠ 0

“…Our resulting stellar M/L of 1.09 ± 0.37 falls below the expected value for a single stellar population having the inferred age and metallicity of NGC 288, M/LSSP = 2.2 ± 0.08, e.g. Kruijssen & Mieske (2009), but is consistent to 1σ with the value predicted for this cluster of 1.42 +0.37 −0.29 by the same authors, after accounting for various dynamical effects such as the selective ejection of low mass stars and the preferential loss of stellar remnants. Indeed, it is expected for present day M/L ratios in globular clusters to fall slightly below single stellar population estimates, due to the dynamical effects mentioned above (Kruijssen & Lamers 2008), which surely also apply under MONDian scenarios, albeit with differences in the detail which remain to be estimated.…”

The evolution of the stellar mass function in star clusters

“…In addition, the effect of a changing MF on cluster photometry has been investigated (Lamers et al 2006;Kruijssen & Lamers 2008;Anders et al 2009). This has been shown to explain the low mass-to-light ratios of globular clusters (Kruijssen 2008;Kruijssen & Mieske 2009) and to have a pronounced effect on the inferred globular cluster mass function (Kruijssen & Portegies Zwart 2009). …”

Gas expulsion in massive star clusters?

“…6. Apart from a possible early gas expulsion event, star clusters lose mass over time for instance as a result of low-mass star depletion (Kruijssen & Mieske 2009), encounters with the environment (Kruijssen et al 2012), or disc passages (Webb et al 2014), which may also lead to expansion through tidal heating. Hence, we generally expect that C 5 will decrease with time, easily by a factor of a few and very likely in different ways for different objects (e.g., Rossi & Hurley 2015b).…”