Globular cluster mass-loss in the context of multiple populations: Figure 1.

Abstract

Many scenarios for the origin of the chemical anomalies observed in globular clusters (GCs; i.e., multiple populations) require that GCs were much more massive at birth, up to 10 − 100×, than they are presently. This is invoked in order to have enough material processed through first generation stars in order to form the observed numbers of enriched stars (inferred to be second generation stars in these models). If such mass loss was due to tidal stripping, gas expulsion, or tidal interaction with the birth environment, there should be clear correlations between the fraction of enriched stars and other cluster properties, whereas the observations show a remarkably uniform enriched fraction of 0.68 ± 0.07 (from 33 observed GCs). If interpreted in the heavy mass loss paradigm, this means that all GCs lost the same fraction of their initial mass (between 95 − 98%), regardless of their mass, metallicity, location at birth or subsequent migration, or epoch of formation. This is incompatible with predictions, hence we suggest that GCs were not significantly more massive at birth, and that the fraction of enriched to primordial stars observed in clusters today likely reflects their initial value. If true, this would rule out self-enrichment through nucleosynthesis as a viable solution to the multiple population phenomenon.

The search for multiple populations in Magellanic Cloud clusters – I. Two stellar populations in the Small Magellanic Cloud globular cluster NGC 121

“…Thus, the result refers to the fraction measured at larger radii. Given our results, the frac-tion of enriched stars appears to vary from cluster to cluster, much more than reported by Bastian & Lardo (2015). This is consistent with the results of Lardo et al (in prep.…”

“…This, however, is much smaller than the expected median value found in Galactic GCs. Bastian & Lardo (2015) collected a sample of 33 GCs and found that the fraction of enriched stars is never smaller than 50% with median value of 68%±7%. Moreover, this fraction seems to be independent of the cluster mass, metallicity or distance to the centre of the Galaxy.…”

“…It is expected that the time it takes to dissolve a star cluster is longer for more massive clusters and in weaker tidal fields (see e.g. Bastian & Lardo 2015, and references therein, for a discussion). Quite extreme assumptions have to be made for the cluster and its environment in order to allow for such high dissolution rates (see D'Ercole et al 2008).…”

Gas expulsion in massive star clusters?

“…If gas expulsion would work in multiple population clusters, one would expect a correlation between the total stellar mass and the fraction of enriched stars, which is, however, not seen (Khalaj & Baumgardt 2015;Bastian & Lardo 2015).…”

NGC 6362: The Least Massive Globular Cluster With Chemically Distinct Multiple Populations*

“…This model suggests that interacting massive stars and binaries can shed enriched material into the cluster, and low-mass pre-main-sequence stars (of the same generation), which are fully convective and have protoplanetary disks, are able to sweep up the enriched material and eventually accrete it. However, all the proposed scenarios present some shortcomings and they are not able to reproduce simultaneously all the observational pieces of evidence (see, e.g., Bastian 2015;Bastian & Lardo 2015;Kruijssen 2015).…”