Properties of stellar generations in globular clusters and relations with global parameters

Abstract

We revise the scenario of the formation of Galactic globular clusters (GCs) by adding the observed detailed chemical composition of their different stellar generations to the set of their global parameters. We exploit the unprecedented set of homogeneous abundances of more than 1200 red giants in 19 clusters, as well as additional data from literature, to give a new definition of bona fide GCs, as the stellar aggregates showing the Na-O anticorrelation. We propose a classification of GCs according to their kinematics and location in the Galaxy in three populations: disk/bulge, inner halo, and outer halo. We find that the luminosity function of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs that includes at least three main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those that led to the formation of dwarf spheroidals, but residing at smaller Galactocentric distances, with the early Galaxy or with other structures); ii) the triggering of a long episode of star formation (the primordial population) from the precursor population; and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal-poor (mainly halo) clusters, while its rôle is minor in small and/or metal-rich (mainly disk) ones. Finally, we use principal component analysis and multivariate relations to study the phase of metal enrichment from first to second generation. We conclude that most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being metallicity, mass, and cluster age. Location within the Galaxy (as described by the kinematics) also plays some rôle, while additional parameters are required to describe their dynamical status.

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

“…If the uncertainty surrounding M92 is purely due to observational spread, then results from homogeneous surveys such as APOGEE will shed light on this issue. However, we note that both NGC 6397 and NGC 5466 are much less massive than M92 and Carretta et al (2010b) andD'Orazi et al (2014) have demonstrated how clusters with greater mass can exhibit a greater spread in their primordial abundances.…”

“…In clusters such as ω Cen, a spread in metallicity and split main-sequences imply a wider span in age, however, these complex systems are in the minority. In most cases, GCs are dominated by second generation stars with an inferred separation in age of at most a few hundred Myr (Carretta et al 2010b). Stellar models representative of these stars only further exacerbate the discrepancies demonstrated in Figure 4.…”

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

“…Using ground-based photometry or spectroscopy measurements, the enriched populations are generally found to be more concentrated (e.g. Carretta et al 2010b;Beccari et al 2013;Larsen et al 2014;Li et al 2014). But due to the crowding in the inner regions these studies usually avoid the central parts of the clusters.…”

Age as a major factor in the onset of multiple populations in stellar clusters

“…Nonetheless, the lack of chemical spreads in relatively young (∼ 1−2 Gyr, Mucciarelli et al 2014), but still massive (few times 10 5 M ) clusters suggests a potentially more complex cause. It has even been proposed that the ancient GCs are intrinsically different from the young clusters with similar masses, as only ancient GCs have been found to host MPs (Carretta et al 2010).…”