Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.

Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, but rather are made up of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second-parameter problem, it also opens new perspectives on the relation between globular clusters and the halo of our Galaxy, and by extension on all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focussing on the most recent studies. Several points remain to become properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.

Multiple populations in globular clusters

Accurate photometry with HST's ACS shows that the main sequence (MS) of the globular cluster NGC 2808 splits into three separate branches The three MS branches may be associated with the complexities of the cluster's horizontal branch and of its abundance distribution We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment Some other massive globulars also appear to have complex populations; we compare them with NGC 2808

https://iopscience.iop.org/article/10.1086/518503/pdf

A triple main sequence in the globular cluster NGC 2808

A new, comprehensive set of low-temperature opacity data has been assembled. From this basic data set, Rosseland and Planck mean opacities have been computed for temperatures between 12,500 and 700 K. In addition to the usual continuous absorbers, atomic line absorption (with more than 8 million lines), molecular line absorption (with nearly 60 million lines), and grain absorption and scattering (by silicates, iron, carbon, and SiC) have been accounted for. The absorption due to lines is computed monochromatically and included in the mean with the opacity sampling technique. Grains are assumed to form in chemical equilibrium with the gas and to form into a continuous distribution of ellipsoids. Agreement of these opacities with other recent tabulations of opacities for temperatures above 5000 K is excellent. It is shown that opacities which neglect molecules become unreliable for temperatures below 5000 K. Triatomic molecules become important absorbers at 3200 K. Similarly, grains must be included in the computation for temperatures below 1700 K.

Low Temperature Rosseland Opacities.

We study the formation and dynamical evolution of clusters with multiple stellar generations. Observational studies have found that some globular clusters host a population of second generation (SG) stars which show chemical anomalies and must have formed from gas containing matter processed in the envelopes of first generation (FG) cluster stars. We study the SG formation process by means of 1D hydrodynamical simulations, starting from a FG already in place and assuming that the SG is formed by the gas ejected by the Asymptotic Giant Branch (AGB) stars. This gas collects in a cooling flow into the cluster core, where it forms SG stars. The SG subsystem emerging from this process is initially strongly concentrated in the cluster innermost regions and its structural properties are largely independent of the FG initial properties. We also present the results of a model in which pristine gas contributes to the SG formation. In this model a very helium-rich SG population and one with a moderate helium enrichment form; the resulting SG bimodal helium distribution resembles that observed for SG stars in NGC 2808. By means of N-body simulations, we then study the two-population cluster dynamical evolution and mass loss. In our simulations, a large fraction of FG stars are lost early in the cluster evolution due to the expansion and stripping of the cluster outer layers resulting from early mass loss associated with FG SN ejecta. The SG population, initially concentrated in the innermost cluster regions, is largely unscathed by this early mass loss, and this early evolution leads to values of the number ratio of SG to FG stars consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. As the cluster evolves and the two populations mix, the local ratio of SG to FG stars, initially a decreasing function of radius, tends to a constant value in the inner parts of the cluster. Until mixing is complete, the radial profile of this number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions.

/pdf/formation-and-dynamical-evolution-of-multiple-stellar-27ohkyaxn1.pdf

Formation and dynamical evolution of multiple stellar generations in globular clusters

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globular clusters. We have tested, with fully consistent stellar evolution calculations, whether theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the color-magnitude diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations and varying initial He mass fractions. Comparisons with "normal" α-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y = 0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the red giant branch tip are also very weakly affected, apart from—depending on the He content of the polluting material—the red giant branch bump region. We also study the distribution of stars along the zero-age horizontal branch and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.

David J. Fusilier

Papers

On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test