Maximum helium content of multiple populations in the globular cluster NGC 6752

Abstract

Context. Multiple populations in globular clusters are usually explained by the formation of stars out of material with a chemical composition that is polluted to different degrees by the ejecta of short-lived, massive stars. But the nature of the “polluters” remains elusive. Different types of stars have been proposed to account for the observed chemical patterns of multiple populations. Among other things, these differ by the amount of helium they spread in the surrounding medium. Aims. In this study we investigate whether the present-day photometric method used to infer the helium content of multiple populations indeed gives the true value or underestimates it by missing very He-rich, but rare stars. This check is important to discriminate between the different polluter scenarios. We focus on the specific case of NGC 6752. Methods. We compute atmosphere models and synthetic spectra along isochrones produced for this cluster for a very broad range of He abundances covering the predictions of the different scenarios, including the extreme case of the fast-rotating massive star (FRMS) scenario. We use the same abundances in isochrones and atmosphere models to ensure consistency. We calculate synthetic photometry in HST filters best suited to study the helium content. We subsequently build synthetic clusters with various distributions of stars. We finally determine the maximum helium mass fraction of these synthetic clusters using a method similar to that applied to observational data. In particular, we select nonpolluted and very He-rich stars from the so-called chromosome map. Results. We re-determine the maximum helium mass fraction Y in NGC 6752, and find a value consistent with published results. We build toy models of clusters with various distributions of multiple populations and ensure that we are able to recover the input maximum Y. We then build synthetic clusters with the populations predicted by the FRMS scenario and find that while we slightly underestimate the maximum Y value, we are still able to detect stars much more He-rich than the current observed maximum Y. This result still holds even in synthetic clusters that contain less He-rich stars than predicted by the FRMS scenario. It is easier to determine the maximum Y on main sequence stars than on red giant branch stars, but qualitatively the results are unaffected by the sample choice. Conclusions. We show that in NGC 6752 it is unlikely that stars more He-rich than the current observational limit of about 0.3 (in mass fraction) are present.

Investigating black hole accretion disks as potential polluter sources for the formation of enriched stars in globular clusters

“…The first step to set up our simulations is to define an initial set of abundances (given as mass ratios). We used the pristine composition of NGC 6752 (see Table 1) because this cluster has been widely studied over the past few years (Carretta 2013;Lapenna et al 2016;Mucciarelli et al 2017Mucciarelli et al , 2019Pancino et al 2017;Lee 2018;Martins et al 2021). It is one of the few globular clusters for which observations of C, N, Na, O, Al, Mg (including its isotopes), Si, and K are available (Yong et al 2003;Carretta et al 2005Carretta et al , 2007Carretta et al , 2012.…”

Could kilomasers pinpoint supermassive stars?

“…AGB stars display O-Na correlation instead of the anticorrelation observed (Forestini & Charbonnel 1997;Denissenkov & Herwig 2003;Karakas & Lattanzio 2007;Siess 2010;Ventura et al 2013;Doherty et al 2014;Renzini et al 2022), furthermore it releases He-burning products, that are not widely detected in GCs (Karakas et al 2006;Decressin et al 2009;Yong et al 2014). FRMSs on the other hand could be able to produce Mg-Al anticorrelations 1 but simultaneously would show a strong He enrichment (Decressin et al 2007;Martins et al 2021). The essential central temperature to activate the MgAl chain is reached by a supermassive star at the very beginning of its evolution when the abundance of He is still low (Prantzos et al 2017).…”

Dissecting the microphysics behind the metallicity-dependence of massive stars radii

“…The primordial composition is particularly important, because of the existence of observational tests comparing stellar populations in M31, various parts of the Galaxy, the Magellanic Clouds, and other dwarf galaxies (Bellazzini et al 2001;Aloisi et al 2007;Annibali et al 2018;Wang et al 2020;Martins et al 2021). Moreover, the kind of binary interactions and the outcome of massive binary evolution, especially for rare paths leading to the formation of gravitational wave sources, are sensitive to the initial composition (e.g., Klencki et al 2021c,b).…”