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Register a new userOn the maximum helium content of multiple populations in the globular cluster NGC6752
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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 of various type. Among other things, these polluters differ by the amount of helium they spread in the surrounding medium. 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. We focus on the specific case of NGC6752. 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 different pollution scenarios, including the extreme case of the fast-rotating massive star (FRMS) scenario. 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. 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. 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. We show that in NGC6752 it is unlikely that stars more He-rich than the current observational limit of about 0.3 are present.
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NGC 4833 is a metal-poor Galactic globular cluster (GC) whose multiple stellar populations present an extreme chemical composition. The Na-O anti-correlation is quite extended, which is in agreement with the long tail on the blue horizontal branch, and the large star-to-star variations in the [Mg/Fe] ratio span more than 0.5 dex. Recently, significant excesses of Ca and Sc with respect to field stars of a similar metallicity were also found, signaling the production of species forged in H-burning at a very high temperature in the polluters of the first generation in this cluster. Since an enhancement of potassium is also expected under these conditions, we tested this scenario by analysing intermediate resolution spectra of 59 cluster stars including the K I resonance line at 7698.98 A. We found a wide spread of K abundances, anti-correlated to Mg and O abundances, as previously also observed in NGC 2808. The abundances of K are found to be correlated to those of Na, Ca, and Sc. Overall, this chemical pattern confirms that NGC 4833 is one of the relatively few GCs where the self-enrichment from first generation polluters occurred at such high temperatures that proton-capture reactions were able to proceed up to heavier species such as K and possibly Ca. The spread in K observed in GCs appears to be a function of a linear combination of cluster total luminosity and metallicity, as other chemical signatures of multiple stellar populations in GCs.
We present a detailed analysis of the radial distribution of light-element multiple populations (LE-MPs) in the massive and dense globular cluster M80 based on the combination of UV and optical Hubble Space Telescope data. Surprisingly, we find that first generation stars (FG) are significantly more centrally concentrated than extreme second generation ones (SG) out to $sim 2.5 r_h$ from the cluster center. To understand the origin of such a peculiar behavior, we used a set of $N$-body simulations following the long-term dynamical evolution of LE-MPs. We find that, given the advanced dynamical state of the cluster, the observed difference does not depend on the primordial relative distributions of FG and SG stars. On the contrary, a difference of $sim 0.05-0.10 M_{odot}$ between the average masses of the two sub-populations is needed to account for the observed radial distributions. We argue that such a mass difference might be the result of the higher He abundance of SG stars (of the order of $Delta Ysim 0.05-0.06$) with respect to FG. Interestingly, we find that a similar He variation is necessary to reproduce the horizontal branch morphology of M80. These results demonstrate that differences in mass among LE-MPs, due to different He content, should be properly taken into account for a correct interpretation of their radial distribution, at least in dynamically evolved systems.
Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and horizontal branch stellar evolution models. Here we have investigated this issue by employing the CUBI= (U-B)-(B-I) index, that previous studies have demonstrated to be very effective in separating multiple populations along both the red giant and asymptotic giant branch sequences. We confirm previous results that the RGB is intrinsically broad in the V-CUBI diagram, with the presence of two components which nicely correspond to the two populations identified by high-resolution spectroscopy. We find that AGB stars are distributed over a wide range of CUBI values, in close analogy with what is observed for the RGB, demonstrating that the AGB of M4 also hosts multiple stellar populations.
We combine MUSE spectroscopy and Hubble Space Telescope ultraviolet (UV) photometry to perform a study of the chemistry and dynamics of the Galactic globular cluster Messier 80 (M80, NGC 6093). Previous studies have revealed three stellar populations that not only vary in their light-element abundances, but also in their radial distributions, with concentration decreasing with increasing nitrogen enrichment. This remarkable trend, which sets M80 apart from the other Galactic globular clusters, points towards a complex formation and evolutionary history. To better understand how M80 formed and evolved, revealing its internal kinematics is key. We find that the most N-enriched population rotates faster than the other two populations at a 2 sigma confidence level. While our data further suggest that the intermediate population shows the least amount of rotation, this trend is rather marginal (1 - 2 sigma). Using axisymmetric Jeans models, we show that these findings can be explained from the radial distributions of the populations if they possess different angular momenta. Our findings suggest that the populations formed with primordial kinematical differences.
Recent work, based on data from the Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first- (1G) and second-generation (2G) stars. In most GCs, both 1G and 2G stars host sub-stellar populations with different chemical composition. We compare multi-wavelength HST photometry with synthetic spectra to determine for the first time the average helium difference between the 2G and 1G stars in a large sample of 57 GCs and the maximum helium variation within each of them. We find that in all clusters 2G stars are consistent with being enhanced in helium with respect to 1G. The maximum helium variation ranges from less than 0.01 to more than 0.10 in helium mass fraction and correlates with both the cluster mass and the color extension of the horizontal branch (HB). These findings demonstrate that the internal helium variation is one of the main (second) parameters governing the HB morphology.
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