The second and third parameters of the horizontal branch in globular clusters

By R. Gratton, E. Carretta, A. Bragaglia, S. Lucatello, Valentina D'Orazi INAF-Osservatorio Astronomico di Padova, I. Bologna, Excellence Cluster Universe. 2010

181 citations recorded by the source corpus.

Context. The second parameter (the first being metallicity) defining the distribution of stars on the horizontal branch (HB) of globular clusters (GCs) has long been one of the major open issues in our understanding of the evolution of normal stars. Large photometric and spectroscopic databases are now available: they include large and homogeneous sets of colour-magnitude diagrams, cluster ages, and homogeneous data about chemical compositions from our FLAMES survey. Aims. We use these databases to re-examine this issue. Methods. We use the photometric data to derive median and extreme (i.e., the values including 90% of the distribution) colours and magnitudes of stars along the HB for about a hundred GCs. We transform these into median and extreme masses of stars on the HB, using the models developed by the Pisa group, and taking into account evolutionary effects. We compare these masses with those expected at the tip of the red giant branch (RGB) to derive the total mass lost by the stars. Results. We find that a simple linear dependence on metallicity of this total mass lost describes quite well the median colours of HB stars. Assuming this mass loss law to be universal, we find that age is the main second parameter, determining many of the most relevant features related to HBs. In particular, it allows us to explain the Oosterhoff dichotomy as a consequence of the peculiar age-metallicity distribution of GCs in our Galaxy, although both Oosterhoff groups have GCs spanning a rather wide range of ages. However, at least an additional – third – parameter is clearly required. The most likely candidate is the He abundance, which might be different in GC stars belonging to the different stellar generations whose presence was previously derived from the Na-O and Mg-Al anticorrelations. Variations in the median He abundance allow us to explain the extremely blue HB of GCs like NGC 6254 (=M 10) and NGC 1904 (=M 79); such variations are found to be (weakly) correlated with the values of the R-parameter (that is the ratio of the number of stars on the HB and on the RGB). We also show that suitable He abundances allow deriving ages from the HB which are consistent with those obtained from the Main Sequence. Small corrections to these latter ages are then proposed. We find that a very tight age-metallicity relation (with a scatter below 4%) can be obtained for GCs kinematically related to the disk and bulge, once these corrections are applied. Furthermore, star-to-star variations in the He content, combined with a small random term, explain very well the extension of the HB. There is a strong correlation between this extension and the interquartile of the Na-O anticorrelation, strongly supporting the hypothesis that the third parameter for GC HBs is He. Finally, there are strong indications that the main driver for these variations in the He-content within GCs is the total cluster mass. There are a few GCs exhibiting exceptional behaviours (including NGC 104=47 Tuc and in less measure NGC 5272=M 3); however, they can be perhaps accommodated in a scenario for the formation of GCs that relates their origin to cooling flows generated after very large episodes of star formation, as proposed by Carretta et al. (2009d).

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

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Citations in the corpus

Multiple populations in globular clusters (2012, background): “…we should then expect that there is a close relation between the extension of the HB (as represented e.g., by the minimum mass along the HB: see Gratton et al. 2010a) and the frequency of AGB stars (as given by the ratio between the number of stars along the AGB and those on the RGB: see…”
Unresolved stellar companions with Gaia DR2 astrometry (2020, background): “The GC data indicate that the primary factor responsible for the HB diversity is the cluster’s metallicity, while the age and the He abundance may act as the second and third parameters (Gratton et al. 2010).”
Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA (2015, methodology): “…efficient mass-loss process, and it is often used e.g. to reproduce the mean colours of horizontal branch stars in Galactic globular clusters, although this morphological feature is affected by other, also poorly constrained, parameters (e.g. Catelan 2009; Gratton et al. 2010; Milone et al. 2014).”
MEASUREMENT OF THE RADIUS OF NEUTRON STARS WITH HIGH SIGNAL-TO-NOISE QUIESCENT LOW-MASS X-RAY BINARIES IN GLOBULAR CLUSTERS (2012, methodology): “In fact, many recent photometric studies of GCs do not quote the amount of uncertainty in the measured distance (Rosenberg et al. 2000; Bica et al. 2006; Gratton et al. 2010).”
The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater (2016, background): “Note, however, that these numbers, and in particular the formal error on the distance, should be taken with caution, given that the behavior of HB stars is still not well understood, therefore rendering the models somewhat uncertain (see e.g. Gratton et al. 2010).”
The bifurcated age–metallicity relation of Milky Way globular clusters and its implications for the accretion history of the galaxy (2013, background): “…in that work are based on the horizontal branch type as a function of metallicity, assuming that age is the main second parameter controlling the HB colour; however, the influence of possible helium enhancements on that metric complicates such divisions (see e.g., Gratton et al. 2010).”
THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED METHOD ALONG WITH COLOR–MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES (2013, background): “…of the horizontal branch — in particular, from the length of the blue HB or the existence of gaps along this feature (e.g., D’Antona & Caloi 2008, Gratton et al. 2010) — are quite speculative as well, though it is possible to differentiate between sub-populations on the HB using uv photometry…”
MULTIPLE STELLAR POPULATIONS IN 47 Tucanae (2011, background): “Multiple stellar populations with different helium abundance also offer an explanation for the complex, extended, and clumpy horizontal branch (HB) morphology exhibited by some clusters (e.g., D’Antona et al. 2005; D’Antona & Caloi 2004; Catelan et al. 2010; Gratton et al. 2010).”
GLOBULAR CLUSTERS IN THE OUTER GALACTIC HALO: NEW HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS IMAGING OF SIX GLOBULAR CLUSTERS AND THE GALACTIC GLOBULAR CLUSTER AGE–METALLICITY RELATION (2011, background): “In this context, it is clear that age is the second parameter influencing HB morphology as concluded by Dotter et al. (2010) and Gratton et al. (2010).”