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What is a globular cluster? An observational perspective


Globular clusters are large and dense agglomerate of stars. At variance with smaller clusters of stars, they exhibit signs of some chemical evolution. At least for this reason, they are intermediate between open clusters and massive objects such as nuclear clusters or compact galaxies. While some facts are well established, the increasing amount of observational data are revealing a complexity that has so far defied the attempts to interpret the whole data set in a simple scenario. We review this topic focusing on the main observational features of clusters in the Milky Way and its satellites. We find that most of the observational facts related to the chemical evolution in globular clusters are described as being primarily a function of the initial mass of the clusters, tuned by further dependence on the metallicity—that mainly affects specific aspects of the nucleosynthesis processes involved—and on the environment, that likely determines the possibility of independent chemical evolution of the fragments or satellites, where the clusters form. We review the impact of multiple populations on different regions of the colour–magnitude diagram and underline the constraints related to the observed abundances of lithium, to the cluster dynamics, and to the frequency of binaries in stars of different chemical composition. We then re-consider the issues related to the mass budget and the relation between globular cluster and field stars. Any successful model of globular cluster formation should explain these facts.

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Adapted from Salaris et al. (2002)

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  1. Globular clusters may have been formed in situ or have been accreted, see the classical paper by Searle and Zinn (1978) and the recent results coming out from the Gaia mission, as presented, e.g., in Gaia Collaboration et al. (2018), Myeong et al. (2018c) and Helmi et al. (2018).

  2. See, however, Sect. 3.6 for the recent extension to lower ages.

  3. For most elements, we adopt the usual spectroscopic notation, i.e., \({[X]}=\log {X_{\mathrm{star}}} -\log {X_\odot }\) for any abundance quantity X, and \(\log {\epsilon (X)} = \log {N_{{X}}/N_{\mathrm{H}}} + 12.0\) for absolute number density abundances. For helium, we use Y, that is the fraction of He in mass.

  4. The interquartile of a distribution is the range of values including the middle 50% of the distribution, leaving out the highest and lowest quartiles.

  5. Alternative estimates of the current masses for MC clusters are provided by other studies, e.g., by McLaughlin and van der Marel (2005), while these last authors did not list values for all the clusters considered here, whenever available the masses agree very well with those given by Mackey and Gilmore (2003a, b), but for the single case of NGC 2257.

  6. The sample of clusters in Milone et al. (2017) may suffer from a selection bias, because only rather nearby and massive GCs have been targeted (the selection is essentially that of the ACS Survey by Sarajedini et al. 2007). On the other hand, these are also those GCs for which more precise data can be obtained. A similar bias can of course be present in case spectroscopy is used to define the populations fractions. It would be interesting to extend the same kind of studies to a sample fully representative of all MW GCs.

  7. NGC 2808 has at least five different populations (Milone et al. 2015b; Carretta et al. 2015). NGC 2419, a very massive cluster with a very large apocenter distance, also shares many characteristics of the chromosome map with NGC 2808, as suggested by the very recent study by Zennaro et al. (2019). However, it is not plotted in Fig. 8, because it actually lacks an explicit classification in Type I/II classes.

  8. It is worth noting that current models do not reproduce the correct zero-point (Cassisi et al. 2011); however, observational studies have been concentrating on differential effects, and we will limit our discussion to those in this text.

  9. NGC 2808, the cluster showing the largest He differences, was not in the calculation, since no star below the RGB bump was observed for this cluster in that survey.

  10. See (Pietrinferni et al. 2004, 2006).

  11. An example of the difficulties in deriving He abundance variations from clusters with red horizontal branch is given by a comparison of the spread in He abundances for the SMCs clusters NGC 121, NGC 339, NGC 416, and Lindsay 1 as determined from the horizontal branch by Chantereau et al. (2019), and from a pseudo-chromosome map by Lagioia et al. (2018). While the first study found variations in the He abundances as large as \(\varDelta Y=0.08\), the second one only found very tiny spreads, with the highest value being \(\varDelta Y=0.010\pm 0.003\). Chantereau et al. (2019) noticed this difference, and attributed it to the different meaning of \(\varDelta Y\) in the two studies—maximum excursion with respect to mean difference between first- and second-generation stars, although it seems quite difficult to justify a factor of almost ten difference between the two results this way. We then think that the spread in He abundances derived for red horizontal branch clusters should be taken with caution.

  12. These calculations are restricted to those stars that have Al abundances, which comprises more than 90% of the total sample.

  13. Note that MacLean et al. (2016) rather use the observed minima in the [Na/H] distribution to separate FG and SG stars along the AGB and the RGB comparison data set

  14. The investigation of the Li discrepancy as measured in Pop ii stars with respect to the standard Big Bang nucleosynthesis is not discussed in this review, since our main focus is the multiple population scenarios. We refer the reader to Sbordone et al. (2010), Mucciarelli et al. (2014b), Fu et al. (2015), and references therein for a specific discussion on this topic.

  15. Given the primordial Li scatter in NGC 104, which is unrelated to the multiple population scenarios, this GC was omitted from the present discussion.

  16. BSS may also be produced by collision in the dense core of GCs. In that case, there should not be large chemical anomalies (Lombardi et al. 1995). However, the majority of BSS in both globular and open clusters are likely the aftermath of the evolution of primordial binaries (see, e.g., Piotto et al. 2004).

  17. There is evidence that Algol systems—that are interacting intermediate-mass binaries—are depleted in C (Tomkin et al. 1993; Sarna and De Greve 1996).

  18. Note that the mass-budget values discussed above should be revised in this scenario because only a fraction of the massive AGB stars should contribute to nucleosynthesis. On the other hand, in this scenario the diluting material was already present in the GC since its birth.