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Multiple populations in massive star clusters under the magnifying glass of photometry: theory and tools

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Abstract

The existence of star-to-star light-element abundance variations in massive Galactic and extragalactic star clusters has fairly recently superseded the traditional paradigm of individual clusters hosting stars with the same age, and uniform chemical composition. Several scenarios have been put forward to explain the origin of this multiple stellar population phenomenon, but so far all have failed to reproduce the whole range of key observations. Complementary to high-resolution spectroscopy, which has first revealed and characterized chemically the presence of multiple populations in Galactic globular clusters, photometry has been instrumental in investigating this phenomenon in much larger samples of stars—adding a number of crucial observational constraints and correlations with global cluster properties—and in the discovery and characterization of multiple populations also in Magellanic Clouds’ intermediate-age clusters. The purpose of this review is to present the theoretical underpinning and application of the photometric techniques devised to identify and study multiple populations in resolved star clusters. These methods have played and continue to play a crucial role in advancing our knowledge of the cluster multiple population phenomenon, and promise to extend the scope of these investigations to resolved clusters even beyond the Local Group, with the launch of the James Webb Space Telescope.

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Notes

  1. Interestingly, it has been found that within these ‘peculiar’ clusters C–N, O–Na anticorrelations are present among stars with the same Fe abundance (see, e.g.Marino et al. 2011).

  2. In the interiors of low-mass models, \({\log {R}}\) reaches values on the order of \(\sim 0.5\) only during the RGB advanced evolutionary phases. However in this regime the electron conduction opacity is the dominant contributor to the opacity in the He-core.

  3. In case of complete ionisation, the mean molecular weight is given by \(\mu =\frac{1}{2X+\frac{3}{4}Y+\frac{Z}{2}}\).

  4. We recall that, as long as the CNO sum (and initial Y) is the same, P1 and P2 theoretical isochrones are identical.

  5. Lee (2017) has introduced the \(\mathrm{cn}_\mathrm{JWL}\) index, defined as \(\mathrm{cn}_\mathrm{JWL}=JWL39-\mathrm{Ca}_\mathrm{new}\), where JWL39 and \(\mathrm{Ca}_\mathrm{new}\) are two filters in the wavelength range between 3800 and 4050 Å. The \(\mathrm{cn}_{JWL}\) index is sensitive to the absorption of the CN band at 3883 Å, and the V-\(cn_{JWL}\) diagram is very similar to the V-\(c_y\) counterpart, but with a better MP resolving power.

  6. The \(\delta _4\)index is also almost insensitive to reddening, given that \(E(\delta _4) \sim 0.01 E(B-V)\).

  7. See also Milone et al. (2015) for a first discussion of this map for the Galactic GC NGC 2808, and alternative versions of chromosome maps. Zennaro et al. (2019) has very recently used another version of chromosome maps to study the Galactic GC NGC 2419, employing the pseudocolour \(C_{\mathrm{F275W}, \mathrm{F343N}, \mathrm{F438W}}=(\mathrm{F275W}-\mathrm{F343N})-(\mathrm{F343N}-\mathrm{F438W})\) and the colour \((\mathrm{F438W}-\mathrm{F814W})\). The construction and general properties of this map are the same as for Milone et al. (2017b) maps. For the same cluster, Larsen et al. (2019) have devised an analogous type of chromosome map, but based on the (F438W–F814W) and (F336W–F343N) pair of colours.

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Acknowledgements

We acknowledge the anonymous referee for his/her helpful suggestions. We warmly thank Adriano Pietrinferni for interesting discussions and the fruitful collaboration over all these years. We wish to acknowledge Giampaolo Piotto for his leading role in the development of photometric studies of MPs in globular clusters. Raffaele Gratton and Alvio Renzini are also warmly acknowledged for interesting discussions and collaborations on this research topic. We thank Nate Bastian and Carmela Lardo for several comments on an early draft of the manuscript. We are grateful to Carmela Lardo also for producing some of the figures. SC acknowledges support from Premiale INAF MITiC, from Istituto Nazionale di Fisica Nucleare (INFN) (Iniziativa specifica TAsP), progetto INAF Mainstream (PI: S. Cassisi), PLATO ASI-INAF agreement n.2015-019-R.1-2018, and grant AYA2013-42781P from the Ministry of Economy and Competitiveness of Spain.

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Authors and Affiliations

  1. INAF, Osservatorio Astronomico d’Abruzzo, Via Maggini sn, 64100, Teramo, Italy

    Santi Cassisi

  2. Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Pisa, Università di Pisa, Largo Pontecorvo 3, 56127, Pisa, Italy

    Santi Cassisi

  3. Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK

    Maurizio Salaris

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  1. Santi Cassisi
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Correspondence to Santi Cassisi.

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Cassisi, S., Salaris, M. Multiple populations in massive star clusters under the magnifying glass of photometry: theory and tools. Astron Astrophys Rev 28, 5 (2020). https://doi.org/10.1007/s00159-020-00127-y

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  • DOI: https://doi.org/10.1007/s00159-020-00127-y

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