A chromosome map to unveil stellar populations with different magnesium abundances. The case of ω Centauri

ABSTRACT Historically, photometry has been largely used to identify stellar populations [multiple populations (MPs)] in globular clusters (GCs) by using diagrams that are based on colours and magnitudes that are mostly sensitive to stars with different metallicities or different abundances of helium, carbon, nitrogen, and oxygen. In particular, the pseudo-two-colour diagram called chromosome map (ChM), allowed the identification and the characterization of MPs in about 70 GCs by using appropriate filters of the Hubble Space Telescope (HST) that are sensitive to the stellar content of He, C, N, O, and Fe. We use here high precision HST photometry from F275W, F280N, F343N, F373N, and F814W images of ω Centauri to investigate its MPs. We introduce a new ChM whose abscissa and ordinate are mostly sensitive to stellar populations with different magnesium and nitrogen, respectively, in monometallic GCs. This ChM is effective in disentangling the MPs based on their Mg chemical abundances, allowing us to explore, for the first time, possible relations between the production of these elemental species for large samples of stars in GCs. By comparing the colours of the distinct stellar populations with the colours obtained from appropriate synthetic spectra we provide ‘photometric-like’ estimates of the chemical composition of each population. Our results show that, in addition to first-generation (1G) stars, the metal-poor population of ω Cen hosts four groups of second-generation stars with different [N/Fe], namely, 2GA–D. 2GA stars share nearly the same [Mg/Fe] as the 1G, whereas 2GB, 2GC, and 2GD are Mg depleted by ∼0.15, ∼0.25, and ∼0.45 dex, respectively. We provide evidence that the metal-intermediate populations host stars with depleted [Mg/Fe].

Impact of a companion and of chromospheric emission on the shape of chromosome maps for globular clusters

Context. Globular clusters (GCs) host multiple populations of stars that are well-separated in a photometric diagram – the chromosome map – built from specific Hubble Space Telescope (HST) filters. Stars from different populations feature at various locations on this diagram due to peculiar chemical compositions. Stars of the first population, with field star-like abundances, sometimes show an unexpected extended distribution in the chromosome map. Aims. We aim to investigate the role of binaries and chromospheric emission on HST photometry of globular clusters’ stars. We quantify their respective effects on the position of stars in the chromosome map, especially among the first population. Methods. We computed atmosphere models and synthetic spectra for stars of different chemical compositions, based on isochrones produced by stellar evolution calculations with abundance variations representative of first and second populations in GCs. From this we built synthetic chromosome maps for a mixture of stars of different chemical compositions. We subsequently replaced a fraction of stars with binaries, or stars with chromospheric emission, using synthetic spectroscopy. We studied how the position of stars is affected in the chromosome map. Results. Binaries can, in principle, explain the extension of the first population in the chromosome map. However, we find that given the binary fraction reported for GCs, the density of stars in the extended part is too small. Another difficulty of the binary explanation is that the shape of the distribution of the first population in the chromosome map is different in clusters with similar binary fractions. Also, the decrease of the binary fraction with radius is not mirrored in the shape of the chromosome map. Additionally, we find that the contribution of chromospheric emission lines to the HST photometry is too small to have an observable impact on the shape of the chromosome map. Continuum chromospheric emission has an effect qualitatively similar to binaries. Conclusions. We conclude that binaries do have an impact on the morphology of the chromosome map of GCs, but they are unlikely to explain entirely the shape of the extended distribution of the first population stars. Uncertainties in the properties of continuum chromospheric emission of stars in GCs prevent any quantitative conclusion. Therefore, the origin of the extended first population remains unexplained.

An extragalactic chromosome map: the intermediate-age SMC cluster Lindsay 1

ABSTRACT The discovery of star-to-star abundance variations (a.k.a. multiple populations – MPs) within globular clusters (GCs), which are generally not found in the field or in lower mass open clusters, has led to a search for the unique property of GCs that allow them to host this phenomenon. Recent studies have shown that MPs are not limited to the ancient GCs but are also found in massive clusters with ages down to (at least) 2 Gyr. This finding is important for understanding the physics of the MP phenomenon, as these young clusters can provide much stronger constraints (e.g. on potential age spreads within the clusters) than older ones. However, a direct comparison between ancient GCs and intermediate clusters has not yet been possible due to the different filters adopted in their studies. Here we present new Hubble Space Telescope UV photometry of the 7.5 Gyr, massive Small Magellanic Cloud cluster, Lindsay 1, in order to compare its pseudo colour–colour diagram to that of Galactic GCs. We find that they are almost identical and conclude that the MPs phenomenon is the same, regardless of cluster age and host galaxy.

Chemical inhomogeneities amongst first population stars in globular clusters

Spreads in light element abundances among stars (also known as multiple populations) are observed in nearly all globular clusters. One way to map such chemical variations using high-precision photometry is to employ a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters (called the “chromosome map”), to maximise the separation between the different multiple populations. For each individual cluster its chromosome map separates the first population (with metal abundance patterns typical of field halo stars) from the second population (which displays distinctive abundance variations among a specific group of light elements). Surprisingly, the distribution of first population stars in chromosome maps of several but not all clusters has been found to be more extended than expected from purely observational errors, suggesting a chemically inhomogeneous origin. We consider here three clusters with similar metallicity ([Fe/H] ~ −1.3) and different chromosome maps, namely NGC 288, M 3, and NGC 2808, and argue that the first population extended distribution (as observed in two of these clusters) is due to spreads of the initial helium abundance and possibly a small range of nitrogen abundances as well. The presence of a range of initial He and N abundances amongst stars traditionally thought to have homogeneous composition, and that these spreads appear only in some clusters, challenges the scenarios put forward so far to explain the multiple population phenomenon.