Photometric signatures of multiple stellar populations in Galactic globular clusters

Abstract

We calculated synthetic spectra for typical chemical element mixtures (i.e., a standard α-enhanced distribution, and distributions displaying CN and ONa anticorrelations) found in the various subpopulations harboured by individual Galactic globular clusters. From the spectra we determined bolometric corrections to the standard Johnson-Cousins and Strömgren filters and finally predicted colours. These bolometric corrections and colour-transformations, coupled to our theoretical isochrones with the appropriate chemical composition, provided us with a complete and self-consistent set of theoretical predictions for the effect of abundance variations on the observed cluster colour-magnitude diagrams. CNO abundance variations affect mainly wavelengths shorter than ∼400 nm owing to the rise of molecular absorption bands in cooler atmospheres. As a consequence, colour and magnitude changes are largest in the blue filters, independently of using broad or intermediate bandpasses. Colour-magnitude diagrams involving uvy and UB filters (and their various possible colour combinations) are therefore best suited to infer photometrically the presence of multiple stellar generations in individual clusters. They are particularly sensitive to variations in the N abundance, with the largest variations affecting the red giant branch (RGB) and lower main sequence (MS). BVI diagrams are expected to display multiple sequences only if the different populations are characterized by variations of the C+N+O sum and/or helium abundance that lead to changes in luminosity and effective temperature, but leave the flux distribution above 400 nm practically unaffected. A variation of just the helium abundance up to the level we investigate here exclusively affects the interior structure of stars, and is largely irrelevant for the atmospheric structure and the resulting flux distribution in the whole wavelength range spanned by our analysis.

NGC 6362: The Least Massive Globular Cluster With Chemically Distinct Multiple Populations*

“…In contrast, in the (V, V -I) CMD the average color spread of RGB and SGB stars is fully compatible with the photometric errors. Theoretical models demonstrate (Sbordone et al 2011) that this behavior is the photometric signature of the CNONa chemical anomalies.…”

The HST large programme on ω Centauri – I. Multiple stellar populations at the bottom of the main sequence probed in NIR–Optical

“…Sbordone et al 2011;Milone 2015;Dotter et al 2015). In contrast, the effect of light elements strongly affects the flux of fainter than the knee stars in the optical and near-infrared bands used in this paper.…”

“…Indeed a C+N+O variation produces a marginal color difference for MS stars (e.g. Cassisi et al 2008;Ventura et al 2009;Sbordone et al 2011). …”

Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models

“…This shortcoming of the current generation of α-enhanced SSP models can be understood when taking properly into account the phenomenon of Multiple Populations (MPs) in Galactic GCs (see Gratton generations, the first one born with a canonical α-enhanced heavy element distribution, the second generation characterised by anti-correlated CN-ONa abundance patterns, and by moderate (and -only -in a few cases very large) He enhancement. Due to the impact on the SED of molecular bands associated with both C and N (see the detailed discussion made by Sbordone et al 2011 and, one might anticipate that the light-element (anti-) correlations characteristic of the second generation stars in Galactic GCs will affect the strength of specific indices such as CN1 and CN2. This issue has been explored by Coelho, Percival & Salaris (2011) given certain assumptions 13 about the chemical abundance patterns of second 13 They adopted extreme -although still realistic -light-element chemical patterns in order to maximise the possible expected impact of these chemical peculiarities on SSP modes.…”