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Nov 4, 2012

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Page maintened by Matteo Monelli
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This page is hosted by the IAC. A list of projects can be found here.

SUMO - the project

SUMO is a multi-wavelength, wide-field photometric survey aimed at discovering and characterizing the multiple populations phenomenon in globular clusters

Contrary to the classical paradigm, globular clusters (GCs) are now known to host not a single, but multiple stellar populations of different chemical composition. This fact is substantiated by both photometric and spectroscopic studies. In particular, the HST - Hubble Space Telescope - provided strong photometric clues that the phenomenon of multiple sequences is not confined to the unusual cluster Omega Centauri: Multiple main sequences (MSs) have been detected in NGC 2808, NGC 6752, and 47 Tucanae and multiple sub-giant branches are present in NGC 1851, M22 and at least six other GCs (see Piotto 2009 for a review). Most of these results come from the analysis of the homogeneous data-set collected within the HST/ACS Survey of Galactic GCs (GO-10775, PI. Sarajedini). This is a Treasury program designed to obtain high-accuracy photometry in the central regions of 66 GCs. Moreover, spectroscopy revealed that the star-to-star variations in light elements (e. g. C, N, O, Na) are mainly associated with different generations of stars, and are not due to evolutionary effects (e. g. Gratton et al. 2001). Driven by the HST results, the study of multiple stellar populations has been extended also to ground-based photometry, where the Johnson U filter turned out to be especially suited to identify multiple stellar sequences in the color-magnitude diagram (CMD), as it is very sensitive to the CN abundance. A direct connection between multiple photometric sequences and the chemical content of stellar populations was found in the RGB stars of M4 (Marino et al. 2008). Two groups of stars with different C, N, Na, and O have been identified, which exhibit different (U - B) colors: Na-poor (CN-weak) giants define a bluer sequence than the one populated by the Na-rich (CN-strong) ones. Multiple RGBs populated by stars with different chemical properties have been found in many other clusters (e. g. Yong et al. 2008).

On the theoretical side, stellar isochrones suggest that multiple MSs should be associated with stars of different He abundance, with the bluer sequences being He-enhanced with respect to the primordial He content (e. g. Norris 2004). Salaris et al. (2006) and Pietrinferni et al. (2009) computed evolutionary stellar models and, hence cluster isochrones - for chemical mixtures accounting for the CN- and ONa-anticorrelations, characteristic of second stellar generations. They demonstrated that in the Johnson-Cousins filters V and I only an extreme He enhancement leads to a significant colour change as compared to a standard population II mixture. Cassisi et al. (2008) used these isochrones to explain the split SGB in NGC 1851 as a consequence of enhanced C+N+O abundances. Therefore, both the spectroscopic and the photometric evidence for multiple populations are intimately connected to the same physical mechanism. However, the observational scenario is complicated by the variety of CMD features and chemical variations found in different GCs, and the origin and early evolution of multiple populations are still controversial (Renzini 2008). Moreover, the different populations might have experienced quite a different dynamical evolution. In particular, recent models by D'Ercole et al. (2008) predict that a large fraction (up to 90% or more) of first generation stars should be lost early in the GC evolution because of the expansion and stripping of the cluster outer layers resulting from the mass loss consequent to the massive SNe explosions. The fact that first generation stars were much more numerous at the time of the cluster formation can also account for the polluting material needed to explain the He enrichment of the second (and third) generations. Signatures of the origin and evolution of the single populations still remains in their present day radial distribution, and a systematic study of the radial gradients provides us with important constraints to understand the nature of the polluters and the processess which brought to the formation of successive stellar generations. The combination of available HST data for the central (crowded) regions and the proposed wide field images will be the basic tool for the radial distribution study of the multiple populations, and one of the main targets of the present project.

We propose here a systematic and uniform investigation of the photometric properties of a large number of GCs. This, coupled with a comparison with theoretical isochrones which take into account the full extent of abundance variations, is strongly required to explain the nature of multiple stellar populations in GCs. Recently, members of our team (Sbordone et al. 2011), developed a self-consistent grid of isochrones computed by accounting for peculiar chemical abundances patterns which can be considered representative of those observed in the galactic GC sample. While previous isochrones treated the actual chemical composition consistently only in the stellar interior models, our new isochrones have been computed with new bolometric corrections and colours derived from synthetic spectra with chemical compositions typical of both first and second generations of stars found in GCs. This is a fundamental advantage because the comparison of our models with the observed CMDs will allow us to estimate, for the first time, the difference in Helium content and overall C+N+O abundance for the various stellar subpopulations from photometry.

The main objectives of this project are:

  1. By using U-band photometry, the identification of multiple stellar populations by searching for a split or broadening of the red and sub-giant branches.
  2. The wide field of view of the INT/WFC will allow the analysis of the spatial distribution of different stellar populations in GCs. These will provide fundamental test to the hydrodynamical models of GC evolution (e. g. D'Ercole et al. 2008).
  3. The new set of theoretical isochrones (Sbordone et al. 2011) will allow us to discriminate in the CMD the multiple stellar populations with different helium and light elements (C, N, O, Na) abundances in this sample of GCs.
  4. Four bands photometry will permit to estimate the effect of differential reddening, if present, following the procedure described in Milone et al. (2009).
  5. The study of blue stragglers and binaries will complement similar works on the same GCs based on HST data and limited to the cluster center (e. g. Milone et al. 2008).
  6. The study of the luminosity function will be performed, investigating any dependence with the distance from the cluster center.

List of clusters: