Project duration: 1 year. Potential to grow into a thesis project (see below)
PI: Annalisa Calamida (INS/WFC3), RW303, 667 218 6428
Galactic globular clusters (GGCs) have long been considered the most simple stellar populations, with their stars located at the same distance, with typically the same extinction, and the same bona fide age and chemical composition. However, during the last two decades, high-resolution spectroscopic studies have shown that most GGCs display a large spread of light elements and anti-correlations.This evidence suggests the presence of more than one generation of stars in these systems, with the younger generation born from a stellar medium enriched by the material ejected by the previous one.
Thanks to the high spatial resolution and depth of Hubble Space Telescope (HST) imaging, it was possible to discover different sequences in the color-magnitude diagrams (CMDs) of several GGCs. A stunning case is Omega Cen, the most massive GGC, where at least 15 stellar sub-populations were identified. However, Omega Cen is very different from all other GGCs, since it shows an iron spread of more than 1 dex and dispersions of most of the heavy elements and a possible age and helium spread. Omega Cen is then thought to be the nucleus of a dwarf galaxy accreted by the Milky Way or the result of the merger of two clusters.
NGC2808 is the second most massive Galactic globular cluster and a very peculiar object as well. HST photometric studies showed that the cluster main-sequence splits in a blue, an intermediate and a red sequence (see Fig. 1) and that stars on the intermediate and blue main sequences are more centrally concentrated compared to stars on the red main-sequence. It was suggested that NGC2808 experienced different episodes of star formation with a significant helium enrichment, with the bluest main-sequence being the most enhanced. However, high-resolution spectroscopic measurements of red-giant stars indicate that, while expected in the case of different star formation episodes, no spread in iron content is present in NGC2808.
A consensus on the origin of the different stellar populations in NGC2808 has not been reached yet. Previous findings are based on data for a few small fields centered on the cluster (HST) or for a field of view covering about half the tidal radius (ground). There is now the need for deep photometry covering the entire cluster (tidal radius rt ~ 16 arcmin)
with the accuracy necessary to enable the identification of the different main sequences. This photometric catalog would allow the homogenous study of the multiple populations in NGC2808 and possibly a clear understanding of the origin and evolution of this peculiar globular cluster.
We have proprietary wide-field DECam (mounted on the 4m-Blanco telescope, NOAO) observations that cover the entire extent of NGC2808 and so will enable us to characterize the spatial distribution of the main-sequence (and red-giant) stars from the core to the tidal radius and obtain a clear picture of the evolutionary history of the cluster. The observations will then be compared to numerical simulations performed by our collaborators to characterize the formation and evolutionary history of this complex cluster.
