Project duration: 1 year. Potential to grow into a thesis project (see below)
PI: Dr. Annalisa Calamida (INS/WFC3), Office: RW303, Email: calamida@stsci.edu, Phone: 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 (red-giant, and horizontal branch) 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.
Fig. 1: Left - HST F814W, F475W - F814W color-magnitude diagram of NGC2808 from Milone et al. (2012, A&A, 537, A77). Right - Hess diagram for the MS region (indicated with a grey box on the left). The three main sequences, blue, intermediate and red, are clearly separated from about the cluster turn-off level at F814W ~ 18.5 down to 22 mag.
Fig. 2: DECam g-i vs u-r vs r color-color-magnitude diagram of Omega Cen member (multiple color dots) and field stars (gray dots) seen from the front (top panel) and the back (bottom). This approach can be applied to separate field and cluster stars thanks to the opportunity to use the u filter, since this band allows a better sensitivity to both effective temperature and metallicity. We will apply this method to separate cluster and field stars in NGC2808. This figure is taken from Calamida et al. (2017, AJ, 153, 175).
Fig. 3: Left: DECam g, g-i color-magnitude diagram of Omega Cen member stars. Right: Zoom of the g, g-i color-magnitude diagram showing the split of Omega Cen main-sequence. It was previously identified only with HST and VLT photometry of the central regions of the cluster. We will identify the different main sequences in NGC2808 by using our DECam data for this cluster. This figure is taken from Calamida et al. (2017, AJ, 153, 175).
Fig. 4: DECam footprint (blue) centered on NGC2808. The yellow square indicates the field of view of the ground-based + HST photometric catalog used in the study of Iannicola et al. (2009, ApJ, 696, L120). We will combine these datasets to DECam photometry to study the property of the different multiple populations across NGC2808.
The student project
DECam data will be reduced by the student, in collaboration with the PI, Dr. Annalisa Calamida (STScI), Dr. Armin Rest (STScI, JHU), and the grad student Giovanni Strampelli (STScI, IAC), using our available pipelines at STScI. DECam photometry will be then combined to the publicly available HST catalog for NGC2808. The merged multi-band deep photometric catalog covering the entire extent of NGC2808 will be used to study the radial distribution of the different multiple stellar populations in the cluster, following the same process adopted in Calamida et al. (2017, AJ, 153, 175).
Numerical simulations will be produced by collaborators Dr. Alessandra Mastrobuono-Battisti (MPIA - Heidelberg) and Dr. Alice Zocchi (ESA-ESTEC) and compared to the observations. The interpretation of the data and models will be done by all the members of the team, and the student will be in charge of writing the paper illustrating the results with the help of the PI. The student will be the first author of the publication.
The main scientific goals we plan to accomplish in this project are:
- Identify NGC2808 three main sequences (MSs) in our merged DECam + HST photometric catalog to characterize their spatial distribution from the core to the tidal radius. In particular, we are interested in investigating if the three MSs, supposedly with different helium abundances, show a similar spatial distribution or if the bluer MS stars are more or less concentrated compared to the red MS stars;
- Characterize the spatial distribution of NGC2808 horizontal branch (HB) stars and eventually confirm previous results from Iannicola et al. (2009), where different components of the HB show a flat radial trend across the cluster;
- Compare observations with numerical simulations to determine the origin and the dynamical evolution of NGC2808.
Possibility of the project to grow into a thesis
DECam u,g,r,i-band images for other GGCs (47 Tuc, NGC362, NGC288, NGC6752, etc) are available, together with public HST data, and similar studies could be performed on these clusters after the project on NGC2808 is over. We selected NGC2808 to start this project because it is the second more massive GGC after Omega Cen and presents both similar properties, such as the main sequence split (compare Fig. 1 and 3) and the extended horizontal branch, and different ones, such as the iron spread. These two stellar systems still constitute an open puzzle that once solved, as we plan to do with this project, will enable us to make a step forward in the understanding of the build-up of GGCs.
Fig. 5: DECam g, u-g color-magnitude diagram for the globular cluster 47 Tuc. Note the Small Magellanic Cloud in the background.
Fig. 6: DECam g, u-g color-magnitude diagram for the globular cluster NGC362. Note the Small Magellanic Cloud in the background.
