The SUMO project I. A survey of multiple populations in globular clusters

Abstract

We present a general overview and the first results of the SUMO project (a SUrvey of Multiple pOpulations in Globular Clusters). The objective of this survey is the study of multiple stellar populations in the largest sample of globular clusters homogeneously analysed to date. To this aim we obtained high signal-to-noise (S/N>50) photometry for main sequence stars with mass down to ∼0.5 M odot in a large sample of clusters using both archival and proprietary U, B, V, and I data from ground-based telescopes. In this paper, we focus on the occurrence of multiple stellar populations in twenty three clusters. We have defined a new photometric index c U,B,I = (U − B) − (B − I), that turns out to be very effective for identifying multiple sequences along the red giant branch (RGB). We found that in the V-c U,B,I diagram all clusters presented in this paper show broadened or multimodal RGBs, with the presence of two or more components. We found a direct connection with the chemical properties of different sequences, that display different abundances of light elements (O, Na, C, N, and Al). The c U,B,I index is also a powerful tool to identify distinct sequences of stars along the horizontal branch and, for the first time in the case of NGC 104 (47 Tuc), along the asymptotic giant branch. Our results demonstrate that i) the presence of more than two stellar populations is a common feature amongst globular clusters, as already highlighted in previous work; ii) multiple sequences with different chemical contents can be easily identified by using standard Johnson photometry obtained with ground-based facilities; iii) in the study of GC multiple stellar populations the c U,B,I index is alternative to spectroscopy, and has the advantage of larger statistics. ⋆ Based on observations made with the INT telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Based on observations made with MPG/ESO 2.2m telescope in La Silla, Chile, proposal 088.A-9012.†

Age as a major factor in the onset of multiple populations in stellar clusters

“…The CUBI pseudo-colour (defined as (U − B)−(B − I)) has been shown to be very effective at unveiling multiple sequences on the RGB (Dalessandro et al 2016;Monelli et al 2013). Here, we take advantage of a similar pseudo-colour, known as CUnBI ≡(U n−B)−(B−I), to provide a direct and quantifiable comparison between all the clusters in our survey.…”

The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)

“…These variations also include contributions not due to multiple populations but to the standard evolution of population II low-mass stars: the first dredge-up and the second mixing episode after the RGB bump both concur to enhance the N abundance, equally in first and second generation stars. From the study of Monelli et al (2013), at face value the changes in the abundances of C and N are comparable in extent for GCs, such as M 80 and M 4 and NGC 288.…”

“…However, also taking the physical limitations imposed by the fibres positioning tool into account, a more suitable approach to study the radial concentration of different stellar generations is offered by photometry of large samples over an extended area on the cluster. The combination (U − B) − (B − I), which Monelli et al (2013) call c U,B,I , is a good way to study the separation of stars according to different populations in a GC, since it does not significantly depend on reddening and distance modulus. There are 16 objects, including M 80, in common between the SUMO sample and our FLAMES survey.…”

“…However, improvements in precision photometry from ground-or space-based facilities produced colour-magnitude diagrams (CMDs) of GCs revealing more and more split SGBs, RGBs, and even multiple main sequences (e.g. Grundahl et al 1998;Han et al 2009;Lee et al 2009;Monelli et al 2013;Milone et al 2012Milone et al , 2013Nardiello et al 2015 and references therein). The synergy between spectroscopy and photometry allows the uncovering of how absorption features due to the involved light elements affect the colour indexes of split sequences (e.g.…”

“…Recently Monelli et al (2013) used private photometry from their SUMO project to study the colour spread of the RGB.…”