Stellar models of multiple populations in globular clusters - I. The main sequence of NGC 6752

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Authors
Dotter, Aaron
Ferguson, Jason W.
Conroy, Charlie
Milone, A. P.
Marino, A. F.
Yong, David
Issue Date
2015-01-11
Type
Article
Language
en_US
Keywords
Stars: abundances , Stars: evolution , Globular clusters: individual: NGC 6752
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Abstract

We present stellar atmosphere and evolution models of main-sequence stars in two stellar populations of the Galactic globular cluster NGC 6752. These populations represent the two extremes of light-element abundance variations in the cluster. NGC 6752 is a benchmark cluster in the study of multiple stellar populations because of the rich array of spectroscopic abundances and panchromatic Hubble Space Telescope photometry. The spectroscopic abundances are used to compute stellar atmosphere and evolution models. The synthetic spectra for the two populations show significant differences in the ultraviolet and, for the coolest temperatures, in the near-infrared. The stellar evolution models exhibit insignificant differences in the Hertzsprung-Russell (H-R) diagram except on the lower main sequence. The appearance of multiple sequences in the colour-magnitude diagrams (CMDs) of NGC 6752 is almost exclusively due to spectral effects caused by the abundance variations. The models reproduce the observed splitting and/or broadening of sequences in a range of CMDs. The ultraviolet CMDs are sensitive to variations in carbon, nitrogen, and oxygen but the models are not reliable enough to directly estimate abundance variations from photometry. On the other hand, the widening of the lower main sequence in the near-infrared CMD, driven by oxygen variation via the water molecule, is well described by the models and can be used to estimate the range of oxygen present in a cluster from photometry. We confirm that it is possible to use multiband photometry to estimate helium variations among the different populations, with the caveat that the estimated amount of helium enhancement is model dependent.

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Dotter, Aaron; Ferguson, Jason W.; Conroy, Charlie; Milone, A. P.; Marino, A. F.; Yong, David. 2015. Stellar models of multiple populations in globular clusters - I. The main sequence of NGC 6752. Monthly Notices of the Royal Astronomical Society, vol. 446:no. 2:pp 1641-1656
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Oxford University Press
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0035-8711
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