http://arxiv.org/abs/1706.05454
We present a new set of horizontal-branch (HB) models computed with the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code. They are used in our HB population synthesis tool to generate theoretical distributions of HB stars in order to describe the multiple stellar populations in the globular clusters 47Tuc, M3, and M13. The observed HB in 47Tuc can be reproduced very well by our simulations for [Fe/H] $= -0.70$ and [$\alpha$/Fe] $= +0.4$ if the initial helium mass-fraction abundance varies by $\Delta Y_0\sim 0.03$ and approximately 21%, 37%, and 42% of the stars have $Y_0 = 0.257$, $0.270$, and $0.287$, respectively. These simulations yield $(m-M)_V = 13.27$, implying an age near 13.0 Gyr. In the case of M3 and M13, our synthetic HBs for [Fe/H] $= -1.55$ and [$\alpha$/Fe] $= 0.4$ match the observed ones quite well if M3 has $\Delta Y_0 \sim 0.01$ and $(m-M)_V = 15.02$, resulting in an age of 12.6 Gyr, whereas M13 has $\Delta Y_0 \sim 0.08$ and $(m-M)_V = 14.42$, implying an age of 12.9 Gyr. Mass loss during giant-branch evolution and $\Delta Y_0$ appear to be the primary second parameters, as differences in these quantities are able to explain the very different HB morphologies of M3 and M13, which have very similar ages and metallicities. New observations for 7 of the 9 known RR Lyrae in M13 are also reported. Periods predicted for the $c$-type variables tend to be too high (by up to $\sim 0.1$ d), but we are unable to provide a satisfactory explanation of this discrepancy.
P. Denissenkov, D. VandenBerg, G. Kopacki, et. al.
Tue, 20 Jun 17
47/72
Comments: 26 pages, 21 figures, 5 tables, submitted to ApJ
You must be logged in to post a comment.