Models for metal-poor stars with different initial abundances of C, N, O, Mg, and Si. II. Application to the colour–magnitude diagrams of the globular clusters 47 Tuc, NGC 6362, M 5, M 3, M 55, and M92

Author:

VandenBerg Don A1,Casagrande Luca2ORCID,Edvardsson Bengt3

Affiliation:

1. Department of Physics and Astronomy, University of Victoria, P.O. Box 1700, STN CSC, Victoria, BC, Canada V8W 2Y2

2. Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, The Australian National University, Canberra, ACT 2611, Australia

3. Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden

Abstract

ABSTRACT Stellar models for −2.5 ≤ [Fe/H] ≤−0.5 that have been computed for variations in the C:N:O abundance ratio (for two different values of [CNO/Fe]) are compared with HST Wide Field Camera 3 (WFC3) observations of the globular clusters (GCs) 47 Tuc, NGC 6362, M 5 M 3, M 55, and M 92. The bolometric corrections (BCs) used to transpose the models to the observed planes are based on new MARCS synthetic spectra that incorporate improved treatments of molecules that involve atoms of C, N, and O. On the assumption of well-supported distance moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the other α elements, which are favoured by binary stars in GCs, generally reproduce the main features of observed colour–magnitude diagrams (CMDs) to within ∼0.03 mag. In particular, they appear to match the spreads in the observed (MF336W − MF438W)0 colours that are spanned by CN-weak and CN-strong stars along the lower giant branch quite well, but not the bluest giants, which are suspected to be N-poor ([N/Fe] ≲ −0.5). Both the absolute (MF438W − MF606W)0 colours and the variations in these colours at a given MF606W magnitude on the giant branch are difficult to explain unless the reddest stars are C-rich ([C/Fe] ≳ +0.5). Allowing for moderate He abundance variations ($\delta \, Y \sim 0.05$) improves the fits to the observations.

Funder

ARC

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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1. A Comparative Study between M30 and M92: M92 is a Merger Remnant with a Large Helium Enhancement;The Astrophysical Journal;2024-01-30

2. Stellar Neutrino Emission across the Mass–Metallicity Plane;The Astrophysical Journal Supplement Series;2023-12-19

3. Zero-age horizontal branch models for −2.5 ≤ [Fe/H] ≤−0.5 and their implications for the apparent distance moduli of globular clusters;Monthly Notices of the Royal Astronomical Society;2023-11-10

4. Isochrone fitting of Galactic globular clusters – V. NGC 6397 and NGC 6809 (M55);Monthly Notices of the Royal Astronomical Society;2023-10-13

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