Effect of Binary Fraction on Horizontal Branch Morphology under Tidally Enhanced Stellar Wind

TL;DR

This study investigates how varying binary star fractions influence the horizontal branch morphology in globular clusters, revealing that higher binary fractions tend to produce bluer HB morphologies when metallicity and age effects are controlled.

Contribution

It demonstrates that binary fraction affects HB morphology independently of metallicity and age, providing a clearer understanding of its role as a second parameter in globular clusters.

Findings

HB becomes bluer with higher binary fractions when metallicity and age are fixed.

Results align with observations for metal-rich and metal-poor GCs.

A weak trend is observed in intermediate-metallicity GCs.

Abstract

Tidally enhanced stellar wind may affect horizontal branch (HB) morphology in globular clusters (GCs) by enhancing the mass loss of primary star during binary evolution. Lei et al. (2013a, 2013b) studied the effect of this kind of wind on HB morphology in details, and their results indicated that binary is a possible secondparameter (2P) candidate in GCs. Binary fraction is an very important fact in the tidally-enhanced-stellar-wind model. In this paper, we studied the effect of binary fraction on HB morphology by removing the effects of metallicity and age. Five different binary fractions (i.e., 10%, 15%, 20%, 30% and 50%) are adopted in our model calculations. The synthetic HB morphologies with different binary fractions are obtained at different metallicities and ages. We found that, due to the great influence of metallicity and age, the effect of binary fraction on HB morphology may be masked by these two parameters. However, when the effects of metallicity and age are removed, the tendency that HB morphologies become bluer with increasing of binary fractions is clearly presented. Furthermore, we compared our results with the observation by Milone et al. (2012). Our results are consistent well with the observation at metalrich and metal-poor GCs. For the GCs with intermediate metallicity, when the effect of age on HB morphology is removed, a weak tendency that HB morphologies become bluer with increasing of binary fractions is presented in all regions of GCs, which is consistent with our results obtained in this metallicity range.