大样本恒星演化研究组知识库

Context. It is believed that some other parameters, except for metallicity (the first parameter), are needed to explain the horizontal branch (HB) morphology of globular clusters (GCs). Furthermore, these parameters are considered to be correlated with the mass loss of the red giant branch (RGB) stars. Unfortunately, the physics of mass loss on the RGB is poorly understood at present. In our previous work, we proposed that tidally enhanced stellar wind during binary evolution may affect the HB morphology by enhancing the mass loss of the red giant primary and that we can reproduce the basic morphology of HB in GCs. 

Aims. We did not consider the effects of other important parameters (e.g., metallicity and age) in our final results there. As a further study, we now investigate the effects of metallicity and age on HB morphology by considering tidally enhanced stellar winds during binary evolution. 

Methods. We incorporated the tidally enhanced-stellar-wind model of Tout & Eggleton into Eggleton's stellar evolution code to study the binary evolution. A group of binary system samples were generated by Monte Carlo simulations. The position of each sample star in a color-magnitude diagram was obtained by transforming temperature and luminosity into B - V color and absolute magnitude. To study the effects of metallicity and age on our final results, we conducted two sets of model calculations: (i) for a fixed age, we used three metallicities, namely Z = 0.0001, 0.001 and 0.02; (ii) for a fixed metallicity, Z = 0.001, we used five ages in our model calculations: 14, 13, 12, 10, 7 Gyr. 

Results. We found that HB morphology of GCs becomes bluer with decreasing metallicity, and old GCs present bluer HB morphology than young ones. These results are consistent with previous work. Although the envelope-mass distributions of zero-age HB stars produced by tidally enhanced stellar wind are similar for different metallicities, the synthetic HB under tidally enhanced stellar wind for Z = 0.02 presented a distinct gap between red and blue HB. However, this feature was not seen clearly in the synthetic HB for Z = 0.001 and 0.0001. We also found that higher binary fractions may make HB morphology become bluer, and we discuss the results with recent observations.