Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters

¹ National Astronomical Observatories/Yunnan Observatory, the Chinese Academy of Sciences, Kunming 650011, PR China
e-mail: lzx2008@ynao.ac.cn; cxf@ynao.ac.cn; zhangfh@ynao.ac.cn; zhanwenhan@ynao.ac.cn
² Key Laboratory for the Structure and Evolution of Celestial Objects, the Chinese Academy of Sciences, Kunming 650011, PR China
³ University of the Chinese Academy of Sciences, Beijing 100049, PR China

Received: 16 August 2012
Accepted: 28 November 2012

Abstract

Context. Metallicity is the first parameter to influence the horizontal branch (HB) morphology of globular clusters (GCs). It has been found, however, that some other parameters may also play an important role in affecting the morphology. While the nature of these important parameters remains unclear, they are believed to be likely correlated with wind mass-loss of red giants, since this mass loss determines their subsequent locations on the HB. Unfortunately, the mass loss during the red giant stages of the stellar evolution is poorly understood at present.

Aims. The stellar winds of red giants may be tidally enhanced by companion stars if they are in binary systems. We investigate the evolutionary consequences of red giants in binaries by including tidally enhanced stellar winds, and examine the effects on the HB morphology of GCs.

Methods. We used Eggleton’s stellar evolution code to study the binary evolution. The tidally enhanced stellar-wind model of Tout & Eggleton is incorporated into this code, where the tidal enhancement parameter, B_w, has various values (e.g., 10 000 and 500) to examine the dependency of the final results on this parameter. A Monte Carlo simulation was performed to generate a group of binary systems. The position of each primary star on the HB in the Hertzsprung-Russell diagram in this sample is obtained through interpolations among the constructed HB evolutionary tracks. Finally, a synthetic HB in the color–magnitude diagram is obtained by transforming the effective temperature and luminosity of each primary star on the HB into B − V colors and absolute magnitude.

Results. We find that red, blue, and extreme horizontal branch stars are all produced under the effects of tidally enhanced stellar wind without any additional assumptions on the mass-loss dispersion. Furthermore, the horizontal branch morphology is found to be insensitive to the tidal enhancement parameter, B_w. We compare our theoretical results with the observed horizontal branch morphology of globular cluster NGC 2808, and find that the basic morphology of the horizontal branch can be well reproduced. The number of blue horizontal branch stars in our calculations, however, is lower than that of NGC 2808.