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

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.

Hot Subluminous Stars

“…An alternative scenario explains the EHB and blue hook stars by the presence of a second generation of helium-enriched stars. Recently, three additional scenarios were added: (i) He white dwarf mergers (Han 2008), (ii) enhanced mass loss by rapid rotation (Tailo et al 2015), and (iii) tidally enhanced mass loss in binaries (Lei et al 2013a(Lei et al , 2013b(Lei et al , 2015.…”

“…However, asteroseismology of red giants showed that angular momentum transfer is a lot faster than previously assumed (Aerts 2015), which may challenge this scenario. Lei et al (2013aLei et al ( , 2013bLei et al ( , 2015 investigated the role of binary evolution for the formation of sdB stars in globular clusters by considering a tidally enhanced wind of a red giant star to cause the required huge mass loss. The model binary was chosen to consist of 0.85 M e star and a 0.53 M e companion.…”

Hot subdwarf Atmospheric parameters, Kinematics, and Origins -- Based on 1587 hot subdwarf stars observed in Gaia DR2 and LAMOST DR7

“…Hot subdwarf stars are interesting objects in many fields of astronomy. They are found in all galactic populations, and they are known to be the main source of the ultraviolet excess radiation of elliptical galaxies and the bulges of spiral galaxies (Han et al 2007), and also responsible for the extended horizontal branch morphology of globular clusters (Han 2008;Lei et al 2013Lei et al , 2015. Hypervelocity hot subdwarf stars allow us to probe the Galactic gravitational potential and put constraints on the mass of the Galactic dark matter halo Németh et al 2016).…”

BayeSED: A GENERAL APPROACH TO FITTING THE SPECTRAL ENERGY DISTRIBUTION OF GALAXIES

“…However, some important issues still remain. For example, some short lived but bright phases of stellar evolution such as the thermally pulsing asymp-totic giant branch, horizontal branch (Catelan 2009;Lei et al 2013) and blue stragglers (Tian et al 2006;Han et al 2007;Chen & Han 2009) are still not well understood, and they potentially have important effects on the resulting SEDs of galaxies. Furthermore, there are issues about the universality of the stellar initial mass function (Padoan et al 1997;Myers et al 2011;Dutton et al 2013), different parameterizations of the star formation history (Maraston et al 2010), the complex effects of interstellar medium (Calzetti et al 2000), the stochastic nature of stellar population modeling (Buzzoni 1993;Cerviño 2013), and any possible contribution from active galactic nuclei (Polletta et al 2007;Murphy et al 2009;.…”