Subdwarf B stars from the common envelope ejection channel

¹ Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, 650216 Kunming, PR China
e-mail: cxf@ynao.ac.cn; zhanwenhan@ynao.ac.cn
² Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, 650216 Kunming, PR China
³ Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, 100012 Beijing, PR China
⁴ University of Chinese Academy of Sciences, 100049 Beijing, PR China
⁵ Department of Astrophysics, University of Oxford, Oxford OX1 3RH, UK

Received: 29 August 2016
Accepted: 5 January 2017

Abstract

Context. Subdwarf B stars (sdB) are important to stellar evolutionary theory and asteroseismology, and they are crucial to our understanding of the structure and evolution of the Galaxy. According to the canonical binary scenario, the majority of sdBs are produced from low-mass stars with degenerate cores where helium is ignited in a way of flashes. Owing to numerical difficulties, the models of produced sdBs are generally constructed from more massive stars with non-degenerate cores. This leaves several uncertainties on the exact characteristics of sdB stars.

Aims. The purpose of this paper is to study the characteristics of sdBs produced from the common envelope (CE) ejection channel.

Methods. We used the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), which can resolve flashes during stellar evolution. To mimic the CE ejection process, we first evolved a single star to a position near the tip of the red giant branch, then artificially removed its envelope with a very high mass-loss rate until the envelope began to shrink. Finally, we followed the evolution of the remnant until it became a helium or a carbon-oxygen white dwarf.

Results. The sdB stars produced from the CE ejection channel appear to form two distinct groups on the effective temperature-gravity diagram. One group, referred to as the flash-mixing sdBs, almost has no H-rich envelope and crowds at the hottest temperature end of the extreme horizontal branch (EHB), while the other group, called the canonical sdBs, has significant H-rich envelope and is spread throughout the entire canonical EHB region. The key factor for the dichotomy of the sdB properties is the development of convection during the first helium flash, that is, that the convection region penetrates the H-rich envelope in the case of the flash-mixing sdBs, but fails to do this in the case of the canonical sdBs.

Conclusions. The dichotomy of the sdB properties from the CE ejection channel is intrinsic and caused by the interior structure of the star after the CE ejection. The modelling of the CE ejection process will greatly change the parameter space for the two typical groups of sdB stars. If the CE ejection stops early for a given initial stellar mass and a given core mass at the onset of the CE, then the star has a relatively massive H-rich envelope, which generally results in a canonical sdB. Observationally, only a few sdB binaries with short orbital periods are located in the flash-mixing sdB region, and there is a lack of He-rich sdBs in binaries with short orbital periods. This indicates that flash mixing is not very frequent in products of the CE ejection. A falling-back process after the CE ejection, similar to what occurs in nova, is an appropriate way of increasing the envelope mass, and it then prevents flash mixing.