Philipp Podsiadlowski

Abstract:

Since the majority of massive stars are members of binary systems, an understanding of the intricacies of binary interactions is essential for understanding the large variety of supernova types and sub-types. I therefore briefly review the basic elements of binary evolution theory and discuss how binary interactions affect the presupernova structure of massive stars and the resulting supernovae. SN 1987A was a highly anomalous supernova, almost certainly because of a previous binary interaction. The most likely scenario at present is that the progenitor was a member of a massive close binary that experienced dynamical mass transfer during its second red-supergiant phase and merged completely with its companion as a consequence. This can naturally explain the three main anomalies of SN 1987A: the blue color of the progenitor, the chemical anomalies and the complex triple-ring nebula.

Abstract:

Context

Subdwarf B stars (sdB) are important to stellar evolutionary theory and asteroseismology, and are crucial to our understanding of the structure and evolution of the Galaxy. From 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. Due 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 use 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 firstly evolve a single star to a position near the tip of red giant branch, then artificially remove its envelope with a very high mass loss rate until the star begins to collapse. Finally, we followed the evolution of the remnant until it becomes 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 as the flash-mixing sdBs, almost has no H-rich envelope and crows at the hottest temperature end of the extremely horizontal branch (EHB), while the other group, called as the canonical sdBs, has significant H-rich envelope and spreads over the whole canonical EHB region. The key factor for the dichotomy of the sdB properties is the development of convection during the first helium flash, i.e. the convection region penetrates into the H-rich envelope for the flash-mixing sdBs but doesnot for 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 change the parameter spaces much for the two typical groups of sdB stars. For a given initial stellar mass and a given core mass at the onset of the CE, if the CE ejection stops early, the star has a relatively massive H-rich envelope, resulting in a canonical sdB generally. The fact of only a few short-orbital-period sdB binaries being in the flash-mixing sdB region and the lack of He-rich sdBs in short-orbital-period binaries indicate that the flash mixing is not very often in the products of the CE ejection. A falling back process after the CE ejection, similar to that happened in nova, is an appropriate way of increasing the envelope mass, then prevents the flash mixing.