Recently, Ph.D. candidate CUI Yingzhen and Prof. MENG Xiangcun, from Yunnan Observatories of the Chinese Academy of Sciences (CAS), performed hydrodynamic simulations on the common-envelope wind model of type Ia supernovae (SNe Ia), and revealed the mass loss mechanism and the main observational features of white dwarf binaries in the common-envelope wind phase, which provide theoretical guidance for the subsequent observational search. This result was published in Astronomy & Astrophysics.

SNe Ia are some of the most energetic events in the Universe and play an important role in many astrophysical fields. They are used as cosmological distance indicators, which have led to the discovery of the accelerating expansion of the Universe. However, its progenitor system is still unclear. One of the most popular progenitor models of SNe Ia is the single-degenerate model, in which a carbon-oxygen white dwarf accretes material from a non-degenerate companion star to increase its mass, and eventually undergoes a thermonuclear explosion. The problem with this model is that when the mass transfer rate exceeds a certain critical value, the accreted envelope of the white dwarf expands and eventually forms a common envelope around the binary system. It is believed that such common envelope can lead the binary to merge rapidly and therefore prevents the occurrence of SNe Ia.

The common-envelope wind model is a modified model to address the above-mentioned shortcomings of the single-degenerate model. This model suggests that a strong mass loss occurs at the surface of the common envelope, resulting in a lower density of the envelope and therefore a longer timescale for the binary to merge. As a result, the white dwarf has enough time to accumulate material for an SN Ia explosion to occur. However, it is not clear how the mass loss at the surface of the common envelope arises and what the observational characteristics of such systems are.

The researchers carried out detailed hydrodynamic simulations of common-envelope wind model and found that such systems are always dynamically unstable and consequently produce dramatic mass loss, resulting in an envelope mass of only a few thousands of solar mass. By analyzing the internal structure, they found that this instability is driven by ionization-recombination processes of hydrogen and helium in the envelope, the same mechanism as the pulsating excitation of classical Cepheids. In the HR diagram, the center of the evolutionary trajectory of the common-envelope wind model is also located within the classical Cepheid instability strip, implying that this system may appear as periodic variable stars. This result can provide theoretical guidance for the subsequent observational search for progenitor system of SNe Ia.

Contact:
CUI Yingzhen
Yunnan Observatories, CAS
Email: cuiyingzhen@ynao.ac.cn