| 其他摘要 | In the 1990s, the discovery of the accelerating expansion of the Universe, using type Ia supernova(SN Ia) as a cosmological distance indicator, implied the presence of dark energy in the Universe and posed a great challenge to the understanding of fundamental physics. Despite the importance of SNe Ia, it is still not clear how such events came from (the progenitor problem), which cold hinder the development of precise cosmological research. It is widely accepted that a SN Ia originates from a thermonuclear runaway of a carbon-oxygen white dwarf (CO WD) in a binary system when its mass reaches its maximum stable limit. However, it is not clear how the mass of WD grows during this process. One of the most popular model is that a WD accretes material from a non-degenerate companion and burns the accreted material steadily on its surface to increase its mass, which is called the single-degenerate model. However, the exact process of the evolution of accreting WDs to SNe Ia is not yet clear. Previous studies have found that if the accretion rate is too high, it is difficult for an accreting WD to increase its mass steadily, and no SN Ia will occur. In order to overcome this difficulty, the optically thick wind (OTW) model was proposed, which suggests that an OTW will be driven on the surface of the WD and can automatically adjust the accretion rate to avoid the above problem. Therefore, the optically thick wind model is considered to be the physical basis for the ability of the single-degenerate model. However, in the last decades, many observations have contradicted the predictions of the OTW model, implying that OTW may not occurre in the progenitor of SNe Ia.The common-envelope wind (CEW) model is a new version of single-degenerate model to address the above shortcomings of the OTW model. This model suggests that the common envelope will undergo strong mass loss on its surface, resulting in a low envelope density and a long time for the binary to merge, so that the WD has enough time to increase its mass and then explode, as an SN Ia. However, it is not clear how the mass loss of the common envelope arises and what the observational characteristics of such systems are.In this paper, we perform detailed hydrodynamical simulations of the above two single-degenerate models using the stellar evolution code, MESA, to address the main problems encountered in these two models. Our main results are shown as follows:(1) We reconsider the conditions for the occurrence of the OTW. We find that the accreted material of the accreting WD can interacts with the outflow on the surface of the WD, and such interaction can effectively prevent the formation of the OTW. The results of 1D hydrodynamical simulations of such interaction show that the occurrence of optically thick star winds strongly depends on the accretion rates. At a sufficiently high accretion rate, it is difficult for an OTW to occur. This result provides an explaination to the contradiction between the OTW model and observations.%, and also raises new questions related to the study of progenitor model of SNe Ia.(2) We carry out detailed hydrodynamic simulations of CEW model and find 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, we find that such instability is driven by ionization-recombination processes of hydrogen and helium in the envelope, which is the same mechanism as the pulsating excitation of classical Cepheids. In the HR diagram, the center of the evolutionary trajectory of the CEW 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. |
修改评论