分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The mergers of double helium white dwarfs are believed to form isolated helium-rich hot subdwarfs. Observation shows that the helium-rich hot subdwarfs can be divided into two subgroups based on whether the surface is carbon-rich or carbon-normal. But it is not clear whether this distribution directly comes from binary evolution. We adopt the binary population synthesis (BPS) to obtain the population of single helium-rich hot subdwarfs according to the channel of double helium white dwarfs merger. We find that the merger channel can represent the two subgroups in the $T_{\rm{eff}}-\log g$ plane related to different masses of progenitor helium white dwarfs. For $Z$ = 0.02, the birth rates and local density of helium-rich hot subdwarf stars by the mergers of two helium white dwarfs is $\sim 4.82 \times 10^{-3}$ $\rm yr^{-1}$ and $\sim$ 290.0 $\rm kpc^{-3}$ at 13.7 Gyr in our Galaxy, respectively. The proportion of carbon-rich and carbon-normal helium-rich hot subdwarfs are 32$\%$ and 68$\%$, respectively.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Over 1500 DBZ or DZ white dwarfs (WDs) have been observed so far, and polluted atmospheres with metal elements have been found among these WDs. The surface heavy element abundances of known DBZ or DZ WDs show an evolutionary sequence. By using Modules for Experiments in Stellar Evolution, we create DB WDs, and simulate the element diffusion due to high gravitational fields and the metal-rich material accretion coming from the planet disrupted by the WD. In our models, the input parameters ($\alpha_{\rm MLT}$, $\alpha_{\rm th}$ and $Z$) have very weak effect on DB WD structures including interior temperatures, chemical profiles and convective zones.The mass-accretion rate and the effective temperature of DB WDs determine the abundances of heavy elements. The evolutionary sequence of Ca element for about 1500 observed DB or DBZ WDs cannot be explained by the model with a constant mass-accretion rate, but is consistent well with the model in which the mass-accretion rate decreases by one power law when $T_{\rm eff}>10$ kK and slightly increases by another power law when $T_{\rm eff}<10$ kK. The observed DB WD evolutionary sequence of heavy element abundances originates from WD cooling and the change of mass-accretion rate.
分类： 天文学 >> 恒星和银河系 提交时间： 2023-10-09
摘要： Symbiotic stars are interacting binary systems, making them valuable for studying various astronomical phenomena, such as stellar evolution, mass transfer, and accretion processes. Despite recent progress in the discovery of symbiotic stars, a significant discrepancy between the observed population of symbiotic stars and the number predicted by theoretical models. To bridge this gap, this study utilized machine learning techniques to efficiently identify new symbiotic stars candidates. Three algorithms (XGBoost, LightGBM, and Decision Tree) were applied to a dataset of 198 confirmed symbiotic stars and the resulting model was then used to analyze data from the LAMOST survey, leading to the identification of 11,709 potential symbiotic stars candidates. Out of the these potential symbiotic stars candidates listed in the catalog, 15 have spectra available in the SDSS survey. Among these 15 candidates, two candidates, namely V* V603 Ori and V* GN Tau, have been confirmed as symbiotic stars. The remaining 11 candidates have been classified as accreting-only symbiotic star candidates. The other two candidates, one of which has been identified as a galaxy by both SDSS and LAMOST surveys, and the other identified as a quasar by SDSS survey and as a galaxy by LAMOST survey.
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