Formation of Carbon-Enhanced Metal-Poor RR Lyrae Stars |
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| Institution: | 1. School of Engineering Science, University of Chinese Academy of Science, Beijing 100049;2. Key Lab of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101;3. Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101;4. School of Artificial Intelligence, Beijing Normal University, Beijing 100875;1. National Time Service Center, Chinese Academy of Sciences, Xi’an 710600;2. Key Laboratory of Time and Frequency Primary Standards, National Time Service Center, Chinese Academy of Sciences, Xi’an 710600;3. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049;4. University of Chinese Academy of Sciences, Beijing 100049;1. School of Physics and Electronic Information Technology, Yunnan Normal University, Kunming 650500;2. Key Laboratory of High Energy Astrophysics of University of Yunnan Province, Kunming 650500;1. Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming 650011;2. University of Chinese Academy of Sciences, Beijing 100049;3. Department of Astronomy, Beijing Normal University, Beijing 100875;4. Center for Astronomical Mega-Science, Chinese Academy of Sciences, Beijing 100012;1. National Space Science Center, Chinese Academy of Sciences, Beijing 100190;2. University of Chinese Academy of Sciences, Beijing 100049;1. Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030;2. University of Chinese Academy of Sciences, Beijing 100049 |
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| Abstract: | Carbon-enhanced metal-poor (CEMP) stars are considered to be related to the first generation of stars, and responsible for the chemical evolution of the early Galaxy. More than half of them are in binaries, and could be explained by the binary evolution, but the formation channel of them is still not fully understood. Among the hundreds of CEMP stars, there are nine CEMP RR Lyrae stars identified, and at least seven of which are very likely not binaries. The usual binary star evolution channel is difficult to produce such a single star, particularly that of carbon enrichment. One way in which such a single star might be produced is the merger of a helium white dwarf with a Hertzsprung gap (HG) star. We use a stellar evolution program to calculate the models of the merger remnants, and find that the models can reproduce the observed distribution of these CEMP single RR Lyrae stars in terms of surface temperature, gravity, and carbon abundance. Hence, it is extremely possible that the helium white dwarf and HG star merger model is one of the formation channels of the metal-poor carbon-rich RR Lyrae stars. |
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| Keywords: | Stars: abundances Stars: variables: RR Lyrae methods: data analysis |
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