Production of Silicon on Mass-increasing White Dwarfs: Possible Origin of High-velocity Features in Type Ia Supernovae

Mariko Kato1, Hideyuki Saio2, and Izumi Hachisu3
The Astrophysical Journal 863, 125 Link to Article [https://doi.org/10.3847/1538-4357/aad327]
1Department of Astronomy, Keio University, Hiyoshi, Yokohama 223-8521, Japan
2Astronomical Institute, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan
3Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however, the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon–oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 M , 1.2 M , and 1.35 M CO WDs accreting helium-rich matter with several mass-accretion rates, ranging from 1 × 10−7 M yr−1 to 7.5 × 10−7 M yr−1. For the 1.35 M WD with the accretion rate of 1.6 × 10−7 M yr−1, the surface layer developed as helium burning ash and consisted of 40% 24Mg, 33% 12C, 23% 28Si, and a few percent of 20Ne by weight. For a higher mass-accretion rate of 7.5 × 10−7 M yr−1, the surface layer consisted of 58% 12C, 31% 24Mg, and 0.43% 28Si. For the 1.2 M WDs, silicon is produced only for lower mass-accretion rates (2% for 1.6 × 10−7 M yr−1). No substantial silicon (<0.07%) is produced on the 1.0 M WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si ii HVFs, its characteristics are consistent with that of mass-increasing WDs. We also discuss possible Ca production on very massive WDs (gsim1.38 M ).

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