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 (1.38 M _☉).

^1,2P. Woitke, ^1,2,5Ch. Helling, ^1,2G. H. Hunter, ^1,2J. D. Millard, ^1,2G. E. Turner, ^1,2M. Worters, ³J. Blecic, ⁴J. W. Stock
Astronomy & Astrophysics 614, A1 Link to Article [https://doi.org/10.1051/0004-6361/201732193]
¹SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
²Centre for Exoplanet Science, University of St Andrews, St Andrews, UK
³New York University Abu Dhabi, Abu Dhabi, United Arab Emirates
⁴Department of Chemistry and Environmental Science, Medgar Evers College – City University of New York, 1650 Bedford Avenue, Brooklyn, NY 11235, USA
⁵Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

We currently do not have a copyright agreement with this publisher and cannot display the abstract here