An Asymmetric Double-Degenerate Type Ia Supernova Explosion with a Surviving Companion Star

Matthew R. Siebert; Ryan J. Foley; Yossef Zenati; Georgios Dimitriadis; Eva Schmidt; Grace Yang; Kyle W. Davis; Kirsty Taggart; César Rojas-Bravo

arXiv · 2306.11788

An Asymmetric Double-Degenerate Type Ia Supernova Explosion with a Surviving Companion Star

Matthew R. Siebert·Ryan J. Foley·Yossef Zenati·Georgios Dimitriadis·Eva Schmidt·Grace Yang·Kyle W. Davis·Kirsty Taggart·César Rojas-Bravo

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Abstract

We present nebular spectroscopy of SN 2020hvf, a Type Ia supernova (SN Ia) with an early bump in its light curve. SN 2020hvf shares many spectroscopic and photometric similarities to the carbon-rich high-luminosity "03fg-like" SNe Ia. At $>$240 days after peak brightness, we detect unambiguous emission from [Ca II] $\lambda\lambda$7291, 7324 which is never observed in normal-SNe Ia and only seen in peculiar subclasses. SN 2020hvf displays "saw-tooth" emission profiles near 7300 A that cannot be explained with single symmetric velocity components of [Fe II], [Ni II], and [Ca II], indicating an asymmetric explosion. The broad [Ca II] emission is best modeled by two velocity components offset by 1,220 km s$^{-1}$, which could be caused by ejecta associated with each star in the progenitor system, separated by their orbital velocity. For the first time in a SN Ia, we identify narrow (${\rm FWHM} = 180\pm40$ km s$^{-1}$) [Ca II] emission, which we associate with a wind from a surviving, puffed-up companion star. Few published spectra have sufficient resolution and signal-to-noise ratio necessary to detect similar narrow [Ca II] emission, however, we have detected similar line profiles in other 03fg-like SNe Ia. The extremely narrow velocity width of [Ca II] has only otherwise been observed in SNe Iax at late times. Since this event likely had a double-degenerate "super-Chandrasekhar" mass progenitor system, we suggest that a single white dwarf (WD) was fully disrupted and a wind from a surviving companion WD is producing the observed narrow emission. It is unclear if this unique progenitor and explosion scenario can explain the diversity of 03fg-like SNe Ia, potentially indicating that multiple progenitor channels contribute to this subclass.

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Matthew R. Siebert, Ryan J. Foley, Yossef Zenati, Georgios Dimitriadis, Eva Schmidt, Grace Yang, Kyle W. Davis, Kirsty Taggart, César Rojas-Bravo. 2023-06-20. An Asymmetric Double-Degenerate Type Ia Supernova Explosion with a Surviving Companion Star. https://arxiv.org/abs/2306.11788

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