A hybrid type Ia supernova with an early flash triggered by helium-shell detonation

Abstract

Type Ia supernovae arise from the thermonuclear explosion of white-dwarf stars that have cores of carbon and oxygen. The uniformity of their light curves makes these supernovae powerful cosmological distance indicators, but there have long been debates about exactly how their explosion is triggered and what kind of companion stars are involved. For example, the recent detection of the early ultraviolet pulse of a peculiar, subluminous type Ia supernova has been claimed as evidence for an interaction between a red-giant or a main-sequence companion and ejecta from a white-dwarf explosion. Here we report observations of a prominent but red optical flash that appears about half a day after the explosion of a type Ia supernova. This supernova shows hybrid features of different supernova subclasses, namely a light curve that is typical of normal-brightness supernovae, but with strong titanium absorption, which is commonly seen in the spectra of subluminous ones. We argue that this early flash does not occur through previously suggested mechanisms such as the companion-ejecta interaction. Instead, our simulations show that it could occur through detonation of a thin helium shell either on a near-Chandrasekhar-mass white dwarf, or on a sub-Chandrasekhar-mass white dwarf merging with a less-massive white dwarf. Our finding provides evidence that one branch of previously proposed explosion models-the helium-ignition branch-does exist in nature, and that such a model may account for the explosions of white dwarfs in a mass range wider than previously supposed.

SNe Ia from double detonations: Impact of core-shell mixing on the carbon ignition mechanism

“…It showed a prominent early optical flash ∼ 0.5 days after explosion, an early red and rapid color evolution, and a light curve typical of normal brightness Type Ia supernovae, but showed strong titanium absorption, which is typically seen in the spectra of sub-luminous supernovae. Jiang et al (2017) attribute the early flash to the decays of 56 Ni and other radioactive isotopes in the outer layers of the ejecta, produced in the helium detonation. They ruled out interaction between the ejecta and either circumstellar material or a companion star as the cause of the early flash due to the early red color, as their models showed a bluer color evolution for these scenarios.…”

“…The spectra are discussed afterwards. We correct the spectra and photometry for SN 2016jhr and SN 2018byg for reddening due to Galactic extinction (given by E(B-V) MW = 0.0263 mag (Jiang et al 2017) and AV = 0.032 mag (De et al 2019), respectively). The total extinction to SN 2011fe was found to be negligible (Nugent et al 2011).…”

A type Ia supernova at the heart of superluminous transient SN 2006gy

“…The formation of BSG/RSG + WD binaries are predicted by binary population synthesis codes (63-65), and candidate systems have been observed(66). The uncertainty in this scenario is whether a He core + WD merger is a robust path to explosion, although the existence of the He detonation-triggered SN Ia has been recently suggested observationally(67).…”

The HSC-SSP Transient Survey: Implications from Early Photometry and Rise Time of Normal Type Ia Supernovae

“…With an even larger sample of early SNe Ia, a large scatter of rise times with a mean value of 18.5 days (without stretch correction) was reported by the Zwicky Transient Factory (ZTF) very recently ). In contrast, EExSNe Ia usually show longer rise times than the mean values obtained from statistics (Cao et al 2015;Marion et al 2016;Jiang et al 2017), suggesting the existence of a long dark phase of some SNe Ia (Piro & Nakar 2013;Mazzali et al 2014).…”

“…In addition to the companioninteraction scenario, an interaction between dense circumstellar matter (CSM) and SN ejecta ("CSM-ejecta interaction"; Levanon et al 2015Levanon et al , 2017 and vigorous mixing of radioactive 56 Ni in the outermost region of SN ejecta ("surface-56 Ni-decay"; Piro & Morozova 2016) may produce similar early light-curve excess to that predicted by the companion interaction. Moreover, radiation from short-lived radioactive elements generated by a precursory detonation at a helium shell of the primary WD ("He-shell detonation"; He-det) can cause a prominent but relatively red early excess (Jiang et al 2017;Maeda et al 2018). In this article, we define "earlyexcess SNe Ia (EExSNe Ia)" as SNe Ia that show luminosity excess compared to "smoothly rising" SNe Ia whose light curves can be roughly fitted by single powerlaw functions, such as the classical fireball model with a power-law index of 2, in the first few days of explosions (see Figure 2 of Jiang et al (2018)).…”