# Initial Flash and Spectral Formation of Type Ia Supernovae with An Envelope: Applications to Over-luminous SNe Ia

## TL;DR

This paper proposes a unified model involving a white dwarf with an envelope to explain the diverse peculiar features of over-luminous Type Ia supernovae, supported by hydrodynamic and radiation transfer simulations.

## Contribution

It introduces a novel scenario with a C+O-rich envelope around an exploding white dwarf to account for peculiar observational features of over-luminous SNe Ia.

## Key findings

- The model explains the initial flash with a 0.5-3 day timescale.
- Strong C II lines are produced within the shocked envelope.
- Late-phase dust formation can cause increased optical depth.

## Abstract

Over-luminous type Ia supernovae (SNe Ia) show peculiar observational features, for which an explosion of a super-massive white dwarf (WD) beyond the classical Chandrasekhar-limiting mass has been suggested, largely based on their high luminosities and slow light-curve evolution. However, their observational features are diverse, with a few extremely peculiar features whose origins have not been clarified; strong and persisting C II lines, late-time accelerated luminosity decline and red spectra, and a sub-day time-scale initial flash clearly identified so far at least for three over-luminous SNe Ia. In the present work, we suggest a scenario that provides a unified solution to these peculiarities, through hydrodynamic and radiation transfer simulations together with analytical considerations; a C+O-rich envelope (~0.01 - 0.1 Msun) attached to an exploding WD. Strong C II lines are created within the shocked envelope. Dust formation is possible in the late phase, providing a sufficient optical depth thereafter. The range of the envelope mass considered here predicts an initial flash with time-scale of ~0.5 - 3 days. The scenario thus can explain some of the key diverse observational properties by a different amount of the envelope, but additional factors are also required; we argue that the envelope is distributed in a disc-like structure, and also the ejecta properties, e.g., the mass of the WD, plays a key role. Within the context of the hypothesized super-Chandrasekhar-mass WD scenario, we speculatively suggest a progenitor WD evolution including a spin-up accretion phase followed by a spin-down mass-ejection phase.