# Numerically Modeling the First Peak of the Type IIb SN 2016gkg

## TL;DR

This study uses numerical models to analyze the early light curve of SN 2016gkg, revealing the properties of its extended envelope and constraining the explosion timing, with implications for understanding Type IIb supernova progenitors.

## Contribution

The paper introduces a comprehensive grid of extended envelope models, including wind profiles, to better interpret early supernova light curves, especially for SN 2016gkg.

## Key findings

- Approximately 0.02 solar masses of extended material with radius 180-260 solar radii fit the light curve.
- The explosion occurred within 2-3 hours of the first observed data point.
- Spherical models overpredict early velocities, indicating asymmetry in the hydrogen-rich material.

## Abstract

Many Type IIb supernovae (SNe) show a prominent additional early peak in their light curves, which is generally thought to be due to the shock cooling of extended hydrogen-rich material surrounding the helium core of the exploding star. The recent SN 2016gkg was a nearby Type IIb SN discovered shortly after explosion, which makes it an excellent candidate for studying this first peak. We numerically explode a large grid of extended envelope models and compare these to SN 2016gkg to investigate what constraints can be derived from its light curve. This includes exploring density profiles for both a convective envelope and an optically thick steady-state wind, the latter of which has not typically been considered for Type IIb SNe models. We find that roughly $\sim0.02\,M_\odot$ of extended material with a radius of $\approx180-260\,R_\odot$ reproduces the photometric light curve data, consistent with pre-explosion imaging. These values are independent of the assumed density profile of this material, although a convective profile provides a somewhat better fit. We infer from our modeling that the explosion must have occurred within $\approx2-3\,{\rm hrs}$ of the first observed data point, demonstrating that this event was caught very close to the moment of explosion. Nevertheless, our best-fitting one-dimensional models overpredict the earliest velocity measurements, which suggests that the hydrogen-rich material is not distributed in a spherically symmetric manner. We compare this to the asymmetries seen in the SN IIb remnant Cas A, and we discuss the implications of this for Type IIb SN progenitors and explosion models.