Shock cooling of a red-supergiant supernova at redshift 3 in lensed images

TL;DR

This study captures the early ultraviolet light curve of a supernova at redshift 3 using gravitational lensing, constraining its progenitor to be a red supergiant with a radius of about 533 solar radii.

Contribution

First measurement of an early supernova light curve at high redshift using gravitational lensing, providing new insights into progenitor star properties in the distant universe.

Findings

Progenitor radius constrained to ~533 solar radii

Supernova consistent with a red supergiant progenitor

Early UV observations within 6 hours of explosion

Abstract

The core-collapse supernova of a massive star rapidly brightens when a shock, produced following the collapse of its core, reaches the stellar surface. As the shock-heated star subsequently expands and cools, its early-time light curve should have a simple dependence on the progenitor's size and therefore final evolutionary state. Measurements of the progenitor's radius from early light curves exist for only a small sample of nearby supernovae, and almost all lack constraining ultraviolet observations within a day of explosion. The several-day time delays and magnifying ability of galaxy-scale gravitational lenses, however, should provide a powerful tool for measuring the early light curves of distant supernovae, and thereby studying massive stellar populations at high redshift. Here we analyse individual rest-frame ultraviolet-through-optical exposures taken with the Hubble Space Telescope that simultaneously capture, in three separate gravitationally lensed images, the early phases of a supernova at redshift $z \approx 3$ beginning within $5.8\pm 3.1$ hr of explosion. The supernova, seen at a lookback time of $\sim11.5$ billion years, is strongly lensed by an early-type galaxy in the Abell 370 cluster. We constrain the pre-explosion radius to be $533^{+154}_{-119}$ solar radii, consistent with a red supergiant. Highly confined and massive circumstellar material at the same radius can also reproduce the light curve, but is unlikely since no similar low-redshift examples are known.