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Wanted: Surviving Companions of Stripped-Envelope Supernovae

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illustration of a supernova and a companion star
Illustration of a star exploding as a supernova and impacting its binary companion. [NASA, ESA, Leah Hustak (STScI)]

The jury’s still out on whether stripped-envelope supernovae arise from single stars or binary pairs. New research explores how to find the surviving binary companion (should it exist) of a recent supernova, offering a potential way to reach a verdict.

A Supernova Subclass

An illustration of the binary origin for stripped-envelope supernovae

An illustration of the binary origin for a stripped-envelope supernova. In the first panel, two main-sequence stars inhabit a binary system. In the second panel, one of the stars swells to become a red supergiant and loses its atmosphere to its companion. Finally, the evolved star goes supernova. [Adapted from NASA, ESA, Leah Hustak (STScI)]

When a massive star expires as a supernova, the light from the explosion can tell us about the star’s life, death, and the environment that surrounds it. Typical supernova light curves show prominent emission lines from hydrogen and helium, but some supernovae lack these characteristic lines.

Researchers believe that supernovae lacking hydrogen or helium lines result from stars that lost their outer envelope of gas before exploding. These events, dubbed stripped-envelope supernovae, might arise from stars that blew away their own atmospheres through ferocious stellar winds, or they might come from stars that have had their atmospheres stolen by a binary companion. Surprisingly, simulations show that a binary companion to a stripped-envelope supernova should actually be able to withstand the explosion, and new research explores whether we might be able to track down these survivor stars.

Imagining an Explosion’s Impact

A team led by Hsin-Pei Chen (陳昕霈) from National Tsing Hua University in Taiwan explored the possibility of tracking down the binary companion of a stripped-envelope supernova that was detected in 2020. Because of the low amount of mass ejected during the explosion, simulations suggest that the supernova, denoted SN 2020oi, likely happened in a binary system.

snapshots of simulated gas density

Gas density at four points in time during a simulation of supernova ejecta impacting a 5.5-solar-mass star. Click to enlarge. [Chen et al. 2023]

To understand what would have happened to SN 2020oi’s binary companion, Chen and collaborators simulated the impact of an exploding 10-solar-mass star on the evolution of a lower-mass stellar companion. The team ran several simulations with companion stars of 3.0, 5.5, and 8.0 solar masses, and they also varied the separation between the exploding star and its companion. After modeling the impact, the team switched to a second simulation to understand how the companion star’s physical properties and observational characteristics might change.

Ready, Set, Observe!

Plot of the luminosity, size, and temperature of the companion star as a function of time after the supernova

Luminosity, size, and temperature of the companion star as a function of time. The gray shaded area indicates the three-day period during which the supernova heats the star. [Chen et al. 2023]

These simulations captured the interaction between the companion star and the cloud of ejecta created during the explosion. When the supernova crashes into the companion star, it heats up the star, peels away a tiny bit of its mass, and kicks it away from the explosion. Over the course of a few years, the energy injected by the supernova makes the star larger and more luminous.

These changes might be observable by JWST and the Hubble Space Telescope, but only under certain circumstances; Chen and teammates estimate that if SN 2020oi’s binary companion has a mass between 3 and 8 solar masses, and if the two stars were initially separated by no more than five times the radius of the star that went supernova, we can hope to identify the companion star by 2030.

It’s always exciting when a prediction can be tested in years rather than gigayears. With luck and a little telescope time, we might track down SN 2020oi’s stellar companion, giving us new insight into the causes of stripped-envelope supernovae.

Citation

“Exploring the Observability of Surviving Companions of Stripped-envelope Supernovae: A Case Study of Type Ic SN 2020oi,” Hsin-Pei Chen et al 2023 ApJ 949 121. doi:10.3847/1538-4357/acc9af