Patient Astronomers Catch Stellar Litterbugs: What Happens Next Is Shocking!

Dr. Melissa L. Graham, LSST Research Scientist and DIRAC Fellow, coauthored paper on a large Hubble survey of supernovae “Delayed Circumstellar Interaction for Type Ia SN 2015cp Revealed by an HST Ultraviolet Imaging Survey”.

Our Hubble Space Telescope image of Type Ia supernova 2015cp, obtained with a near-ultraviolet filter at 686 days after the original explosion. The point source labeled “A” is light from the explosion of a white dwarf star hitting material within 0.1 light-years of the progenitor star system. The very existence of this material indicates that the exploding star had was in a binary system, and likely had either a sun-like or red giant star companion – not another white dwarf star.

The supernova explosions of white dwarf stars are thought to be caused by the influence of a binary companion star. Most evidence points towards a merger with another white dwarf star, but accreting too much mass from a sun-like or red giant star could also cause a supernova. It is very difficult to directly observe the companion star caught in a supernova explosion, but luckily, some companion stars are very messy. Red giants are so big and bright that material blows off their surface and is scattered around the vicinity like litter in the wind. If the fast-moving front of material ejected by a supernova hits this residual gas, it sends a shock through the material that creates luminous ultraviolet emission. This has only been seen a few times before, when the residual gas was very close to the explosion, and it was unknown how often these stellar systems had gas littering their environment, which had been blown out to further distances, and thus gone unnoticed. By patiently waiting, and using HST to take NUV snapshots of the sites of ~70 supernovae at 1-3 years after the explosion, we caught one white dwarf explosion in a stellar system with a litterbug companion star: Supernova 2015cp. Our team used ground- and space-based facilities (including Keck, VLT, VLA, HST, and Swift) to obtain additional data and study SN 2015cp in detail. Our analysis limits the fraction of white dwarf explosions with messy companion stars to be <6%.

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