Free Astronomy Magazine September-October 2022

SEPTEMBER-OCTOBER 2022 T his infographic shows the evolution astronomers propose for supernova (SN) 2013ge. Panels 1-3 show what has already occurred, and panels 4-6 show what may take place in the future. 1) A binary pair of massive stars orbit one another. 2) One star ages into its red giant stage, getting a puffy outer envelope of hydro- gen that its companion star siphons off with gravity. Astronomers propose this is why Hubble found no trace of hydrogen in the supernova debris. 3) The stripped- envelope star goes supernova (SN 2013ge), jostling but not destroying its compan- ion star. After the supernova, the dense core of the former massive star remains either as neutron star or black hole. 4) Eventually the companion star also ages into a red giant, maintaining its outer envelope, some of which came from its com- panion. 5) The companion star also undergoes a supernova. 6) If the stars were close enough to each other not to be flung from their orbits by the supernova blast wave, the remnant cores will continue to orbit one another and eventually merge, creating gravitational waves in the process. [NASA, ESA, Leah Hustak (STScI)] The cause of the hydrogen loss had been a mystery, and astronomers have been using Hubble to search for clues and test theories to explain these stripped supernovae. The new Hubble observations provide the best evidence yet to support the the- ory that an unseen companion star siphons off the gas envelope from its partner star before it explodes. “This was the moment we had been waiting for, finally seeing the evi- dence for a binary system progenitor of a fully stripped supernova,” said astronomer Ori Fox of the Space Tel- escope Science Institute in Baltimore, Maryland, lead investigator on the Hubble research program. “The goal is to move this area of study from theory to working with data and see- ing what these systems really look like.” Fox’s team used Hubble’s Wide Field Camera 3 to study the region of supernova (SN) 2013ge in ultra- violet light, as well as previous Hub- ble observations in the Barbara A. Mikulski Archive for Space Tele- scopes (MAST). Astronomers saw the light of the supernova fading over time from 2016 to 2020 — but an- other nearby source of ultraviolet light at the same position main- tained its brightness. This underlying source of ultraviolet emission is what the team proposes is the surviving binary companion to SN 2013ge. Previously, scientists theorized that a massive progenitor star’s strong winds could blow away its hydrogen gas envelope, but observational evi- dence didn’t support that. To ex- plain the disconnect, astronomers developed theories and models in which a binary companion siphons off the hydrogen. “In recent years many different lines of evidence have told us that stripped super- novae are likely formed in binaries, but we had yet to actually see the companion. So much of studying cosmic explosions is like forensic sci- ence — searching for clues and see- ing what theories match. Thanks to Hubble, we are able to see this di- rectly,” said Maria Drout of the Uni- versity of Toronto, a member of the Hubble research team. In prior observations of SN 2013ge, Hubble saw two peaks in the ultra- violet light, rather than just the one typically seen in most supernovae. Fox said that one explanation for this double brightening was that the second peak shows when the super- nova’s shock wave hit a companion star, a possibility that now seems much more likely. Hubble’s latest ob- servations indicate that while the companion star was significantly jos- tled, including the hydrogen gas it had siphoned off its partner, it was not destroyed. Fox likens the effect to a jiggling bowl of jelly, which will eventually settle back to its original form. While additional confirmation and similar supporting discoveries need to be found, Fox said that the