Free Astronomy Magazine July-August 2018

23 JULY-AUGUST 2018 SPACE CHRONICLES sion. This type of exploding star was first identified in 1987 by team member Alex Filippenko of the Uni- versity of California, Berkeley. How stripped-envelope supernovas lose that outer envelope is not en- tirely clear. They were originally thought to come from single stars with very fast winds that pushed off the outer envelopes. The problem was that when astronomers started looking for the primary stars from which supernovas were spawned, they couldn’t find them for many stripped-envelope supernovas. “That was especially bizarre, be- cause astronomers expected that they would be the most massive and Prior to the supernova explosion, the orbit of the two stars around each other took about a year. When the primary star exploded, it had far less impact on the surviving companion than might be thought. Imagine an avocado pit — repre- senting the dense core of the com- panion star — embedded in a gelatin dessert — representing the star’s gaseous envelope. As a shock wave passes through, the gelatin might temporarily stretch and wob- ble, but the avocado pit would re- main intact. In 2014, Fox and his team used Hubble to detect the companion of another Type IIb su- pernova, SN 1993J. However, they captured a spectrum, not an image. The case of SN 2001ig is the first time a surviving companion has been photographed. “We were fi- nally able to catch the stellar thief, confirming our suspicions that one had to be there,” said Filippenko. Perhaps as many as half of all stripped-envelope supernovas have companions — the other half lose their outer envelopes via stellar winds. Ryder and his team have the ultimate goal of precisely determin- ing how many supernovas with stripped envelopes have compan- ions. Their next endeavor is to look at completely stripped-envelope su- pernovas, as opposed to SN 2001ig and SN 1993J, which were only about 90 percent stripped. These completely stripped-envelope supernovas don’t have much shock interaction with gas in the sur- rounding stellar environment, since their outer envelopes were lost long before the explosion. Without shock interaction, they fade much faster. This means that the team will only have to wait two or three years to look for surviving companions. In the future, they also hope to use the James Webb Space Telescope to continue their search. the brightest progenitor stars,” ex- plained team member Ori Fox of the Space Telescope Science Insti- tute in Baltimore. “Also, the sheer number of stripped-envelope super- novas is greater than predicted.” That fact led scientists to theorize that many of the primary stars were in lower-mass binary systems, and they set out to prove it. Looking for a binary companion after a supernova explosion is no easy task. First, it has to be at a rel- atively close distance to Earth for Hubble to see such a faint star. SN 2001ig and its companion are about at that limit. Within that dis- tance range, not many su- pernovas go off. Even more importantly, astronomers have to know the exact position through very precise measurements. In 2002, shortly after SN 2001ig exploded, scien- tists pinpointed the pre- cise location of the su- pernova with the Euro- pean Southern Observa- tory’s Very Large Tele- scope (VLT) in Cerro Paranal, Chile. In 2004, they then followed up with the Gemini South Observatory in Cerro Pachón, Chile. This obser- vation first hinted at the presence of a surviving bi- nary companion. Knowing the exact coordi- nates, Ryder and his team were able to focus Hubble on that location 12 years later, as the supernova’s glow faded. With Hub- ble’s exquisite resolution and ultraviolet capability, they were able to find and photograph the surviving companion — something only Hubble could do. ! S hortly after SN 2001ig exploded, scientists pho- tographed the super- nova with the European Southern Observatory’s Very Large Telescope (VLT) in 2002. Two years later, they followed up with the Gemini South Observatory, which hinted at the presence of a surviving binary companion. As the su- pernova’s glow faded, scientists focused Hub- ble on that location in 2016. They pinpointed and photographed the surviving companion, which was possible only due to Hubble’s exquisite resolution and ultraviolet sensi- tivity. Hubble observa- tions of SN 2001ig provide the best evi- dence yet that some supernovas originate in double-star systems. [NASA, ESA, S. Ryder (Australian Astronomi- cal Observatory), and O. Fox (STScI)]