On December 10, 2001 Australian amateur astronomer Robert Evans discovered a supernova in the outer edge of NGC 7424, a spiral galaxy located in the southern constellation Grus, about 40 million light-years away. Shortly after, professional astronomers photographed the supernova, named SN 2001ig, with ESO’s Very Large Telescope in 2002. Two years later, they followed up with the Gemini South Observatory, which hinted at the presence of a surviving binary companion. As SN 2001ig’s glow faded, they focused the NASA/ESA Hubble Space Telescope on that location in 2016. They pinpointed and photographed the surviving companion — an early B-type main-sequence star — which was possible only due to Hubble’s exquisite resolution and UV sensitivity. The Hubble image is the most compelling evidence that some supernova explosions originate in binary systems.
Ryder et al used Hubble to get the first image of a surviving companion to a supernova; this is the most compelling evidence that some supernova explosions originate in binary star systems. Image credit: NASA / ESA / S. Ryder, Australian Astronomical Observatory / O. Fox, STScI.
“We know that the majority of massive stars are in binary pairs. Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close together,” said lead author Dr. Stuart Ryder, of the Australian Astronomical Observatory in Sydney, Australia.
“The companion to the supernova’s progenitor star was no innocent bystander to the explosion. It siphoned off almost all of the hydrogen from the doomed star’s stellar envelope, the region that transports energy from the star’s core to its atmosphere.”
“Millions of years before the primary star went supernova, the companion’s thievery created an instability in the primary star, causing it to episodically blow off a cocoon and shells of hydrogen gas before the catastrophe.”
SN 2001ig is categorized as a Type IIb stripped-envelope supernova. This type of supernova is unusual because most, but not all, of the hydrogen is gone prior to the explosion.
This type of exploding star was first identified in 1987 by team member Dr. Alex Filippenko of the University of California, Berkeley.
How stripped-envelope supernovae lose that outer envelope is not entirely 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 supernovae were spawned, they couldn’t find them for many stripped-envelope supernovae.
“That was especially bizarre, because astronomers expected that they would be the most massive and the brightest progenitor stars,” said team member Dr. Ori Fox, from the Space Telescope Science Institute.
“Also, the sheer number of stripped-envelope supernovae is greater than predicted.”
That fact led astronomers 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,” the astronomers said.
“First, it has to be at a relatively close distance to Earth for Hubble to see such a faint star. SN 2001ig and its companion are about at that limit. Within that distance range, not many supernovae go off.”
“Even more importantly, we have to know the exact position through very precise measurements.”
This graphic illustrates the scenario for the processes that create a Type IIb stripped-envelope supernova, in which most, but not all, of the hydrogen envelope is lost prior to the primary star’s explosion. The four panels show the interaction between the SN 2001ig progenitor star, which ultimately exploded, and its surviving companion: 1) two stars orbit each other and draw closer and closer together; 2) the more massive star evolves faster, swelling up to become a red giant. In this late phase of life, it spills most of its hydrogen envelope into the gravitational field of its companion; as the companion siphons off almost all of the doomed star’s hydrogen, it creates an instability in the primary star; 3) the primary star explodes in a supernova; 4) as the supernova’s glow fades, the surviving companion becomes visible to Hubble; the faint remnant of the supernova, at lower left, continues to evolve but in this case is too faint to be detected by Hubble. Image credit: NASA / ESA / A. Field, STScI.
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 — representing the dense core of the companion star — embedded in a gelatin dessert — representing the star’s gaseous envelope. As a shock wave passes through, the gelatin might temporarily stretch and wobble, but the avocado pit would remain intact,” the explained.
In 2014, Dr. Fox and colleagues used Hubble to detect the companion of another Type IIb supernova, 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 finally able to catch the stellar thief, confirming our suspicions that one had to be there,” Dr. Filippenko said.
“It was tremendously exciting to have our theory confirmed after all these years, in an object which was first discovered right here in Australia,” Dr. Ryder said.
The findings were published online in the Astrophysical Journal.
_____
Stuart D. Ryder et al. 2018. Ultraviolet Detection of the Binary Companion to the Type IIb SN 2001ig. ApJ 856, 83; doi: 10.3847/1538-4357/aaaf1e
