Astronomers find picture of hefty star before it blew up

Supernovas are the deathly explosions of massive stars. One of the ways that astronomers look for clues about how these stars blow up is to go hunting for what's known as the progenitor to a supernova - the star before it died. They comb through archival telescope images and try to pinpoint the location and identity of the star before it blasted apart. Now, for the first time, a Caltech-led team has likely found such a progenitor for a supernova class known as "Type Ic" (pronounced "one-C"). Of all the classes of supernovas, this is the only one that did not have a known progenitor until now and thus its identification was thought of as something of a Holy Grail by astronomers. The Type Ic supernova, called SN 2017ein, was initially discovered in May 2017 by researchers using the Tenagra Observatories in Arizona. It is located in a spiral galaxy called NGC 3938, about 65 million light-years away. The Caltech astronomy team was able to track down this supernova's progenitor using archival images from NASA's Hubble Space Telescope, taken in 2007. "An alert was sent out when the supernova was initially found," says Schuyler Van Dyk, a staff scientist at IPAC, a science and data center for astronomy at Caltech. "You can't sleep once that happens and have to mobilize to try to find the progenitor to the explosion. Within a few weeks after the supernova was discovered, we found a candidate using both new and archival Hubble images."

 Van Dyk is lead author of a paper about the findings, published this summer in The Astrophysical Journal. "The new images were essential for pinpointing the candidate progenitor's location."

The progenitor is hot and luminous and is thought to be either a single hefty star 48 or 49 times the mass of our Sun or a massive binary star system in which the star that exploded weighs between 60 and 80 solar masses.

"Type Ic supernovas occur with the most massive of stars," says Van Dyk. "But we were surprised by how massive this one appears to be, and especially by the possibility of a massive double-star system as the progenitor. Although theories have existed for the last three decades that Type Ic supernovas could be the explosions of very massive single stars, alternative, more recent theories point toward stars of lower mass in binary systems as being the origins of these explosions."

Other supernova classes include Type Ia, which occur when white dwarfs in binary star systems explode (cosmologists used these to discover that our universe is not only expanding but accelerating apart). Type II, Type Ib, and Ic supernovas occur when massive stars collapse at the end of their lives, forming neutron stars or black holes. Type Ib and Ic differ from Type II in that their progenitor stars lose outer envelopes of material around their central cores before exploding. Type Ib and Ic supernovas differ from each other slightly in chemical composition.

Piecing together how each of these supernova types occurs provides a better understanding of the evolution of the most massive stars in our universe.

"The origins of such explosions are relevant to the entire astronomical community, not just supernova researchers," says Ori Fox from the Space Telescope Science Institute (STScI), a co-author on the study. "The results have implications on ideas from star formation to stellar evolution and feedback into the galaxy."

"Astronomers have been trying to find this progenitor for some 20 years," says Van Dyk. "Humans wouldn't be here without supernovas - they make the chemical elements from which we are made."

The astronomers say that they should be able to confirm with certainty whether they have identified the correct progenitor to the Type Ic explosion within a few years, using Hubble or the upcoming NASA James Webb Space Telescope, set to launch in 2021.

As the supernova fades as expected, the astronomers will have a clearer view of the area around it. If the luminous progenitor candidate was correctly identified in archival images, then it will have vanished and should not be seen in the new images. If the scientists still see the candidate progenitor, that means it was misidentified and some other hidden star was the culprit.

Astronomers find possible elusive star behind supernova

Astronomers may have finally uncovered the long-sought progenitor to a specific type of exploding star by sifting through NASA Hubble Space Telescope archival data. The supernova, called a Type Ic, is thought to detonate after its massive star has shed or been stripped of its outer layers of hydrogen and helium. These stars could be among the most massive known - at least 30 times heftier than our Sun. Even after shedding some of their material late in life, they are expected to be big and bright. So it was a mystery why astronomers had not been able to nab one of these stars in pre-explosion images. Finally, in 2017, astronomers got lucky. A nearby star ended its life as a Type Ic supernova. Two teams of astronomers pored through the archive of Hubble images to uncover the putative precursor star in pre-explosion photos taken in 2007. The supernova, cataloged as SN 2017ein, appeared near the center of the nearby spiral galaxy NGC 3938, located roughly 65 million light-years away. This potential discovery could yield insight into stellar evolution, including how the masses of stars are distributed when they are born in batches. "Finding a bona fide progenitor of a supernova Ic is a big prize of progenitor searching," said Schuyler Van Dyk of the California Institute of Technology (Caltech) in Pasadena, lead researcher of one of the teams. "We now have for the first time a clearly detected candidate object." His team's paper was published in June in The Astrophysical Journal.

A paper by a second team, which appeared in the Oct. 21, 2018, issue of the Monthly Notices of the Royal Astronomical Society, is consistent with the earlier team's conclusions.

"We were fortunate that the supernova was nearby and very bright, about 5 to 10 times brighter than other Type Ic supernovas, which may have made the progenitor easier to find," said Charles Kilpatrick of the University of California, Santa Cruz, leader of the second team.

"Astronomers have observed many Type Ic supernovas, but they are all too far away for Hubble to resolve. You need one of these massive, bright stars in a nearby galaxy to go off. It looks like most Type Ic supernovas are less massive and therefore less bright, and that's the reason we haven't been able to find them."

An analysis of the object's colors shows that it is blue and extremely hot. Based on that assessment, both teams suggest two possibilities for the source's identity. The progenitor could be a single hefty star between 45 and 55 times more massive than our Sun.

Another idea is that it could have been a massive binary-star system in which one of the stars weighs between 60 and 80 solar masses and the other roughly 48 suns. In this latter scenario, the stars are orbiting closely and interact with each other. The more massive star is stripped of its hydrogen and helium layers by the close companion and eventually explodes as a supernova.

The possibility of a massive double-star system is a surprise. "This is not what we would expect from current models, which call for lower-mass interacting binary progenitor systems," Van Dyk said.

Expectations on the identity of the progenitors of Type Ic supernovas have been a puzzle. Astronomers have known that the supernovas were deficient in hydrogen and helium and initially proposed that some hefty stars shed this material in a strong wind (a stream of charged particles) before they exploded. When they didn't find the progenitors stars, which should have been extremely massive and bright, they suggested a second method to produce the exploding stars that involves a pair of close-orbiting, lower-mass binary stars.

In this scenario, the heftier star is stripped of its hydrogen and helium by its companion. But the "stripped" star is still massive enough to eventually explode as a Type Ic supernova.

"Disentangling these two scenarios for producing Type Ic supernovas impacts our understanding of stellar evolution and star formation, including how the masses of stars are distributed when they are born, and how many stars form in interacting binary systems," explained Ori Fox of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, a member of Van Dyk's team. "And those are questions that not just astronomers studying supernovas want to know, but all astronomers are after."

Type Ic supernovas are just one class of exploding star. They account for about 20% of massive stars that explode from the collapse of their cores.

The teams caution that they won't be able to confirm the source's identity until the supernova fades in about two years. The astronomers hope to use either Hubble or the upcoming NASA James Webb Space Telescope to see whether the candidate progenitor star has disappeared or has significantly dimmed. They also will be able to separate the supernova's light from that of stars in its environment to calculate a more accurate measurement of the object's brightness and mass.

SN 2017ein was discovered in May 2017 by Tenagra Observatories in Arizona. But it took the sharp resolution of Hubble to pinpoint the exact location of the possible source. Van Dyk's team imaged the young supernova in June 2017 with Hubble's Wide Field Camera 3. The astronomers used that image to pinpoint the candidate progenitor star nestled in one of the host galaxy's spiral arms in archival Hubble photos taken in December 2007 by the Wide Field Planetary Camera 2.

Kilpatrick's group also observed the supernova in June 2017 in infrared images from one of the 10-meter telescopes at the W. M. Keck Observatory in Hawaii. The team then analyzed the same archival Hubble photos as Van Dyk's team to uncover the possible source.