Nov 16 2018
By sifting through the archival data of NASA Hubble Space Telescope, astronomers might have finally exposed the much-needed progenitor to a particular type of exploding star. It is believed that the supernova, known as a Type Ic, will detonate after its gigantic star has been stripped of or shed its outer layers of helium and hydrogen.
This is an artist’s concept of a blue supergiant star that once existed inside a cluster of young stars in the spiral galaxy NGC 3938, located 65 million light-years away. It exploded as a supernova in 2017, and Hubble Space Telescope archival photos were used to locate the doomed progenitor star, as it looked in 2007. The star may have been as massive as 50 suns and burned at a furious rate, making it hotter and bluer than our Sun. It was so hot, it had lost its outer layers of hydrogen and helium. When it exploded in 2017, astronomers categorized it as a Type Ic supernova because of the lack of hydrogen and helium in the supernova’s spectrum. In an alternative scenario (not shown here), a binary companion to the massive star may have stripped off its hydrogen and helium layers. (Image credit: NASA, ESA, and J. Olmsted (STScI))
Perhaps among the most massive known stars, these gigantic celestial bodies are at least 30 times heavier than the Sun. Even after the stars shed some of their material, they are anticipated to be bright and big. Hence, it was strange why one of these stars was not captured by astronomers in pre-explosion images.
However, in 2017, astronomers were able to solve this mystery. A close by star ended its life as a Type Ic supernova. Following this, two groups of astronomers sifted through the archive of Hubble Space Telescope images to expose the supposed precursor star in pre-explosion photos captured in 2007. Catalogued as SN 2017ein, the supernova appeared close to the middle of the nearby NGC 3938—a spiral galaxy situated about 65 million light-years away. This promising finding could provide a better understanding about stellar evolution, such as how the groups of stars, when born in batches, is distributed.
Finding a bona fide progenitor of a supernova Ic is a big prize of progenitor searching. We now have for the first time a clearly detected candidate object.
Schuyler Van Dyk, Lead Researcher, California Institute of Technology.
His team’s paper has been reported in The Astrophysical Journal.
The second team’s paper, which was featured in the Monthly Notices of the Royal Astronomical Society, is consistent with the conclusions of the previous team.
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. 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.
Charles Kilpatrick, Team Leader, University of California.
An examination of the colors of the object reveals that it is blue and very hot. On the basis of that assessment, two possibilities for the source’s identity were proposed by both groups. The candidate progenitor star might be a single heavy star that is 45 to 55 times more gigantic than the Sun. Another concept is that the progenitor may have been a huge binary-star system, wherein one of the stars weighs roughly 60 to 80 solar masses and the other weighs about 48 suns. In this latter situation, the stars are closely orbiting and interacting with one another. The more gigantic star is stripped of its helium and hydrogen layers by the neighboring companion, ultimately exploding as a supernova.
The potential of an enormous double-star system was indeed a surprise.
“This is not what we would expect from current models, which call for lower-mass interacting binary progenitor systems,” said Van Dyk.
Expectations regarding the identity of the progenitors of Type Ic supernovas have been rather mysterious. Astronomers were aware that the supernovas lacked helium and hydrogen, and originally suggested that before exploding, certain heavy stars shed this material in a powerful wind (a stream of charged particles). When the astronomers didn’t locate the progenitors stars, which should have been quite bright and gigantic, they proposed a second approach for producing the exploding stars, which involves a couple of lower mass, close-orbiting binary stars. In this situation, helium and hydrogen of the heavier star are stripped by its companion; however, the “stripped” star is still sufficiently large to ultimately 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. And those are questions that not just astronomers studying supernovas want to know, but all astronomers are after.
Ori Fox, Space Telescope Science Institute.
As only one class of exploding stars, the Type Ic supernovas account for roughly 20% of gigantic stars that explode from the collapse of their centers. The groups, however, cautioned that until the supernova fades in approximately a couple of years, the identity of the source cannot be validated.
The astronomers are hoping to apply either the upcoming NASA James Webb Space Telescope or the Hubble Space Telescope to check whether the long-sought progenitor star has considerably dimmed or vanished completely. The astronomers will also be able to isolate the light of the supernova from that of stars in its environment so as to measure a more precise measurement of the mass and brightness of the object.
Tenagra Observatories in Arizona discovered SN 2017ein was in May 2017; however, it took the Hubble’s sharp resolution to pinpoint the precise location of the potential source. Using Hubble’s Wide Field Camera 3, Van Dyk’s group successfully captured the young supernova in June 2017. The team utilized that image to show the candidate progenitor star dwelling in one of the spiral arms of the host galaxy in the archival Hubble photos captured by the Wide Field Planetary Camera 2 in December 2007.
In June 2017, Kilpatrick’s team was also able to view the supernova in infrared images from one among the 10-meter telescopes at the W. M. Keck Observatory located in Hawaii. To expose the potential source, the group subsequently examined the same archival Hubble images as Van Dyk’s group.