Astronomers have watched as N6946-BH1, a red supergiant with a mass of 25 solar masses in the spiral galaxy NGC 6946, was likely reborn as a black hole.
N6946-BH1 should have exploded in a bright supernova. Instead, it fizzled out — and then left behind a black hole.
“Massive ‘fails’ like this one in a nearby galaxy could explain why astronomers rarely see supernovae from the most massive stars,” said Prof. Christopher Kochanek, of Ohio State University.
“As many as 30% of such stars, it seems, may quietly collapse into black holes.”
“The typical view is that a star can form a black hole only after it goes supernova,” he said.
“If a star can fall short of a supernova and still make a black hole, that would help to explain why we don’t see supernovae from the most massive stars.”
Among the galaxies Prof. Kochanek and co-authors have been watching is NGC 6946, a medium-sized, face-on spiral galaxy approximately 22 million light years away from Earth.
Starting in 2009, one particular star, N6946-BH1, in this galaxy began to brighten weakly.
By 2015, N6946-BH1 appeared to have winked out of existence.
After the Large Binocular Telescope (LBT) survey for failed supernovas turned up the star, the astronomers aimed the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope to see if it was still there but merely dimmed.
They also used Spitzer to search for any infrared radiation emanating from the spot. That would have been a sign that the star was still present, but perhaps just hidden behind a dust cloud.
All the tests came up negative. The star was no longer there.
By a careful process of elimination, the team eventually concluded that N6946-BH1 must have become a black hole.
It’s too early in the project to know for sure how often stars experience massive fails, but the scientists were able to make a preliminary estimate.
“N6946-BH1 is the only likely failed supernova that we found in the first seven years of our survey,” said Dr. Scott Adams, of Caltech and Ohio State University.
“During this period, six normal supernovae have occurred within the galaxies we’ve been monitoring, suggesting that 10 to 30% of massive stars die as failed supernovae.”
“This is just the fraction that would explain the very problem that motivated us to start the survey, that is, that there are fewer observed supernovae than should be occurring if all massive stars die that way.”
Details of the research were recently published in the Monthly Notices of the Royal Astronomical Society. The article is also publicly available at arXiv.org.
S.M. Adams et al. 2017. The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star. MNRAS 468 (4): 4968-4981; doi: 10.1093/mnras/stx816