aSTRONOMY
The big star that couldn't become a supernova
One
star's 'massive fail' could help solve a mystery
Date: May 25, 2017
Source: Ohio State University
Summary: For the first time in
history, astronomers have been able to watch as a dying star was reborn as a
black hole. It went out with a whimper instead of a bang.
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STORY
In the failed supernova of a red
supergiant, the envelope of the star is ejected and expands, producing a cold,
red transient source surrounding the newly formed black hole, as illustrated by
the expanding shell (left to right). Some residual material may fall onto the
black hole, as illustrated by the stream and the disk, potentially powering
some optical and infrared emissions years after the collapse.
Credit: NASA, ESA, P. Jeffries
(STScI)
For the first time in history,
astronomers have been able to watch as a dying star was reborn as a black hole.
It went out with a whimper instead
of a bang.
The star, which was 25 times as
massive as our sun, should have exploded in a very 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 Christopher Kochanek, professor of astronomy at
The Ohio State University and the Ohio Eminent Scholar in Observational
Cosmology.
As many as 30 percent of such stars,
it seems, may quietly collapse into black holes -- no supernova required.
"The typical view is that a
star can form a black hole only after it goes supernova," Kochanek
explained. "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."
He leads a team of astronomers who
have been using the Large Binocular Telescope (LBT) to look for failed
supernovae in other galaxies. They published their latest results in the Monthly
Notices of the Royal Astronomical Society.
Among the galaxies they've been
watching is NGC 6946, a galaxy 22 million light-years away that is nicknamed
the "Fireworks Galaxy" because supernovae frequently happen there --
indeed, SN 2017eaw, discovered on May 14th, is shining near maximum brightness
now. Starting in 2009, one particular star in the Fireworks Galaxy, named
N6946-BH1, began to brighten weakly. By 2015, it appeared to have winked out of
existence.
The astronomers aimed the Hubble
Space Telescope at the star's location to see if it was still there but merely
dimmed. They also used the Spitzer Space Telescope 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 researchers
eventually concluded that the star must have become a black hole. It's too
early in the project to know for sure how often stars experience massive fails,
but Scott Adams, a former Ohio State student who recently earned his Ph.D.
doing this work, was 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. During
this period, six normal supernovae have occurred within the galaxies we've been
monitoring, suggesting that 10 to 30 percent of massive stars die as failed
supernovae," he said.
"This is just the fraction that
would explain the very problem that motivated us to start the survey."
To study co-author Krzystof Stanek,
the really interesting part of the discovery is the implications it holds for
the origins of very massive black holes -- the kind that the LIGO experiment
detected via gravitational waves. (LIGO is the Laser Interferometer
Gravitational-Wave Observatory.)
It doesn't necessarily make sense,
said Stanek, professor of astronomy at Ohio State, that a massive star could
undergo a supernova -- a process which entails blowing off much of its outer
layers -- and still have enough mass left over to form a massive black hole on
the scale of those that LIGO detected.
"I suspect it's much easier to
make a very massive black hole if there is no supernova," he concluded.
Story Source:
Materials provided by Ohio State University. Original written by Pam Frost Gorder. Note: Content may
be edited for style and length.
Journal Reference:
- S. M. Adams, C. S. Kochanek, J. R. Gerke, K. Z. Stanek.
The search for failed supernovae with the Large Binocular Telescope:
constraints from 7 yr of data. Monthly Notices of the Royal
Astronomical Society, 2017; 469 (2): 1445 DOI: 10.1093/mnras/stx898
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