SN 2011fe in the Pinwheel Galaxy (M101) at maximum brightness, a composite of optical data from the Las Cumbres Observatory Global Telescope Network 0.8-meter Byrne Observatory Telescope at the Sedgwick Reserve and (purple) hydrogen emission data from the Palomar Transient Factory. Image: B.J. Fulton (LCOGT) / PTF
An international team
of scientists, including astrophysicists from University of California,
Santa Barbara, has discovered that a supernova that exploded in
August––dubbed the supernova of a generation––was a “white dwarf” star,
and that its companion star could not have been a “red giant,” as
previously suspected. The findings are published in two papers in the
journal Nature this week.
dwarf stars are small but very dense stars, and red giants are stars
that swell to massive proportions when they approach middle age.
new “type Ia” thermonuclear supernova, known as PTF 11kly, exploded on
August 24th in the Pinwheel galaxy, located in the “Big Dipper,” also
known as Ursa Major. These supernovae are used to measure dark energy,
which scientists believe is related to the expansion of the universe.
The discovery of the supernova was made by an international team of
astronomers known as the Palomar Transient Factory.
21 million light-years away, this supernova was practically next door,
in cosmic terms, and could be seen in early September with binoculars.
The explosion gave scientists their best chance yet to study a
thermonuclear supernova up close, with modern instruments.
the past 50 years, astrophysicists have discovered that type Ia
supernovae are part of binary systems––two stars orbiting each other.
The one that exploded was theorized to be a white dwarf star.
what our sun will be at the end of its life,” said Andy Howell, a
member of the UCSB team. “It will have the mass of the sun crammed into
the size of the Earth.” Howell is a staff scientist at the
UCSB-affiliated Las Cumbres Observatory Global Telescope Network LCOGT,
an assistant adjunct professor of physics at UCSB, and co-author of both
Scientists are upbeat about the finding that the supernova is a white dwarf.
been nearly 50 years since the original theoretical suggestions were
made that these supernovae were caused by white dwarfs,” said co-author
Lars Bildsten. “The observational proof is very satisfying to see!”
Bildsten is a permanent member of UCSB’s Kavli Institute of Theoretical
Physics (KITP) and is UCSB’s Wayne Rosing, Simon and Diana Raab Chair in
white dwarf stars would normally be dead forever, slowly cooling and
freezing solid over cosmic time. However, if it has a companion star,
then the white dwarf can steal its matter, and return to life. If they
steal too much matter, the carbon atoms will fuse so rapidly that the
burning cannot be stopped, leading to an explosion as a Type Ia
has long been the leading theory, although proof has remained elusive
for decades. One of the papers shows that the exploding star had to be
smaller than a tenth of the radius of the sun. That rules out normal
stars, and for the first time provides direct evidence that white dwarfs
are responsible for Type Ia supernovae. The lead author is Peter
Nugent, who discovered the supernova, and is a senior staff scientist at
Lawrence Berkeley National Laboratory, and an adjunct professor at UC
have not yet ascertained the type of the companion star to the
white-dwarf-turned supernova. However, they have ruled out the type of
star they expected––a red giant. Previous studies of RS Ophiuci, a
binary star system in our own Milky Way galaxy that is similar to the
one being studied, has a white dwarf near the limit that will cause it
to explode. And, it is being fed by a companion red giant star. So
scientists were somewhat surprised that they did not find a red giant
next to the supernova that exploded in August.
second paper regarding the companion star to the white dwarf was led by
Weidong Li, a research scientist at the University of California,
Berkeley. He explained: “This is the first time through direct imaging
of the explosion site, we were able to rule out certain types of stars
as the companion to a Type Ia supernova. The second star couldn’t have
been a massive red giant.”
decades of hunting the origins of Type Ia supernovae, scientists were
finally able to make progress in this case for two reasons. In the case
of the Li paper, it is because this was the closest thermonuclear
supernova since sensitive modern instruments, like those on the Hubble
Space Telescope, have been available.
the Nugent paper, while closeness was necessary, another factor was
even more important––the speed of the discovery. The team discovered the
supernova only 11 hours after it exploded, allowing for the first
estimate of the size of the star when it blew up.
only is this the closest Type Ia supernova in the last 25 years, it is
the youngest and brightest ever discovered in the digital age,” said
Nugent. “Observations with ground- and space-based telescopes from the
radio through X-ray wavelengths have provided unprecedented constraints
on how the supernova exploded.”
scientists noted that these rapid observations were not due to luck;
they were possible because the Palomar 48-inch telescope, which was used
to discover the supernova, is effectively a robot. Given regions of the
sky to scan, Palomar 48 observes all night long without a human driving
it. The data are then automatically processed by computers, and new
potential supernovae are presented to the discovery team when they wake
In fact, LCOGT is building a global network of telescopes to take this idea to the next level.
you have telescopes spread out in longitude, it is always dark
somewhere, so you can observe targets around the clock,” said Howell.
“We like to say that the sun will never rise on the LCOGT empire.”
already has telescopes in Haleakala, Hawaii, and Siding Spring,
Australia, as well as at the Sedgwick reserve near Santa Ynez, Calif. In
2012 it will expand to Texas, Chile, and South Africa. Last August,
astronomers at LCOGT were able to use the fledgling network to monitor
the brightness of the supernova as it rose in the weeks after explosion.
add to the robotic data, B.J. Fulton, a recent UCSB graduate and
astronomer at LCOGT, remotely controlled the 0.8-meter Byrne Observatory
Telescope at the Sedgwick Reserve from his home in Santa Barbara 35
soon as I got word of the young supernova from the PTF collaboration I
knew that this was a rare opportunity,” said Fulton. “I immediately
slewed the telescope to M101 and monitored the supernova nearly every
night for the next two months. We’re still in the process of automating
the Sedgwick telescope, so I operated it remotely, but in the future
observations like this will happen automatically, as they did at our
telescope in Hawaii.”
the supernova reached its peak brightness, Fulton used the Byrne
Observatory to take a series of shots, which he composed to make an
image that is as aesthetically pleasing as it is scientifically useful.
is the supermodel of supernovae,” said Howell. “The image B.J. created
of SN 2011fe is the most beautiful image ever of a thermonuclear
supernova and its host galaxy. And the fact that he did it with a modest
0.8-meter telescope is incredible. The previous poster child for type
Ia supernovae, SN 1994D, was obtained with the Hubble Space Telescope.
This image will be in all the textbooks.”
from Bildsten, Fulton, and Howell, other Santa Barbara contributors to
the findings include Federica Bianco, Benjamin Dilday, Melissa Graham,
and David Sand with UCSB and LCOGT; and Jerod Parrent with LCOGT.
Palomar Transient Factory (PTF) is based on the 48-inch Oschin Schmidt
telescope and the 60-inch telescope of the Palomar Observatory of the
California Institute of Technology, and is a collaboration among
California Institute of Technology; Columbia University; LCOGT; Lawrence
Berkeley Laboratory; Oxford University; University of California,
Berkeley; and the Weizmann Institute for Science.
Nature articles follow on the heels of the Dec. 10 presentation of the
Nobel Prize in physics to three astrophysicists. This Nobel Prize
recognized the scientists for the discovery that the universe is
accelerating its expansion, a finding that they made using type Ia
supernovae as a measuring tool, as “standard candles.”