Berkeley
Lab researchers make historic observation of rare Type 1a Supernova
Berkeley, Calif.—Exploding stars called
Type 1a supernova are ideal for measuring cosmic distance because they are
bright enough to spot across the Universe and have relatively the same
luminosity everywhere. Although astronomers have many theories about the kinds
of star systems involved in these explosions (or progenitor systems), no one
has ever directly observed one—until now.
In the August 24 issue of Science, the
multi-institutional Palomar Transient Factory (PTF) team presents the first-ever
direct observations of a Type 1a supernova progenitor system. Astronomers have
collected evidence indicating that the progenitor system of a Type 1a
supernova, called PTF 11kx, contains a red giant star. They also show that the
system previously underwent at least one much smaller nova eruption before it
ended its life in a destructive supernova. The system is located 600 million
light years away in the constellation Lynx.
By comparison, indirect observations of
another Type 1a supernova progenitor system (called SN 2011fe, conducted by the
PTF team last year) showed no evidence of a red giant star. Taken together,
these observations unequivocally show that just because Type 1a supernovae look
the same, that doesn’t mean they are all born the same way.
“We know that Type 1a supernovae vary
slightly from galaxy to galaxy, and we’ve been calibrating for that, but this
PTF 11kx observation is providing the first explanation of why this happens,”
says Peter Nugent, a senior scientist at the Lawrence Berkeley National
Laboratory (Berkeley Lab) and a co-author on the paper. “This discovery gives
us an opportunity to refine and improve the accuracy of our cosmic
measurements.”
“It’s a total surprise to find that
thermonuclear supernovae, which all seem so similar, come from different kinds
of stars,” says Andy Howell, a staff scientist at the Las Cumbres Observatory
Global Telescope Network (LCOGT) and a co-author on the paper. “How could these
events look so similar, if they had different origins?”
A
One in a Thousand Discovery, Powered by Supercomputers
The supernova PTF 11kx can be seen as
the blue dot on the galaxy. The image was taken when the supernova was near
maximum brightness by the Faulkes Telescope North. The system is located
approximately 600 million light years away in the constellation Lynx. (BJ
Fulton, Las Cumbres Observatory Global Telescope Network)
Although Type 1a supernovae are rare,
occurring maybe once or twice a century in a typical galaxy, Nugent notes that
finding a Type 1a progenitor system like PTF 11kx is even more rare. “You maybe
find one of these systems in a sample of 1,000 Type 1a supernovae,” he says.
“The Palomar Transient Factory Real-Time Detection Pipeline was crucial to
finding PTF 11kx.”
The PTF survey uses a robotic telescope
mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory in
southern California to scan the sky nightly. As the observations are taken, the
data travels more than 400 miles via high-speed networks–including the National
Science Foundation’s High Performance Wireless Research and Education Network
and the Department of Energy’s Energy Sciences Network (ESnet)–to the National
Energy Research Scientific Computing Center (NERSC), located at Berkeley Lab.
There, the Real-time Transient Detection Pipeline uses supercomputers, a
high-speed parallel filesystem and sophisticated machine learning algorithms to
sift the data and identify events for scientists to follow up on.
According to Nugent, the pipeline
detected the supernova on January 16, 2011. He and UC Berkeley postdoctoral
researcher Jeffrey Silverman immediately followed up on the event with
spectroscopy observations from the Shane telescope at the University of
California’s Lick Observatory. These observations revealed incredibly strong
calcium signals in the gas and dust surrounding the supernova, which is
extremely unusual.
The signals were so peculiar that Nugent
and his UC Berkeley colleagues, Alex Filippenko and Joshua Bloom, triggered a
Target of Opportunity (ToO) observation using the Keck Telescope in Hawaii. “We
basically called up a fellow UC observer and interrupted their observations in
order to get time critical spectra,” Nugent explains.
From the Keck observations, astronomers
noticed that the clouds of gas and dust surrounding PTF 11kx were moving too
slowly to be coming from the recent supernova, but moving too quickly to be
stellar wind. They suspected that maybe the star erupted, or went nova,
previously propelling a shell of material outwards. The material, they
surmised, must be slowing down as it collided with wind from a nearby red giant
star. But for this theory to be true, the material from the recent supernova
should eventually catch up and collide with gas and dust from the previous
nova. That’s exactly what the PTF team eventually observed.
In the months following the supernova,
the PTF team watched the calcium signal drop and eventually vanish. Then, 58
days after the supernova went off, Berkeley Lab Scientist Nao Suzuki who was
observing the system with the Lick telescope noticed a sudden, strong burst in
calcium coming from the system, indicating that the new supernova material had
finally collided with the old material.
“This was the most exciting supernova
I’ve ever studied. For several months, almost every new observation showed
something we’d never seen before,” says Ben Dilday, a UC Santa Barbra
postdoctoral researchers and lead author of the study.
A
New Kind of Type 1a Supernova
According to Dilday, it is not unusual
for a star to undergo nova eruptions more than once. In fact, a “recurrent nova” system called RS Ophiuchi
exists within our own Milky Way Galaxy. Located about 5,000 light years away,
the system is close enough that astronomers can tell that it consists of a
compact white dwarf star (the corpse of a sun-like star) orbiting a red giant.
Material being blown off the red giant star in a stellar wind lands on the
white dwarf. As the material builds up, the white dwarf periodically explodes,
or novas, in this case, about every 20 years.
Astronomers predict that in recurring
novas, the white dwarf loses more mass in the nova eruption than it gains from
the red giant. Because Type 1a supernovae occur in systems where a white dwarf
accretes mass from a nearby star until it can’t grow any further and explodes,
many scientists concluded that recurrent nova systems could not produce Type 1a
supernovae. They thought the white dwarf would lose too much mass to ever
become a supernova. PTF 11kx is the first observational evidence that Type 1a
supernovae can occur in these systems.
“Because we’ve looked at thousands of
systems and PTF 11kx is the only one that we’ve found that looks exactly like
this, we think it is probably a rare phenomenon. However, these systems could
be somewhat more common, and nature is just hiding their signatures from us,”
says Silverman.
The Palomar Transient Factory’s
Real-Time detection pipeline is made possible with support from the DOE Office
of Science, NASA, and the National Science Foundation.
###
The Palomar Transient Factory is an
international collaboration of scientists and engineers from Berkeley Lab,
California Institute of Technology (Caltech), NASA’s Infrared Processing and
Analysis Center, UC Berkeley, Las Cumbres Observatory Global Telescope Network,
the University of Oxford, Columbia University, the Weizmann Institute of
Science in Israel, and Pennsylvania State University.
http://www.astro.caltech.edu/ptf/.
About
Berkeley Lab
Lawrence Berkeley National Laboratory
addresses the world’s most urgent scientific challenges by advancing
sustainable energy, protecting human health, creating new materials, and
revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s
scientific expertise has been recognized with 13 Nobel prizes. The University
of California manages Berkeley Lab for the U.S. Department of Energy’s Office
of Science. For more, visit www.lbl.gov.
About
NERSC
The National Energy Research Scientific
Computing Center (NERSC) is the primary high-performance computing facility for
scientific research sponsored by the U.S. Department of Energy’s Office of
Science. Located at Lawrence Berkeley National Laboratory, the NERSC Center
serves more than 4,000 scientists at national laboratories and universities
conducting fundamental research in a wide range of disciplines.
http://www.nersc.gov
About
ESnet
The Energy Sciences Network (ESnet)
provides the high-bandwidth, reliable connections that link scientists at
national laboratories, universities and other research institutions, enabling
them to collaborate on some of the world’s most important scientific challenges
including energy, climate science, and the origins of the universe. Funded by
the U.S. Department of Energy’s (DOE) Office of Science and located within the
Scientific Networking Division at Lawrence Berkeley National Laboratory, ESnet
provides scientists with access to unique DOE research facilities and computing
resources. http://es.net
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