A star in a distant galaxy explodes as a supernova: while observing a galaxy known as UGC 9379 (left; image from the Sloan Digital Sky Survey; SDSS) located about 360 million light years away from Earth, the team discovered a new source of bright blue light (right, marked with an arrow; image from the 60-inch robotic telescope at Palomar Observatory). This very hot, young supernova marked the explosive death of a massive star in that distant galaxy.
A detailed study of the spectrum (the distribution of colors composing
the light from the supernova) using a technique called “flash
spectroscopy” revealed the signature of a wind blown by the aging star
just prior to its terminal explosion, and allowed scientists to
determine what elements were abundant on the surface of the dying star
as it was about to explode as a supernova, providing important
information about how massive stars evolve just prior to their death,
and the origin of crucial elements such as carbon, nitrogen and oxygen.
The Palomar 48 inch telescope
Photo by: Iair Arcavi, Weizmann Instiute of Science
Berkeley Lab Researchers Help Catch a Wolf-Rayet Hours After it Goes Supernova
Our Sun may seem pretty impressive: 330,000 times as massive as
Earth, it accounts for 99.86 percent of the Solar System’s total mass;
it generates about 400 trillion trillion watts of power; and it has a
surface temperature of about 10,000 degrees Celsius. Yet for a star,
it’s a lightweight.
The real cosmic behemoths are Wolf-Rayet stars, which are more than
20 times as massive as the Sun and at least five times as hot. Because
these stars are relatively rare and often obscured, scientists don’t
know much about how they form, live and die. But this is changing,
thanks to an innovative sky survey called the intermediate Palomar Transient Factory (iPTF), which uses resources at the National Energy Research Scientific Computing Center (NERSC) and Energy Sciences Network (ESnet), both located at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), to expose fleeting cosmic events such as supernovae.
For the first time ever, scientists have direct confirmation that a
Wolf-Rayet star—sitting 360 million light years away in the Bootes
constellation—died in a violent explosion known as a Type IIb supernova.
Using the iPTF pipeline, researchers at Israel’s Weizmann Institute of Science
led by Avishay Gal-Yam caught supernova SN 2013cu within hours of its
explosion. They then triggered ground- and space-based telescopes to
observe the event approximately 5.7 hours and 15 hours after it
self-destructed. These observations are providing valuable insights into
the life and death of the progenitor Wolf-Rayet.
“Newly developed observational capabilities now enable us to study
exploding stars in ways we could only dream of before. We are moving
towards real-time studies of supernovae,” says Gal-Yam, an
astrophysicist in the Weizmann Institute’s Department of Particle
Physics and Astrophysics. He is also the lead author of a recently
published Nature paper on this finding.
“This is the smoking gun. For the first time, we can directly point to an observation and say that this type of Wolf-Rayet star leads to this kind of Type IIb supernova,” says Peter Nugent, who heads Berkeley Lab’s Computational Cosmology Center (C3) and leads the Berkeley contingent of the iPTF collaboration.
“When I identified the first example of a Type IIb supernova in 1987,
I dreamed that someday we would have direct evidence of what kind of
star exploded. It’s refreshing that we can now say that Wolf-Rayet stars
are responsible, at least in some cases,” says Alex Filippenko,
Professor of Astronomy at UC Berkeley. Both Filippenko and Nugent are
also co-authors on the Nature paper.
Elusive Signatures Illuminated in a Flash of Light
Some supermassive stars become Wolf-Rayets in the final stages of
their lives. Scientists find these stars interesting because they enrich
galaxies with the heavy chemical elements that eventually become the
building blocks for planets and life.
“We are gradually determining which kinds of stars explode, and why,
and what kinds of elements they produce,” says Filippenko. “These
elements are crucial to the existence of life. In a very real sense, we
are figuring out our own stellar origins.”
All stars—no matter what size—spend their lives fusing hydrogen atoms
to create helium. The more massive a star, the more gravity it wields,
which accelerates fusion in the star’s core, generating energy to
counteract gravitational collapse. When hydrogen is depleted, a
supermassive star continues to fuse even heavier elements like carbon,
oxygen, neon, sodium, magnesium and so on, until its core turns to iron.
At this point, atoms (even subatomic particles) are packed in so
closely that fusion no longer releases energy into the star. It is now
solely supported by electron degeneracy pressure—the quantum mechanical
law that prohibits two electrons from occupying the same quantum state.
When the core is massive enough, even electron degeneracy won’t
support the star and it collapses. Protons and electrons in the core
merge, releasing a tremendous amount of energy and neutrinos. This, in
turn, powers a shockwave that tears through the star ejecting its
remains violently into space as it goes supernova.
The Wolf-Rayet phase occurs before the supernova. As nuclear fusion
slows, the heavy elements forged in the star’s core rise to the surface
setting off powerful winds. These winds shed a tremendous amount of
material into space and obscure the star from prying telescopes on
Earth.
“When a Wolf-Rayet star goes supernova, the explosion typically
overtakes the stellar wind and all information about the progenitor star
is gone,” says Nugent. “We got lucky with SN 2013cu—we caught the
supernova before it overtook the wind. Shortly after the star exploded,
it let out an ultraviolet flash from the shock wave that heated and lit
up the wind. The conditions that we observed in this moment were very
similar to what was there before the supernova.”
Before the supernova debris overtook the wind, the iPTF team managed
to capture its chemical light signatures (or spectra) with the
ground-based Keck telescope
in Hawaii and saw the telltale signs of a Wolf-Rayet star. When the
iPTF team performed follow-up observations 15 hours later with NASA’s Swift satellite,
the supernova was still quite hot and strongly emitting in the
ultraviolet. In the following days, iPTF collaborators rallied
telescopes around the globe to watch the supernova crash into material
that had been previously ejected from the star. As the days went by, the
researchers were able to classify SN 2013cu as a Type IIb supernova
because of the weak hydrogen signatures and strong helium features in
the spectra that appeared after the supernova cooled.
“With a series of observations, including data I took with the Keck-I
telescope 6.5 days after the explosion, we could see that the
supernova’s expanding debris quickly overtook the flash-ionized wind
that had revealed the Wolf-Rayet features. So, catching the supernova
sufficiently early is hard—you’ve got to be on the ball, as our team
was,” says Filippenko.
“This discovery was totally shocking, it opens up a whole new
research area for us,” says Nugent. “With our largest telescopes you
might have a chance of getting a spectrum of a Wolf-Rayet star in the
nearest galaxies to our Milky Way, perhaps 4 million light years away.
SN 2013cu is 360 million light years away—further by almost factor of
100.”
And because the researchers caught the supernova early—when the
ultraviolet flash lit up the progenitor’s stellar wind—they were able to
take several spectra. “Ideally, we’d like to do this again and again
and develop some interesting statistics, not just for supernovae with
Wolf-Rayet progenitors but other types as well,” says Nugent.
Pipeline Upgrade Leads to Unexpected Discoveries
Since February 2014, the iPTF survey has been scanning the sky
nightly with a robotic telescope mounted on the 48-inch Samuel Oschin
Telescope at Palomar Observatory in Southern California. As soon as
observations are taken, the data travel more than 400 miles to NERSC in
Oakland via the National Science Foundation’s High Performance Wireless
Research and Education Network and the Department of Energy’s ESnet. At
NERSC, the Real-Time Transient Detection Pipeline sifts through the
data, identifies events to follow up on and sends an alert to iPTF
scientists around the globe.
The survey was built on the legacy of the Palomar Transient Factory
(PTF), designed in 2008 to systematically chart the transient sky by
using the same camera at Palomar Observatory. Last year Nugent and
colleagues at Caltech and UC Berkeley made significant modifications to
the transient detection pipeline for the iPTF project. Working with
NERSC staff, Nugent upgraded the pipeline’s computing and storage
hardware. The iPTF team also made improvements to the machine learning
algorithms at the heart of the detection pipeline and incorporated the Sloan Digital Star Survey III star and galaxy catalogs so the pipeline could immediately reject known variable stars.
They even added an asteroid rejection feature to the automated
workflow, which calculates the orbit of every known asteroid at the
beginning of the night, determines where the asteroids are in an
individual image, and then rejects them.
“All of our modifications significantly sped up our real-time
transient detection; we now send high quality supernova alerts to
astronomers all around the globe in less than 40 minutes after taking an
image at Palomar,” says Nugent. “In the case of SN 2013cu, that made
all the difference.”
***
The automated real-time detection pipeline was created under the DOE
Office of Science’s Scientific Discovery through Advanced Computing
(SciDAC) program and through additional support from NASA.
NERSC
provided the storage and systems infrastructure. NERSC and ESnet are
also supported by the DOE Office of Science.
Led by Shri Kulkarni of Caltech, iPTF has discovered more than 2000
supernovae during its four and a half years of observations, including
many rare and exotic types of cosmic outbursts. The iPTF survey is a
scientific collaboration among Caltech, Los Alamos National Laboratory,
the University of Wisconsin, Milwaukee, the Oskar Klein Center, the
Weizmann Institute of Science, the TANGO Program of the University
System of Taiwan, and the Kavli Institute for the Physics and
Mathematics of the Universe.
This research was supported by the I-CORE Program “The Quantum
Universe” of the Planning and Budgeting Committee and The Israel Science
Foundation; grants from the ISF, BSF, GIF, Minerva, the FP7/ERC, and a
Kimmel Investigator award; support from the Hubble and
Carnegie-Princeton Fellowships; support from the Arye Dissentshik career
development chair and a grant from the Israeli MOST; support from the
NSF; support from an NSF Postdoctoral Fellowship; support from the
TABASGO Foundation, the Christopher R. Redlich Fund, and NSF grant
AST-1211916. Some of the data were obtained at the W. M. Keck
Observatory, which was made possible by the generous financial support
of the W. M. Keck Foundation.
