The 1.2-meter Samuel Oschin Telescope at Palomar Observatory that was used to discover four supernovae of a new class. Inset: one of the newly discovered supernovae, PTF09cnd. Image: Caltech/Scott Kardel/Robert Quimby/modified from Nature |
They’re bright and blue—and a bit strange.
They’re a new type of stellar explosion that was recently discovered by a team
of astronomers led by the California Institute of Technology (Caltech). Among
the most luminous in the cosmos, these new kinds of supernovae could help
researchers better understand star formation, distant galaxies, and what the
early universe might have been like.
“We’re learning about a whole new
class of supernovae that wasn’t known before,” says Robert Quimby, a
Caltech postdoctoral scholar and the lead author on a paper to be published in Nature. In addition to finding four
explosions of this type, the team also discovered that two previously known
supernovae, whose identities had baffled astronomers, also belonged to this new
class.
Quimby first made headlines in 2007 when—as
a graduate student at the University of Texas, Austin—he discovered what was
then the brightest supernova ever found: 100 billion times brighter than the
sun and 10 times brighter than most other supernovae. Dubbed 2005ap, it was
also a little odd. For one thing, its spectrum—the chemical fingerprint that
tells astronomers what the supernova is made of, how far away it is, and what
happened when it blew up—was unlike any seen before. It also showed no signs of
hydrogen, which is commonly found in most supernovae.
At around the same time, astronomers using
the Hubble Space Telescope discovered a mysterious supernova called SCP 06F6.
This supernova also had an odd spectrum, though there was nothing that
indicated this cosmic blast was similar to 2005ap.
Shri Kulkarni, Caltech’s John D. and
Catherine T. MacArthur Professor of Astronomy and Planetary Science and a
coauthor on the paper, recruited Quimby to become a founding member of the
Palomar Transient Factory (PTF). The PTF is a project that scans the skies for
flashes of light that weren’t there before—flashes that signal objects called
transients, many of which are supernovae. As part of the PTF, Quimby and his
colleagues used the 1.2-meter Samuel Oschin Telescope at Palomar Observatory to
discover four new supernovae. After taking spectra with the 10-meter Keck
telescopes in Hawaii, the 5.1-meter telescope
at Palomar, and the 4.2-meter William Herschel Telescope in the Canary Islands, the astronomers discovered that all four
objects had an unusual spectral signature.
Quimby then realized that if you slightly
shifted the spectrum of 2005ap—the supernova he had found a couple of years
earlier—it looked a lot like these four new objects. The team then plotted all
the spectra together. “Boom—it was a perfect match,” he recalls.
|
The astronomers soon determined that
shifting the spectrum of SCP 06F6 similarly aligned it with the others. In the
end, it turned out that all six supernovae are of the same type, and that they
all have spectra that are very blue—with the brightest wavelengths shining in
the ultraviolet.
According to Quimby, the two mysterious
supernovae—2005ap and SCP 06F6—had looked different from one another because
2005ap was 3 billion light-years away while SCP 06F6 was 8 billion light-years
away. More distant supernovae have a stronger cosmological redshift, a
phenomenon in which the expanding universe stretches the wavelength of the
emitted light, shifting supernovae spectra toward the red end.
The four new discoveries, which had
features similar to 2005ap and SCP 06F6, were at an intermediate distance,
providing the missing link that connected the two previously unexplained
supernovae. “That’s what was most striking about this—that this was all one
unified class,” says Mansi Kasliwal, a Caltech graduate student and
coauthor on the Nature paper.
Even though astronomers now know these
supernovae are related, no one knows much else. “We have a whole new class
of objects that can’t be explained by any of the models we’ve seen
before,” Quimby says. What we do know about them is that they are bright
and hot—10,000 to 20,000 Kelvin; that they are expanding rapidly at 10,000
kilometers per second; that they lack hydrogen; and that they take about 50
days to fade away—much longer than most supernovae, whose luminosity is often
powered by radioactive decay. So there must be some other mechanism that’s
making them so bright.
One possible model that would create an
explosion with these properties involves a pulsating star about 90 to 130 times
the mass of the sun. The pulsations blow off hydrogen-free shells, and when the
star exhausts its fuel and explodes as a supernova, the blast heats up those
shells to the observed temperatures and luminosities.
A second model requires a star that
explodes as a supernova but leaves behind what’s called a magnetar, a rapidly
spinning dense object with a strong magnetic field. The rotating magnetic field
slows the magnetar down as it interacts with the sea of charged particles that
fills space, releasing energy. The energy heats the material that was
previously blown off during the supernova explosion and can naturally explain
the brightness of these events.
The newly discovered supernovae live in
dim, small collections of a few billion stars called dwarf galaxies. (Our own
Milky Way has 200–400 billion stars.) The supernovae, which are almost a
hundred times brighter than their host galaxies, illuminate their environments
like distant street lamps lighting up dark roads. They work as a kind of
backlight, enabling astronomers to measure the spectrum of the interstellar gas
that fills the dwarf galaxies in which the supernovae reside, and revealing
each galaxy’s composition. Once an observed supernova fades a couple of months
later, astronomers can directly study the dwarf galaxy—which would have
remained undetected if it weren’t for the supernova.
These supernovae could also reveal what
ancient stars might have been like, since they most likely originate from stars
around a hundred times more massive than the sun—stars that would have been
very similar to the first stars in the universe.
“It is really amazing how rich the night
sky continues to be,” Kulkarni says. “In addition to supernovae, the
Palomar Transient Factory is making great advances in stellar astronomy as well.”
