From the “Backyard Supernova,” the Berkeley Lab-led Nearby Supernova Factory has built a benchmark atlas for normal Type Ia’s
August, 2011, saw the dazzling appearance of the closest and brightest Type Ia supernova since Type Ia’s were established as “standard candles” for measuring the expansion of the universe. The brilliant visitor, labeled SN 2011fe, was caught by the Palomar Transient Factory less than 12 hours after it exploded in the Pinwheel Galaxy in the Big Dipper.
Easy
to see through binoculars, 2011fe was soon dubbed the Backyard
Supernova. Major astronomical studies from the ground and from space
followed close on its heels, recording its luminosity and colors as it
rapidly brightened and then slowly faded away.
The
international Nearby Supernova Factory (SNfactory), led by Greg
Aldering of the U.S. Department of Energy’s Lawrence Berkeley National
Laboratory (Berkeley Lab), has now released a unique dataset based on 32
nights of repeated observations of 2011fe with the SuperNova
Integral Field Spectrograph (SNIFS), built by the SNfactory’s partners
in Lyon and Paris, France, and mounted on the University of Hawaii’s
2.2-meter telescope on Mauna Kea. The observations began
two weeks before the supernova reached its peak brightness and
continued for over three months after maximum light had passed.
“We’d never before seen a Type Ia supernova this early,” says Aldering, a cosmologist in Berkeley Lab’s Physics Division. “Our measurements showed how remarkably normal 2011fe is.”
SNfactory member Rui Pereira of the Institut de
Physique Nucléaire de Lyon says that the collected data “will be the
benchmark atlas for all future studies of Type Ia’s.” Pereira is the
lead author of the article presenting the observations in the journal Astronomy & Astrophysics.
Why a perfectly normal supernova is so odd
Type Ia supernovae aren’t so much standard
candles as “standardizable” ones. Graphs of how their brightness and
spectral features change over time – their light curves – vary, but
because timing and brightness are related, the light curves can be
stretched (or squeezed) to match the standard. SN 2011fe’s light curve
falls right in the peak of the distribution – as astrophysicists say, it
has “stretch 1.”
Rollin Thomas, of Berkeley Lab’s Computational
Research Division, was deeply involved in the 2011fe analysis. As new
data arrived from the telescope each night he recalls thinking “please
don’t be peculiar, please don’t be peculiar,” and was pleased to find
that the supernova was so normal.
2011fe not only looks like a textbook case, it
passes important tests. Its brightness at different times (epochs) could
be accurately recorded because the distance to its home galaxy had been
measured independently, and there was little or no dust in the line of
sight to affect color or brightness.
Normal as it is, however, 2011fe’s light curve
doesn’t match the leading computational models, none of which fit the
SNfactory data. Given the unavoidable uncertainties of supernova
observation, says Aldering, “to date it has been a little too easy to
cobble data together, depending on what you think it should be.” The
SNfactory’s benchmark atlas raises the bar. “From now on researchers
won’t be able to arbitrarily tweak knobs in their models.”
The 2011fe gold-standard atlas will help answer
many longstanding questions about Type Ia supernovae, including the
progenitors of these titanic thermonuclear explosions and the mechanisms
of the explosions themselves.
The single degenerate model of Type Ia
progenitors posits a single white dwarf that steals extra mass from a
large companion star. (Electron “degeneracy” is a result of tight
packing of atoms in a white dwarf.) In the resulting supernova explosion
there should be signs of interaction with the companion, or what’s left
of it. In the double degenerate model, two white dwarfs collide. The
resulting supernova would show no signs of interaction with a companion.
“The 2011fe observations can be used to test
these models,” says Aldering. “For 2011fe, the existing models of the
double-degenerate scenario agreed best at some epochs, but the
single-degenerate scenario was better at others. And for some epochs
both agreed very poorly with the data, suggesting these models have a
way to go.”
The 2011fe data also point to unburned carbon
as characteristic of the spectrum of a normal Type Ia. The finding adds
support for a particular model, “pure turbulent deflagration,” compared
to two-stage explosions that would eliminate most excess carbon.
Carbon surviving from the original white dwarf
indicates that different supernovae burn material with a range of
different efficiencies when they explode. Methods for detecting unburned
carbon, which may often have been missed in the past, are suggested by
the 2011fe data.
In sum, says Aldering, “The SN 2011fe atlas
offers unprecedented detail and a solid point of reference for Type Ia
physics. We’ve never had data like this. It’s a dream opportunity to
stimulate deeper thinking about these markers of the expansion of the
universe.”
This work was supported by DOE’s Office of
Science; the Gordon and Betty Moore Foundation; the French National
Center for Scientific Research (CNRS) National Institute of Nuclear and
Particle Physics (IN2P3) and National Institute for Earth Sciences and
Astronomy (INSU); the French National Program of Cosmology and Galaxies
(PNCG); and the Transregional Research Center, “The Dark Universe” (TRR33), of the German Research Foundation (DFG).
* * *
“Spectrophotometric
time series of SN 2011fe from the Nearby Supernova Factory,” by Rui
Pereira, Rollin C. Thomas, Greg Aldering, Pierre Antilogus, Charles
Baltay, Sandra Benitez-Herrera, Sébastien Bongard, Clement Buton, Arnaud
Canto, Flora Cellier-Holzem, Juncheng Chen, Mike Childress, Nicolas
Chotard, Yannick Copin, Hannah K. Fakhouri, Michael Fink, Dominique
Fouchez, Emmanuel Gangler, Julien Guy, Wolfgang Hillebrandt, Eric Y.
Hsiao, Matthias Kerschhaggi, Marek Kowalski, Markus Kromer, Jakob
Nordin, Peter Nugent, Kerstin Paech, Reynald Pain, Emmanuel Pécontal,
Saul Perlmutter, David Rabinowitz, Mickael Rigault, Karl Runge, Clare
Saunders, Gerard Smadja, Charling Tao, Stefan Taubenberger, André
Tilquin. and Chao Wu (The Nearby Supernova Factory), will appear in Astronomy & Astrophysics and is available online at http://www.aanda.org/articles/aa/abs/2013/06/aa21008-12/aa21008-12.html.
The SNfactory has constructed an animation of
SN 2011fe’s light curve, showing a wide range of wavelengths varying in
brightness at different epochs as the spectrum evolves. Available online
at http://www.youtube.com/watch?v=8t9ZWOrUtCc, the animation is an exemplar of a normal Type Ia light curve and an ideal teaching tool.
SN 2011fe was discovered on August 24, 2011, by
Peter Nugent of Berkeley Lab’s Computational Research Division, who
leads the Lab’s collaboration in the multi-institutional Palomar
Transient Factory (PTF), while he was searching PTF data arriving at
Lab’s National Energy Research Scientific Computing Center (NERSC). See http://newscenter.lbl.gov/news-releases/2011/12/14/sn-2011fe/.
Nearby Supernova Factory partners include the Lawrence Berkeley National Laboratory, the Institut de Physique Nucleaire
de Lyon, the Laboratoire de Physique Nucleaire et
des Hautes Energies, the Centre de Recherche Astronomique de Lyon, Yale
University, Universitat Bonn, the Tsinghua Center for Astrophysics, the
Max Planck Institute for Astrophysics, and the Centre de Physique des
Particules de Marseille.
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