Type Ia supernovae result from the explosions of white dwarf stars. These supernovae vary widely in peak brightness, how long they stay bright, and how they fade away, as the lower graph shows. Theoretical models (dashed black lines) seek to account for the differences, for example why faint supernovae fade quickly and bright supernovae fade slowly. A new analysis by the Nearby Supernova Factory indicates that when peak brightnesses are accounted for, as shown in the upper graph, the late-time behaviors of faint and bright supernovae provide solid evidence that the white dwarfs that caused the explosions had different masses, even though the resulting blasts are all “standard candles.” (Click here for best resolution)
The Nearby Supernova Factory based at Berkeley Lab shows that Type Ia supernovae have a surprisingly large range of masses
Sixteen years ago two teams of supernova hunters, one led by Saul Perlmutter of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the other by Brian Schmidt of the Australian National University, declared that the expansion of the universe is accelerating – a Nobel Prize-winning discovery tantamount to the discovery of dark energy. Both teams measured how fast the universe was expanding at different times in its history by comparing the brightnesses and redshifts of Type Ia supernovae, the best cosmological “standard candles.”
These dazzling supernovae are remarkably similar in brightness, given
that they are the massive thermonuclear explosions of white dwarf
stars, which pack roughly the mass of our sun into a ball the size of
Earth. Based on their colors and how fast they brighten and fade away,
the brightnesses of different Type Ia supernovae can be standardized to
within about 10 percent, yielding accurate gauges for measuring cosmic
distances.
Until recently, scientists thought they knew why Type Ia supernovae
are all so much alike. But their favorite scenario was wrong.
The assumption was that carbon-oxygen white dwarf stars, the
progenitors of the supernovae, capture additional mass by stripping it
from a companion star or by merging with another white dwarf; when they
approach the Chandrasekhar limit (40 percent more massive than our sun)
they experience thermonuclear runaway. Type Ia brightnesses were so
similar, scientists thought, because the amounts of fuel and the
explosion mechanisms were always the same.
“The Chandrasekhar mass limit has long been put forward by
cosmologists as the most likely reason why Type Ia supernovae
brightnesses are so uniform, and more importantly, why they are not
expected to change systematically at higher redshifts,” says cosmologist
Greg Aldering, who leads the international Nearby Supernova Factory
(SNfactory) based in Berkeley Lab’s Physics Division. “The Chandrasekhar
limit is set by quantum mechanics and must apply equally, even for the
most distant supernovae.”
But a new analysis of normal Type Ia supernovae, led by SNfactory
member Richard Scalzo of the Australian National University, a former
Berkeley Lab postdoc, shows that in fact they have a range of masses.
Most are near or slightly below the Chandrasekhar mass, and about one
percent somehow manage to exceed it.
The SNfactory analysis has been accepted for publication by the Monthly Notices of the Royal Astronomical Society and is available online as an arXiv preprint.
A new way to analyze exploding stars
While white dwarf stars are common, Scalzo says, “it’s hard to get a
Chandrasekhar mass of material together in a natural way.” A Type Ia
starts in a two-star (or perhaps a three-star) system, because there has
to be something from which the white dwarf accumulates enough mass to
explode.
Some models picture a single white dwarf borrowing mass from a giant
companion. However, says Scalzo, “The most massive newly formed
carbon-oxygen white dwarfs are expected to be around 1.2 solar masses,
and to approach the Chandrasekhar limit a lot of factors would have to
line up just right even for these to accrete the remaining 0.2 solar
masses.”
If two white dwarfs are orbiting each other they somehow have to get
close enough to either collide or gently merge, what Scalzo calls “a
tortuously slow process.” Because achieving a Chandrasekhar mass seems
so unlikely, and because sub-Chandrasekhar white dwarfs are so much more
numerous, many recent models have explored how a Type Ia explosion
could result from a sub-Chandrasekhar mass – so many, in fact, that
Scalzo was motivated to find a simple way to eliminate models that
couldn’t work.
He and his SNfactory colleagues determined the total energy of the
spectra of 19 normal supernovae, 13 discovered by the SNfactory and six
discovered by others. All were observed by the SNfactory’s unique SNIFS
spectrograph (SuperNova Integral Field Spectrograph) on the University
of Hawaii’s 2.2-meter telescope on Mauna Kea, corrected for ultraviolet
and infrared light not observed by SNIFS.
A supernova eruption thoroughly trashes its white dwarf progenitor,
so the most practical way to tell how much stuff was in the progenitor
is by spectrographically “weighing” the leftover debris, the ejected
mass. To do this Scalzo took advantage of a supernova’s layered
composition.
A Type Ia’s visible light is powered by radioactivity from nickel-56,
made by burning carbon near the white dwarf’s center. Just after the
explosion this radiation, in the form of gamma rays, is absorbed by the
outer layers – including iron and lighter elements like silicon and
sulfur, which consequently heat up and glow in visible wavelengths.
But a month or two later, as the outer layers expand and dissipate,
the gamma rays can leak out. The supernova’s maximum brightness compared
to its brightness at late times depends on how much gamma radiation is
absorbed and converted to visible light – which is determined both by
the mass of nickel-56 and the mass of the other material piled on top of
it.
The SNfactory team compared masses and other factors with light
curves: the shape of the graph, whether narrow or wide, that maps how
swiftly a supernova achieves its brightest point, how bright it is, and
how hastily or languorously it fades away. The typical method of
“standardizing” Type Ia supernovae is to compare their light curves and
spectra.
“The conventional wisdom holds that the light curve width is
determined primarily or exclusively by the nickel-56 mass,” Scalzo says,
“whereas our results show that there must also be a deep connection
with the ejected mass, or between the ejected mass and the amount of
nickel-56 created in a particular supernova.”
Exploding white dwarf stars, the bottom line
Greg Aldering summarizes the most basic result of the new analysis:
“The white dwarfs exploding as Type Ia supernovae have a range of
masses, and the resulting light-curve width is directly proportional to
the total mass involved in the explosion.”
For a supernova whose light falls off quickly, the progenitor is a
lot less massive than the Chandrasekhar mass – yet it’s still a normal
Type Ia, whose luminosity can be confidently standardized to match other
normal Type Ia supernovae.
The same is true for a Type Ia that starts from a “classic”
progenitor with Chandrasekhar mass, or even more. For the heavyweights,
however, the pathway to supernova detonation must be significantly
different than for lighter progenitors. These considerations alone were
enough to eliminate a number of theoretical models for Type Ia
explosions.
Carbon-oxygen white dwarfs are still key. They can’t explode on their
own, so another star must provide the trigger. For super-Chandrasekhar
masses, two C-O white dwarfs could collide violently, or one could
accrete mass from a companion star in a way that causes it to spin so
fast that angular momentum supports it beyond the Chandrasekhar limit.
More relevant for cosmolology, because more numerous, are models for
sub-Chandrasekhar mass. From a companion star, a C-O white dwarf could
accumulate helium, which detonates more readily than carbon – the result
is a double detonation. Or two white dwarfs could merge. There are
other surviving models, but the psychological “safety net” that the
Chandrasekhar limit once provided cosmologists has been lost. Still,
says Scalzo, the new analysis narrows the possibilities enough for
theorists to match their models to observations.
“This is a significant advance in furthering Type Ia supernovae as
cosmological probes for the study of dark energy,” says Aldering,
“likely to lead to further improvements in measuring distances. For
instance, light-curve widths provide a measure of the range of the star
masses that are producing Type Ia supernovae at each slice in time, well
back into the history of the universe.”
This work was supported by DOE’s Office of Science and the Gordon and
Betty Moore Foundation, and in France by CNRS/IN2P3 (National Center
for Scientific Research, National Institute of Nuclear and Particle
Physics), CNRS/INSU (CNRS National Institute for Earth Sciences and
Astronomy), and PNC (Programme National de Cosmologie).
###
“Type Ia Supernova Bolometric Light Curves and Ejected Mass
Estimates from the Nearby Supernova Factory,” by Richard Scalzo, Greg
Aldering, Pierre Antilogus, Cecilia Aragon, Stephen Bailey, Charles
Baltay, Sébastien Bongard, Clement Buton, Flora Cellier-Holzem, Mike
Childress, Nicolas Chotard, Yannick Copin, Hannah K. Fakhouri, Emmanuel
Gangler, Julien Guy, Alex Kim, Marek Kowalski, Markus Kromer, Jakob
Nordin, Peter Nugent, Kerstin Paech, Reynald Pain, Emmanuel Pécontal,
Rui Pereira, Saul Perlmutter, David Rabinowitz, Mickael Rigault, Karl
Runge, Clare Saunders, Stuart Sim, Gerard Smadja, Charling Tao, Stefan
Taubenberger, Rollin Thomas, and Benjamin Alan Weaver (The Nearby
Supernova Factory), will appear in the Monthly Notices of the Royal Astronomical Society and is available online as an arXiv preprint.
The researchers acknowledge the assistance of the University of Hawaii 2.2-meter telescope, the W. M. Keck Observatory, Lick Observatory, Southern Astrophysical Research telescope (SOAR), the Palomar Observatory, QUEST-II collaboration, and High Performance Wireless Research and Education Network (HPWREN) in obtaining the data, and DOE’s National Energy Research Scientific Computing Center (NERSC) for storage and computing time.
The Nearby Supernova Factory
is a scientific collaboration between the Centre de Recherche
Astronomique de Lyon, Institut de Physique Nucléaire de Lyon,
Laboratoire de Physique Nucléaire et des Hautes Energies, Lawrence
Berkeley National Laboratory, Yale University, University of Bonn, Max
Planck Institute for Astrophysics, Tsinghua Center for Astrophysics, and
the Centre de Physique des Particules de Marseille. The ARC Centre of Excellence for All‐sky Astrophysics (CAASTRO)
is a collaboration between The University of Sydney, The Australian
National University, The University of Melbourne, Swinburne University
of Technology, The University of Queensland, The University ofb Western
Australia, and Curtin University.
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 http://www.lbl.gov.
The DOE Office of Science is the single largest supporter of basic
research in the physical sciences in the United States and is working to
address some of the most pressing challenges of our time. For more
information, please visit science.energy.gov.
Paul Preuss
Email: paul_preuss@lbl.gov
