Supernovae Shape Up for Cosmology
A collaboration among the U.S. Department
of Energy's Lawrence Berkeley National Laboratory, the
European Southern Observatory, and the University of
Texas has yielded the discovery that Type 1a supernovae
do not explode spherically. This discovery marks the
first time that the intrinsic polarization of a normal
Type 1a supernova has been detected.
![Supernova 2001el](/peth04/20041025102236im_/http://sc.doe.gov/Science_News/feature_articles_2003/september/cosmology/2001el.jpg) |
Supernova 2001el appeared
in September 2001, in galaxy NGC 1448 in the southern
constellation Horologium (the Clock) |
September 22, 2003Scientists
at the DOE Office of Science's Lawrence Berkeley National
Laboratory, working with colleagues at the European
Southern Observatory (ESO) and the University of Texas
at Austin, have established that the extraordinarily
bright and remarkably similar astronomical "standard
candles" known as Type Ia supernovae do not explode
in a perfectly spherical manner.
Led by Lifan Wang, an astronomer and astrophysicist
in Berkeley Lab's Physics Division, the researchers
used the ESO's Very Large Telescope (VLT) in Chile to
measure the polarization of light emitted by supernova
2001el as it brightened and dimmed. This is the first
time the intrinsic polarization of a normal Type Ia
supernova has been detected.
The researchers were able to show that
at peak brightness the exploding star was slightly flattened,
with one axis shorter by about 10 percent. By a week
later, however, the visible explosion was virtually
spherical.
"For the first time we have actually measured
the asymmetry of a Type Ia supernova," says Wang. Not
only can this information be used to test models of
how Type Ia supernovae originate and explode, he says,
it also helps to underline "how valid supernovae are
for doing cosmology."
It was by comparing the brightness and
redshift of Type Ia supernovae that the international
Supernova Cosmology Project, based at Berkeley Lab,
discovered the accelerating expansion of the universe;
confirmed by other researchers, their finding was announced
in 1998. Accelerating expansion implies the existence
of a "cosmological constant" or other form of so-called
dark energy, now known to constitute some 75 percent
of the density of the universe.
Yet while Type Ia supernovae are by far
the best standard candles for measuring cosmological
distances, and hence for investigating dark energy,
a small measurement uncertainty persists.
"The asymmetry we have measured in SN
2001el is large enough to account for a large part of
this intrinsic uncertainty," says Wang. "If all Type
Ia supernovae are like this, it would account for a
lot of the dispersion in brightness measurements. They
may be even more uniform than we thought."
The spectropolarimetry program led by
Wang has previously established that other types of
supernovae show considerably higher degrees of polarization,
and therefore asphericity, than does Type Ia. Not only
is the asphericity of Type Ia supernovae small by comparison,
its effect on brightness measurements may be readily
correctable.
Asymmetry and How to Measure
It
Wang and his colleagues use the example
of a carton of eggs to explain how asymmetry can affect
brightness measurements. All the eggs in the carton
are similar, but the egg shape is only apparent when
they are viewed from the side; viewed end-on, an egg
looks round.
Likewise, if supernovae are not spherically
symmetric, they will shine more brightly in one direction
than in others. Even with a telescope as powerful as
the VLT, however, distant supernovae appear only as
point-sources of light, so asymmetric shapes cannot
be seen directly. Instead they must be inferred from
the way the light is polarized.
Polarization refers to the orientation
of the plane of the electric wave component of light
and other electromagnetic radiation. Polaroid sunglasses,
for example, "measure" polarization by blocking or absorbing
much of the light polarized by reflection from horizontal
surfaces.
![Telescope](/peth04/20041025102236im_/http://sc.doe.gov/Science_News/feature_articles_2003/september/cosmology/telescope.jpg) |
The Very Large Telescope
at the European Southern Observatory in Paranal,
Chile. |
In the light from a spherically symmetric
star, however, all orientations are equally represented,
and there is no net polarization. Not so for an asymmetric
star or explosion. Light emitted along the longer side
shows a net excess of a particular polarization.
"The differences are very small," says
Dietrich Baade, a scientist with the European Southern
Observatory and a member of the team that did the spectropolarimetry.
"Measuring them requires an instrument that is very
sensitive and very stable."
To study SN 2001el, the team used the
FORS1 spectropolarimetry instrument in conjunction with
the VLT, the world's largest optical telescope array.
They analyzed the polarization of various parts of the
supernova's spectrum, beginning immediately after its
discovery in September 2001. They made repeated measurements
as the supernova grew brighter, reached maximum brightness,
and then slowly faded.
Following the Light Curve
"Distance measurements of Type Ia supernovae
have typically been calculated at maximum brightness,"
says Wang. "Our observations of SN 2001el show that
asymmetry persists up to and beyond maximum brightness."
As spherical symmetry begins to dominate,
about a week after maximum, "it's not because the supernova
is changing shape, but because we are seeing different
layers of it," says Wang. Outer layers expanding at
thousands of kilometers a second grow diffuse and become
transparent, allowing the inner layers to become visible.
"When it explodes, the outer part is aspherical, but
as we see lower down, the dense inner core is spherical."
The spectropolarimetry of SN 2001el suggests
that information from the fading part of Type Ia supernovae
light curves can be used to reduce uncertainties in
the relation between their distance and brightness.
![Polarimetric studies](/peth04/20041025102236im_/http://sc.doe.gov/Science_News/feature_articles_2003/september/cosmology/2001el2.jpg) |
Polarimetric studies of
SN 2001el favor the model in which a Type Ia supernova
begins as a white dwarf star accreting matter from
a companion. |
Polarimetry also has much to say about
a Type Ia supernova's progenitors and the way it burns
when it explodes. The observations of SN 2001el provide
evidence for the model in which a white dwarf star accretes
material from an orbiting companion until it reaches
the Chandrasekhar limit, a critical mass about 1.4 times
the mass of our sun.
To fully understand the resulting thermonuclear
explosion it may be essential, as the spectropolarimetry
of SN 2001el shows, to incorporate polarimetric data
in three-dimensional models of the process. Typical
one and two-dimensional computer modeling is inadequate.
In addition to polarimetry, analyzing
the spectrum of a Type Ia supernova during the rising
part of its light curve can reveal specific information
about its elemental composition. "This shows that we
need to follow the rise and fall of the light curve
completely," Wang says.
In the future, very high-precision projects
like the SuperNova/Acceleration Probe (SNAP) satellite,
now under development, will insure that Type Ia supernovae
are compared like-to-like, thus averaging out any asymmetries.
In order to investigate dark energy, SNAP is designed
to find thousands of new supernovae and perform detailed
studies of their spectra as they brighten and fade.
Meanwhile the ground-based Nearby Supernova
Factory, another collaboration based at Berkeley Lab,
will study hundreds of Type Ia supernovae including,
in some cases, polarimetric examinations like those
pioneered by the FORS1 observation of SN 2001el. The
ESO VLT's extraordinary achievement stands as a paradigm
in a new age of supernova observations.by
Paul Preuss
Media contact: Paul Preuss,
LBNL science writer, (510) 486-6249, mailto:paul_preuss@lbl.gov
Technical contact: Lifan Wang, LBNL
astronomer and astrophysicist, (510) 495-2733, mailto:LWang@lbl.gov
Related Links
"Spectropolarimetry
of SN 2001el in NGC 1448: asphericity of a normal Type
Ia supernova." Lifan Wang, Dietrich Baade, Peter
Hoeflich, Alexei Khokhlov, J. Craig Wheeler, D. Kasen,
Peter E. Nugent, Saul Perlmutter, Claes Fransson, and
Peter Lundqvist. Astrophysical Journal, vol 591, p 1110
(10 July, 2003)
The
VLT Measures the Shape of a Type Ia Supernova (European
Southern Observatory press release)
Supernova
Cosmology Project
European
Southern Observatory
Supernova
Research Group at the University of Texas
SNAP satellite
Funding:
This research is supported by the DOE
Office of
Science's High-Energy
Physics program and the European
Southern Observatory.
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Author: Paul Preuss
has been a science writer at Lawrence Berkeley
National Laboratory for the last four years, covering
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etc. Before joining LBNL, Preuss spent 20 years
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even collaborating with Arthur C. Clarke on his
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a pale reflection." |
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