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IPMU Scientists Found Supernovae Are Not Round (Forwarded)



 
 
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Old February 13th 08, 05:45 AM posted to sci.space.news
Andrew Yee[_1_]
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Default IPMU Scientists Found Supernovae Are Not Round (Forwarded)

Institute for the Physics and Mathematics of the Universe

Contact:
Keiichi Maeda
Assistant Professor
Institute for the Physics and Mathematics of the Universe (IPMU)
University of Tokyo
Kashiwano-ha 5-1-5, Kashiwa City, Chiba 277-8582, Japan
Tel: +81-4-7136-4945 Fax: +81-4-7136-4944

2008-02-01

IPMU Scientists Found Supernovae Are Not Round with an international team
and Subaru telescope

Kashiwa, Japan -- An international team has uncovered the shape of
core-collapse supernovae. They used the Subaru Telescope to discover that
supernovae are not round but rather pencil-like. The result sheds light on
actively debated unsolved topics in astrophysics: the explosion mechanisms
of supernovae and gamma-ray bursts. The result was published on Science
Online.

Massive stars (more than 10 times the Sun) end their lives with a bang. The
massive stars' death is triggered by the gravitational collapse of the inner
core. The central region becomes superdense by this collapse, leaving a
neutron star or even a black hole. A neutron star is a compact star as
massive as the Sun within about 10 km of radius, and a blackhole has such a
strong gravitational pull that even light cannot escape. As an outcome of
the collapse, the energy of the falling matter is somehow transferred to the
outer part of the star leading to a supernova explosion. However, there has
been a big problem in this theory: astrophysicists have not been successful
in understanding the detailed mechanism that turns the collapse into an
explosion as we can see in telescopes. In recent years, interesting theories
have been proposed to understand the explosion mechanism. The key is to make
the collapse not round and the explosion pencil-like, like two cannons
placed back to back.

Materials thrown out by the explosion actually keep expanding even years
later. If one can see the shape of the expanding materials, it will tell
astrophysicists details about the original explosion. However, a supernova
explosion gives out such a dense gas that it has been impossible to see what
is going on at its core. In addition, most supernovae take place in other
galaxies, namely about a few hundred million light years away, and it is
impossible to see the image of their shape directly. It has been a great
challenge to astrophysicists to observe the shape of very core of a
supernova explosion.

Keiichi Maeda of IPMU and his colleagues had predicted theoretically that a
line profile of light emitted by oxygen could tell them the shape of
expanding materials. Most importantly, they realized that it is possible to
judge whether a supernova is round by studying the color (spectrum) of a
supernova at late-times, namely 200 days after the explosion (Figs. 1, 2).
Then the outer part of the exploded star becomes thin enough so that we can
see directly into the expanding core. The light emitted from the expanding
oxygen atoms becomes slightly bluer or redder by Doppler effects depending
on if the gas is moving towards or away from us. Studying the small change
in color of the light, they proposed to determine the shape of the expanding
core.

The research team proceeded, and has finally collected late-time spectra of
15 supernovae using the 8.2 m Subaru telescope (operated by National
Astronomical Observatory of Japan, Fig. 3). They also used in part the Very
Large Telescope (European Southern Observatory). Even with these largest
telescopes, the observation was challenging, because a supernova is
extremely faint at such a late phase after 200 days since the explosion.
Indeed, such observations had been available only for a few supernovae
before the present work and it was not possible to tell if they were special
in some way or of garden-variety types.

A large sample is essential in their investigation. It is difficult to
clearly distinguish the two cases -- a round explosion or a pencil-like
explosion viewed from the pointed direction -- for a single object. With
more than 10 supernovae, however, it becomes possible to derive the
detailed, generic shape, because the random orientation relative to the
viewing angle from the Earth allows removing the uncertainty by a
statistical analysis. They found 5 supernovae showing a clear signature of a
pencil-like explosion viewed from the side, and 4 supernovae with hints of
such a feature. Considering that pencil-like explosion appear round if these
are viewed from the pointed direction, the probability of seeing the feature
of pencil-like explosion indicates that all supernovae are not round. This
is the first observational confirmation that supernovae are in general not
round.

Their result supports recent theoretical scenarios of pencil-like supernova
explosion, namely by hydrodynamic instability (i.e., the vibration of the
shock waves), or rotation plus a magnetic field. "Also important is," says
Maeda, the first author of the paper, "the finding that the deviation from
spherical symmetry looks smaller in usual supernovae than in extreme ones
associated with gamma-ray bursts." This conclusion is also reported in the
same paper. He adds, "And it strongly indicates that the explosion
mechanisms of the supernovae linked to gamma-ray bursts and other usual
supernovae are intrinsically different." According to him, the team plans to
look into more details of individual theoretical scenarios and compare those
with the observations. "This is even more challenging than the present
study, both in theory and observation. But we believe this is within the
reach," says Maeda.

The international research team consists of researchers from the following
institute: IPMU, Hiroshima University, School of Science of U. Tokyo,
National Astronomical Observatory of Japan, Max-Planck-Institute for
Astrophysics, Trieste Observatory, European Southern Observatory, National
Observatory of China, National Optical Astronomical Observatory, University
of California Berkeley, Japan Aerospace Exploration Agency.

Keiichi Maeda was born in 1976. He received Ph. D degree from School of
Science, University of Tokyo, in 2004. He then worked at graduate school of
Arts and Science, University of Tokyo, as a JSPS (Japanese Society of
Promotion of Science) postdoctoral researcher. He moved to the
Max-Planck-Institute for Astrophysics (Garching; Germany) on April, 2004 as
a JSPS postdoctoral fellow for Research Abroad. He then moved back to
University of Tokyo as an assistant professor to join in the new institute,
the Institute for the Physics and Mathematics of the Universe (IPMU). "This
work," mentions Keiichi Maeda about the study reported here, "represents one
of the main goals we have been trying to achieve, during the long period in
which I have been at Tokyo, Garching, and then Kashiwa. Without discussion
with many researchers in these institutes, not only coauthors, we would not
have made it this far. We have just started thinking about applications of
the result of this study in a context of the cosmological study. There are
several interesting possibilities. They are not easy, I'm afraid, but not
impossible," mentions Maeda.

IPMU has been founded on 1 October, 2007, to solve many mysteries of the
Universe, such as what the mysterious Dark Energy is, as one of the WPI
initiative centers by Ministry of Education, Culture, Sports, Science and
Technology. "Acrucial concept behind this new institute is," says the
director, Hitoshi Murayama, "that we respect and support initiatives by
young researchers. This study is a very nice example, and proves that our
concept of the institute bears fruit."

Original Work:
Science Express (online edition of Science), 31 January 2008 issue.
Title: "Asphericity in Supernova Explosions from Late-Time Spectroscopy"
Authors: Keiichi Maeda, Koji Kawabata, Paolo A. Mazzali, Masaomi Tanaka,
Stefano Valenti, Ken'ichiNomoto et al.

Related sites:

* IPMU
http://www.ipmu.jp/
* Subaru telescope
http://www.naoj.org/
* WPI initiative
http://www.jsps.go.jp/english/e-toplevel/index.html

IMAGE CAPTIONS:

[Fig. 1:
http://www.ipmu.jp/press/img20080201/fig1e.jpg (130KB)
Fig. 2:
http://www.ipmu.jp/press/img20080201/fig2e.jpg (70KB)]
If the explosion is more or less round, the oxygen-rich core of a supernova
progenitor star is thrown out as a shell, resulting in a "single-peaked"
oxygen emission line when intensity of light is shown as a function of its
color. On the other hand, if the explosion is pencil-like (or, canon balls),
the oxygen-rich materials are ejected mostly along the equatorial direction
as a "doughnut" (Fig. 1). The oxygen emission line profile is then predicted
to be "single-peaked" for an on-axis observer (Fig. 2, upper), but
"double-peaked" for an observer looking at the supernova sideways (Fig. 2,
lower). This stems from different Doppler shift -- longer/shorter wavelength
detected for materials moving away from/toward the observer.

[Fig. 3:
http://www.ipmu.jp/press/img20080201/fig3.jpg (591KB)]
Images of supernovae at the late-phase observed by the Subaru telescope. A
few supernovae were also observed with the Very Large Telescope.
Spectroscopy was performed for these objects plus SN 2002ap (for which we
did not perform imaging observations).
 




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