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Rethinking last century's closest, brightest supernova (Forwarded)



 
 
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Old January 11th 07, 01:28 AM posted to sci.space.news
Andrew Yee[_1_]
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Default Rethinking last century's closest, brightest supernova (Forwarded)

Media Relations
University of California Berkeley

Media Contacts:
Robert Sanders
(510) 643-6998, (510) 642-3734

Additional Resources:

Nathan Smith, UC Berkeley
(206) 219-4651, (510) 642-6931

FOR IMMEDIATE RELEASE: Tuesday, January 09, 2007

Rethinking last century's closest, brightest supernova

Twenty years ago next month, the closest and brightest supernova in four
centuries lit up the southern sky, wowing astronomers and the public alike.

Ongoing observations of the exploded star, called supernova 1987A, provided
important tests for theories of how stars die, but it also raised some new
questions. Principal among these was how a bizarre, triple-ring nebula
surrounding the supernova -- ejected by the star a few thousand years before
it exploded -- originated. Astronomers devised a complicated theory that,
within a relatively short period of time, the original star, a red
supergiant, merged with a companion and started spinning rapidly, then
underwent a transition to a blue supergiant, and finally exploded.

University of California, Berkeley, astronomer Nathan Smith has proposed a
different theory for the origin of the nebula, arguing instead that
SN1987A's progenitor star may have been in a class of unstable blue
supergiant stars, called luminous blue variables, which eject material from
their surfaces in recurring, volcano-like eruptions before they finally die
in a supernova explosion.

Smith recently discovered two such blue supergiant stars with nebulae
closely resembling the peculiarly shaped cloud of dust and gas around
SN1987A. A third such nebula was already known.

"Taken together, the three closest analogs of SN1987A in our galaxy are all
around blue supergiants; two of them have not gone through a red supergiant
phase at all, and one was ejected as a luminous blue variable," said Smith,
a UC Berkeley postdoctoral researcher. "This makes a pretty solid case that
we should rethink models for how the rings around SN1987A were formed.

"If these other stars with rings are likely to explode, it may hint that
LBVs and blue supergiants can explode even before becoming red supergiants,
which would be a bit of a shock to our understanding of stellar evolution."

Smith will present his findings today (Tuesday, Jan. 9) at a 10:30 a.m.
press conference and an all-day poster session during the American
Astronomical Society (AAS) meeting in Seattle.

The proximity of SN1987A, only 168,000 light years away in the Large
Magellanic Cloud, and the availability of pre-existing data provided the
first chance for astronomers to posthumously identify the star that
exploded. Astronomers were surprised to find that it had been a hot blue
supergiant -- not a cooler red supergiant, as most theories predicted at the
time.

Adding to the mystery, images taken in the early 1990s by instruments like
NASA's Hubble Space Telescope revealed a bizarre, triple-ring nebula. The
origin of this nebula and its shaping mechanism are still difficult to
understand. The merger theory with conversion from red supergiant to blue
supergiant before exploding has become the prevailing view because it
accounts for both the blue supergiant and the shape of the nebula.

The surprise, Smith said, is that analysis of these new objects in our
galaxy that resemble SN1987A provide good reasons to suspect that they
ejected and shaped their nebulae while they were still blue supergiants, and
not in the transition from red to blue as has been proposed for SN1987A.

Furthermore, none of the three stars is spinning rapidly, as one might
expect if it had recently merged with a close orbiting companion star. A
merger and the subsequent red-to-blue transition are the key ingredients in
the prevailing explanation for the nebula around SN1987A, but the three
stars discussed by Smith apparently formed similar nebulae without either
mechanism.

"We are seeing these nebulae before the stars blow up, and they look quite
similar to the nebula around SN1987A," said Smith. "The trouble is, they may
contradict how we think the nebula around SN1987A was formed."

According to Smith, the unusual nebula around SN1987A, looking like a figure
8, was originally interpreted to mean that the star had recently been a red
supergiant that had shed its outer envelope in an expanding shell, but then
turned into a blue supergiant before exploding. The blue supergiant
generated a faster wind that overtook the earlier wind and became distorted.

"In that picture, the equatorial ring formed because the slow wind of the
red supergiant had more material in the equator, so the waist of the blue
supergiant wind was pinched," Smith said. "The fly in the ointment is that
in order to get the enhanced density in the equator of the red supergiant,
you need it to be spinning rapidly -- but red supergiant stars don't do that
because they are so big. So the only solution would be if the progenitor of
SN1987A swallowed a companion star and the two merged, while the added
angular momentum made the red supergiant spin to make a disk."

"This requires that the nearest and best observed supernova in modern
history just happens to also be a freak, resulting from a coincidental
merger event," he added.

While looking through images taken by NASA's Infrared Array Camera on the
Spitzer Space Telescope, however, Smith noticed a similarly weird nebula
around a nearby star designated HD168625. This star is a luminous blue
variable (or LBV), an unstable massive star that burps from time to time and
ejects a bipolar nebula as a blue supergiant, not a red supergiant. A
well-known LBV is Eta Carinae, the brightest and most massive star in our
Milky Way galaxy, weighing in at more than 100 solar masses.

"This new twin of the SN1987A nebula around an LBV gives us an alternative
to the binary merger hypothesis for how these form," Smith said. "It hints
that SN1987A may have ejected the nebula as a blue supergiant or an LBV, and
not as a red supergiant."

Later, Smith identified a second ring nebula, identical in size to the
equatorial ring around SN1987A but surrounding another blue supergiant in
our galaxy. He found this in the Carina Nebula in the southern Milky Way in
data taken by the 4-meter Blanco telescope at Chile's Cerro Tololo
Inter-American Observatory, part of the National Optical Astronomy
Observatory, and in images taken by one of two 6.5-meter Magellan telescopes
in Chile.

The second star, called SBW1, has almost the same spectral type as the
progenitor of SN1987A, but the chemical abundances in the nebula imply that
it has not yet been a red supergiant. This directly contradicts the old
picture for how the rings around SN1987A were formed, he said. A third
similar object in our galaxy, called Sher 25, was already known, and it has
chemical abundances that also suggest it has not yet been a red supergiant.

Smith's research was supported by NASA.

NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., manages the
Spitzer Space Telescope mission for NASA's Science Mission Directorate in
Washington, D.C. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology, also in Pasadena. Caltech
manages JPL for NASA.

[NOTE: Images supporting this release are available at
http://sscws1.ipac.caltech.edu/Image...name=sig07-003 ]
 




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