[astro] whoops, is that a GRB in your pocket?

[Eugen Leitl]

(Just 8 kLYrs? Scary).

http://www.news.com.au/dailytelegraph/story/0,22049,23310103-5001021,00.html

Is this the real Death Star?

March 03, 2008 12:00am

The Earth may be in the firing line when one of the sky's most beautiful
objects explodes, a Sydney astronomy expert claims.

University of Sydney astronomer Peter Tuthill discovered the elegant rotating
pinwheel system, named WR104, eight years ago in the constellation
Sagittarius.

It includes a highly unstable star known as a Wolf-Rayet, widely regarded by
astronomers as ticking bombs – the last stop in a star’s life before a
cataclysmic supernova explosion.

“When it finally explodes as a supernova, it could emit an intense beam of
gamma rays coming our way”, says Dr Tuthill, whose work is published in the
latest edition of the Astrophysical Journal.

In the article Dr Tuthill reveals stunning new images of WR 104 taken with
the Keck Telescope in Hawaii, showing a glowing plume of hot dust and gas
flung out into a whirling spiral as the two stars at the centre of the system
orbit one another every eight months.

But something odd about the images caught the attention of Dr Tuthill and his
team: “Viewed from Earth, the rotating tail appears to be laid out on the sky
in an almost perfect spiral. It could only appear like that if we are looking
nearly exactly down on the axis of the binary system.”

Dr Tuthill and his team worry this box-seat view might put us in the firing
line when the system finally explodes.

“Sometimes, supernovae like the one that will one day destroy WR104, focus
their energy into a narrow beam of very destructive gamma-ray radiation along
the axis of the system. If such a `gamma-ray burst’ happens, we really do not
want Earth to be in the way.” warns Dr Tuthill.

At only 8000 light years distance, WR104 is just down the road in galactic
terms, only ¼ of the way to the centre of our Milky Way Galaxy.

“Earlier research has suggested that a gamma-ray burst – if we are
unfortunate enough to be caught in the beam – could be harmful to life on
Earth out to these distances. Scientists have speculated that, eons ago, a
gamma-ray burst from a distant star could explain mass extinctions seen in
the fossil record,” he said.

“I used to appreciate this spiral just for its beautiful form, but now I
can’t help a twinge of feeling that it is uncannily like looking down a rifle
barrel.”

For more on the Death Star, go here.

http://www.physics.usyd.edu.au/~gekko/pinwheel.html

WR 104: The prototype Pinwheel Nebula

The most extensive and detailed imaging study to date of the beautiful and
exotic colliding-wind binary Pinwheel system Wolf-Rayet 104 has caught the
motion of this stellar performer in a series of time-lapse movie frames taken
over an interval of more than six years.

A sequence of 11 sharp frames show the elegant spiral nebula in the
constellation of Sagittarius to be rotating in a circle every 8 months,
keeping precise time like a jewel in a cosmic clock. In the image to the
left, we have rotated the camera frame of each of the 11 images so as to
follow the motion, and as a result we can stack all our images into a single
false-colour composite.

So why does WR 104 appear as a spiral? Powering the heart of the system are a
pair of luminous hot stars locked in a binary orbit. As they circle one
another, winds driven from the two stellar surfaces collide in the
intervening space. Plumes of streaming gas blown off by the stars eventually
become entangled and twisted by the orbital motion of the binary star.
Glowing hot dust forming along the length of the gas tail acts as a vivid
tracer of all this turbulence and activity, painting a spiral banner on the
sky in excess of 200 AU long (thirty billion kilometers, or about one
light-day).

The new data, reported in the March 1st, 2008 issue of the prestigious
Astrophysical Journal, were obtained with specialized techniques in
near-infrared light (2.2 microns) at the Keck telescope , the world's largest
optical observatory. In another way to visualize the data, frames covering
the rotation of the spiral plume over its 8-month orbit have been stitched
together into a time-lapse movie (red colour scale image to the right).

Exceeding previous studies both for clarity of detail and for coverage of the
temporal changes, the new data allow a fascinating exploration of the
prehistory and possible future evolution of the Wolf-Rayet binary system. The
high degree of circularity of the orbit of the massive stars at the heart of
WR 104 speaks of a violent and turbulent past, in which the O-star companion
stripped away the outer layers of gas from the Wolf-Rayet.

But the fireworks which lie ahead for WR 104 should dwarf any performances
from the past. As the last known stable stellar phase before a supernova
explosion, Wolf-Rayet stars are regarded by Astronomers as ticking bombs. For
a star such as WR 104, the fuse is now very short (to an astronomer) and it
may explode any time within the next few hundred thousand years. Although an
eventual supernova explosion in a system with the proximity of WR 104 would
no doubt put on a very impressive show for future earthbound stargazers, such
explosions occurring at the vast interstellar distances are normally thought
to be harmless.

However, the enigmatic WR 104 system has one final card to play, which gives
Astronomers just a little pause for thought. The new data show that the
orientation of the spin-axis of the system, with respect to our vantage point
from Earth, is almost exactly aligned. As nearly as we can tell, we are
looking directly down upon the polar direction of the spinning binary stars.
	
Is the spiral nebula WR 104 a view down the gunbarrel?

The problem with explosions in rapidly spinning stars is that they are likely
to be quite different from the simple spherical blast-wave of their
slowly-rotating kin. It is widely believed among astronomers that such
supernova explosions exhibit a preferred axis aligned with the spin of the
progenitor star. Just how strong the degree of asymmetry in the explosion,
with energy and matter beamed along the axis, is something astronomers are
still trying to decide.

The worst-case scenario, however, is something witnessed by networks of
sensitive space telescopes nearly every day. The powerful Gamma-Ray Bursts
(GRBs), first discovered in the 1960's by cold war satellites eavesdropping
on nuclear tests, are believed to be among the most powerful and asymmetric
explosions in the cosmos. The tightly-beamed energy of these stellar
explosions, occupying a cone of only 12 degrees opening angle, could well
leave a narrow trail of destruction across a considerable swath of the galaxy
(several kiloparsecs).

Could WR 104 be the Death Star?

So our accidental priveliged pole-on view onto this system may result in
something beyond a pretty spiral apparition in the sky. We are looking along
the axis of one of one of the nearest imminent Wolf-Rayet supernova
progenitors known. If there is a significant beaming of energy along this
axis, could the Earth be in harm's way?

There is still much work to do before the idea that WR 104 could pose a
threat to Earth's biosphere should be given too much credence. For starters,
the degree to which the system is pointing exactly in our direction needs to
be firmed up with spectroscopic observations. Data so far are just not
accurate enough to nail down the orientation precisely.

But perhaps the biggest uncertainty of all is the exact nature of WR 104's
future supernova explosion. The detailed makeup of gamma-ray bursts and
asymmetric supernova explosions is a very hot area of current research: ideas
are changing and evolving all the time. Our best present guess is that a
full-fledged Gamma-Ray Burst from WR 104 is unlikely, although just how much
energy might be beamed our way when it does go supernova is uncertain.

More research to firm up the orientation, and to predict the degree of
directed energy beaming in the coming supernova explosion, may help give us
just a little more peace of mind as we gaze upwards into the spiral coils of
this uniquely beautiful system.

Further web pages devoted to these fascinating spiral systems can be found
for WR 104 (original discovery page); WR 98a the second Pinwheel; and also
the beautiful Quintuplet Cluster.

Principal Researchers:

Peter Tuthill is a researcher in the School of Physics at the University of
Sydney, Australia.  John Monnier works at the University of Michigan

These links are for download of images and press release materials:

  Large Rainbow-Color Image file (2000px gif format).

  Large Rainbow-Color Image file (2000px jpeg format).

  Large high-quality red-scale "filmstrip" image.

  Movie of the 8-month time-lapse rotation.

  Coords: RA/Dec = 18:02:04.07 -23:37:41.2 (J2000) V=14mag

This research appears in:

# "The prototype colliding-wind pinwheel WR 104"

Peter Tuthill, John Monnier, Nicholas Lawrance, William Danchi, Stan Owocki
and Kenneth Gayley 01 March 2008, Astrophysical Journal

For an Abstract and full preprint of the paper, click here (link to Astro-PH)

For FULL TEXT of the paper, click here (link to Astrophysical Journal)


This research was supported by grants from the Australian Research Council
and the US National Science Foundation Stellar Astronomy and Astrophysics
Program.

For more information, contact: Dr Peter Tuthill

Astronomy Department

School of Physics, University of Sydney

N.S.W. 2006, AUSTRALIA

Email: p.tuthillphysics.usyd.edu.au)

phone: +61 2 9351 3679