Astronomers see our origins in 20-year-old star explosion (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Patrice Bouchet, Observatoire de Paris, GEPI
Tél: 33 1 45 07 79 21
Fax: 33 1 45 07 79 39

14 April 2006

Astronomers see our origins in 20-year-old star explosion

Nearly 20 years after its explosion in the Large Magellanic Cloud, we
hear again about the supernova SN1987A. An international team of
astronomers [1], led by Patrice Bouchet from Paris Observatory (GEPI)
has just detected a ring of dust in the infrared with the Gemini 8m
telescope in Chile. Spectra of the dust by the Spitzer satellite show
that it is indeed thermal emission from silicate grains, which have
condensed from the red giant stellar wind of the precursor star.

In 1987 a massive star in a neighboring galaxy exploded, an event called
a supernova. It was the closest supernova to Earth since the invention
of the telescope centuries ago. All the major observatories and millions
of people worldwide turned to watch the death of this star.

Now, nearly 20 years after the star's death, the explosion is revealing
signs of life -- in the form of dust particles that are the building
blocks of rocky planets and all living creatures. And astronomers once
again are captivated.

"Supernova 1987A is changing right before our eyes," said Dr. Eli Dwek,
a cosmic dust expert at NASA Goddard Space Flight Center in Greenbelt,
Maryland. Dwek and his colleague Dr. Patrice Bouchet of the Observatoire
de Paris have been following for several years this rapidly changing
supernova, named SN 1987A for the year it was discovered in the Large
Magellanic Cloud, a dwarf galaxy. "What we are seeing is a milestone in
the evolution of a supernova."

Using infrared telescopes, Bouchet, Dwek and their colleagues [1]
detected silicate dust created by the star from before it exploded. This
dust survived the explosion; was swept up and pushed out by shock waves;
and is now, nearly 20 years later, finally approaching a ring of gas
surrounding the embers of the dead star, making it "visible" to infrared
detectors.

Dust -- chemical particles and crystals finer than beach sand -- is both
a frustration and a fascination for astronomers. Dust can obscure
observations of distant stars. Yet dust is the stuff from which all
solid bodies are formed. This is why dust, as bland as it sounds, is one
of the most important topics in astronomy and astrobiology.

Dust is everywhere in the universe, yet astronomers know so little about
its origin. Clearly dust is made in stars and hurled into space by
supernovae. But the devil is in the details. How much dust is made in a
star? How much is vaporized by the star explosion and how much survives?
And how do wispy dust clouds form planets and ultimately life?

These are the questions that scientists such as Patrice Bouchet and Eli
Dwek want to answer. With 1987A, they have a perfect laboratory to watch
the process unfold.

This is new territory for astronomers, said Bouchet, who led the SN
1987A infrared observations with the Gemini South telescope in Chile and
NASA's Spitzer Observatory. His team is witnessing processes never
before seen. This is the first time scientists have direct evidence of
dust from a large star surviving a supernova; the first time they detect
cold dust intermingled in hot, X-ray-emitting gas of millions of
degrees; and the first time they are witnessing sputtering, the process
in which dust is broken down into smaller particles.

They frankly don't know what to expect, and they have already stumbled
upon a few surprises.

Infrared detectors are crucial for this kind of observation. Infrared is
a less-energetic form of radiation than visible light. The infrared
detectors are similar to night-vision goggles. The dust is over a
hundred degrees below zero and too cold to emit visible light. Optical
telescopes can see gas but not the dust.

Through infrared imaging, the science team determined that the dust is
in the region of the equatorial ring of gas around SN 1987A. Thus, it
took many years for the dust to travel out from the explosion and into
the interstellar medium, a distance of a little less than one
light-year. "This much was expected," said Bouchet. "The collision
between the ejecta of SN 1987A and the equatorial ring predicted to
occur sometime in the interval of 1995 to 2007 is now underway."

The silicate dust detected could not have formed in the supernova blast
itself, because of the high energies involved, and could only have come
from the progenitor star. Detection of silicates was a bit of a surprise
and thrill, Dwek said.

The key finding, however, is that the team has detected far less dust
than expected. This could mean that a supernova blast wave can destroy
more dust than thought possible. If confirmed, this will have broad
implications for determining dust concentrations throughout the universe.

Yet this is a work in progress. "Overall, we are witnessing the
interaction of the supernova blast wave with its surrounding medium,
creating an environment that is rapidly evolving at all wavelengths,"
said Bouchet.

To that effect, scientists are planning a series of new observations of
SN 1987A now that it has once again become very interesting. Who knows
what will be revealed once the dust settles.

[1] The team includes Patrice Bouchet (GEPI, Paris Observatory), Eli
Dwek (NASA, GSFC), John Danziger (Trieste Observatory, Italy), Richard
Arendt (NASA, SSAI), James De Buizer (Gemini Observatory), Sangwook Park
(Pennsylvania State University, USA), Nicholas Suntzeff (Cerro Tololo
Interamerican Observatory, Chile), and Robert Kirshner and Peter Challis
(Harvard-Smithsonian, CfA, Cambridge MA, USA).

Reference

Article accepted in Astrophysical Journal,
http://arxiv.org/abs/astro-ph/0601495

IMAGE CAPTION:
[http://www.obspm.fr/actual/nouvelle/...SN1987A-f1.jpg (37KB)]
The image shows the dust ring observed with the Gemini 8-m Telescope in
Chile (red) superimposed on the image obtained in the visible by the
Hubble Space Telescope (yellow).