Much as new parents excitedly share photographs of their baby’s first
smile and first steps, an international team of astronomers is
presenting images of their first-ever look at the early development
of an infant supernova remnant. Presented this Tuesday (06/04/02) at
the 200th meeting of the American Astronomical Society in Albuquerque,
NM, these images document for the first time the emergence and
movement of “hot spots” at the points of impact between debris from
an exploded star and surrounding gas clouds.
In this case the “baby” is the remains of Supernova 1987A, a stellar
explosion discovered in February of 1987. The images are both
confirming general theoretical expectations, yet providing several
surprises as a cosmic “smoke ring” puffed off by a dying star is
obliterated by a tremendous shock wave traveling at millions of
miles per hour.
A supernova occurs when a massive star collapses in on itself,
sending its outer layers, called ejecta [1], off in a titanic
explosion that can briefly outshine an entire galaxy filled with
billions of stars. But the explosion itself is far from the end of
the story. The ejecta, moving at millions of miles per hour, collide
with and shock surrounding interstellar gas clouds, superheating
them to millions of degrees. As this gas cools, it produces
spectacular displays of X-rays, radio waves and visible light in
what is known as a supernova remnant. Unfortunately supernovae are
quite rare, and all nearby supernova remnants are many centuries old.
Much of their explosion energy and debris have already been absorbed
and diluted by swept-up interstellar gas. The details of the
transition from supernova to remnant have simply never been seen
before.
That was before Supernova 1987A entered the picture. As the closest
and brightest supernova in nearly four centuries, it is the only
such object ever to be resolved in any significant detail. Supernova
1987A took place in the Large Magellanic Cloud, a small galaxy
orbiting our home Milky Way galaxy, estimated to lie 160,000
light-years distant in the southern constellation of Dorado.
Surrounding the supernova are three glowing rings of gas, which
the doomed star expelled some 20,000 years prior to exploding (See
Figure 1). Observations indicate that the rings are approximately
flat and circular, and appear elliptical due to the 45 degree tilt
of our viewing angle. The rings were initially lit up by the
ultraviolet radiation released during the explosion and have been
gradually fading ever since.
Although “close” by cosmic standards, the supernova remnant is by
no means easy to study. The apparent size of the innermost ring as
viewed from Earth is the same as an American quarter seen at a
distance of 2 miles. Only the Hubble Space Telescope (HST) and the
most advanced ground-based telescopes are able to monitor the
system’s detailed evolution.
When the star exploded, its launched its ejecta outward at speeds
over 50 million miles per hour. Astronomers expected this ejecta
would collide with the innermost ring first, crushing it with
tremendous shock waves and raising the gas temperature to millions
of degrees. Evidence for this impact came in 1997, when a single
brightening “hot spot” was detected on the inner ring in visible-
light HST images. Using sensitive data-processing techniques,
astronomers determined this first spot appeared very faintly two
years earlier, making March 1995 the “birthday” of Supernova Remnant
1987A at visible wavelengths.
Initially thought to mark the beginning of a full-scale collision,
astronomers expected this first hot spot to spread out and rapidly
light up the entire inner ring. Instead, over the next two years,
the first spot remained a single point and no new spots were
detected, leading astronomers to speculate that the large-scale
collision was still to come.
And come it has, as reported at the AAS meeting by Ben Sugerman, a
doctoral candidate from Columbia University, representing an
international collaboration of astronomers known as the Supernova
Intensive Survey, or SInS. Sugerman and several members of the SInS
team also present their work in the most recent (June 10, 2002)
issue of The Astrophysical Journal.
Beginning in early 1999, new hot spots began appearing at various
sites around the ring with each new HST observation: six in 1999,
six more in 2000, and four more as of late 2001. The continuing
trend of individual spots rather than extended bright arcs suggests
that the hot spots mark the impact of the ejecta with narrow,
inward-pointing protrusions distributed around the visibly clumpy
ring. “These protrusions are being struck first, much as the piers
of a coastal city are the first things destroyed by a tidal wave,
just before it reaches the shore,” notes Dr. Stephen Lawrence of
Hofstra University.
“The hot spots have popped up one-by-one, much like a baby cutting
her first teeth,” describes Dr. Arlin Crotts of Columbia University.
“What’s surprising is that the spots haven’t appeared in any
regular or predictable order, and they still haven’t spread out
into resolved arcs.”
At this point there are hot spots scattered around almost the full
circumference of the ring. “The tidal wave of ejecta is close to
colliding with the edge of the ring itself, and then the real
fireworks will begin, as the total amount of shocked, superheated
gas suddenly skyrockets,” Lawrence adds. (The appearance of new
hot spots is shown in Figure 2 and more dramatically in animated
Figure 3)
As with any cosmic phenomenon observed for the first time, unexpected
surprises tell astronomers the most. Roughly twice as many hot spots
have appeared on the eastern side of the ring as compared to the
west, and the eastern spots have tended to appear earlier. This
indicates that the ejecta have reached this side of the ring first,
suggesting the ring was not exactly centered on the former star.
While the oldest spots are typically brightest, the ninth spot to
appear (located at the 12 o’clock position in the images) has
brightened so quickly in two years that it is now second brightest.
This indicates there are significant differences in the shapes or
densities of the shocked protrusions, details too small to resolve
with even Hubble’s exquisite vision.
Sugerman has also discovered that many of the hot spots appear to
move outward in time (See Figure 4) “The precise center of the first
hot spot is traveling away from the site of the supernova at over
five million miles per hour,” he notes. “At this speed it would take
less than two seconds to travel from New York City to Los Angeles.”
But the spot motion is an optical illusion of sorts. Because of the
tremendous distance of this system, astronomers cannot directly
observe any detail in the shocked protrusions, even with HST.
Rather, what they see as a single hot spot is actually the
unresolved emission from all of the shocked gas within a large,
clumpy protrusion. “This is similar to your nighttime view of a
distant town from the window of an airliner,” notes Crotts. “The
individual lights can be blurred by the distance into a single,
unresolved glowing patch.”
The illusion of motion is created as the shock sweeps past more
of the protrusion, making the average position of all the light-
emitting material move outward as time passes. “This is the smoking
gun of the ejecta’s invisible front edge,” says Sugerman, “and the
velocity of the spot’s apparent motion gives us a minimum speed for
the shock wave itself.”
Astronomers are hopeful that the combination of pre-hot spot images
with the locations, birth dates, motions and rates of brightening
of the hot spots will allow them to infer details about the shapes
and properties of ring structures too small to see even with HST.
“Being witness to the birth of a supernova remnant is certainly a
once-in-a-lifetime opportunity,” says Sugerman. One that could have
significant results in many fields of astronomy. With the exception
of hydrogen and helium, all other elements in the Universe were
created in the interiors of massive stars and expelled into space
in supernova explosions. These newly created elements are then
mixed into the general interstellar medium during the evolution of
a supernova remnant. “All the oxygen we breath, the calcium in our
bones, the iron in our blood, the carbon in our DNA — all of these
atoms came from massive stars that lived and died long ago,”
describes Lawrence. Some astronomers have proposed that the shock
wave from a supernova remnant triggered the collapse of the
interstellar cloud that formed our Solar System five billion years
ago. “In watching Supernova 1987A give birth to a supernova
remnant,” he adds, “we are literally looking at the fires of
creation.”
The SInS collaboration is lead by Dr. Robert Kirshner of Harvard
University. SInS members Dr. Peter Challis of Harvard and Dr. Peter
Garnavich of Notre Dame have also been an integral part of this
research. Sugerman’s doctoral research has been funded in part by
grants from the National Science Foundation and the National
Aeronautics and Space Administration.
[1] Note to editors: ejecta is a plural noun but is colloquially used
as singular. We have preserved the correct grammatical sense here.
Supernovae is the correct plural of supernova.
Related Links
* Additional information on and images of infant supernova remnant
http://www.astro.columbia.edu/~ben/87Apr.html
For more information, please contact:
Ben Sugerman, Columbia University
(212) 854-6864, ben@astro.columbia.edu
Dr. Stephen Lawrence, Hofstra University
(516) 463-5584, Stephen.Lawrence@hofstra.edu
Dr. Arlin Crotts, Columbia University
(212) 854-7899, arlin@astro.columbia.edu
Dr. Robert Kirshner, Harvard University
(617) 495-7519, kirshner@cfa.harvard.edu
Dr. Peter Garnavich, University of Notre Dame
(574) 631-7262, pgarnavi@miranda.phys.nd.edu
Dr. Peter Challis, Harvard University
(617) 469-5203, pchallis@cfa.harvard.edu
