June 6, 2005
Astronomers say exploding star left no visible
core
By Christine Pulliam
In 1987, earthbound observers saw a star explode in the nearby
dwarf galaxy called the Large Magellanic Cloud. Astronomers
eagerly studied this supernova--the closest seen in the past
300 years--and have continued to examine its remains.
The remnant of supernova 1987A shows no sign of the neutron
star scientists believe is lurking at its heart. The Hubble
Space Telescope took this image in December 2004.
Photo: P. Challis and R. Kirshner (Harvard-Smithsonian Center for
Astrophysics) |
Although its blast wave lit up surrounding clouds of gas and
dust, the supernova appears to have left no core behind. Astronomers
now report that even the sharp eyes of the Hubble Space Telescope
failed to locate the black hole or ultracompact neutron star
they believe was created by the star's death 18 years ago.
"We think a neutron star was formed. The question is:
Why don't we see it?" said Genevieve Graves, a graduate
student in astronomy at UCSC and first author on the paper announcing
these results.
"Therein lies the mystery--where is that missing neutron
star?" mused coauthor Robert Kirshner of the Harvard-Smithsonian
Center for Astrophysics (CfA). Graves worked on the study as
an undergraduate at CfA.
When a massive star explodes, it leaves behind some sort of
compact object, either a city-sized ball of subatomic particles
called a neutron star, or a black hole. The outcome depends
on the mass of the progenitor star. Smaller stars form neutron
stars while larger stars form black holes.
The progenitor of supernova (SN) 1987A weighed 20 times as
much as the Sun, placing it right on the dividing line and leaving
astronomers uncertain about what type of compact object it produced.
All observations to date have failed to detect a light source
in the center of the supernova remnant, leaving the question
of the outcome unanswered.
Detecting a black hole or neutron star is challenging. A black
hole can be detected only when it swallows matter, because the
matter heats up and emits light as it falls into the black hole.
A neutron star at the distance of the Large Magellanic Cloud
can be detected only when it emits beams of radiation as a pulsar,
or when it accretes hot matter like a black hole.
"A neutron star could just be sitting there inside SN
1987A, not accreting matter and not emitting enough light for
us to see," said Peter Challis, an astronomer at CfA and
second author on the study.
Observations have ruled out the possibility of a pulsar within
SN 1987A. Even if the pulsar's beams were not aimed at the Earth,
they would light the surrounding gas clouds. However, theories
predict that it can take anywhere from 100 to 100,000 years
for a pulsar to form following a supernova, because the neutron
star must gain a sufficiently strong magnetic field to power
the pulsar beam. SN 1987A may be too young to hold a pulsar.
As a result, the only way astronomers might detect the central
object is to search for evidence of matter accreting onto either
a neutron star or a black hole. That accretion could happen
in one of two ways: spherical accretion, in which matter falls
in from all directions, or disk accretion, in which matter spirals
inward from a disk onto the compact object.
The Hubble data rule out spherical accretion because light
from that process would be bright enough to detect. If disk
accretion is taking place, the light it generates is very faint,
meaning that the disk itself must be quite small in both mass
and radial extent. Also, the lack of detectable radiation indicates
that the disk accretion rate must be extremely low--less than
about one-fifth the mass of the Moon per year.
In the absence of a definitive detection, astronomers hope
to learn more about the central object by studying the dust
clouds surrounding it. That dust absorbs visible and ultraviolet
light and reradiates the energy at infrared wavelengths.
"By studying that reprocessed light, we hope to find out
what's powering the supernova remnant and lighting the dust,"
said Graves. Future observations by NASA's Spitzer Space Telescope
should provide new clues to the nature of the hidden object.
Additional observations by Hubble also could help solve the
mystery. "Hubble is the only existing facility with the
resolution and sensitivity needed to study this problem,"
said Kirshner.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between
the Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists, organized into six research divisions,
study the origin, evolution, and ultimate fate of the universe.
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