Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994eso..pres....1.&link_type=abstract
ESO Press Release, 01/1994
Astronomy and Astrophysics
Astronomy
Scientific paper
On 23 February 1994, it will be exactly seven years since
the explosion of Supernova 1987A in the Large Magellanic
Cloud [1] was first observed, at a distance of
approx. 160,000 light-years. It was the first naked-eye
supernova to be seen in almost four hundred years. Few
events in modern astronomy have met with such an
enthusiastic response by the scientists and this famous
object has been under constant surveillance ever since.
After several years of relative quiescence, things are now
beginning to happen in the immediate neighbourhood of SN
1987A. Recent observations with the ESO 3.5 m New Technology
Telescope (NTT) indicate that interaction between the
stellar material which was ejected during the explosion and
the surrounding ring-shaped nebulae has started. This
signals the beginning of a more active phase during which
the supernova is likely to display a number of new and
interesting phenomena, never before observed.
SEVEN YEARS IN THE LIFE OF A SUPERNOVA
After brightening to maximum light at about magnitude 3 a
few months after the explosion, the long period of steady
fading which is typical for supernovae, set in by
mid-1987. The matter ejected by the explosion took the form
of an expanding fireball, which began to spread through the
nearly empty space around the supernova with a velocity of
almost 10,000 km/sec. As it cooled, the temperature and
therefore the total brightness decreased and the supernova
became fainter and fainter. At the present moment, the
magnitude of SN 1987A is about 18.5, that is almost 2
million times fainter than it was at maximum.
Various phenomena have been observed around SN 1987A during
the past years. Already in early 1988, light echoes were
seen as concentric, slowly expanding luminous circles; they
represent the reflections of the explosion light flash in
interstellar clouds inside the Large Magellanic Cloud,
between the supernova and us. In 1989, high-resolution
observations with the NTT showed an elliptical
``ring-nebula'', only two arcsec across, surrounding SN
1987A; it was interpreted as interactions between
pre-existing circumstellar material and a shell of matter
which was thrown off a few thousand years ago when a red
giant star evolved into the blue star that eventually
exploded. The best images of this nebula were first
obtained by the Hubble Space Telescope in 1990.
There are also faint outer nebular loops around SN 1987A. It
is thought that they trace the outer rim of a large bubble
that was formed by the outflowing ``wind'' of matter from
the star that later exploded.
During the past years, astronomers working at large
telescopes in the southern hemisphere have conducted
unsuccessful searches for a pulsar inside SN 1987A. Although
most theories predict the emergence of a very compact object
at the centre of a supernova, even very detailed
investigations reaching very faint light levels have so far
not been able to prove the existence of such an object in SN
1987A.
RECENT CHANGES IN THE RING
But the development of SN 1987A is not yet over. After the
first seven years, it is now about to enter a new phase.
In a Circular of the International Astronomical Union,
astronomers Li-Fan Wang (Beijing Observatory) and E. Joseph
Wampler (European Southern Observatory) have just reported
that changes are seen in the inner ring nebula around SN
1987A when the latest NTT observations are compared with
those carried out over the past two years.
The distribution of light along the ring has recently
changed dramatically. It is now found to be gradually
increasing in brightness at several locations.
This is most easily seen on images obtained in the light of
ionised nitrogen which enhances the contrast between the SN
1987A ring nebulae and their surroundings.
Following computer sharpening of December 1993 CCD pictures
to a resolution of 0.2 arcseconds - corresponding to the
angle subtended by a coin of 1 cm diameter at a distance of
10 km - it is clear that the ring emission regions are now
highly clumped.
BEGINNING OF THE ``FIREWORKS'' ?
It is most interesting that these new bright patches in the
inner ring coincide roughly with the recently observed
structure of the radio emission received from SN 1987A. The
astronomers believe that these changes in the ring may
herald the beginning of the predicted collision between the
matter in the expanding fireball and the nebular material
which was ejected from the star during the evolutionary
phase that preceded the explosion. The supernova shell is
``catching up'' with the material that was ejected earlier.
This interpretation is also supported by the recent
observation of weak X-ray emission from the supernova with
the ROSAT satellite. It probably signifies a beginning
heating of the gas inside the nebular ring when the
particles collide at high speeds.
Further observations at ESO show that no significant amount
of the expanding matter has yet reached the ring. Detailed
spectra, obtained with the NTT by the same astronomers,
still do not show the violent motions that would signal a
collision between the main mass of the expanding supernova
envelope and the ring material.
These important observations have alerted astronomers to
watch out for sudden, possibly quite dramatic changes in the
ring. As a result, SN 1987A will now be monitored much more
intensively. Never before has it been possible to observe
such an event directly; the observed phenomena will
undoubtedly provide completely new information about the
chemical and physical state of the matter in the colliding
clouds.
FIRST IMAGE OF THE FIREBALL FROM THE GROUND
The NTT has scored another first during these observations:
thanks to its excellent optical properties, high-resolution
images of the supernova in near-infrared light with a
ground-based telescope for the first time show the exact
extension of the fireball. Until now, this had only been
possible with the Hubble Space Telescope.
The measured diameter in the sky is only 0.37 arcseconds. At
the distance of the supernova, this corresponds to a size of
about 0.29 light-years. The mean expansion rate during the
past seven years is therefore about 0.04 light-years per
year, or about 6000 km/sec, a value which is very close to
that obtained from spectroscopic observations.
[1] See ESO Press Releases 04/87, 05/87, 06/87, 07/87,
11/87, 2/88, 2/89 and 01/90.
FIGURE CAPTION
ESO PR PHOTO 01/94-1: CHANGES IN THE RING NEBULA AROUND SN 1987A
This computer-processed picture is an average of two
15-min exposures, obtained in the light of ionised
nitrogen during the morning of December 20, 1993,
with the Super Seeing Instrument (SuSI) at the 3.58 m
ESO New Technology Telescope.
It shows the complex structure of the faint nebulae
around SN 1987A in the Large Magellanic Cloud. The
inner ring was formed from interaction with matter
ejected during an earlier phase by the star that
later exploded as a supernova. Spectra and
theoretical models suggest that the faint outer
nebular loops trace the outer rim of a large bubble
which was also formed by outflowing matter. The
supernova itself, at the centre of the inner ring,
emits very little light in the narrow spectral band
used for this observation and is not visible on this
picture.
Because of the very large range of intensities in
these nebulosities, and in order to better show the
structure, the intensities of the low surface
brightness parts of the image have been greatly
enhanced, relative to the brighter parts.
Most of the light from the inner ring is concentrated
in a narrow, bright band which has been given
relatively little prominence in this modified
picture. However, the uneven structure, in particular
the greater intensity in the North-West (upper right
part), is clearly visible. This picture shows very
faint structures and therefore a greater overall
width of the ring than do earlier pictures.
The nominal resolution of this picture as determined
by the size of the stellar images in the field, is
0.
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