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OCR for page 237
Observing SN 1987A with
the International Ultraviolet Explorer
ROBERT P. KIRSHNER
Harvard-Smithsonian Center for Astrophysics
INTRODUCTION
The International Ultraviolet Explorer (IUE) satellite has played a
leading role in elucidating the nature of SN 1987A, providing a unique
ultraviolet perspective on the brightest supernova since 1604. One funda-
mental properly of the IUE project proved essential: it is a satellite whose
program can be rapidly changed to take advantage of scientific opportu-
nities. On both sides of the Atlantic, there was a target-of-opportunity
proposal in place, so that an orders, though very exciting, series of ob-
servatior~s was carried out starting within 4 hours of the first report, on
February 24, 1987, as recounted by de Vorken (1988) and by Kirshner
(1988~.
IVE observations of SN 1987A began promptly after the discovery and
have been frequent through 1988 and 1989, using the FES for photometry,
low dispersion spectra for the supernova spectrum as described in the next
section, high dispersion observations for the interstellar medium when the
supernova was bright (see below), and for circumstellar gas surrounding the
supernova as the initial event faded (see below). The UV data have been
especially useful in determining which star exploded, assessing the ionizing
pulse produced as the shock hit the surface of the star, and in constraining
the stellar evolution that preceded the explosion through observations of
a circumstellar shell. These discoveries are placed in a broader context by
the review of Arnett et al (1989) and earlier reports on the ultraviolet
data are summarized by Kirshner (19~. The ultraviolet spectrum of the
supernova itself is produced by the superposition of many lines of Fe, Co,
and other elements in the stellar photosphere, and it has remained opaque
237
OCR for page 238
238
AMERICAN AND SOVIET PERSPECTIVES
long after the infrared and optical have changed to emission-line spectra.
High-dispersion IUE observations provide a detailed look at the ionization
structure of the line-of-sight to the supernova, both in our Galaxy and in the
LMC. Future observations will include a UV observation of the light echo,
monitoring the decay of the circumstellar emission, and perhaps a glimpse
into the enriched material that was formerly the interior of Sanduleak ~9
20Z
Supernova 1987A in the Large Magellanic Cloud has moved the sub-
ject of supernovae from plausible argument to observational demonstration
~ a number of areas and the IUE observations have helped in essen-
tial ways. While the supernova was the first visible to the unaided eye
since Kepler's 16~ supernova, retinal observations have not proved the
most novel Instead, the advances in technology, including geosynchronous
satellites, have provided the data for real insight. The Large Magellanic
Cloud is ideally placed for observation: circumpolar for the outstanding
observatories of the Southern hemisphere, it is also near the ecliptic pole,
facilitating observations with the IUE at almost any time of the year. The
observations gathered over the entire spectrum from radio to gamma rays
and the direct detection of neutrinos from SN 1987A have helped sketch
the most complete picture of the life and death of a massive star.
A combination of stellar evolution theory and astronomical obsena-
tion supports the picture that one class of supernova explosions (13 pe II)
results from massive stars, which release 1053 ergs of neutrinos as their
iron cores collapse to become neutron stars (Woosley and Weaver 1986~.
In a remarkable leap of scientific intuition, the essence of this picture was
sketched by Baade and Zwicky (1933), shortly after the discovery of the
neutron. Testing this picture for SN 1987A required neutnno detectors,
which caught enough of the neutrinos to make a convincing case that we
understand the binding energy of a neutron star, as well as the tempera-
ture and duration of the neutrino emission (Bahcall 1989~. The neutrino
observations provide a fiducial point: the moment of core collapse at 1987
FebruaIy 23.316. One interesting sidelight (see section Future Observations
of SN 1987A) is that UV observations of a light echo may provide a way to
obsene the flux emitted from the surface of the star in the hours between
the arrival of the shock and the discovery of the supernova by Ian Shelton
on February 24.23 (Madore and Kunkel 1987~. A key part of this picture
was the identification of the star which exploded. As descried in the
section "Circumstellar Matters," IUE provided essential data to identifier
directly the massive progenitor. This is the first time that a pre-supernova
star has been observed, and the star is Sanduleak ~9 202, a B3 ~ star
of about 20 solar masses. Many of the unique features of SN 1987A as
observed in the ultraviolet trace their origin to the explosion of a blue
supergiant, rather than the red supergiants favored for most extragalactic
OCR for page 239
HIGH-ENERGY ASTROPHYSICS
239
SN II's. IUE observations of a nitrogen-nch circumstellar shell help trace
the stellar evolution of SK -69 202 into the recent past, as described in the
section on the Ultraviolet Echo.
Supernovae are essential players in the chemical enrichment of the
universe. When the star destroys itself, the accumulated products of stel-
lar energy generation such as helium, nitrogen, carbon, oxygen, calcium
and silicon are dispersed into the interstellar gas along with the elements
synthesized in the explosion, such as the radioactive isotopes of the iron
peat The chemistry of the stellar interior can now be probed by infrared
observations (Rank et al. 1988), and the gamma ray detections (Matz et
al. 1988) provide strong proof that radioactive 56Ni is produced in the
explosion. The indirect effects of the energy release are seen in the light
curve, measured with the Fine Error Sensor on IUE, as presented in in
the first section, but the direct measurement of the interior composition
through ultraviolet lines will occur only after the opaque atmosphere turns
transparent Clues to a possible neutron star remnant may also be embed-
ded in the light curio for SN 1987A, but the resolution of these tantalizing
questions lies in the future.
OBSERVATIONS
The first IUE spectra of SN1987A were taken from Goddard on the
afternoon of Tuesday, February 24, about 4 hours after the report from
the Central Bureau for Astronomical Telegrams. The first frame of 15
seconds duration was heavily overexposed, and good low dispersion spectra
were eventually obtained with 1.5 second exposures. lithe initial spectra
were unlike the other IUE spectra of supernovae (Blair and Panagia 1987,
Benvenuti e! al. 19823, and changed very rapidly in the first few days of
observation. Interestingly, by February 26 the TJV spectrum of SN 1987A
resembled the spectrum of a lope I supernova (SN I) as seen in the
ultraviolet, while the combined optical and UV spectrum showed that the
supernova had distinct hydrogen Balmer lines: the iden~ing criterion for
SN II. The solution to this paradox is straightforward: in SN I, as show
by Branch and Venkataknshna (19863 and in the atmosphere of the blue
supergiant SK ~9 202, as shown by Luc y (1987) the strong blended lines of
Fe II and Co II dominate the opacity in the ultraviolet region of interest.
In SN II, like SN 1979C, the analysis of Fransson et al. (1984) shows that
the slow, dense wind of a red supergiant plays a key role in determining
the UV spectrum.
The conspicuous P-gygni lines in the UV and optical indicated an
initial expansion velocity near 30,000 km/see in the first hours of observation
and a temperature near 14,000 K Model atmospheres for SN 1987A have
been calculated by Eastman and Kirshner (1989) which provide a good
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AMERICAN AND SOVIET PERSPECTIVES
understanding of the effects of scattering and spherical geometry in the
expanding atmosphere. These models allow the distance to the LMC to be
determined by the Expanding Photosphere Method (Kirchner and Kwan
1974~: the result of 50 ~ 6 kpc is in good accord with the distances found
from Cepheids and from RR Lyrae stars (Walker 1987; WaLker and Mack
1988), and raises the prospect of using Sly II as an important tool in
establishing the extragalactic distance scale.
The apparent velocity declined rapidly as the fastest-moving layers
turned transparent, and the temperature declined rapidly as the supernova
atmosphere expanded and cooled adiabatically. The effect on the ultravi-
olet flux was profound: both the cooling and the onset of powerful line
blanketing combined to reduce the UV flux in the SWP range by a factor
of 1,000 in the first three days. The level flux at later times is due to the
presence of the two B stars near SK -69 202 which are in the IUE aperture.
The plummeting UV seen on Feb 24 is presumably the cooling tail of
a much hotter photosphere which must have been present on Feb 23, when
the shock from the stellar interior first reached the surface of Sk -69 202
about 2 hours after core collapse. Theoretical calculations show that the
temperature might have reached the range of 2-5 x 106 K for a brief time
on Feb 23. This ultraviolet flash is the source of photoionization of the
circumstellar matter, as described in the section on circumstellar matters,
where ionization up to N V is observed. Another way to detect the
ultraviolet hash emitted before the supernova was discovered is through its
UV echo from interstellar dust, as described in the section on the Ultraviolet
Echo. Empincal evidence that the surface of the Sanduleak star was very
hot comes from the photograph of the LMC taken by McNaught (1987)
on February 23.443, which showed that SN 1987A was already at about
mag 6. The product of velocity and age requires a temperature of order
100,000 K to produce the observed flub 1b press the UV obsenations
back to the earliest possible moment, we have begun to examine the badly
over-exposed images taken in the first attempts to get IUE spectra of SN
1987A In the depths of the absorption lines, and in the regions of the
spectrum where {UK has the least sensitivity, some reliable measurements
may be recovered which will help trace the arrival of the shock at the
surface of the Sanduleak star.
Following the dramatic changes of the first days, the IUE spectrum of
SN 1987A has remained remarkably constant. There are changes in the
UV flux through this penod, but the spectrum, set by the atomic physics
of photosphexic iron and cobalt shows only subtle variations. Although
the optical spectrum is now dominated by strong emission lines which
arise from material that was originally far below the photosphere of the
star, the UV photosphere has remained opaque. Although there have
been predictions of a "UV Renaissance" when the ultraviolet finally turns
OCR for page 241
HIGH-ENERGY ASTROPHYSICS
241
transparent (McCray e! al. 1987), Figure 1 indicates that we must first
endure the ultraviolet Dark Ages of 1989. 1b obtain Figure 1, the LWP
spectra have been integrated over broad wavelength intervals. Ultraviolet
observations of the material from the stellar interior should eventually
prove very helpful in determining the mass of carbon, magnesium, and
silicon produced by SN 1987A, but that time has not yet arrived. These
elements are important in comparing the observed composition for SN
1987A with the theoretical results for massive star evolution, and they are
difficult to observe at optical wavelengths.
Measurements from the Fine Error Sensor have proved surprisingly
accurate and useful in monitoring the flux from SN 1987N Even though
the FES was never intended as an accurate photometer, we have found
that careful attention to calibration by a standard star during the same
observing session produces a marked decrease in the random errors of
FES measurements. While the carefully integrated bolometric measures of
the Cerro lblolo workers (Suntzeff et al. 19~3) and of the South African
group (Whitelock 1988) are the primary data for comparing the radiative
output of SN 1987A with models, the FES data are instructive, illustrating
every major feature of the bolometnc light curve, as shown in Figure 2.
The familiar features of the rise to maximum in late May 1987, the long
exponential tail from age 110 days to 300 days, and the subsequent drop
below the extrapolated output of 56Co are all illustrated in the FES light
cube. The most recent data show that the steepening decline in the FES
light curse has abated. One possible interpretation is that a constant source
at a luminosity of 5 x 1~37 erg/see is now contributing to the SN 1987A
light curve. Whether this is related to the putative pulsar (Kristian et al.
1989) or possible accretion onto the neutron star (Chevalier 1989) remains
to be seen. The FES measurements will continue, unaffected by weather or
seeing and never vulnerable to the errors that accrue at large hour angles!
Ultimately, contamination from the neighboring stars will become a serious
problem, and only the superb images of HST will permit a light curve for
SN 1987A into the 1990's.
The UV light curies of Figure 1 show a distinctly different behavior.
While they share the rise to maximum seen in the FES data, they did not
experience the long exponential decline. Evidently, the small fraction of
the energy coming out in the UV rose during that period, presumably as
a result of decreasing line blanketing. The future of the UV light curio
is hard to predict, but it may show a dramatic change when the spectrum
changes to emission lines.
IN INTERSTELLAR MEDIUM TOWARDS SN 1987A
Although SN 1987A reached its maximum bolometric luminosity in
OCR for page 242
242
-
tQ
C\2
c)
sit
a)
cot
-
o
~ 1
_
2000
1 000
500
rig
200
1 ~
to
AMERICAN AND SOVIET PERSPECTIVES
1 j 1 1 1 ' 1 1 1 1 1 1 1 1 1 1 1 1 ' 1 ~_
-oh
a
A
~-
I' In'
100
50
~SN 1 987A
F
LONG WAVELENGTH
ULTRAVIOLET
/\2000 ~ - 2450
02500 ~ - 3000
03000 ~ - 3200
1
1987.5 1988 1988.5 1989 1989.5
Year
FIGURE 1 1be long wavelength light come of SN 1987A as observed with IUE.
late May 1987, the rapid decline in the UV flux made the fruitful time for
high resolutions observations very brief. During February 24 and 25, IUE
high resolution spectra were obtained which provided the best signal-tin
noise for study of the interstellar gas from Earth to the Sanduleak star.
The brightness of the supernova as a background source allowed much
shorter exposures than previous studies of the ISM toward the LMC were
compelled to use, and the resulting particle background in the data was
much lower. Because the dine span for obtaining these exquisite data was
OCR for page 243
HIGH-ENERGY ASTROPHYSICS
4
6 _
_
To
~8
0
243
~ i I ~ i I I ~ I i I T 1- 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i ~ I I J
-
-a
1
SN 1 987A
12 _
1 1 1 1 1 1 1, I 1 1 1 1,, 1 1 1 1 , 1 ~
300 400 500 600
-
,~, ,
JO
.~1
I I 1 1,,,, 1 1 1 ~ ~ 1
_ -
APT
, , 1 , . . .
700 800 900
0 1 Do 200
Days from v-Burst
FIGURE 2 An optical light curve for SN 1987A as observed with the Fine Error Sensor
on IUE.
short, there was little opportunity to study the time-dependent effects of
the ionizing flash from SN 1987A, but the very high quality of the data that
were obtained not only allowed them to confirm kinematic results obtained
at higher resolution from the ground but to extend them by providing the
chemical composition of the intervening absorbers.
Several velocity components were detected in a wide range of ionization
stages ranging from neutral gas to triply ionized carbon (de Boer et al. 1987,
Dupree et al. 19g7, Blades et al. 1988a,b, Savage e! al. 1989~. The observed
UV absorption components to SN 1987A have a velocity distribution which
is similar to that observed on the lines of sight to other stars in the LMC
(e.g., Savage and de Boer 1979; Savage 1986) and which agrees with the
main optical absorption systems found by Andreani et al. (1987) in the
spectrum of SN 1987N
The absorption features over the velocity range from O to +3001~/sec
arise in the disk and halo of our Galaxy and in the EMC. While it is not
controversial to assign the adsorptions with v > 200 km/see to gas in the
LMC and those with v < 70 hn/sec to our Galaxy, intermediate velocities
require more discussion. An abundance analysis of the intermediate velocity
components at 129 and 171 km/see by Blades et al. (1988a) shows that they
are not intergalactic clouds, but belong to the LMC. These clouds may
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244
AMERICAN AND SOVIET PERSPECTIVES
have their origins in gas that is stepped by tidal interactions or may arise
from w~nd-dr~ven shells or supernova remnants in the LMC.
The absence of N V absorption in the spectrum of SN 1987A has been
used by Fransson et al. (1987) to constrain the luminosity of the UV flash.
From the observed upper limit to N V absorption, they derived an upper
limit to the N V column density of 3 x 10~4 cm-2 and inferred a limit to
the number of ionizing photons S < 1.6 x 105 7/n2 (where n is the ambient
density for a temperature Teff = 5 x 105 K Comparison with models
for the shock arriving at the surface of a star (Klein and Chevalier 1978)
shows that a red supergiant would produce too much ionization, but a blue
supergiant, such as SK -69 202 would be a good match to the observational
constraint.
CIRCUMSTELLAR MATTERS
Blue supergiants like SK -69 202 often have low density, high velocity
stellar winds, but IUE observations of SN 1987A show that this star had
a dense circumstellar shell that resulted from an interesting stellar history.
The weak radio emission from SN 1987A ~Ibrtle et al. 1987) was inter-
preted (Chevalier and Fransson 1987) as arising from a shock in the low
density blue supergiant wind of SK ~9 202. After 1987 May 24, the short
wavelength IUE spectra began to show evidence for narrow emission lines,
as shown in Figure 3. Here the flux from the two neighboring stars as
observed in March 1987 has been subtracted from the subsequent spectra.
The observed lines Include He II, C III, N III, N IV, N V and O ITI,
and they increased in strength with lime. The observed velocities are low,
and the velocity widths of the lines are unresolved at the low dispersion,
implying velocities less than 1,000 hn/sec. All of these clues point toward
a circumstellar origin for the emission lines. First, the fact that we can see
the emission, while the supernova photosphere is opaque to the UV implies
that the source of the emission is outside the expanding star. Second, the
low velocities do not correspond to the debris, where the characteristic
velocities are a few thousand parsec. The great strength of the nitrogen
lines is consistent with the CNO-ennched composition of material that
results from a massive star's mass loss (Chevalier 1987; Fransson et al.
1989) and constrains the history of SK -69 20Z
The excitation of this circumstellar shell results from the UV flash that
took place when the shock traversing the Sanduleak star hit the surface.
This initial pulse of energy would have been very hot (T > 105 K) and
brief (<1 hour). Since the supernova was not discovered on the day of the
neu~ino burst, but the day after, the declining UV seen on 24 February
was just the tail of this violent UV flash.
The observed UV flux from the circNmstellar shell increased with time
OCR for page 245
HIGH-ENERGY ASTROPHYSICS
1.0
0.8
0.6
x
0.4
0.0
245
I ' 1' ' ' ~ I,,, ~ I ~ ~ ' ' I ' ' ' ' I ' ' ' I I I I I
- SN 1987A ~
8 Oct 1987 - 21 Apr 1988 Averaged - _
March 1987 Subtracted v ~
02 t
_
1200 1300 1400 1500 1600 1700 1800 1900 2000
WAVELENGTH (A)
FIGURE 3 Averaged spectrum of the narrow emission lines from the circumstellar shell
of SN 1987^ The spectrum of stars 2 and 3, as observed in March 1987 is subtracted from
the subsequent spectra.
until 400 days after the explosion, then began a symmetric decline, at least
in some lines. A plausible geometrical picture for the fluorescent material
is a shell at a distance of 200 light days from the supernova site. Light travel
times are important in determining the observed flux, and this dimension
of order 5 x 10~7 cm is indicated by the duration of the increase. The
spatial extent of this shell would be about one arc second, not measurable
with IUE, but well within the reach of HST.
Because the flux increased, high dispersion TUE measurements of the
circumstellar lines were possible. They remain unresolved at 30 km/see
resolution. The observed line ratios are consistent with a density of order
104 in the emitting gas, and ground based observations of narrow tO
III] help determine the temperature at about 45,000 K (Wampler and
Richichi 1988~. Win the physical conditions reasonably well determined,
the chemical abundances result from a nebular analysis. Fransson et al. find
N/C = 7.8 it 4 and N/O = 1.6 ~ 0.8. These are respectively factors of 37
and 12 higher man the solar values, implying that the gas has undergone
substantial CNO processing. To reveal CNO-processed material at the
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246
AMERICAN AND SOVIET PERSPECTIVES
surface, the progenitor of SN 1987A is likely to have lost much of its
hydrogen envelope before the explosion. This, and the existence of the
shell, are consistent with models where a red supergiant evolves to the blue
supergiant stage before exploding.
High nitrogen abundance was also found in the c~rcumstellar matter of
an earlier SN II with IUE (Fransson et; al. 19841. There, the explosion took
place while He star was a red supergiant: here, the circumstellar matter
was evident ejected from the star as a red supergiant, but the star evolved
to the blue before exploding. Thus the IUE observations help establish the
history of SK ~9 202 for the 2O,000 years before it exploded.
Matching the path in the H-R diagram and the chemical composition
of the circumstellar matter has proved a challenging task for theorists, who
were already struggling with the question of why the star exploded as a blue
supergiant. The evolution from blue (on the main sequence) to red (as a
mass-losing red supergiant) back to the blue (to explode as a B3 Ia star) has
been examined, for example, by Saio et aL (1988~. Key ingredients seem
to be the lower heavy element abundance in the LMC, thorough mixing of
hydrogen-burning products, and substantial mass loss as a red supergiant.
One prediction based on the presence of a circumstellar shell is that
the rapidly expanding debris, moving at 1/10 c, will strike the shell, at
1/2 light year, in the next several years. So for the end of the century,
we may expect a recrudescence of SN 1987A, with a hot shock interaction
producing copious X-rays and perhaps renewed nonthermal radio emission.
TlIE ULTRAVIOLET ECHO?
The discovery of two echo rings in the optical (Crofts 1988; Rosa 1988;
Heathcote et al. 1988; and a third echo ring reported by Bond et al. 1989)
attributed to dust scattering of light from the supernova by matter in the
LMC, has provided the opportunity for an interesting {UK investigation.
In the optical, the rings reflect light from the optical maximum observed In
May 1987. This is demonstrated by spectra of the nags taken in 19~, in
which the light from the rings has the spectrum of the supernova in May,
1987. The expected UV ring would be the result of the UV maximum, the
brief flash of UV emitted in the first hours of the event This means that, if
detected, the UV echo could provide direct information on the supernova
spectrum at the time of shock breakout: the observations would show the
properties of the supernova before discovery'
From an inspection of the optical images of the echo rings and a
comparison with preplan images, the brightest patch of the inner nag was
selected for the JUE observation (Gilmozzi 1988), since a simple calculation
following Chevalier and Emmering (1988) shows that the UV echo should
be just a few arcsec external to the optical ring. The ring was observed
OCR for page 247
HIGH-ENERGY ASTROPHYSICS
247
on 1 May, 1988, with the long axis of the IUE aperture perpendicular to
the ring, to obtain spatial information on the distribution of UV light. A
second, longer exposure on May 25 confirms the presence of a weak UV
signal Although the feature near 1550 A may be spurious, the rise around
125OA Is reaL
No background star contamination is expected, since the slit location
includes no stars brighter than 18 may. The coincidence of the emission and
the calculated position for the echo are consistent with the flux observed
arising from a UV echo. However, there is still the possibility that the
emission is due to diffuse matter scattering the light from nearby hot stars.
The key test is to observe the same location in 1989: if the flux is still
present, it is not due to the echo ring, which will have expanded to a larger
diameter.
If the detected signal is the UV echo, the spatial extent of the emission
(about 5 arc minutes) is a good measure of the thickness of the scattering
cloud, since the UV emission is the echo of a very brief event (Chevalier and
Emmering 1988~. The derived cloud thickness is about 40 pc, which agrees
well with the upper limit of 50 pc derived from the optical observations.
The flux from the UV echo, if confirmed, will also be instructive. If
it is of the same order as the optical echo, this implies that the energy
emitted in the UV represents about 10% of the total energy radiated by the
supernova. Since the optical maximum lasted at least 100 tunes as long as
the UV peak (two months compared to less than a day), and the scattering
efficiency in the UV is about 10 times better (Chevalier and Emmenng
1988), then equal observed flmes would imptr that me integrated UV
luminosity was about 10% of the bolometric luminosity radiated near the
SN peak If this observation Is confirmed, it will provide a useful constraint
on theoretical models of the outburst.
While the case is not yet proven, the IUE results provide the tantalizing
possibility of detecting a signal which was emitted two days before the IUE
first pointed at SN 1987A' and which may prove useful in understanding
the physics of supernova explosions.
FUTURE OBSERVATIONS OF SN 1987A
IUE observations of SN 1987A in 1989 and beyond will depend on the
behavior of the supernova, but will surely include a diligent monitoring of
the UV spectrum, with the hope that the opaque atmosphere will begin to
turn transparent and reveal the internal composition of the now-vanished
Sanduleak ~9 202. The changes in ionization of the circumstellar shell
should provide a time-lapse view of the recombination of that gas, and
an improved understanding of the physical setting for the emission. A
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248
AMERICAN AND SOVIET PERSPECTIVES
carefully planned observation of the UV echo position should provide the
decisive test for that possible observation.
Continued ultraviolet investigation of the supernova should continue
with HST. Its powerful UV spectrometers will allow the interstellar medium
near SN 1987A to be studied by looking in absorption at the nearby stars 2
and 3. The UV flux from the supernova itself will be safely resolved from
those neighbors so it can be followed down to much fainter levels both ~
spectroscopic and photometric observations. The arcumstellar shell that
IUE detects spectroscopically should be a good target for HST imaging.
While {UK observations of the debris from the supernova hitting the
circumste31ar shell would be interesting, it is reasonable to hope that when
this event occurs in 1999 we will have another instrument to use! Finally,
HST should be an effective tool for studying the expanding debris itself,
and perhaps the pulsar within SN 1987N Even though we can anticipate
these desirable observations, the most intriguing possibilities may be the
observations that we have not yet conceived. The ability to change the
observing program, sometimes on short notice, in response to events in the
LMC rather than constraints imposed from Earth, is an essential part of
studying an evolving object.
The fact the target-of-opportunity proposals were in place, and that
interested observers were ready to carry out a planned program of ob-
senation is only half the story of the UV observations of SN 1987N The
target~f-opportunity proposals focused on He aspects of supernovae which
had been important in previous investigations: the explosion physics and
the chemical analysis of the debris. But astronomy is an observational sci-
ence and the observed objects have rarely read the proposals. In the case of
SN 1987A, the contributions of IUE turned out to be especially important
in areas that were not anticipated, using the satellite in ways which were
not customary. For example, the identification of the progenitor by using
the astrometry and the imaging properties of IUE was a useful contribu-
tion that required novel use of IUE. Employing the FES as an accurate
photometer was not anticipated, but new calibration methods make those
measurements quite helpful. No one predicted that IUE short-wavelength
observations of narrow emission lines from a fluorescent circumstellar shell
would be a major constraint on the late stages of stellar evolution for
the EMC supernova, but a careful background subtraction technique has
made this a reality. While the jury is still out on the UV echo, there is a
chance that the IUE observations may provide a glimpse of the supernova
explosion's flux before it was discovered.
The key ingredient in the success of the IUE observations of SN
1987A has been the ability to modify the observing program in response
to the behavior of the supernova. On the first day, this meant a rapid
change in schedule and real-time adjustment of exposure times. Later, it
OCR for page 249
HIGH-ENERGY ASTROPHYSICS
249
implied changing the balance of long and short wavelength exposures, and
combing shifts for very long exposures. For the echo observations, it
required precise choice of dates of observation In every case, the IUE
Observatory has had the gentility to accommodate these requirements,
and a deeper understanding of this unique event has been the result
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Astrophys. 27.
Baade, W., and F. Zwicly. 1933. Phys. Rev 45: 138.
Bahcall, J.N. 1989. Neutrino Astrophysics Cambridge University Press, Cambridge.
Benvenuti, P., et al. 198Z ESA SP-1046.
Blades, J.C, J.M. Wheatley, N. Panagia, M. Grewing, M. Pettim, and W. Wamsteker 1988a.
Ap. J (Lett.) 332: L75.
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Representative terms from entire chapter:
circumstellar shell
