-7- 

SEMI-ANNUAL STATUS REPORT 

August 1988-February 1989 

NASA GRANT NAG 5-841 

ROBERT P. KIRSHNER 

PRINCIPAL INVESTIGATOR 

HARVARD COLLEGE OBSERVATORY 

60 GARCEN STREET 

CAMBRIDGE, MA 02138 

(NASA-CB-184973) [ IUE IBlVESTIGATIONS OF S# N 89-25079 

1987A] Semiannual Status Beport, Aug., 1988 - 

Peb. 1989 (Harvard Coll. Observatory) 36 p 

CSCL 03A Unclas 

G3/89 0206414 
SEMI-ANNUAL REPORT 

NASA GRANT NAG 5-841 

May 4, 1989 

The purpose of this grant is to support IUE investigations 

of SN 1987A, the brightest supernova observed in 383 years. A 

detailed summary of the extensive scientific results from the IUE 

observations has been prepared for inclusion in the revised 

version of ItExploring the Universe with the IUE Satellite" which 

is edited by Yoji Kondo. Since that book chapter provides a 

comprehensive summary of the work to date, it is being submitted 

as the semi-annual report for this period. 
PUBLICATIONS 

AUGUST 1988-FEBRUARY 1989 

NASA GRANT NAG 5-841 

Fransson, C., Cassatella, A., Gilmozzi, R., Panagia, N., 

Wamsteker, W., Kirshner, R.P., and Sonneborn, G. 1988, 

"Narrow W Emission Lines from SN 1987A: Evidence for CNOProcessing in the Progenitor" Ap.J. 336, 429. 

Arnett, W.D., Bahcall, J.N., Kirshner, R.P., and Woosley, S.E. 

1989, "Supernova 1987A" Ann. Rev. Astron. Astrophys. in 

press. 

Kirshner, R.P. and Gilmozzi, R. 1989, IIObserving SN 1987A with 

IUE" in Explorina the Universe with the IUE Satellite, 2nd 

edition, Y. Kondo (editor), Kluwer Academic Publishing, in 

press. 
OBSERVING SN 1987A WITH IUE 

Robert P. Kirshner 

Harvard-Smithsonian Center for Astrophysics 

Cambridge, MA, U.S.A. 

Roberto Gilmozzil 

Space Telescope Science Institute 

Baltimore, MD, U.S.A. 

1. Introduction 

The 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 

fundamental property of the IUE project proved essential: it is a 

satellite whose program can be rapidly changed to take advantage 

of scientific opportunities. On both sides of the Atlantic, 

there was a target-of-opportunity proposal in place, so that an 

orderly, though very exciting, series of observations 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). 

IUE 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 section 2, and high dispersion 

observations for the interstellar medium when the supernova was 

bright (section 3) and for circumstellar gas surrounding the 

supernova as the initial event faded (section 5). The UV data 

have been especially useful in determining which star exploded 

(section 4), 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, Bahcall, Kirshner and Woosley 

(1989) and earlier reports on the ultraviolet data are summarized 

by Kirshner (1988). 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 

'Affiliated to the AstrophySics Division, Space Sciences 

Division, ESA 
Kirshner t Gilmozzi: Observing SN 1987A with IUE 

remained opaque 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 lineof-sight to the supernova, both in our Galaxy and in the LMC. 

Future observations will include a W 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 -69 202. 

Supernova 1987A in the Large Magellanic Cloud has moved the 

subject of supernovae from plausible argument to observational 

demonstration in a number of areas and the IUE observations have 

helped in essential ways. While the supernova was the first 

visible to the unaided eye since Kepler's 1604 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 

observation supports the picture that one class of supernova 

ex losions (Type 11) results from massive stars, which release 

log3 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 neutrino 

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 temperature and duration of the 

neutrino emission (Bahcall 1989). The neutrino observations 

provide a fiducial point: the moment of core collapse at 1987 

February 23.316. On interesting sidelight (section 6) is that W 

observations of a light echo may provide a way to observe 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 described in section 4, IUE provided essential data 

to identify directly the massive progenitor. This is the first 

time that a pre-supernova star has been observed, and the star is 

Sanduleak -69 202, a B3 I star of about 20 solar masses. Many of 

the unique features of SN 1987A as observed in the ultraviolet 

2 

Y 
Kirshner &I Gilmozzi: Observing SN 1987A with IUE 

trace their origin to the explosion of a blue supergiant, rather 

than the red supergiants favored for most extragalactic SN 11's. 

IUE observations of a nitrogen-rich circumstellar shell help 

trace the stellar evolution of SK -69 202 into the recent past, 

as described in Section 5. 

Supernovae are essential players in the chemical enrichment 

of the universe. When the star destroys itself, the accumulated 

products of stellar 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 peak. 

The chemistry of the stellar interior can now be probed by 

infrared observations (Rank et al. 1988), and the gamma ray 

detections et al. 1988) provide strong proof that 

radioactive &%? 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 

section 2, but the direct measurement of the interior composition 

through ultraviolet lines will occur only after the opaque 

atmosphere turns transparent, as suggested in section 7. Clues 

to a possible neutron star remnant may also be embedded in the 

light curve for SN 1987A, but the resolution of these tantalizing 

questions lies in the future. 

2. 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. The initial spectra were unlike the other IUE 

spectra of supernovae (Blair and Panagia 1987, Benvenuti et al. 

1982), and changed very rapidly in the first few days of 

observation as shown in Figure 1. Interestingly, by February 26 

the W spectrum of SN 1987A resembled the spectrum of a Type I 

supernova (SN I) as seen in the ultraviolet, while the combined 

optical and W spectrum in Figure 2 shows that the supernova had 

distinct hydrogen Balmer lines: the identifying criterion for SN 

11. The solution to this paradox is straightforward: in SN I, 

as shown by Branch and Venkatakrishna (1986) and in the 

atmosphere of the blue supergiant SK -69 202, as shown by Lucy 

(1987) the strong blended lines of Fe I1 and Co I1 dominate the 

opacity in the ultraviolet region of interest. In SN 11, like SN 

3 
Kirshner 61 Gilmozzi: Observing SN 1987A with IUE 

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 W spectrum. 

The conspicuous P-Cygni lines in the W and optical 

indicated an initial expansion velocity near 30 000 km/sec 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 understanding of the 

effects of scattering and spherical geometry in the expanding atmosphere, as Figure 2 indicates. These models allow the 

distance to the LMC to be determined by the Expanding Photosphere 

Method (Kirshner 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 SN I1 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 ultraviolet 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 1000 in the first three 

days, as shown in Figure 3. 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, as described in section 4. 

The plummeting W 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 lo6 K for a brief time on Feb 23. 

This ultraviolet flash is the source of photoionization of the 

circumstellar matter, as described in section 5, where ionization 

up to N V is observed. Another way to detect the ultraviolet 

flash emitted before the supernova was discovered is through its 

UV echo from interstellar dust, as described in section 6. 

Empirical 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 flux. To 

press the W observations 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 

IUE has the least sensitivity, some reliable measurements may be 

4 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

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. This is 

illustrated in Figure 4, which shows the LWP spectrum, like the 

stable dog in the Sherlock Holmes story, is remarkable for what 

it does do from February of 1987 through 1988. There are 

changes in the W flux through this period, but the spectrum, set 

by the atomic physics of photospheric 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 W 

photosphere has remained opaque. Although there have been 

predictions of a ttW Renaissancett when the ultraviolet finally 

turns transparent (McCray, Shull and Sutherland 1987), Figure 5 

indicates that we must first endure the ultraviolet Dark Ages of 

1989. To obtain Figure 5, 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 

1987A. 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 Tololo 

workers (Suntzeff et al. 1988) 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 bolometric light curve, 

as shown in Figure 6. 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 56C~ are all illustrated in the FES light curve. In 

Figure 7, the most recent data show that the steepening decline 

in the FES light curve has abated. One possible interpretation 

is that a constant source at a luminosity of 5 x erg/sec is 

now contributing to the SN 1987A light curve. Whether this is 

related to the putative pulsar (Kristian et a1 1989) or possible 

accretion on to 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 

5 
Kirshner &I Gilmozzi: Observing SN 1987A with IUE 

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 W light curves of Figure 5 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 W 

rose during that period, presumably as a result of decreasing 

line blanketing. The future of the W light curve is hard to 

predict, but it may show a dramatic change when the spectrum 

changes to emission lines. 

3. The Interstellar Medium Towards SN 1987A 

Although SN 1987A reached its maximum bolometric luminosity 

in late May 1987, the rapid decline in the W 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-to-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 time span for obtaining 

these exquisite data was 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. 1987, Blades et al. 

1988a,b, Savage et al. 1989). The observed W 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 (eg. 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 1987A. 

Table 1, adapted from Blades et al. (1988b) shows the 

velocity structure of the interstellar species detected with IUE, 

and Table 2 summarizes the same information for various Fe I1 

6 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

lines. Figure 8 shows an example of the IUE data for some of the 

higher ionization stages. Note that the high velocity component 

is highly saturated. 

The absorption features over the velocity range from 0 to 

+300 km/sec arise in the disk and halo of our Galaxy and in the 

LMC. While it is not controversial to assign the absorptions 

with v >/= 200 km/sec to gas in the LMC and those with v </= 70 

km/sec to our Galaxy, intermediate velocities require more 

discussion. An abundance analysis of the intermediate velocity 

components at 129 and 171 km/sec by Blades et al. (1988a) shows 

that they are not intergalactic clouds, but belong to the LMC. 

These clouds may have their origins in gas that is stripped by 

tidal interactions or may arise from wind-driven shells or 

supernova remnants in the LMC. 

Although there are some weak and unidentified features in 

the spectra, there is no evidence for narrow lines in the 

absorbing material at high velocity associated with the supernova 

explosion itself. Similarly, over the limited time span of the 

high resolution observations, no variations in the spectra have 

been found which could be attributed to time dependent effects in 

the neighborhood of the supernova. However, one absorption 

component, at 206 km/sec, is unusual with only high ionization 

species (C IV, Si IV, and possibly A1 111) and is not typical of 

absorption spectra seen toward other LMC stars. It is tempting 

to speculate that this unique component might be related to the 

supernova or the effects of its progenitor. One way to tell will 

be to study the absorption spectra of the Sanduleak star's close 

neighbors, star 2 and star 3, with HST. By examining these 

neighboring lines of sight, the extent of the peculiar absorbing 

region can be pinned down. 

Another special feature is the cloud observed at 281 km/sec 

in which the highly ionized species of C IV and Si IV have 

equivalent widths which are significantly larger than found in 

other LMC stars (see Figure 8). One possible origin for this 

increased strength is that the W flash increased the ionization 

of the region out to 1 pc. If this region has an average density 

greater than 10 ~rn'~, it could account for the observed line 

strengths. As detailed in section 5, circumstellar emission 

lines have been observed with IUE (Fransson et al. 1989) , and 

high dispersion observations of these lines show they have a 

velocity near 281 km/sec. It is tempting to identify the 

absorption component seen in the first days with the gas that 

produced the emission seen many months later: circumstellar gas 

at the interface between the slow, dense red supergiant wind, and 

the fast, low density wind of the blue supergiant. However, the 

analysis by Savage et al. (1989) places limits on the density of 

7 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

material very near the supernova by examining the 1264A line from 

Si I1 in an excited fine-structure level. They conclude that it 

is unlikely that the ultraviolet flash from the shock breakout 

could explain the strong Si IV and C IV features seen at LMC 

velocities. 

The absence of N V absorption in the spectrum of SN 1987A 

has been used by Fransson et a1 (1987) to constrain the 

luminosity of the W flash. From the observed upper limit to N V 

absorption they derived an upper limit to the N V column density 

of 3 x 101'4 cm'2 and inferred a limit to the number of ionizing 

photons S < 1.6 x 1057/n2 where n is the ambient density) for a 

temperature Teff = 5 x lob 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. 

4. Ultraviolet Clues to the Progenitor of 6N 1987A 

The ultraviolet data from IUE played an interesting role in 

determining that the B3 I supergiant Sanduleak -69 202 (Sanduleak 

1974) was the progenitor of SN 1987A. Initial position measures 

(McNaught 1987) indicated that this star was coincident with the 

supernova, and indicated the presence of a companion about 3" 

away. As the supernova plummeted in the ultraviolet (see Figure 

5), IUE observers found the surprising result that five days 

after the explosion the spectrum resembled an early B star. This 

suggested the possibility that SK -69 202 had survived the 

explosion, and was not itself the supernova progenitor (Gry et 

al. 1987). 

The situation began to become clearer when careful 

astrometry of plates in the region of the supernova was carried 

out by West et a1 (1987) and by Walborn et a1 (1987). They 

showed that there were actually three stars within three arc sec 

of the supernova position. The excellent positional agreement 

between the SN and the Sanduleak star (B31, V=12.36) made this 

the best candidate for the progenitor, while the position of star 

2 (3" at PA=315', V=15.3) made it clear that this star could be 

excluded. A third star (1.5" at PA= 120°, V=15.7) was also 

detected. The possibility of two stars not connected with the 

supernova explosion prompted a re-analysis of the IUE spectra 

which showed two objects were present in the IUE aperture 

(Sonneborn and Kirshner 1987). Both Panagia et a1 (1987) and 

8 
Kirshner &I Gilmozzi: Observing SN 1987A with IUE 

Kirshner et a1 (1987) concluded that the two spectra seen by IUE 

were the spectra of stars 2 and 3. Both of these stars are hot 

enough to be important in the IUE SWP spectral range, especially 

as the supernova faded so rapidly at those wavelengths. 

Even though the IUE is not an imaging instrument, there is 

some information on the spatial distribution of light in the 

aperture which can be extracted from the line-by-line files. 

A quantitative reanalysis of the two-dimensional spectra (see 

Figure 9) was performed in the l300A to 1500A region where the 

Point spread function of the instrument is good and the supernova 

flux was negligible to examine which two stars were contributing 

the W flux (Gilmozzi et a1 1987, Sonneborn, Altner, and Kirshner 

1987). The spatial profile was fit using the skewed point spread 

function of the SWP camera (Cassatella et al. 1985). This 

deconvolution showed that the two stars were separated by 4.5" 

+/-0.2", and the intensity ratio was 1.75 +/- 0.12. These values 

are in good accord with the projected separation of stars 2 and 3 

(4 .,I1) and demonstrably different from the 3" separation between 

the Sanduleak -69 202 star and star 2. Further tests of the flux 

from the stars and changes in the projected separation as the 

satellite changed orientation confirmed these findings. The 

stars seen by IUE were stars 2 and 3, and the 12 mag B3 I star 

Sanduleak -69 202 had disappeared because it was the progenitor 

of SN 1987A. 

The spectra of the two stars could be extracted separately. 

Gilinozzi et a1 (1987) measured the flux from star 3 and provided 

a classification of BO V for star 2 and B1.5V for star 3, based 

on a comparison with W standard stars. To examine this more 

closely, Gilmozzi and Panagia (1989) and Gilmozzi (1988) have 

constructed a useful color-magnitude diagram [M(V) , m(1ambda) -VJ 

with the loci of luminosity classes V to 111 for the spectral 

classes BO to B5, as shown for two wavelengths in Figure 10. A 

consistent result for stars 2 and 3 is reached only for 

luminosity class V, with a reddening of E(B-V) of 0.18. The 

galactic contribution, assumed to be 0.05 (Panagia et a1 1987) 

can be seen in the figure as the steeper portion of the reddening 

line. While the intrinsic ultraviolet colors for stars come from 

a galactic sample and the metallicity of the LMC is lower, which 

may introduce some uncertainty, it appears that star 2 is a 

genuine BOV star, while star 3 may be slightly earlier than the 

B1.5V type derived by Gilmozzi et a1 (1987). 

The identification of SK -69 202 as the progenitor was 

surprising, since conventional wisdom holds that SN I1 come from 

red supergiants. But the destruction of this star, a B3 Ia 

supergiant, radiating lo5 solar luminosities with surface 

temperature near 15 000 K, and an extent of only 40 solar radii, 

9 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

would produce the observed unusual velocities, color changes, and 

luminosity as modelled by Woosley et al. (1987). It is to the 

everlasting credit of the theorists that even when the IUE 

results appeared to show that the Sanduleak star might have 

survived, they adhered to their conviction that the progenitor of 

SN 1987A must have been just like that star! It is a satisfying 

outcome of the observational work that the star which disappeared 

in the star which exploded! 

5. 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 (Turtle et a1 1987) was interpreted (Chevalier and 

Fransson 1987) as arising from a shock in the low density blue 

supergiant wind of SK -69 202. After 1987 May 24, the short 

wavelength IUE spectra began to show evidence for narrow emission 

lines, as shown in Figure 11. 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 11, C 111, N 111, N IV, N V 

and 0 111, and they increased in strength with time. The 

observed velocities are low, and the velocity widths of the lines 

are unresolved at the low dispersion, implying velocities less 

than 1000 km/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 W 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 km/sec. The great strength of the nitrogen lines 

is consistent with the CNO-enriched composition of material that 

results from a massive star's mass loss (Chevalier 1987, Fransson 

et a1 1989) and constrains the history of SK -69 202. 

The excitation of this circumstellar shell results from the 

W 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 > lo5 K) and brief ( c1 hour). Since the 

supernova was not discovered on the day of the neutrino burst, 

but the day after, the declining UV seen on 24 February was just 

the tail of this violent W flash. 

10 
Kirshner &I Gilmozzi: Observing SN 1987A with IUE 

The observed UV flux from the circumstellar shell 

increasing with time until 400 days after the explosion, then 

began a symmetric decline, at least in some lines, as shown in 

Figure 12. 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 1017 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 IUE measurements 

of the circumstellar lines were possible. They remain unresolved 

at 30 km/sec resolution. The observed line ratios are consistent 

with a density of order lo4 in the emitting gas, and ground based 

observations of narrow [O 1111 help determine the temperature at 

about 45 000 K (Wampler and Richichi 1988). With the physical 

conditions reasonably well determined, the chemical abundances 

result from a nebular analysis. Fransson et al. find N/C = 7.8 

+/- 4 and N/O = 1.6 +/- 0.8. These are respectively factors of 

37 and 12 higher than the solar values, implying that the gas has 

undergone substantial CNO processing. To reveal CNO-processed 

material at the 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 circumstellar 

matter of an earlier SN I1 with IUE (Fransson et a1 1984). 

There, the explosion took place while the star was a red 

supergiant: here, the circumstellar matter was evidently 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 -69 202 for the 20 000 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, Kato, and Nomoto (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. 

11 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

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. 

6. The Ultraviolet Echo? 

The discovery of two echo rings in the optical (Crotts 1988, 

Rosa 1988, Heathcote et a1 1988, and a third echo ring reported 

by Bond et a1 1989) attributed to dust scattering of light from 

the supernova by matter in the LMC, has provided the opportunity 

for an interesting IUE investigation. In the optical, the rings 

reflect light from the optical maximum observed in May 1987. 

This is demonstrated by spectra of the rings taken in 1988, in 

which the light from the rings has the spectrum of the supernova 

in May, 1987. The expected W ring would be the result of the W 

maximum, the brief flash of W emitted in the first hours of the 

event. This means that, if detected, the W 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 pre-SN images, the brightest patch of the 

inner ring was selected for the IUE observation (Gilmozzi 1989), 

since a simple calculation following Chevalier and Emmering 

(1988) shows that the W echo should be just a few arcsec 

external to the optical ring. The ring was observed on 1 May 

1988 with the long axis of the IUE aperture perpendicular to the 

ring, to obtain spatial information on the distribution of W 

light. A second, longer exposure on May 25 confirms the presence 

of a weak W signal, as illustrated in Figure 13. Although the 

feature near 1550 A may be spurious, the rise around 1250A is 

real. 

No background star contamination is expected, since the slit 

location includes no stars brighter than 18 mag. The coincidence 

of the emission and the calculated position for the echo are 

consistent with the flux observed arising from a W echo. 

However, there is still the possibility that the emission is due 

to diffuse matter scattering the light from nearby hot stars. 

They 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. 

12 
. 

Kirshner & Gilmozzi: Observing SN 1987A with IUE 

If the detected signal is the W echo, the spatial extent of 

the emission (about 5 arc minutes) is a good measure of the 

thickness of the scattering cloud, since the W 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 W 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 times as long as the W peak 

(two months compared to less than a day), and the scattering 

efficiency in the W is about 10 times better (Chevalier and 

Emmering 1988), then equal observed fluxes would imply that the 

integrated W luminosity was about 10% of the bolometric 

luminosity radiated near the SN peak. If this observation is 

confirmed, it 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 proved useful in understanding the physics of supernova 

explosions. 

7. 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 W spectrum, with the hope that the 

opaque atmosphere will begin to turn transparent and reveal the 

internal composition of the now-vanished Sanduleak -69 202. The 

changes in ionization of the circumstellar shell should provide a 

time-lapse view of the recombination of that gas, and an improved 

carefully planned observation of the W echo position should 

provide the decisive test for that possible observation. 

Continued ultraviolet investigation of the supernova should 

continue with HST. Its powerful W spectrometers will allow the 

interstellar medium near SN 1987A to be studied by looking in 

absorption at the nearby stars 2 and 3. The W flux from the 

supernova itself will be safely resolved from those neighbors so 

it can be followed down to much fainter levels both in 

spectroscopic and photometric observations. The circumstellar 

understanding of the physical setting for the emission. A 

13 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

shell that IUE detects spectroscopically should be a good target 

for HST imaging. While IUE observations of the debris from the 

supernova hitting the circumstellar 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 1987A. 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 observation is only half the story of the W 

observations of SN 1987A. The target-of-opportunity proposals 

focussed on the 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 

science 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 contribution 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 shortwavelength observations of narrow emission lines from a 

fluorescent circumstellar shell would be a major constraint on 

the late stages of stellar evolution for the LMC supernova, but a 

careful background subtraction technique has made this a reality. 

While the jury is still out on the W 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 implied changing the balance of long 

and short wavelength exposures, and combining 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 flexibility to accommodate these 

requirements, and a deeper understanding of this unique event has 

been the result. 

14 
Kirshner & Gilmozzi: Observing SN 1987A with IUE 

Acknowledgements 

One of the great pleasures of studying SN 1987A has been the 

lively and stimulating discussion among the participants, and the 

cooperation of so many who supported the observations by adapting 

to a revised schedule. We are especially grateful to the 

dedicated crews at Goddard and at VILSPA for their diligent 

cooperation in gathering the data, and to our many colleagues who 

cheerful changed their schedules so that we could make these 

once-in-a-lifetime observations. This generous spirit places a 

special obligation on those of us who are working on the data to 

do the best job we can, and to leave a unique set of data for 

future astronomers. Collaborators on these observations and 

their interpretation include Angelo Cassatella, Nino Panagia, 

George Sonneborn, and Willem Wamsteker. RPK's research on 

supernovae is supported by NASA grants NAG5-645 and NAG5-841, and 

by NSF grant AST85-16537. 

15 
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17 
VELOCITY STllUCrUllr OF ImTELM SPrClCS TOWARD SN 1987A 

MEAN HELIOCENTRIC VELOCim (H THE 10 INTERSTELLAR FEATU~ (kms-')~ 

ION (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) 

CI .............. ... 18 ... ... ... ... .., ... ... 280 

01 nr nr nr 63 127 166 ... 233 ... 286 

Mg I -1s 27 79 131 174 ... 226 ... 28 I 

a1 11 ... 216 

Mg 11 nr nr nr nr 139 185 ... nr nr nr 

AI II I1 67 123 166 ... 22 1 ... 270 

Si II 16 67 126 168 ... 228 ... 285 

s I1 16 78 ... ... ... 270 

Cr 11 2s ... ... ... 23 1 ... 292 

Mn II 21 72 ... ... ... 223 258 288 

Ni 11 I5 72 167 ... 228 ... 288 

Zn II 13 ... ... 223 ... 28 I 

Fe 11' -19 18 72 126 I70 ... 219 261 288 

16 69 ... 213 ... ... 279 

Si IV 29 72 135: 1% ... ... 28 1 

c IV 14 63 ... 209 ... ... 280 

c I* 27 ... ... 273 ... c 1.0 16: ... ... ... 268 ... c II* 26 1 ... Si ti* .. ... ... 262 ... 

............. 

............ ... ............. ... ... ... ... ... ... ... ........... 

............ ... ... ............. ... ... ............. ... ... ... ... ............ ... ... ... ........... ... ... ............ ... ... ... ............ ... ... ... ... ........... ... ... ... ... ............ ... ... ... ............ ... ... ... ............ ... ... ... ... ... ........... ... ... ... ... ............ ... ... ... ... ... ... ... ... ........... ... ... ... ... ... ~~ 

' Details are given in Table 4. ' An .. nr " means the line is present but not resolved. 

A SUMMARY OF THE Fe 11 I~LW Llm TOWARD SN 1987A 

ION 1 ~ ~~ ~~ 

Fen 2599 

2585 

2382 

2373 

2366 

2343 

2260 

HELIOCE~C Vm (km s-'Y 

(1) (2) (4) (5) (6) (8) (9) (10) 

-18 18 74 130 175 nr ar 286 

-17 19 73 127 172 221 257 294 

nr nr 71 126 171 nr nr nr -4: 18 75 126 173 217 258 289 

... .................. 283 

-18 21 72 127 167 220 261 295 

... 23 ................. 288 

1611 ... 15 ............... 283 

1608 -22 12 68 120 159 219 nr 282 

Non~-The numbers in parentheses der to nine of the 10 velocity features 

given in Trble)(fcature 3 is not included in this table). An "nr" means the line is 

present but not resolved. 
Figure Legends 

Figure 1: The rapidly changing LWP IUE spectrum of SN 1987A as 

observed in the first weeks after discovery. 

Figure 2: Combined W and optical spectrum for SN 1987A on 24 

February 1987. Also shown are model spectra by Eastman and 

Kirshner (1989) . 

Figure 3: The short wavelength IUE light curve for SN 1987A. The 

precipitous decline observed in the first few days halted at the 

level contributed by stars 2 and 3, then increased slightly due 

to flux from the fluorescent echo of circumstellar matter. 

Figure 4: The spectral history of SN 1987A in the IUE long 

wavelength region. Except for rapid evolution in the first week, 

the spectrum remained almost unchanged in the following year. 

Figure 5: The long wavelength light curve of SN 1987A as 

observed with IUE. 

Figure 6: An optical light curve for SN 1987A as observed with 

the Fine Error Sensor on IUE. 

Figure 7: Detail of the late light curve for SN 1987A as 

observed with the Fine Error Sensor on IUE. The solid line 

indicates the decay energy of 0.07 solar masses of 56C0. Note 

the recent levelling out which may indicate the onset of a new 

energy source. 

Figure 8: Velocity profiles of the high ionization components 

for interstellar lines towards SN 1987A. 

Figure 9: Composite line-by-line spectrum between 1210A and 

1510A, obtained after the supernova had faded in this band. Two 

objects are present in the aperture (bottom). For comparison, 

the spectrum of a single point source is shown (top). 

Figure 10: Ultraviolet color-magnitude diagram for stars 2 and 

3..The rightmost dot for each star represents the observed 

values, while each successive dot represents a correction of 0.05 

in E(B-V) . The steeper correction corresponds to the galactic 

extinction law. A value E(B-V)= 0.18 is indicated as the 

simultaneous solution for both stars and both wavelengths. 

Figure 11: Averaged spectrum of the narrow emission lines from 

the circumstellar shell of SN 1987A. The spectrum of stars 2 and 

3, as observed in March 1987 is subtracted from the subsequent 

spectra. 

18 
. 

Figure 12: Flux measured in the N V emission line of the 

circumstellar shell. The increase to age 400 days is attributed 

to light travel time effects in a shell of dimension 200 light 

days. 

Figure 13: A 12 hour SWP spectrum obtained at the predicted 

position of the W echo on May 25, 1988. 

I 

19 
24 Feb 1987 

2000 

1500 

1000 

500 

1800 2000 2200 2400 2600 2800 3000 3200 3400 

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