Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.eso.org/~bleibund/papers/ASI450.ps Äàòà èçìåíåíèÿ: Sun Jan 7 21:58:56 2001 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:15:16 2012 Êîäèðîâêà:

OBSERVATIONS OF SUPERNOVAE 1
BRUNO LEIBUNDGUT
University of California
Astronomy Department
Berkeley, CA 94720
and
European Southern Observatory
Karl­Schwarzschild­Strasse 2
D­85748 Garching
Germany
Abstract.
The available observations of the various types of supernovae are described and their connection
with the current theoretical understanding outlined. We critically discuss the classification scheme
employed for supernovae and point out its limits.
The observational homogeneity of type Ia supernovae has been challenged by recent, well­
observed events. Nevertheless, the general characteristics are rather well described by models
employing explosive carbon burning in white dwarfs. The most important input which observations
should provide at this point is the mass of the progenitor star and the amount of radioactive nickel
produced in the explosion. With reliable absolute magnitudes and improved model calculations
becoming available such determinations appear in reach.
While the separate character of SNe Ib/c is now firmly established, too few objects have been
observed detailed enough to probe the physical nature of these supernovae. The subclassification
into SNe Ib and SNe Ic has not convincingly been proven. Paradoxically, the most valuable infor­
mation on these supernovae has been provided by the observations of SNe II which switched to
SNe Ib/c at late phases confirming the link between these two classes.
Observations of SNe II have revealed several distinct phases which are governed by different
radiation mechanisms. Many details have been learned from SN 1987A and applied to other SNe II.
The influences of size and mass of the progenitor star as well as the explosion energy on the evolution
of the luminosity have been established. The additional powering of SNe II by conversion of kinetic
energy into electromagnetic radiation in a dense circumstellar medium has revealed a new phase for
these objects which starts several years after the explosion. Few cases have been observed where
this energy source appears to be dominant at all times.
Key words: Supernovae, Formation of Neutron Stars
1. Introduction
The astrophysical significance of supernovae (SNe) is reflected in the many views
astronomers have of them. These explosions mark the end of the nuclear energy
production in massive (?10 M fi ) stars and the start of the life as a neutron star,
or are the disruption of a cooling white dwarf forced into explosive carbon burn­
ing. This for the star catastrophic event also affects the circum­ and the interstellar
medium through ionization and shock heating. The morphology of galaxies is influ­
enced by the deposited energy, the chemistry changed by enrichment in metals, and
1 To appear in The Lives of Neutron Stars, eds. J. van Paradijs and A. M. Alpar, (Dordrecht:
Kluwer)

2 BRUNO LEIBUNDGUT
stellar material recycled. The nucleosynthesis of all elements heavier than oxygen is
provided by supernovae. Finally, supernovae have been employed as light beacons
for cosmological distance measurements.
Observations of supernovae hence bear on many astrophysical issues, like stellar
evolution, stellar mass loss, collapse and explosion physics, radiative hydrodynamics,
shock physics, galactic structure and chemical evolution, and cosmology. There have
been many excellent reviews in recent years. General overviews on supernovae are
provided in the books by Petschek (1990), Wheeler et al. (1990), Woosley (1991),
Danziger & Kj¨ar (1991), and McCray (1994). Woosley and Weaver (1986) have
presented the most complete description of the explosion physics of supernovae.
Reviews dealing with more specific topics include Wheeler & Harkness (1990) on
SNe I, Weiler & Sramek (1988) for radio supernovae, Arnett et al. (1989) and
McCray (1993) for SN 1987A, van den Bergh & Tammann (1991) on supernova rates,
Bartel (1985), Branch & Tammann (1992), and Schmidt (1993) for supernovae as
distance indicators.
The observational tools available for the investigation of supernovae are also very
diverse. Optical photometry and spectroscopy have contributed most information
during the first decades of supernova studies. These data are now supplemented
with observations at other wavelengths and, in the case of SN 1987A, even a neutrino
detection. The different techniques provide insight into the various processes that
occur in the explosion. The evolution of the spectrum (optical and near­IR) carries
information on the chemical composition of the material taking part in the explosion.
Some of this matter is not burned but merely accelerated, such as the envelope of
a massive star after the core collapse. In other cases, the spectrum is composed of
emission from the ashes of matter processed in the explosion. The line velocities can
be used as diagnostic of the explosion energies. Optical photometry of supernovae
provides insights into the overall energy radiated from the explosion and can disclose
the power source. It is also sensitive to the energy source, the mass of the envelope,
and the size of the progenitor star. Radio observations probe the circumstellar
environment around the supernova and can provide important information on the
newly­born pulsar through the evolution of the spectral index. X­ray observations
have some bearing on the radioactive source powering the emission and indicate
interaction with circumstellar material. Finally, fl­rays are direct measurements
of the radioactive power source, the mass of the envelope in which the radioactive
material is buried, and mixing of the stellar material with the ashes of the explosion.
Additional information has been gathered from supernova statistics and the mor­
phology of, as well as the supernova position in, the parent galaxy. Within the limits
of the spatial resolutions attained it is possible to derive parameters of the class of
progenitor stars for different supernova types. Most recently the direct observations
of the progenitors of SN 1987A and, possibly, SN 1993J have provided very impor­
tant clues to what kind of stars end their lives in these explosive events. These
observations provide much needed constraints on the explosions themselves, not to
mention the surprises they represented.
Next I will describe the current classification scheme for supernovae and some
secondary signatures of the various types (x2). The observational status of each
of the different classes will be outlined separately. The evidence of white dwarfs

OBSERVATIONS OF SUPERNOVAE 3
as progenitors of SNe Ia is presented in x3. In section 4, a brief status on the
observations of SNe Ib and SNe Ic is given. The discussion of SN 1993J then leads
to the observations of SNe II (x4) and the link between the latter two classes. The
summary (x5) discusses open questions.
2. Classification
Classification of supernovae should be based on a simple observational signature to
distinguish different types. The physical implications of such a scheme can, however,
be misleading and might not be too profound. Moreover, it is important to establish
various examples of similar behavior, before a subclass can be created. Another
aspect is to keep the scheme simple and not to involve too many observationally
distinct methods for the definition of a class. The more complete information that
is sought for the understanding of the individual objects or a closely defined group
is stepping beyond any means of the classification scheme. In the above sense,
classification of supernovae is a crude observational instrument for the separation of
individual object groups.
Classification of supernovae was introduced by Minkowski (1941) to distinguish
the two different spectral appearances of supernovae. The two classes were identi­
fied with well­observed supernovae, namely SN 1937C for Type I and SN 1940B for
Type II. The latter had suspected emission of Hff near maximum and later ``resem­
bles'' normal novae. The spectra of SNe I were not well understood but recognizable
by their ``very wide emission bands.'' The classification system was further refined
by Zwicky (1965) with three additional classes. Since these three classes had only
one known representative each, they were dismissed by Minkowski (1964) and Oke
& Searle (1973).
Modern supernova classification has been summarized by Harkness & Wheeler
(1990). It is based on spectroscopic observations near maximum light. Three main
types are distinguished (cf. Figure 1): type Ia, defined by the absence of hydrogen
lines in their spectra and the presence of a Si II absorption near 6100 š A, type Ib
and Ic both lacking H lines and the Si feature but displaying, in the case of SNe Ic
very weak, He lines, and type II which encompass all SNe with hydrogen lines. The
spectral evolution of SNe Ia produces spectra a few weeks past maximum which
resemble SNe Ib near peak light. This has caused confusion in a few cases and
points to the need of careful analysis of the spectra for classification. A separation
between SNe Ib, helium­rich, and SNe Ic, helium­poor, is proposed (Wheeler et al.
1987, Wheeler & Swartz 1994). Whether there is a continuous variation from one
subclass to the other or a clear dichotomy of objects is still undecided.
Secondary indicators for the classification schemes are becoming more apparent
with more data and better theoretical understanding of the explosions and their
stellar progenitors. So are the spectra several months past maximum rather homo­
geneous for SNe Ia with many broad lines of Fe­peak elements, while SNe II display
distinctive emission lines of Hff, [O I] and [O III]. SNe Ib and SNe Ic, with broad
forbidden lines of oxygen and calcium, are indistinguishable from each other at this
epoch. There have been, however, intriguing cases of SNe II changing to what nor­
mally are typical spectra of SNe Ib/c at this phase. The two well­documented cases

4 BRUNO LEIBUNDGUT
4000
6000
8000
l
(Angstrom)
log
F
l
+
const.
Na
I?
He
I
Fe
II
Fe
II
Ca
II
Type
Ic
(SN
1990B)
Si
II
S
II
Si
II
Si
III
Mg
II
Ca
II
Type
Ia
(SN
1990O) Ca
II
Ha
He
I
Na
I
Hb
Type
II
(SN
1991H)
Fig. 1. Sample spectra near maximum for the main supernova types
are SN 1987K (Filippenko 1988) and SN 1993J (e.g. Wheeler & Filippenko 1994).
These objects represent important links between different classes for the physical
understanding of supernovae.
While the optical light curves of SNe II have been rather distinctive and prompted
a subclassification into linear and plateau­like light curves (Barbon et al. 1979),
SNe Ia and SNe Ib/c display nearly indistinguishable light curve shapes (Ensman
& Woosley 1988, Leibundgut 1988). Although the colors are much redder in the
case of SNe Ib/c, this selection criterion is spoiled by the often unknown extinction
toward the objects. The shape of the near­infrared (JHK) light curves appears to
separate the two subclasses of SNe I (Elias et al. 1985), but some peculiar SNe Ia
have shown intermediate shapes (Frogel et al. 1987).
Radio emission from supernovae is normally associated with circumstellar mate­
rial (Chevalier 1990). No SN Ia has been observed at these wavelengths. A clear sep­
aration between SNe Ib/c and SNe II has been established (Van Dyk et al. 1993a).
There appears to be a subgroup of SNe II with clearly distinct radio light curve
shapes (Weiler et al. 1990, Van Dyk et al. 1993b).
The connection of the classification scheme with the physics of the explosions is
not straightforward. As described below, SNe II are thought to be displays of core

OBSERVATIONS OF SUPERNOVAE 5
4
2
0
Dm
B
SN
1972E
SN
1980N
SN
1981B
SN
1981D
0
50
100
150
4
2
0
t
0
(days
past
B
maximum)
Dm
H
Fig. 2. Optical and near­infrared light curves of SNe Ia (from Leibundgut 1988)
collapses in massive stars, while SNe Ia are more likely the explosive burning of a
white dwarf. The observations of SN 1993J and SN 1987K link the SNe Ib/c with
SNe II and indicate that the former are also core collapses rather than variations of
explosions in white dwarfs, despite the similarities observed in the optical spectra
and the photometry. These cases demonstrate the limitations of the classification
scheme employed for supernovae for the underlying physics.
3. Type Ia Supernovae
Historically these supernovae have been described as extremely uniform in their
appearance. This has been based on the light curve shapes, the observed colors, the
peak luminosities, and the spectral evolution. In the last few years this homogeneity
has cracked under new investigations and peculiar events of the class.
The light curve shapes of SNe Ia are remarkably similar in the optical and near­
infrared broad­band photometry (UBVJHK; Leibundgut 1988). Many events adhere
to a unique light curve shape (Leibundgut et al. 1991b). Peculiar objects, like
SN 1939B (e.g. Minkowski 1964) and SN 1986G (Phillips et al. 1987, Frogel et al.
1987), even though exhibiting clearly discrepant light curves, had been disregarded.
Recently, Phillips (1993a) pointed out that also the optical light curves are not as
uniform as assumed. The decline during the first 15 days after maximum ranges
from 1.5 mag to 2.5 mag for a set of nine supernovae, an effect confirmed in an
independent analysis by Vacca & Leibundgut (1994) for a much larger sample and a
more detailed fitting procedure. Furthermore, Suntzeff (1994) has shown that the R

6 BRUNO LEIBUNDGUT
and I light curves of SNe Ia do not follow a unique shape, but rather are individual
for most SNe Ia. The colors of SNe Ia display some variation, but the scatter
is remarkably small (e.g. oe(B­V)=0.22 mag for 24 supernovae, Leibundgut 1991)
considering extinction which probably affects some of the supernovae. The value of
the intrinsic color (e.g. (B­V) 0 ) is still a matter of debate (Pskovskii 1968, Cadonau
et al. 1985, Capaccioli et al. 1990, Hamuy et al. 1991, Sandage & Tammann 1993)
with the more recent determinations favoring a value of (B­V) 0 ú 0:0 mag.
The spectra of SNe Ia are dominated by many lines of intermediate­mass elements
like Si, S, Mg, and Ca near maximum light (Branch 1990). With more accurate ob­
servations in recent years it has become obvious that significant variations exist in
the line strengths among individual objects near maximum (e.g. Filippenko et al.
1992a, b, Phillips et al. 1992, Leibundgut et al. 1993b, Phillips 1993b, Branch et
al. 1993). Figure 3 illustrates this point. About a month later the continuum emis­
sion disappears and is replaced by broad emission lines of many iron­peak elements
(Axelrod 1980, Pinto 1988). At about seven weeks past peak light the spectra ap­
pear virtually identical (Filippenko et al. 1992b, Leibundgut et al. 1993b). This
is a strong indication that the ashes are very similar and the explosion mechanism
rather unique.
A ``standard'' model was devised to explain most of the above observations and
also account for the fact that SNe Ia were observed in elliptical galaxies and in the
halos of spiral galaxies, which are presumably composed of old stars. In this model
(cf. Woosley & Weaver 1986, Wheeler & Harkness 1990 for reviews) a white dwarf
is incinerated by explosive carbon burning. The triggering of the explosion takes
place when the white dwarf is nudged over the Chandrasekhar mass (¸1.4 M fi )
and the density and temperature in the center of the star become high enough to
start the burning. The optical display of the supernova is then entirely due to
heating from radioactive decays of 56 Ni and 56 Co nuclei. This model has been very
successful in describing most of the available observations. With the basically unique
configuration of the precursor system a satisfactory homogeneity in appearance is
guaranteed. The fact that only ``processed'' material is observed in the spectra
is explained by the burning front converting most material into nuclear statistical
equilibrium, i.e. iron­peak elements. The tracers of intermediate­mass elements
observed in the early­time spectra are due to the dying out of the subsonic flame
and incomplete burning near the surface of the white dwarf. The differences in the
maximum­light spectra can be explained by small variations as the flame peters out
and might arise from only tiny amounts of matter. At late times, when the explosion
becomes transparent, we are truly looking at the ashes. No circumstellar material
is expected around a white dwarf and thus the lack of radio detections (Weiler et
al. 1986) is accounted for naturally. As end­products of stellar evolution white
dwarfs represent an old population which concurs with their non­association with
star forming regions (Maza & van den Bergh 1976).
The above model is further supported by the agreement reached from spectral
modeling at early and late epochs (Harkness 1991a, b, Jeffery et al. 1992, Mazzali
et al. 1993, Axelrod 1980, Pinto 1988). The fast decay of the late­time bolometric
light curve is readily explained by the escape of fl­rays from the ejecta, pointing to
a small mass (Leibundgut & Pinto 1992). Detailed calculations of the photospheric

OBSERVATIONS OF SUPERNOVAE 7
3000
4000
5000
6000
7000
8000
9000
l
(Angstrom)
log
F
l
+
const
SN
1991bg
(+1
days)
SN
1991T
(­5
days)
SN
1989B
(0
days)
SN
1986G
(0
days) SN
1981B
(­3
day)
Si
II
Fe
III
Fe
III
Mg
II
Si
III
Ca
II
Ca
II
Mg
II
Si
II
S
II
Si
II
Si
II
Fig. 3. Spectra of SNe Ia near maximum (from Leibundgut et al. 1993b)
peak phase of supernova light curves have recently succeeded in reproducing the
observations (Khokhlov et al. 1993, H¨oflich et al. 1993). A crucial parameter is
the rise time from explosion to maximum light and certain models now can match
the observations (Leibundgut 1994a). The most direct evidence of the radioactive
power source, however, comes from a study of Co and Fe lines several weeks past
maximum (Kuchner et al. 1994). The line ratios follow exactly the predictions of
the decay times determined by nuclear physics.
Although the ``standard'' model can explain many aspects of SNe Ia, it has its
shortfalls. Since the white dwarf has to accrete matter, there must be a companion
star. The lack of radio emission indicates that not much matter is lost by this star,
which excludes giant star companions. Any hydrogen and helium accreted onto the

8 BRUNO LEIBUNDGUT
white dwarf has to be processed quickly enough or it would show up in early time
spectra. Unfortunately, there are no good limits on the H mass on top of a SN Ia from
the observations. Double­degenerate systems, in which two degenerate white dwarfs
merge have been adopted instead as the canonical model (Iben & Tutukov 1984,
Webbink 1984, Tutukov et al. 1992). The dynamical time scales and the formation
of a disk in the merger are not easily explained in these models. Furthermore, there
have been no candidate systems observed despite extensive searches (Bragaglia et al.
1990, Renzini 1994). Another problem with the Chandrasekhar­mass white dwarfs
is the observed mass distribution of white dwarfs (Bergeron et al. 1992). There is
only one white dwarf with a mass above 1M fi , which probably is already a merged
object. Thus, at least 0.4M fi would have to be accreted to build a Chandrasekhar­
mass white dwarf.
Other problems include the IR light curves which display conspicuous minima
about a week after the optical maxima (Fig. 2). Similar dips are also observed in
R and I light curves (Ford et al. 1993, Hamuy et al. 1993, Suntzeff 1994). The
reason for this flux redistribution has not yet been addressed in model calculations.
Direct determinations of the Ni mass from late­time spectra also point to rather
large differences in the Ni production contrary to the predictions from models (Ruiz­
Lapuente & Lucy 1992). Large variations in the expansion velocities as measured
from Doppler displacement of absorption lines (Branch et al. 1988, Branch & van
den Bergh 1993) indicate variations in the explosion energies.
The most severe challenge for the ``standard'' model arouse recently from detailed
observations of SN 1991T (Filippenko et al. 1992a, Ruiz­Lapuente et al. 1992,
Phillips et al. 1992) and SN 1991bg (Filippenko et al. 1992b, Leibundgut et al.
1993b). Both objects displayed differences in the light curve shapes and spectral
peculiarities. In the case of SN 1991bg there are very severe discrepancies in the
late time spectrum as well (Filippenko et al. 1992b, Leibundgut et al. 1993b, Ruiz­
Lapuente et al. 1993). Such variations cannot be explained within the standard
framework of the explosion of a Chandrasekhar­mass white dwarf.
An important conclusion from the observations is that most of the material has
been burned in the explosion and hydrogen and helium are absent, the radioactive
power source appears now well established, and a low ejecta mass is deduced from the
decline in the late­time light curve. All this points toward a white dwarf progenitor
for SNe Ia, but none of the above signatures provides a quantitative measure of
the total mass or the amount of radioactive 56 Ni synthesized. This has prompted
models which employ small­mass white dwarfs in the explosions (Shigeyama et al.
1992, Woosley & Weaver 1994). With the physics available for off­center explosions
(Livne 1990, Livne & Glasner 1991, Woosley & Weaver 1994) it might be possible
to explain simultaneously the apparent homogeneity of the bulk of the objects and
the deviations of a few supernovae from this norm.
A frequent application of SNe Ia has been the determination of cosmological
distances and the derivation of the Hubble constant H 0 (Branch & Tammann 1992,
and references therein). To achieve meaningful results two conditions have to be met.
First, the narrow range of peak luminosities of SNe Ia has to be established reliably,
and, second, the absolute value of this peak luminosity has to be determined.
Establishing the standard candle character of supernovae primarily means that

OBSERVATIONS OF SUPERNOVAE 9
they reach the same luminosity at maximum. Further indications of the homogeneity
of the class in its appearance provide additional arguments, but are not essential for
the use as distance indicators. Several groups have measured the observed luminosity
distribution of SNe Ia at maximum (Tammann & Leibundgut 1990, Della Valle &
Panagia 1992, Branch & Miller 1993, Sandage & Tammann 1993) and agree fairly
well with each other. There is little doubt that by appropriate treatment of ``outliers''
a tight distribution around a mean absolute magnitude can be established. There
has been some concern about difficult it is to sort out the ``peculiar'' supernovae,
but Branch et al. (1993) argue that the spectroscopic signatures are clear enough
for a separation. For many SNe Ia observed in the 1960's and 1970's, however, not
enough data are available to assess their status and most of the above derivations
rely on data samples which include many of those SNe. An additional complication
is the often unknown extinction toward SNe Ia. The effect of this has been discussed
in Leibundgut & Pinto (1992). Unfortunately, errors of up to 40% can be incurred in
the determination of the mean of the distribution of the peak luminosity of SNe Ia.
The most reliable measurement of the absolute luminosity of a SN Ia is by Saha
et al. (1994) for SN 1937C. They have measured cepheids within IC 4182 and
established the distance to this galaxy. A different route is the calculation of the
luminosity from models pioneered by Arnett et al. (1985). Branch (1992) has
gathered what he considered the best determinations of this kind. An independent
investigation involving a variety of models has been presented by Leibundgut & Pinto
(1992). They conclude that as long as we are not able to select the ``best'' model,
a large uncertainty has to be accepted. All models considered, however, employed
the explosion of Chandrasekhar­mass white dwarfs. With smaller mass explosions
the luminosities are decreased. A slightly different approach was used by M¨uller
& H¨oflich (1994) who find a suitable model for each individual supernova and thus
individual luminosities to determine H 0 . No assumption on the standard character
of SNe Ia is made.
No value of H 0 is quoted here and the reader referred to the literature. Discussion
of the various error sources is desperately needed to solve some of the outstanding
problems.
4. Type Ib/c Supernovae
Only recognized about a decade ago as a separate subclass among SNe I (Wheeler
& Levreault 1985, Uomoto & Kirshner 1985, Panagia 1985) this group of objects
might represent the ``cross­dressing'' case of supernovae. Around maximum they
resemble SNe Ia in their photometric and spectroscopic appearance, but their late­
time spectra and radio signatures are related to SNe II. With two examples of SNe II
which actually have ``changed type'' (SN 1987K and SN 1993J) the latter connection
has been further strengthened.
The observational data for this class are rather scanty. The general characteristics
have been summarized by Porter & Filippenko (1987). The question whether there
is the need for a further subclassification remains open. Swartz et al. (1993) argue
from model calculations for a clear distinction between SNe Ib (He­rich) and SNe Ic
(He­poor) on the basis of the He mass observed in the spectra. In addition, Wheeler

10 BRUNO LEIBUNDGUT
2
0
SN
1993J
SN
1962L
SN
1984I
Dm
U
4
2
0
Dm
B
­20
0
20
40
60
80
100
4
2
0
Dm
V
t
0
(days)
Fig. 4. Optical light curves of the type Ib/c SN 1962L (Bertola 1964) and SN 1984I (Leibundgut
et al. 1990) compared with SN 1993J (Richmond et al. 1994); the line represents a mean curve for
SNe Ib/c from Leibundgut (1988)
& Swartz (1994) find SNe Ib of higher mass than SNe Ic.
The spectroscopic evolution is dominated by He lines for the Ib supernovae while
in SNe Ic absorption lines of predominantly C, O, Ca, and Fe are observed (Wheeler
et al. 1987). Faint Balmer lines of H are suspected, but not a clear signal has been
detected (Jeffery et al. 1991, Filippenko 1992). After about 6 months the nebular
spectra show strong Ca and O emission lines (Filippenko et al. 1986, Filippenko et
al. 1989).
The optical light curves are very similar to the ones of SNe Ia (Ensman & Woosley
1988, Leibundgut 1988). The infrared light curves, however, do not exhibit the
second maximum and are rather distinct (Elias et al. 1985).
If SNe Ib/c emerge from massive stars and are caused by a core collapse, as is
suggested by their connection with SNe II, the light curves are expected to display a
sharp early peak from the shock break out at the surface of the star. This has never
been observed in a SN Ib/c proper, but SN 1993J had been detected early enough
(Figure 4). It is clear from this figure that the light curve shapes of SN 1993J are
indeed very similar to the ones of SNe Ib/c. A similar conclusion has been found
for the bolometric light curve of SN 1993J and SN 1983N (Nomoto et al. 1993,

OBSERVATIONS OF SUPERNOVAE 11
Richmond et al. 1994). For the comparison in Fig. 4 the curves have been shifted to
coincide at the maximum of the broad peak. The peak for SN 1962L and SN 1984I
is evidently broader in all bands and the slope of the decline 30 days past maximum
steeper. The occurrence of the maximum in U seems also shifted to later times,
although the data are really not sufficient for a clear result. The first B observations
of SN 1984I and SN 1962L, an estimate of a non­standard band photograph however,
are very interesting as they lie at the bottom of the dip in the SN 1993J light curve.
Both these supernovae might have been caught shortly after the the initial peak of
shock breakout. A duration of a few days is not excluded by the observations; an
upper limit about 2 mag fainter than the B peak brightness was determined 14 days
before the first detection of SN 1984I (Leibundgut et al. 1990). The broader peak of
the SN Ib/c indicates a slower release of the energy diffusing out of the core than for
SN 1993J. The steeper slope at late times on the other hand points toward a faster
increase in fl­ray transparency of the envelope. With similar explosion energies this
implies a smaller mass in the envelopes of SN 1962L and SN 1984I than for SN 1993J.
SNe Ib/c are not clearly stronger associated with star forming regions (Van Dyk
1992). It appears as if they are not the very massive stars (? 30 M fi ) which were
suspected in the beginning (e.g. Ensman & Woosley 1988, Schaefer et al. 1987).
A model where the star has lost its hydrogen envelope due to binary interaction
appears more likely (Shigeyama et al. 1990) and is further supported by the apparent
connection of SN 1993J with SNe Ib/c. Additional support for such a model, as
opposed to one with white dwarfs, is the radio emission from these objects (Van Dyk
et al. 1993a). If the connection to SNe II is confirmed, this means that SNe Ib/c
contribute to the population of neutron stars.
5. Type II Supernovae
This class of supernovae comprises a great many individual examples of objects
displaying hydrogen in their spectra. The additional subclassification into plateau
(II­P) and linear (II­L) events (Barbon et al. 1979, Doggett & Branch 1985, Patat
et al. 1993) is based on photometry rather than spectroscopy. A connection of
these photometric types with spectroscopic features has been proposed and consists
mainly in the shape of the line profiles in maximum­light spectra, but has not yet
been unambiguously proven (Harkness & Wheeler 1990). Also grouped in this class
are objects with rather narrow (¸1000 km/s) emission lines of mainly hydrogen and
helium. These events have been proposed for a separate subtype (Schlegel 1990), but
only a few examples have been observed near maximum light. It might also be that
the emission of these objects is not powered by energy deposited by the explosion
in the envelope, but rather originates in the interaction of the explosion shock with
surrounding material (Chevalier 1990, Filippenko 1991, Stathakis & Sadler 1991,
Chugai & Danziger 1993, Leibundgut 1994b). A conjecture supported by the strong
radio emission from these objects (Weiler et al. 1990, Van Dyk et al. 1993b).
In the following we will discuss the physical processes dominating the light curve
of a ``generic'' type II supernova and point out the most prominent deviations ob­
served to date. Much of the details have been learned from SN 1987A and the reader
is referred to the relevant literature.

12 BRUNO LEIBUNDGUT
The optical display of a type II supernova starts with the shock breaking out
at the surface of the progenitor star a few hours after core collapse. Up to this
moment the star looks like any evolved supergiant, since at advanced burning stages
the envelope does not have enough time to adjust to the rapid changes in the core.
With the initial X­ray and UV burst the light curve peaks within hours of the core
collapse before the cooling of the atmosphere decreases the flux very rapidly. The
only observations of this phase we have to date are from SN 1987A (Kirshner et
al. 1987, Arnett et al. 1989) and SN 1993J (cf. Wheeler & Filippenko 1994). For
stars with expanded envelopes (red supergiants) this phase lasts longer than for
more compact progenitors (Falk & Arnett 1977, Klein & Chevalier 1978). The rapid
fading following the peak is only stopped when the photosphere is balanced by the
expansion of the ejecta. This results in the plateau phase lasting as long as the
photosphere recedes through the envelope. The length of the plateau is mainly a
function of the depth of the envelope, i.e. envelope mass, and the explosion energy,
and thus the expansion velocity of the ejecta (Chugai 1991, Popov 1993, Hsu et
al. 1993). During this phase the colors of the supernova tend to remain constant
implying a constant temperature (Schmidt et al. 1992). Once the photosphere has
receded deep enough to encounter the additional heating from the radioactive decay
of 56 Ni and 56 Co the plateau is extended for a short time (Woosley & Weaver 1986,
Hsu et al. 1993), before the light curve starts declining. The transition between
the plateau and the decline phase is determined by the decreasing optical depth to
optical photons. For stars with massive envelopes all the energy released by the 56 Ni
-- 56 Co -- 56 Fe decay chain is converted into optical emission and the bolometric light
curve approaches a decline rate corresponding to the life­time of 56 Co (Suntzeff &
Bouchet 1990, Turatto et al. 1990, Schmidt et al. 1993).
The deviations of SN 1987A and SN 1993J from this general picture point to
different progenitor stars. SN 1987A was the explosion of a compact blue supergiant
and much of the explosion energy went into overcoming the large potential energy.
Thus, the photosphere receded very rapidly and no plateau was formed. The second
broad maximum in the light curve was produced by the heating from the 56 Ni
and 56 Co decays. The large envelope mass resulted in the extended peak for this
supernova. In the case of SN 1993J a rather small envelope mass prevented the
formation of an extended recombination period and the second maximum was again
formed by the diffusion wave from the radioactive decays. The reduced length of
this peak also indicates a small envelope.
The decline rate at late times might be changed by dust formation (Lucy et
al. 1991, Meikle et al. 1993) and the ionization freeze­out (Fransson & Kozma
1993) as observed in SN 1987A. The former shifts the flux from optical wavelengths
into the infrared by heating of newly formed dust grains, while the second effect
arises from the fact that the time of de­excitation of atoms becomes comparable to
the expansion time of the supernova and this additionally ``stored'' energy becomes
visible. Although probably not uncommon in other supernovae, these processes have
been observed only in SN 1987A where they occurred ¸6 and ¸12 magnitudes below
peak brightness, respectively.
Figure 5 displays the spectral evolution of SN 1990E, a rather typical SN II with
well­defined P Cygni profiles of the hydrogen lines around maximum (Schmidt et

OBSERVATIONS OF SUPERNOVAE 13
4000
6000
8000
l
(Angstrom)
log
F
l
+
const.
1
day
8
days 17
days
32
days
138
days 270
days
Fig. 5. Spectra of SN 1990E (from Schmidt et al. 1993). The phases are given as days past the
optical maximum.
al. 1993). In the first spectrum only helium and hydrogen lines are present as the
supernova still cools from the shock­heating of the envelope. During the plateau
phase, which lasted from about 15 to 100 days past explosion, the spectrum changes
only little with weaker lines becoming more apparent as the photosphere moves
inward to slower material. Calcium and iron lines appear when the recombination
front moves through layers rich in these elements. The line profiles persist for the
time the supernova remains on the plateau of the light curve. SN 1990E disappeared
behind the sun about one month after discovery and was observable only 100 days
later (Schmidt et al. 1993, Gomez & Lopez 1993, Benetti et al. 1994). Entering the
nebular phase the supernova displayed very strong Hff emission along with weaker

14 BRUNO LEIBUNDGUT
[O I] emission also observed in other SNe II (Branch et al. 1981 (SN 1979C], Uomoto
& Kirshner 1986 [SN 1980K], Phillips & Williams 1991, Spyromilio et al. 1991
[SN 1987A]). For a few months the Na D lines form a very strong P Cygni line. The
emission near 7300 š A is either from [Ca II] or [O II] lines and is fairly typical for
SNe II.
The evolution of the line luminosity in Hff is a very sensitive measure of additional
energy sources which might become important at late phases (Chugai 1990). For
most supernovae this implies that there is indeed an additional power source (Chugai
1990, Schmidt et al. 1993, Gomez & Lopez 1993, Benetti et al. 1994). Prime
candidates are long­lived radio­isotopes like 57 Co, 22 Na, and 44 Ti (Woosley et al.
1989), a buried pulsar loosing angular momentum and ionizing a plerion (Pacini
& Salvati 1981, Chevalier & Fransson 1992), light echos (Chevalier 1986, Chugai
1992), and shock­interaction with circumstellar matter (Chevalier 1990, Chugai 1992,
Chevalier & Fransson 1994). Such changes have been observed in a few supernovae
several years past explosion, most prominently in SN 1980K (Fesen & Becker 1990,
Leibundgut et al. 1991a, Uomoto 1991, Leibundgut et al. 1993a) where the emission
has remained steady at Vú23 mag for the last three years. Other supernovae with
similar light curves might be SN 1979C (Fesen & Matonick 1993) and SN 1970G
(Fesen 1993). The most likely energy source in these cases is the shock heating
the circumstellar material lost by the progenitor star as a stellar wind (Chevalier
& Fransson 1994, Leibundgut 1994b). In these objects the strong explosion shock
is traveling in the dense circumstellar environment and excites this material. At
the same time the shock is decelerated and a second shock front emerges where
the expanding debris encounters the slower, shocked matter (Chevalier 1990). One
of the main signatures of such a process is its radio emission (Chevalier 1982),
which has been observed for all supernovae with optical detection a few years past
maximum (Leibundgut 1994b). The spectra display broad (¸5000 km s \Gamma1 ) lines
of hydrogen, oxygen, and possibly calcium (Fesen & Becker 1990, Leibundgut et al.
1991a). SN 1980K has reached a constant flux level about 2 years after explosion and
remained a this luminosity for at least a decade (Leibundgut et al. 1993a). It appears
that only the kinetic energy (¸10 51 erg) can provide the energy pool required. A
pulsar could sustain the emission at the same level, but narrow (¸1000 km s \Gamma1 )
lines are predicted from such a power source (Chevalier & Fransson 1992). Other
signatures for shock interaction with circumstellar matter include narrow emission
lines at early times, when the circumstellar material is ionized by the shock breakout,
observed from the ring in SN 1987A (Fransson et al. 1989) and SN 1993J (cf.
Wheeler & Filippenko 1994), infrared echoes, observed for SN 1980K (Dwek 1983),
as well as blue continua, UV, and X­ray emission at early phases as proposed by
Fransson (1984). Many questions remain unanswered at this time. The constancy of
the emission from SN 1980K cannot be accommodated by the models, at the same
time a sudden drop in luminosity observed in SN 1957D (Long et al. 1992) remains
unexplained. Potentially this late emission probes the composition and conditions
in the stellar wind, but we are simultaneously observing the heated debris of the
explosion. To disentangle the contribution of the two emission sites from the inner
and outer shock requires additional modeling.
A small group of SNe II displays remarkable deviations from the above scenario.

OBSERVATIONS OF SUPERNOVAE 15
The peaks of their light curves have never been observed and their decline rates
are much smaller than what is expected from 56 Co decays. The best observed
examples to date are SN 1978K (Ryder et al. 1993), SN 1986J (Rupen et al. 1987,
Leibundgut et al. 1991a), SN 1987F (Filippenko 1989), and SN 1988Z (Stathakis
& Sadler 1991, Filippenko 1991, Turatto et al. 1993). Although only SN 1988Z
has a well­observed light curve (Turatto et al. 1993), the slow decline rates are also
reflected in the evolution of line fluxes (Filippenko 1989, Leibundgut et al. 1991a,
Turatto et al. 1993). The nature of the emission and its power source are not
resolved, but interaction with a very dense material close to the supernova is most
likely (Filippenko 1991, Chugai and Danziger 1993). All known objects of this class
are evolving very slowly (Leibundgut 1994b). It appears as if the interactions with
the pre­supernova wind dominates the light curve from the start thus obliterating
other processes. For SN 1986J mostly narrow (¸1000 km s \Gamma1 ) lines of hydrogen,
helium, and Fe were observed. At the same time the oxygen lines are broader
(¸2000 km s \Gamma1 ; Leibundgut et al. 1991a). The line ratios in the oxygen lines
indicated very dense (n O I ú 10 9 cm \Gamma3 ) and cold (Tú4000 K) material in the ejecta
with only a very small filling factor (!10 \Gamma3 ). Interestingly, the oxygen emission is
not fading, although the Hff flux decreases. While SN 1986J has been discovered
probably only several years after the explosion (Weiler et al. 1990, Bartel et al. 1991)
and suffers from a large, and undetermined, extinction within its parent galaxy,
SN 1988Z has been detected at the latest half a year after explosion and lent itself
to more detailed studies (Stathakis & Sadler 1991, Filippenko 1991, Turatto et al.
1993). At early times a broad component of the Hff lines indicates high expansion
velocities (¸40000 km s \Gamma1 ) and a narrow component of only a few 1000 km s \Gamma1 . As
in SN 1986J narrow lines of H, He, N, and Fe were observed, but also broad lines
of Ca were detected. Many Fe lines are present in a broad emission feature near
5300 š A (Chugai 1994). The strong, narrow Hff lines reached a maximum only ¸400
days past discovery at ¸10 41 erg s \Gamma1 and declined only very slowly (Turatto et al.
1993). The radiated energies are very high for all these objects (?10 39 erg s \Gamma1 ).
They are also the brightest supernovae observed in the radio which indicates very
strong shocks in dense material (Weiler et al. 1990, Van Dyk et al. 1993b). It is as
yet unclear from which stellar progenitor systems these supernovae arise.
The use of SNe II for distance measurements has been boosted considerably with
the new models which became available stimulated by the explosion of SN 1987A.
This object at a known distance provided a perfect opportunity to test the ability
of models to match the observations and in addition predict the luminosity evolu­
tion. Using SNe II for distance determinations had been proposed a few decades ago
(Kirshner & Kwan 1974), but had to await improvements in the spectral modeling
to yield accurate results (Schmidt et al. 1992). The application is known as Baade­
Wesselink method for the determination of distances toward pulsating stars. For
supernovae it is modified to calculate the total flux from hydrodynamical models of
the explosion and assumes homologous expansion of the photosphere. The observed
flux is proportional to the absolute luminosity divided by the square of the distance
to the object. The luminosity of the supernova depends on the size of the photo­
sphere and its temperature (assuming a radiation close to a blackbody), which in
turn changes with the expansion of the photosphere. Thus, a relation can be formed

16 BRUNO LEIBUNDGUT
that links the time since explosion with the distance and the observable quantities
expansion velocity, color, and apparent magnitude (Schmidt et al. 1992). For a suc­
cessful application the models had to provide information on how to determine the
expansion velocities and how the radiation deviates from a pure blackbody. The first
problem has been solved with the recognition that lines with small optical depths
arise much closer to the photosphere than strong lines of hydrogen (Eastman & Kir­
shner 1989). For the second problem a whole suite of models has to be calculated
and the corrections determined for different temperatures. During the plateau (pho­
tospheric) phase of SNe II these calculations have been established reliably. This
procedure can easily be tested with supernovae at known distances and has produced
very convincing results. A warning on the limits of the method was issued by the
determination of the distance toward SN 1993J (see Wheeler & Filippenko 1994 for
a summary). In this case specific models complying with the peculiar nature of the
supernova had to be constructed in order to achieve reasonable results.
The method has been applied to nearly 20 supernovae out to a distance of
180 Mpc (Schmidt 1993). Major limitations are the faintness of the objects and
the lack of sufficient photometric and spectroscopic coverage during the plateau
phase. The value of H 0 derived by this method is 75\Sigma10 km s \Gamma1 Mpc \Gamma1 (Schmidt
1993).
6. Summary
The expression ``supernova'' really stands for two different kinds of explosive events
at the end stages of stellar evolution. The explosive burning of a white dwarf,
probably triggered by mass transfer, does not leave a compact remnant. On the
other hand the core collapse in massive stars initiates the violent display called type
II supernovae, and is probably also responsible for SNe Ib/c. The main difference
between the latter classes might be the amount of hydrogen left in the envelope at
the time of explosion. The classification system, as it is used today, has reached its
limits for it no longer reflects the underlying physical processes.
The conditions of a star when it is disrupted strongly influence the appearance
of the supernova. Size, mass, and chemical composition imprint themselves into the
electromagnetic display of the explosion.
SNe Ia appear to be fairly well understood and most of the observations can be
readily interpreted. Important questions at this point include the stellar evolution
leading to progenitor systems, the mass of the exploding white dwarf, and how
much radioactive fuel is produced in the burning. Recent direct determinations
of absolute magnitudes (Saha et al. 1994), in connection with improved model
calculations (H¨oflich et al. 1993), will provide much needed insight. The variations
among SNe Ia will have to be explained, but there is room within the framework
of explosive burning of white dwarfs for such peculiar objects. The implications of
these results for the distance determinations are profound.
SNe Ib and SNe Ic are in the poorest observational state of all classes. Not
enough objects have been sufficiently observed for stringent conclusions. It is not
even clear whether the subtype Ic is appropriate or whether these objects are just
variants of SNe Ib. The link with SNe II has become very strong with at least two

OBSERVATIONS OF SUPERNOVAE 17
known supernovae evolving from Type II to Type Ib/c at late epochs. Nevertheless,
we do not know much about the stars that end their lives in these explosions, nor
how frequent these supernovae are (cf. Strom 1994, Muller et al. 1992, Cappellaro
et al. 1993 for this debate). More and better data are desperately needed.
SNe II are well explained as the results of core collapses. A new phase for these
supernovae is observed through their interaction with the circumstellar medium they
are placed in. The conversion of kinetic energy into radiation sustains the emission
from these objects for decades. The detailed physics have still to be worked out,
especially the contributions from the inner and outer shock have to be disentangled.
Some rare events do show prominent deviations from the general picture. Their
high X­ray, optical, and radio luminosities, slow evolution, and narrow lines indicate
interaction with a very dense circumstellar environment. First interpretations of the
observations have been advanced, but we are still far from a detailed understanding.
And then there are new surprises in store. The more supernovae with detailed
observations become available the more nature proves to be more innovative than
human mind. The best­observed supernovae do not fit the previously defined classes.
SN 1885A (S And; de Vaucouleurs & Corwin 1985), SN 1987A (Arnett et al. 1989),
SN 1993J (Wheeler & Filippenko 1994), and SN 1991T and SN 1991bg can be
mentioned here. Each displayed new and unexpected behavior.
Acknowledgements
This research has received financial support from the Swiss National Science Foun­
dation and from the American NSF through grant AST­9115174.
References
Arnett, W. D., Bahcall, J. N., Kirshner, R. P., & Woosley, S. E. 1989, ARA&A, 27, 629
Arnett, W. D., Branch, D., & Wheeler, J. C. 1985, Nature, 314, 337
Axelrod, T. S. 1980, Type I Supernovae, ed. J. C. Wheeler, (Austin: University of Texas), 80
Barbon, R., Ciatti, F., & Rosino, L. 1979, A&A, 72, 287
Bartel, N., ed., 1985, Supernovae as Distance Indicators, (Berlin: Springer)
Bartel, N, Rupen, M. P., Shapiro, I. I., Preston, R. A., & Ruis, A. 1991, Nature, 350, 212
Benetti, S., Cappellaro, E., Turatto, M., Della Valle, M., Mazzali, P. A., & Gouiffes, C. 1994, A&A,
in press
Bergeron, P., Saffer, R. A., & Liebert, J. 1992, ApJ, 394, 228
Bertola, F. 1964, Ann. Astrophys. 257, 319
Bragaglia, A., Greggio, L., Renzini, A., & D'Odorico, S. 1990, ApJ, 365, L13
Branch, D. 1990, Supernovae, ed. A. G. Petschek, (New York: Springer), 30
Branch, D. 1992, ApJ, 392, 35
Branch, D., Drucker, W., & Jeffery, D. J. 1988, ApJ, 330, L117
Branch, D., Fisher, A., & Nugent, P. 1993, AJ, 106, 2383
Branch, D., & Miller, D. L., 1993, ApJ, 405, L5
Branch, D., & Tammann, G. A. 1991, ARA&A, 30, 359
Branch, D., & van den Bergh, S. 1993, AJ, 105, 2231
Branch, D., Falk, S. W., McCall, M. L., Rybski, P., Uomoto, A. K., & Wills, B. J. 1981, ApJ, 244,
780
Cadonau, R., Sandage, A., & Tammann, G. A. 1985, Supernovae as Distance Indicators, ed. N.
Bartel, (Berlin: Springer), 151
Capaccioli, M., Cappellaro, E., Della Valle, M., D'Onofrio, M., Rosino, L., & Turatto, M. 1990,
ApJ, 350, 110

18 BRUNO LEIBUNDGUT
Cappellaro. E., Turatto, M., Benetti, S., Tsvetkov, D. Y., Bartunov, O. S., & Makarova, I. N. 1993,
A&A, 273, 383
Chevalier, R. A. 1982, ApJ, 258, 790
Chevalier, R. A. 1986, ApJ, 308, 225
Chevalier, R. A. 1990, Supernovae, ed. A. G. Petschek, (New York: Springer), 91
Chevalier, R. A., & Fransson, C. 1992, ApJ, 395, 540
Chevalier, R. A., & Fransson, C. 1994, ApJ, 420, 268
Chugai, N. N. 1990, SovAL, 16, 457
Chugai, N. N. 1991, SovAL, 17, 210
Chugai, N. N. 1992, SovA, 36, 63
Chugai, N. N., & Danziger, I. J., 1993, MNRAS, in press
Chugai, N. N. 1994, Circumstellar Media in Lates Stages of Stellar Evolution, eds. R. Clegg, P.
Meikle, & I. Stevens, (Cambridge: Cambridge University Press), in press
Danziger, I. J., & Kj¨ar, K., eds., 1991, SN 1987A and Other Supernovae, (Garching: ESO)
Della Valle, M., & Panagia, N. 1992, AJ, 104, 696
Doggett, J. B., & Branch, D. 1985, AJ, 90, 2303
Dwek, E. 1983, ApJ, 274, 175
Eastman, R. G., & Kirshner, R. P. 1989, ApJ, 347, 771
Elias, J. H., Matthews, K., Neugebauer, G., & Persson, S. E. 1985, ApJ, 196, 379
Ensman, L. M., & Woosley, S. E. 1988, ApJ, 333, 754
Falk, S. W., & Arnett, W. D. 1977, ApJS, 33, 515
Fesen, R. A. 1993, ApJ, 413, L109
Fesen, R. A., & Becker, R. H. 1990, ApJ, 351, 437
Fesen, R. A., & Matonick, D. M. 1993, ApJ, 407, 110
Filippenko, A. V. 1988, AJ, 96, 1941
Filippenko, A. V. 1989, AJ, 97, 726
Filippenko, A. V. 1991, SN 1987A and Other Supernovae, (Garching: ESO), eds. Danziger, I. J.,
& Kj¨ar, K., (Garching: ESO), 343
Filippenko, A. V. 1992, ApJ, 384, L37
Filippenko, A. V., Porter, A. C., & Sargent, W. M. W. 1990, AJ, 100, 1575
Filippenko, A. V., & Sargent, W. L. W. 1986, AJ, 691
Filippenko, A. V., et al. 1992a, ApJ, 384, L15
Filippenko, A. V., et al. 1992b, AJ, 104, 1534
Ford, C. H., Herbst, W., Richmond, M. W., Baker, M. L., Filippenko, A. V., Treffers, R. R., Paik,
Y., & Benson, P. J. 1993, AJ, 106, 1101
Fransson, C. 1984, A&A, 133, 264
Fransson, C., Cassatella, A., Gilmozzi, R., Kirshner, R. P., Panagia, N., Sonneborn, G., &
Wamsteker, W. 1989, ApJ, 336, 429
Fransson, C. & Kozma, C. 1993, ApJ, 408, L25
Frogel, J. A., Gregory, B., Kawara, K., Laney, D., Phillips, M. M., Terndrup, D., Vrba, F., &
Whitford, A. E. 1987, ApJ, 315, L129
Gomez, G. & Lopez, R. 1993, MNRAS, 263, 767
Hamuy, M., Phillips, M. M., Maza, J., Wischnjewsky, M., Uomoto, A., Landolt, A. U., & Kathwani,
R. 1991, AJ, 102, 208
Hamuy, M., et al. 1993, AJ, 106, 2392
Harkness, R. P. 1991a, Supernovae, ed. S. E. Woosley, (New York: Springer), 454
Harkness, R. P. 1991b, SN 1987A and Other Supernovae, eds. I. J. Danziger & K. Kj¨ar, (Garching:
ESO), 447
Harkness, R. P., & Wheeler, J. C. 1990, Supernovae, ed. A. G. Petschek, (New York: Springer), 1
H¨oflich, P., Khokhlov, A., & M¨uller, E. 1993, A&A, 268, 570
Hsu, J. J., et al. 1994, ApJ, in press
Iben, I. Jr., & Tutukov, A. V. 1984, ApJS, 54, 335
Jeffery, D. J., Branch, D., Filippenko, A. V., & Nomoto, K. 1991, ApJ, 377, L89
Jeffery, D. J., Leibundgut, B., Kirshner, R. P., Benetti, S., Branch, D., & Sonneborn, G. 1992,
ApJ, 397, 304
Khokhlov, A., M¨uller, E., & H¨oflich, P. 1993, A&A, 270, 223
Kirshner, R. P., & Kwan, J. 1974, ApJ, 193, 27
Kirshner, R. P., Sonneborn, G., Crenshaw, D. M., & Nassiopoulos, G. E. 1987, ApJ, 320, 602

OBSERVATIONS OF SUPERNOVAE 19
Klein, R. I., & Chevalier, R. A. 1978, ApJ, 223, L109
Kuchner, M. J., Kirshner, R. P., Pinto, P. A., & Leibundgut, B. 1994, ApJP, submitted
Leibundgut, B. 1988, PhD. Thesis, University of Basel
Leibundgut, B. 1991, Supernovae, ed. S. E. Woosley, (New York: Springer), 751
Leibundgut, B. 1994a, IAU Colloquium 145: Supernovae and Supernova Remnants, ed. McCray,
(Cambridge: Cambridge University Press), in press
Leibundgut, B. 1994b, Circumstellar Media in Lates Stages of Stellar Evolution, eds. R. Clegg, P.
Meikle, & I. Stevens, (Cambridge: Cambridge University Press), in press
Leibundgut, B., Kirshner, R. P., Pinto, P. A., Rupen, M. P., Smith, R. C., Gunn, J. E., & Schneider,
D. P. 1991a, ApJ, 372, 531
Leibundgut, B., Kirshner, R. P., & Porter, A. C. 1993a, BAAS, 25, 834
Leibundgut, B., Phillips, M. M., & Graham, J. A. 1990, PASP, 102, 898
Leibundgut, B., & Pinto, P. A. 1992, ApJ, 401, 49
Leibundgut, B., Tammann, G. A., Cadonau, R., & Cerrito, D. 1991b, A&AS, 89, 537
Leibundgut, B., et al. 1993b, AJ, 105, 301
Livne, E. 1990, 354, L53
Livne, E., & Glasner, A. S. 1991, 370, 272
Long, K. S., Winkler, P. F., & Blair, W. P. 1992, ApJ, 395, 632
Lucy, L. B., Danziger, I. J., Gouiffes, C., & Bouchet, P. 1991, Supernovae, ed. S. E. Woosley, (New
York: Springer), 82
Maza, J., & van den Bergh, S. 1976, ApJ, 519
Mazzali, P. A., Lucy, L. B., Danziger, I. J., Gouiffes, C., Cappellaro, & Turatto, M. 1993, A&A,
269, 423
McCray, R. 1993, ARA&A, 31, 175
McCray, R., ed., 1994, IAU Colloquium 145: Supernovae and Supernova Remnants, (Cambridge:
Cambridge University Press)
Meikle, W. P. S., Spyromilio, J., Allen, D. A., Varani, G.­F., & Cumming, R. J. 1993, MNRAS,
261, 535
Minkowski, R. 1941, PASP, 53, 224
Minkowski, R. 1964, ARA&A, 2, 247
M¨uller, E., & H¨oflich, P. 1994, A&A, 281, 51
Muller, R. A., Marvin Newberg, H. J., Pennypacker, C. R., Perlmutter, S., Sasseen, T. P., & Smith,
C. K. 1992, ApJ, 384, L9
Nomoto, K., Suzuki, T., Shigeyama, T., Kumagai, S., Yamaoka, & H. Saio, H 1993, Nature, 364,
507
Oke, J. B., & Searle, L. 1974, ARA&A, 12 315
Pacini, F., & Salvati, M. 1981, ApJ, 245, L107
Panagia, N. 1985, Supernovae as Distance Indicators, ed. N. Bartel (Berlin: Springer), 14
Patat, F., Barbon, R., Cappellaro, E., Turatto, M. 1993, A&AS, 98, 443
Petschek, A. G., ed., 1990, Supernovae, (New York: Springer)
Phillips, M. M. 1993a, ApJ, 413, L105
Phillips, M. M. 1993b, BAAS, 25, 834
Phillips, M. M., Wells, L. A., Suntzeff, N. B., Hamuy, M., Leibundgut, B., Kirshner, R. P., & Foltz,
C. B. 1992, AJ, 103, 1632
Phillips, M. M., & Williams, R. E. 1991, Supernovae, ed. S. E. Woosley, (New York: Springer), 36
Phillips, M. M., et al. 1987, PASP, 99, 592
Pinto, P. A. 1988, PhD. Thesis, University of California, Santa Crux
Popov, D. V. 1993, ApJ, 414, 712
Porter, A. C., & Filippenko, A. V. 1987, AJ, 93, 1372
Pskovskii, Y. P. 1968, SovA, 11, 570
Richmond, M. W., Treffers, R. R., Filippenko, A. V., Paik, Y., Leibundgut, B., Schulman, E., Cox,
C. V. 1994, AJ, in press
Renzini, A. 1994, IAU Colloquium 145: Supernovae and Supernova Remnants, ed. R. McCray,
(Cambridge: Cambridge University Press), in press
Ruiz­Lapuente, P., & Lucy, L. B. 1992, ApJ, 400, 127
Ruiz­Lapuente, P., Cappellaro, E., Turatto, M., Gouiffes, C., Della Valle, M., & Lucy, L. B 1992,
ApJ, 387, L33
Ruiz­Lapuente, P., Jeffery, D. J., Challis, P. M., Filippenko, A. V., Kirshner, R. P., Ho, L. C.,

20 BRUNO LEIBUNDGUT
Schmidt, B. P., Sanchez, F., & Canal, R. 1993, Nature, 365, 728
Rupen, M. P., van Gorkom, J. H., Knapp, G. R., Gunn, J. E., & Schneider, D. P. 1987, AJ, 94, 61
Ryder, S., Staveley­Smith, L., Dopita, M. A., Petre, R., Colbert, E., Malin, D., & Schegel, E. 1993,
ApJ, 416, 167
Saha, A., Labhardt, L., Schwengeler, H., Macchetto, F. D., Panagia, N., Sandage, A., & Tammann,
G. A. 1994, ApJ, in press
Sandage, A., & Tammann, G. A. 1993, ApJ, 415, 1
Schaeffer, R., Cass'e, M., & Cahen, S. 1987, ApJ, 316, L31
Schlegel, E. M. 1990, MNRAS, 244, 269
Schmidt, B. P., 1993, Ph.D. Thesis, Harvard University
Schmidt, B. P., Kirshner, R. P., & Eastman, R. G. 1992, ApJ, 395, 366
Schmidt, B. P., et al. 1993, AJ, 105, 2236
Shigeyama, T., Nomoto, K., Tsujimoto, T., & Hashimoto, M. 1990, ApJ, 361, L23
Shigeyama, T., Nomoto, K., Yamaoka, H., & Thielemann, F.­K. 1992, ApJ, 386, L13
Spyromilio, J., Stathakis, R. A., Cannon, R. D., Waterman, L., Couch, W. J., & Dopita, M. A.
1991, MNRAS, 248, 456
Stathakis, R. A., & Sadler, E. M. 1991, MNRAS, 250, 786
Strom, R. 1994, this volume
Suntzeff, N. B. 1994, IAU Colloquium 145: Supernovae and Supernova Remnants, ed. R. McCray,
(Cambridge: Cambridge University Press), in press
Suntzeff, B. N., & Bouchet, P. 1990, AJ, 99, 650
Tammann, G. A., & Leibundgut, B. 1990, A&A, 236, 9
Turatto, M., Cappellaro, E., Barbon, R., Della Valle, M., Ortolani, S., & Rosino, L. 1990, AJ, 100,
771
Turatto, M., Cappellaro, E., Danziger, I. J., Benetti, S., Gouiffes, C., & Della Valle, M. 1993,
MNRAS, 262, 128
Tutukov, A. V., Yungelson, L. R., & Iben, I. 1992, ApJ, 386, 197
Uomoto, A., 1991, AJ, 101, 1275
Uomoto, A., & Kirshner, R. P. 1986, ApJ, 308, 685
Vacca, W. D., & Leibundgut, B. 1994, in preparation
van den Bergh, S., & Tammann, G. A. 1991, ARA&A, 29, 363
Van Dyk, S. D. 1992, AJ, 103, 1788
Van Dyk, S. D., Sramek, R. A., Weiler, K. W., & Panagia, N. 1993a, ApJ, 409, 162
Van Dyk, S. D., Weiler, K. W., Sramek, R. A., & Panagia, N. 1993a, ApJ, 419, L69
Webbink, R. F. 1984, ApJ, 277, 355
Weiler, K. W., & Sramek, R. A. 1988, ARA&A, 26, 295
Weiler, K. W., Sramek, R. A., Panagia, N., van der Hulst, J. M., & Salvati, M. 1986, ApJ, 301,
790
Weiler, K. W., Panagia, N., & Sramek, R. A. 1990, ApJ, 364, 611
Wheeler, J. C., & Filippenko, A. V. 1994, IAU Colloquium 145: Supernovae and Supernova Rem­
nants, ed. R. McCray, (Cambridge: Cambridge University Press), in press
Wheeler, J. C., & Harkness, R. P. 1990, Rep. Prog. Phys., 53, 1467
Wheeler, J. C., Harkness, R. P., Barker, E. S., Cochran, A. L., & Wills, D. 1987, ApJ, 313, L69
Wheeler, J. C., & Levreault, R. 1985, ApJ, 294, L17
Wheeler, J. C., Piran, T., & Weinberg, S., eds., 1990, Supernovae, (London: World Scientific)
Wheeler, J. C., & Swartz, D. A. 1994, Evolution of Massive Stars: Confrontation Between Theory
and Observation, ed. D. Vanbeveren, in press.
Woosley, S. E., ed., 1991, Supernovae, (New York: Springer)
Woosley, S. E., Pinto, P. A., & Hartmann, D. 1989, ApJ, 346, 395
Woosley, S. E., & Weaver, T. A. 1986, ARA&A, 24, 205
Woosley, S. E., & Weaver, T. A. 1994, ApJ, in press
Zwicky, F. 1965, Stellar Structure, eds. L. H. Aller & D. B. McLaughlin, (Chicago: University of
Chicago Press), 367