Light Curves (II)
A professional perspective
Supernovae of both types
reach maximum optical brightness 2 to 3 weeks after their explosions.
At their peaks, Type I SNe are brighter than Type II by about one
magnitude. The peak absolute magnitude depends on the extragalactic
distance scale, which is known only to a factor of 2. On a
"long-distance" scale corresponding to a Hubble Constant
of 50 km/sec/Mpc, Type I SNe reach an absolute magnitude of (~) -20
and have a peak luminosity of 2 x 1043
ergs/sec, 5 x 109 times the luminosity of the sun.
a "short scale" corresponding to 100 km/sec/Mpc, the Type
I absolute magnitude is -18.5 and the luminosity is lower by a
factor of 4. Observations made across the electromagnetic spectrum
show that Type I SNe emit almost all of their energy in the optical
part of the spectrum while Type II SNe emit a significant fracton at
ultraviolet wavelengths. Consequently, Type II SNe, although fainter
optically, have nearly the same peak luminosity as Type I. The total
time-integrated emission of electromagnetic energy of both types is
on the order of 1049
ergs, much less than the kinetic energy of 1051
ergs. Thus the radiative efficiency of SN explosions is low.
mean Type I curve, which is well defined by observations, consisting
of a initial rise and fall (the peak) lasting until 30 days after
maximum light, and a subsequent, slowly fading tail. The tail
appears to be nearly linear when magnitudes are plotted against
time, but magnitude is a logarithmic measre of brightness and the
tail actually corresponds to an exponential decay of brightness with
time. The decay rate of the Type I corresponds to a half-life of 50
II SNe are subdivided into II-P and II-L on the basis of their light
curve shape. Two-thirds of the observed Type II SNe are II-P, which
interrupt the initial declines from their peaks to enter a plateau
phase of nearly constant brightness until 80 days after maximum
light. A Type II-L shows nearly a linear decline from its peak for
80 days after maximum light. The able data on the later phases of Type II light curves are not well defined, but both II-P and II-L do
appear to have slowly fading, linear tails,with a decay rate
corresponding to a half-life of 100 days.
of SN 1987A and M1
left, simulation of Sanduleak -69°202, alias SN
1987A explosion. At right, for comparison
purpose, simulation of M1, the famous Crab nebula
that exploded in 1054. Mpeg files of 2.1 MB and
.Mov of 3.9 MB. Documents STSCI
and Space Telescope
of Type Ia SNe light and color curves
recent years a great deal of photometric data on SNe I has
become available, giving the possibility of assembling a fairly
large and homogeneous sample of light curves for this class of
object....Due to the importance of the Ia SNe as possible distance
indicators, it seems worthwhile to determine as accurately as
possible the mean characteristics of these curves.
may reasonably infer the SNe Ia explosion is a well defined
phenomenon occuring nearly on the same scale, which allow the
fitting of all the best (well monitored) light curves,which are
considered to be relatively representative in shape and absolute
light curve also offers the possibility of finding some general
properties of the Ia event from a photometric point of view
(although observers in this program will be accomplishing this task
in the visual). It allows also the determination of the epoch and
brightness of the maximum for a supernova which has not been well followed,
simply by fitting its magnitudes to the average curve.
below table expresses the mean characteristics of most Type I SNe :
Mean deviation at maximum : +/- 0.15 m
Duration of maximum : 18-19 days (the point where the
event rises and falls a half degree).
Number of days to rise of 3,2,1 magnitudes (to max.) : (t-3)
=15d, (t-2) =13d, (t-1) =10d.
Number of days to fall of 3,2,1 magnitudes (from max.) : (t-3)
=55d, (t-2) =23d, (t-2) =14d.
Rate of decline (mag/day over the first 30 days from max.) : 0.087
Final rate of decline (mag per day from 50-300 days past
max.) : 0.016
peculiar feature of the Type I SNe is the Inflection point, about 30d
past maximum light, and ~2.7 magnitudes below it, which is where the
steep early decline ends and a slower final decline (linear) ensues.
The change in the rate of decline at this point is rather sharp.
In assembling the average light curve some difficulties have
been encountered in obtaining a good fit for all the available
models of light curves.
fact, some of the SNe showed peak maxima with large amplitude
variations, while others have had broad and shallow maxima, these
differances being apparently beyond the range of photometric errors.
This leads to the possibility of a further subdivision of Type Ia
SNe in to two subclasses: "fast and slow", henceforward
also called (in this paper reference) "a" and
"b" (we will not get into the energetics that power these
subdivisions at this time) characterized by different values of the
following parameters :
= width of the light curve two magnitudes below maximum light 32d 38d
= amplitude from maximum light to the inflection point
R = rate of decline from 30 days to 150 days past maximum light
Note: Recent studies, after this article was written have been able
to determine that there does exist different types of Type I SNe;
the Type Ib, and Ic. We will not attempt to describe these events in
detail here, but rather give a broad description of each due to the
fact that there is still a lot of ongoing research being conducted
(see below), although the importance of discovering supernova
events early in their apparition will enable professional astronomers
to better define these events.
our knowledge of the brightness, color, and spectra of SNe has grown
substantially, investigators have therefore endeavored to
reconstruct the light curves from elements that can readily be
determined. These elements include the point at which the decline in
brightness begins to slow down and the mean rate (B) [or Beta, which
denotes a value of magnitude decline per 100 days] of decline in
photographic brightness from maximum light to that point. Without
question, maximum light represents one of the chief distinquishing
points, although not the only one on the photographic light curve of
all practical purposes the color index of a supernovae is
represented by the differance in magnitude of the light curve of the
photographic and the visual modes....thusly, a value of (-) would
indicate that the SN event is diplaying more of a blue value, whilst
the (+) value would be indicative of a more redder scenario.
in context to the color indices of many main sequence stars, ie.
-0.31= a star of spectral class BO, while a value of +0.71 would be
of a K5 star and so on...these differances occur within the
explosive element of the doomed star and are usually synonymous with
its spectral characteristics at a particular point in its
color index which some days before maximum light is negative (-0.30)
steadily increases becoming zero about six days past maximum light
and attaining its largest value, nearly +1.00 (or more), at phase
(time) +30-35d. Later on the color index declines to nearly 0.6 at
phase +80d, approaching zero about 150d past maximum.
after phase +80d the color curve, even with the new data remains
undefined, the dispersion points being fairly large. It is
imperative that many accurate measures at that phase are necessary
to reach certain conclusions. It
may be interesting to remark that, during the first 30 days from
Max., the color curve and light curve of Type I SNe show an opposite
trend, B-V steadily increasing while the brightness declines. About
35 days from
maximum, just at the inflection point of the light curve, the
color index reaches its maximum and then declines at a rate of ~0.01
mag/day, going roughly parellel to the corresponding light curve.
The observed changes in the color curve at the end of the slow
decline, are certainly related to spectral variations.
besides the gradual increase in intensity the red relative to the
blue-violet features, which begins 30-60d from maximum, and the
general fading of all the emmisions, no other relative changes which
may explain the peculiarities of the color curve are found in the
spectra. It may be noted that the progressive red-shifting of some
real physical changes in these stars seem to occur when the color
curve turns down....(Note: For our studies, it is suggested that a
color index of 0.00 in V will occur when the event is at maximum
light in the visual).
The author has attempted a rise/decay time, magnitude chart utilizing data from Pg
(photographic), V-band, and pv (photo visual) mean light curves (by
eye). This will hopefully assist the observer in creating his/her
own mean light curves. In this way their observations can be plotted
against mean values of a Type Ia SNe, hopefully with enough data the
observer could elliviate some degrees of his/her own personal
equation. The Pg will relate to CCD imagers, whilst the V, and pv
hopefully will assist the visual observer...the error here is: +/-
1.5d. For all practical purposes, the pv maximum is usually 2.5d
later than V, and 0.02m brighter.
to Max :
Decay from Maximum
note: As one can see, the V band and pv decay times follow a rather
close relationship, thusly when using charts, refer to the V band
photometry, as more data is available on this value, than on pv
values. A more detailed description of this utility is available in
the program SNCURV.GIF, which can be accessed from the
ISN (International Supernovae Network) home page.
of Type Ib SNe
on optical spectroscopy the authors have initiated the following
characteristcs for this subclass of supernova... A supernova is classified as Type
Ib if it meets the following spectral properties.... There must be a
general resemblence to the spectra of SNe 1a during the first few
months past maximum light (ed. note: The term used is that the supernova is
'born old'). In particular, this includes an absence of hydrogen
emission or absorption lines. Unlike SNe 1a, the spectrum < 25d after maximum light should
not display absorption lines at 6150 Å.
Additionally, or alternatively, if the supernova is observed long after
maximum, the late-time spectrum must be dominated by strong emission lines of
[OI] 6300, [Ca II] 7300, and other intermediate-mass metals. The
spectrum must not show strong emission lines of hydrogen seen in
late-time spectra of SNII... (ed. note: this paper was one of the
first to reveal the appearance of the type Ib SNe, the prototypes
that satisfied all of the above criteria at the time were: SN 1983N,
and SN 1984L).
another event, SN 1984I in the galaxy ESO 323-G99, the authors
contend..."In spite of its red color and apparently low
luminosity of SN 1984I might have been mistaken for a Type Ia SNe if
the spectrum had not been obtained. This seems to emphasize the
importance of spectroscopic observations in accurately sorting out
Type Ia and Type Ib events..." Other additional information on these events..."The
hypothesis we favor for the origin of SN Ib is that they are
basically the same as SNe II but have their hydrogen envelope
of Type Ic SNe
class of SNe is a relative "newcomer" on the scene, more
events and analysis of same are required to place a more difinitive
handle on this subclass. Here is a preliminary thought on the
characteristics of this class. "....SNe Ic spectroscopically
resemble SNe Ib, except that the He I lines are absent at early
times. Thus, SNe Ic are sometimes called "helium-poor SNe
Note: As studies continue on this subclass, it might be noted that
there appears to be a subluminous nature [compared to the above
classes/subclasses] associated with the Type Ic SNe.
of Type II SNe
et. al has classified the light curves of Type II SNe into two
fairly homogeneous subclasses entitled "plateau" (SNe
II-P), and "linear" (SNe II-L). Mean decay rates for SNe
II, are considerably slower than their Type I
counterparts...."II-P"= 0.0075, and "II-L"=
0.012 mag per day. A phase of nearly constant brightness prior to
day 80 is so distinctive, that it may be safe to classify SNe II-P
on a light curve shape alone. Another interesting comparison is the
similarity of SNe I and SNe II-L, which exhibit almost a
similiar linear decay posture, although it is noted that these
events should not be classified by light curve shape alone.
Spectoscopy is still the method of choice in determining various SN
physical characteristics attributed to plateau SNe (II-P) is the
relative high amount of ejected mass (~10 solar masses). Because of
the high ejected mass, the early pre-plateau light curve may be
powered by internal energy that was deposited prompltly into the
envelope by the explosion, and the early luminosity depends
primarily on the radius of the progenitor star. Radioactivity cannot
help to power the early luminosity because the time required for
energy to diffuse from the deep interior to the surface is too long.
Thus the wide range in the early luminosity is caused mainly by
differances in progenitor radius e.g. SN 1987A was the explosion of
a blue supergiant while SN 1983K was the explosion of a red one.
the plateau phase the light curve still can be powered by the
diffuse release of internal energy if the progenitor radius was
sufficiently large, but if it was small, as for SN 1987A, the
initial internal energy is adiabatically degraded by expansion, and
the plateau is powered only by the energy that is deposited slowly
by 56Co decay. If the early luminosity was sufficiently low, the
"plateau" will appear as a 'secondary peak'.
do the linears show a narrower range in peak luminosity than the
plateau's? We suggest that because SN II-L's eject relatively little
mass (otherwise,they would have plateaus) and have short diffusion
times, their light curves are powered even at early times by
radioactivity, just like SN Ia's and SN Ib's. When the diffusion
time is short, the peak luminosity will depend primarily on the
ejected mass of 56Ni, unless the progenitor radius is very large.
The role of radioactivity in powering the late tails of SN II light
curves was suggested, and supported, and confirmed by SN 1987A. The
first suggestion that radioactivity powers even the early light
curves of linear SN II's was based on a resemblence between
composite light curves of SN II-L's and SN I's.
more specific comparison between the light curves of Type II-L 1979C
and the type Ia SN 1972E. These two light curves were practically
indistinquishable during the dirst six magnitudes of decline. The
close resemblance may be partially coincidental since SN 1979C was
radiating strongly at Ha,
while SN 1972E was not; nevertheless if SN 1972E was powered by
radioactivity, so was SN 1979C. SN 1979C was significantly brighter
than the other SN II-L's, the possibility of a large distance error
for SN 1979C relative to all others is unlikely, because SN 1979C
occured in M100, an apparent member of the Virgo cluster.
most obvious possibility for the brightness differance is that SN
1979C was analogous to a type Ia explosion (inside some hydrogen),
ejecting about 0.6 solar mass of 56Ni, while fainter linears were
like SNe Ib, which eject only about 0.1 solar mass of 56Ni. (ed
note: As one can summize, the Type II SNe do not display a
difinitive continuity in either subtype, for example the absolute
magnitude for these events can range from -14.1 (1987A) to -20.3 (SN
1979C and SN 1978K).
report has been published with the courtesy of the International
Supernova Newtork, ISN.
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