The IUE observations of supernova 1979c in M 100 are presented and discussed. The main results are: (1) the bulk of the energy is in the form of continuous emission which is radiated by the main SN envelope; (2) the absorption features originate mostly in both the disks and the haloes of our Galaxy and M 100; and (3) the emission lines are produced in a highly ionized shell which has a radius greater than twice the radius of the main envelope and consists of compressed circumstellar material in which the abundance ratio N/C is about 30 times higher than solar

Panagia, N.

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OBSERVATIONS OF SUPERNOVA 1979c IN M iO0
Nino Panagia
Istituto di Radioastron0mia CNR
Bologna, Italy
ABSTRACT
The IUE observations of supernova 1979c in M IO0 are presented and dis-
cussed. The main results are:
i) The bulk of the energy is in the form of continuous
emission which is radiated by the main SN envelope.
2) The absorption features originate mostly in both
the disks and the haloes of our Galaxy and M iOO.
3) The emission lines are produced in a highly ionized
shell which has a radius greater than twice the
radius of the main envelope and consists of com-
pressed circumstellar material in which the
abundance ratio N/C is about 30 times higher than solar.
INTRODUCTION
On April 19, 1979 Johnson (Ref. i) discovered a bright supernova (denoted
as 1979c, mB = 12m) in the spiral galaxy M I00 (= NGC 4321).
The behavior of the light curve and the optical spectrum have indicated
this SN to be a Type II (Ref. 2).
In the framework of a joint ESA-SRC target-of-opportunity program, on
April 22, 1979 a series of low resolution spectra were taken in both short
and long wavelength ranges. Observations were repeated at several subsequent
epochs (April 24 and 27, May I, 7 and 18, June 4, 15 and 28, and August 4).
In additiQn, simultaneous observations in the visual, radio and X-ray do-
mains were also made resulting in a rather complete coverage and a thorough
follow-up of the SN explosion and its time evolution. Detailed account of the
first six weeks observations can be found in an article presently in the
press (Ref. 3)
Here, I will present and briefly discuss the IUE observations with some
reference to the optical results.
THE CONTINUUM AND THE TOTAL LUMINOSITY
The UV spectrum of the SN taken on April 22 is shown in Fig. i. The bulk
of the energy is radiated in the form of continuous emission which runs smooth-
ly from 1600 to 3200 A corresponding to a color temperature of Tc=ll000 K.
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The emission lines are estimated to contribute less than _ 15% of the total
flux.
As seen in Fig. 2, which presents the observations made during the first
four weeks, the continuum decreases steadily and becomes steeper with time,
corresponding to a decrease of the color temperature from about ii000 K on
April 22 to _ 7400 K on May 7. Starting in mid-May the decline in the UV
bands becomes shallower and becomes similar to that observed in the optical
range. This suggests an almost constant photospheric temperature after the
middle of May.
Integrating over the whole optical and UV spectrum of April 22, after
correction for extinction, the total flux is estimated to be Fto t = 7.9 x i0-IO
_gA_-_i s2-_i I_;P_n_ _A_a_ _f2_ _P_O_r eMgl_O_ =the6.3abs°lu_exiO_Lo .lumin°sityByin-
tegrating the observed spectra over time and frequency the total radiative
energy released _ the Supernova explosion has been estimated to be Era d
(1979c) = 7 x I0_ erg.
THE ABSORPTION LINES
Superimposed on the continuum are many absorption and emission features.
In absorption one can easily identify (cf. Fig. I and 2) lines of low ioniza-
tion ions such as CI, CII, 01, Nal, Mgll, Sill, SII, as well as some resonance
transitions of highly ionized species (e.g. SilV, CIV, AIIII).
The absorptions extend from approximately zero velocity up to velocities
of abQ_ 16OO km s-I and can be produced in the interstellar media of both
M IOO and our own Galaxy. The interstellar origin of all these absorption
features is confirmed by the constancy of their widths and strengths in spec-
tra taken at different epochs (cf. Fig. 2).
The velocity dispersion implied by the absorptionofeatures due to neutral
and once ionized atoms observed between 1250 and 1350 A is found to be inthe
range of 14-26 km s- . This estimate agrees very well with the more direct
determination by Penston & Blades (Ref. 4) of 15 (+II, -6) km s-I obtained from
measurements of the absorption of the Ca II 3934-68 and Na 1 5890-96 doublets.
Similarly, the absorption lines of highly ionized atoms imply the presence,
in both galaxies, of a medium where the gas is highly ionized and the velocity
dispersion is 50 km s-I at least. Evidence for the presence of such a halo in
our Galaxy has been found in the direction of 3C 273 and towards some high z
stars (Ref. 5) as well as towards stars of the LMC (Ref. 6).
THE EMISSION LINES
The strongest emission features present in the April 22 spectrum can be
identified with resonant transition of N V (1238.8-42 A), Si IV (1393.7-
1402.7 A ), C IV (1548.2-50.8 _) and the intercombination transitions of N IV]
522
0 0 o
1486 A, N II_ 1750 A, C.III] 1909 A. The intercombination lines of O III]
1663 _ and of O IV] 1406 A, cannot be discerned in the spectrum. The He II
1640 _ line is neither clearly present in this spectrum nor in those taken at
later e_ochs. On the other hand, in the spectrum one can clearly see the line
1718.5 A of N IV which corresponds to a permitted transition between excited
levels (2p IpO-2p2 IS). This immediately tells us that nitrogen must be pre-
sent largely in the form of N IV in order to produce such a strong line.
As seen in Fig. 2, the spectrum appears to contain several other lines
which are somewhat weaker. However, they are severely blended with each other
so that the identification of individual lines and a quantitative estimate of
their intensities is impossible.
The same crowding of lines makes it difficult to properly determine the
profile of the major features and to measure their intensities to accuracies
better than, say, 30%. Nevertheless, by studying the spectrum observed on
April 22 as well as those obtained at later epochs (cf. Fig. 2) the following
is apparent:
i) The line profiles are sharply peaked at a velocity displacement
of 1800 ± 300 km s-I with respect to the laboratory wavelength.
This velocity is only marginally higher than the radial velocity
of M i00 and confirms that the emission lines originate in the
SN envelope.
2) The wings of th individual emission features extend to no more
than 4000 km s-_.
3) The emission profiles appear to be symmetric with respect to the
emission peak.
Thepeaked profiles indicate that the lines are formed in an expanding
envelope in which a velocity gradient exists and_the maximum expansion veloc-
ity is Vmax(UV) = 4000 km s-I .
The symmetric extension of the llne wings to the red and to the blue in-
dicates that the UV emitting layers are far enough from the SN "photosphere"
that any occultation of the receding part of the shell is negligible. From
simple geometric arguments the average distanceof the UV emitting shell from
the SN surface can be estimated to be at least 2 photospheric radii (i.e.
R (UV shell) > 2.5 x 1015 cm). Therefore, the UV emission llne layers must be
well separated from those which produce the continuous spectrum, i.e. the SN
photosphere. Moreover, inspection of Fig. 2 shows that the line-to-continuum
ratio increases with time. This is further confirmation that they evolve in-
dependently ands thus, are formed in different zones.
On the other hand, the optlcal llnes dlsplay very asymmetric profiles
(Ref. 3), with the blue portion being twice as extended as the red one. This
indicates that the optical lines are formed in the mainoenvelope which has
been ejected in the SN explosion. Also the MglI _28OO A line presents similar
characteristics in both profile and time evolution. Thus, this line too
originates in the top layers of the SN photosphere.
The UV shell is likely to consist of gas originally ejected by the stellar
523
progenitor, a red supergiant, as a more or less continuous wind. The wind
material must have subsequently been compressed and accelerated as a result l_f
the SN explosion, possibly by the radiation pressure of an initial soft X-ra_r
burst (Ref. 7). The UV shell mass can be estimated to be approximately
M(UV shell) = IO-2M e. This may be the result of steady mass loss of
M = 10-4 Me/year. From the intensities of the emission lines of C and N, as
well as the upper limits to the O lines, it is estimated that the number
abundance ratios of N/C and O/C are 7 and <4, respectively. This correspond_
to a strong overabundance of nitrogen relative to both carbon and oxygen by a
factor of 10-30. The high enrichment of N and/or the possible depletion of C
and O are indicative of nuclear processing through CNO cycle. This can have
occurred in an H-burning shell during the red giant phase of the progenitor
star. Subsequently the material has been brought up to the surface and
ejected from the star in the form of stationary wind. It is clear, then, that
the anomalous CNO abundances found in the UV shell provide further evidence
for the accumulation of circumstellar material prior to the SN explosion.
REFERENCES
i. Johnson, G.E.: 1979, IAU Circular No 3348.
2. Ciatti, F. et al.: 1979, IAU Circular No 3361 & 3371.
3. Panagia, N. et al.: 1980, Mon.Not.Roy.Astr.Soc., in press.
4. Penston, M.V. & Blades, J.C.: 1980, Mon.Not.Roy.Astr.Soc. 19___O,51P-57P.
5. Ulrich, M.H. et al.: 1980, Astron.Astrophys., in press.
6. Savage, B.D. & de Boer, K._.: 1980, Astrophys.J., in press.
7. Klein, R.I. & Chevalier, R.A.:I978, Astrophys.J.(Letters) 22___3,LIO9-LII2.
S24
- LT I I I I I I I T I r ] I I I I I I I I I I I
_ _' =_ A., cm] April 22, 1979 _al
g
_ _.e
E
0,0 I I I I I I I I I I I 1 I I I I I I I ..... 1 I I
1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200
Wavelength (A}
Figure I. Ultraviolet spectrum of the supernova taken on April 22.3, 1979.
The identifications of some relevant features either in absorp-
tion or in emission are given. The average wavelengths for
components at zero velocity and at the M iOO radial velocity
(z = 0.0054) are shown for absorption features. Only the red-
shifted wavelength is indicated for emission lines.
1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200
Wavelength (A)
Figure 2. Evolution of the ultraviolet spectrum from April 22 to May 18.
Note the increasing prominence of the far UV lines with time.
Also noteworthy, is the development of the Hg II %2800 A line.
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Observations of supernova 1979c in M 100

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