SN 1993J in M81 (NGC 3031) was discovered
by Garcia on March 29, 1993 (Ripero et al. 1993).
It is the brightest supernova visible in the northern hemisphere in the last
21 years and the second brightest only inferior to SN 1987A in the whole world.
The explosion probably occurred around March 28, and it was soon classified
as a type II supernova (Filippenko et al. 1993b; Garnavich & Ann 1993).
The spectra obtained in the first few weeks
show typical Type II features, such as strong and broad H 
emission
(Wheeler et al. 1993a; Wheeler & Clocchiatti 1993). After April 22,
the emission peak of the H line change to a double-peaked
structure (Filippenko & Matheson 1993a; Hu et al. 1993,1993a) which was
recognized as the emergence of the He I 
6678 line
(Filippenko & Matheson 1993a; Filippenko
et al. 1993; Swartz et al. 1993). The spectra of SN 1993J are predicted to
evolve with time from Type II, which is hydrogen rich, towards those of
Type Ib, which is helium rich and hydrogen absent (Nomoto et al. 1993),
and in the late nebula phase helium lines will
disappear and the spectra will be dominated by emission lines of
[O I], [Ca II] and Ca II, with H very weak or absent (Filippenko et al.\
1993). SN 1993J is then classified as a type IIb
supernova (Woosley et al. 1988; Filippenko et al. 1993; Shigeyama
et al. 1994). Observations in August and November, 1993, do
show such changes (Wang et al. 1993).
SN 1987K (Filippenko 1988) is the only other supernova to have behaved
similarly with SN 1993J. Figures 9 may give the impression
of the evolution of spectra. Spectral synthesis have given detailed
information of this particular supernova. For example, the modeling of the
peculiar shape of H at early times, as shown in figure 10, implies that
there is an outer shallow density gradient layer in addition to the inner
steep atmosphere in the supernova ejecta (Zhang et al. 1995).
Figure 9: The early-time spectral evolution of SN 1993J as observed
with the 2.16m reflector of Beijing Astronomical Observatory. The spectra
are flux and wavelength calibrated, and are removed arbitrarily for clarity.
They are not shifted for the heliocentric velocity of M81 since it is
negligibly small.
The light curves of SN 1993J
exhibit a rapid rise to a first maximum around March 28, a decline of
about 1 week in all bands, a subsequent rise to a second maximum near April
19, and an exponential decline (Schmidt et al. 1993), which correspond
neither to a typical Type II-P nor
to a Type II-L pattern (Doggett and Branch 1985; Patat et al. 1993).
The progenitor candidate may be a Type Ia supergiant of approximate
spectral class K0 (Filippenko 1993). It is also suggested from the modeling of
the light curves that the progenitor
of SN 1993J was a massive star with only a low-mass (about 3-4 solar mass;
Shigeyama et al. 1994) outer skin of helium rich hydrogen envelope, and/or a
thin layer of hydrogen covering it (i.e. about 0.4M 
atmosphere; Swartz et al.\
1993). Most of the mass was lost either through winds of a single star
( 
M ; Höflich, Langer, Duschinger 1993) or,
more likely, through
mass transfer in a binary system ( 
M for the progenitor; Nomoto
et al. 1993; Shigeyama et al. 1994; Ray, Singh, Sutaria 1993). The early
appearance of X-rays (Zimmermann et al. 1993; Tanaka 1993) and radio
radiation (Pooley, Green 1993; Weiler et al. 1993; Van Dyk et al. 1993),
show evidence for interaction between supernova ejecta and circumstellar gas.
Jeffery et al. (1994) explored the smooth UV part of the spectrum on April 15
with a LTE approach,
and found that SN 1993J may have an outskirt thick envelope, which may have
changed the shape of the ejecta density profile due to the interaction
between the ejecta and circumstellar matter.
Figure 10: A synthetic spectrum (dashed line) from a single power law
density structure is compared to the April 7 spectrum (thick line) observed
with the 2.16m reflector of BAO. A solar composition is adopted. From
Zhang et al. (1995).