DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

supernovae, 1991bg
pergiant stars. The neutrino signal (the first supernova
neutrino detection) agreed well with the
predicted flux from core-collapse models.
However, SN 1987A brought with it many
more puzzles than it did answers. SN 1987A
peaked twice at much lower magnitudes than
most Type II supernovae. The progenitor was
indeed a supergiant, but it was a blue supergiant,
not a red supergiant as was predicted by
theorists. The neutron star which should have
formed in the collapse mechanism has yet to
be detected. In addition, images and spectra of
SN 1987A revealed the presence of interstellar
rings, likely to be caused by asymmetric mass
loss from the progenitor star due to a binary companion.
The progenitor of SN 1987A may have
been a merged binary which provides an explanation
for the rings and for the fact that it was a
blue supergiant.
supernovae, 1991bg A peculiar Type Ia supernova.
Although most Type Ia supernovae exhibit
very little scatter in their peak luminosity
(0.3 to 0.5 magnitudes in V and B), the peak luminosity
for SN 1991bg was extremely subluminous
(1.6 magnitudes less in V, 2.5 magnitudes
less in B with respect to normal Type Ia supernovae).
The late-time light curve decay was consistent
with an explosion ejecting only 0.1M⊙
of nickel. In addition, its expansion velocity
(10, 000 km s −1 ) was slightly lower than typical
Type Ia supernovae. SN 1991bg, and a growing
list of additional low-luminosity (low nickel
masses) supernovae (e.g., 1992K,1997cn), may
make up a new class of supernovae which are
better explained by alternative Type Ia mechanisms
(sub-Chandrasekhar thermonuclear explosions
or accretion induced collapse of white
dwarfs).
supernovae, 1993J Supernova 1993J’s
early-time spectra had hydrogen lines and
hence, is officially a Type II supernova. However,
its light curve peaked early, dipped, and
increased again, marking it as a peculiar supernova.
Its late-time spectra exhibited strong oxygen
and calcium lines with little hydrogen, very
similar to the late-time spectra of Type Ib supernovae.
This transition of supernova 1993J (and
the similar 1987K) from Type II to Type Ib spectra
suggests that Type II and Type Ib supernovae
© 2001 by CRC Press LLC
458
are caused by a common core-collapse mechanism.
The progenitor of 1993J was probably a
massive star that went into a common envelope
phase with a binary companion, removing most
of its hydrogen envelope.
supernovae, accretion induced collapse (AIC)
Rapidly accreting C/O white dwarfs and most
OMgNe white dwarfs, which accrete sufficient
material to exceed the Chandrasekhar limit, collapse
into neutron stars. The collapse proceeds
similarly to the core collapse mechanism of Type
Ib/Ic and Type II supernovae. These collapses
eject up to a few tenths of a solar mass and
may explain the low-luminosity Type Ia supernovae
such as Supernova 1991bg. Like the corecollapse
of massive stars, AICs produce neutron
star remnants, but at a rate < 1% that of corecollapse
supernovae. However, in special cases,
such as globular cluster where it is difficult to retain
neutron stars from Type II supernovae, AICs
may form most of the neutron star population.
supernovae, classification Supernovae are
classified in two major groups: those with hydrogen
lines in their spectra (Type II) and those
without hydrogen lines (Type I). Type I supernovae
are further subclassified by their spectra:
Type Ia supernovae have strong silicon lines (Si
II) whereas Type Ib/c supernovae do not. Type
Ib supernovae exhibit helium lines (He I) which
are absent in Type Ic supernovae. Type II supernovae
are differentiated by their light curves: the
luminosity of Type II-Linear supernovae peak
and then decay rapidly whereas Type II-Plateau
supernovae peak, drop to a plateau where the luminosity
remains constant for ∼ 100 days and
then resume the light curve decay. Type Ia are
more luminous than Type II supernovae (∼ 3
magnitudes) and occur in all galaxies. Type Ib/c
and Type II supernovae do not occur in ellipticals.
Type Ia are thought to be the thermonuclear
explosions of white dwarfs, whereas Type
Ib/c and Type II supernovae are caused by the
collapse of massive stars.
This classification scheme collapses the 5
class Zwicky system (Zwicky 1965) to these two
separate classes (the Zwicky III, IV, and V explosions
are all placed in the category of peculiar
Type II supernovae). In addition, supernovae
1987K and 1993J exhibited hydrogen features