DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

supernovae, Type Ib/Ic
in the peak magnitude have been correlated to
the decline rate of the supernova luminosity (see
supernovae, M15/Phillips relation). Using this
correlation, the peak magnitude variations can
be removed, and Type Ia can thus be used as
standard candles for measuring the Hubble constant,
and other cosmological parameters.
The luminosity is powered by the decay of
56 Ni produced in the explosion. Some of the
low luminosity outbursts (e.g., 1991bg) may be
explained by the accretion induced collapse of
white dwarfs (see supernovae, accretion induced
collapse). Type Ia supernovae do not occur in
extremely young stellar populations but do occur
in all types of galaxies at a rate of 0.005 yr −1
per Milky Way-sized galaxy, consistent with the
assumption that the progenitors of these systems
come from low-mass stars. At peak brightness,
a Ia SN can be as bright as its entire host galaxy.
The precise nature of the progenitors is not clear.
One popular candidate is a pair of white dwarfs
whose total mass exceeds the Chandrasekhar
limit in a binary system with orbit period less
than about one day. Such a pair will spiral together
in less than the age of the universe and explode
as required when they merge, but we have
not yet actually seen any white dwarf binaries
with the required properties. Tycho’s and Kepler’s
supernovae were probably Type Ia events.
supernovae, Type Ib/Ic Type Ib/Ic supernovae
exhibit neither hydrogen lines nor silicon
lines. Type Ib supernovae are characterized by
the existence of helium lines, absent in Type Ic
supernovae. These two types of supernovae are
otherwise very similar (both have oxygen and
calcium in their late-time spectra) and are generally
lumped together. They occur at a rate of
0.002 yr −1 per Milky Way-sized galaxy.
Type Ib/Ic supernovae are similar to Type II
supernovae in that they are caused by the collapse
of massive stars (> 10M⊙). However, at
the time of collapse, Type Ib/Ic supernovae have
lost most/all of their hydrogen envelope either
through stellar winds or during binary evolution
through a common envelope phase. Type
Ic supernovae have lost not only their hydrogen
envelope but most of their helium envelope as
well. The connection between Type II supernovae
and Type Ib/Ic supernovae comes from
supernovae 1987K and 1993J which both ex-
© 2001 by CRC Press LLC
462
hibited hydrogen in their spectra but mimicked
Type Ib supernovae at late times. The two progenitors
of these supernovae appear to have lost
most, but definitely not all, of their hydrogen
before collapse, implying a smooth continuum
between Type Ib/Ic and Type II supernovae.
supernovae, Type II The kind of supernova
that results when the core of a massive star collapses
to a neutron star or black hole (see core
collapse). They are the primary source of new
heavy elements (those beyond hydrogen and helium)
made by nuclear reactions over the life
of the star and expelled when energy from the
core collapse blows off the outer layers of the
star. The spectrum is dominated by lines of hydrogen
gas, from the envelope of the star, but
oxygen and other heavy elements are also seen.
Core collapse, when the star has already lost its
hydrogen envelope, produces hydrogen-free supernovae
of Types Ib and Ic. It is estimated that
about one SNII occurs in our galaxy each century.
This class of supernovae is subdivided into
two major groups based on their light curves:
Type II-Linear (II-L) supernovae peak and then
decay quickly, and Type II-Plateau (II-P) which,
after their peak, decay only ∼ 1 magnitude and
then reach a plateau for ∼ 100 days before a latetime
decay similar to type II-L supernovae. The
expansion velocities inferred from the spectra
are roughly ∼ 7000 km s −1 . The absolute peak
visual magnitudes of Type II supernovae have
considerable scatter (〈MV〉 =−17 ± 2 mag.).
However, models of Type II-P supernovae allow
a physical calibration of these objects, allowing
them to be used as “standard” candles without
relying upon a local calibrator such as Cepheid
Variables.
Type II supernovae are caused by the core collapse
of massive stars (> 10M⊙) and are powered
by the potential energy released during this
collapse (see supernovae, core collapse mechanism).
Because their progenitors are short-lived,
they only occur in young stellar populations, and
none have been observed in elliptical galaxies.
Type II (and Type Ib/Ic) supernovae form the
bulk of the neutron stars in the universe and occur
at a rate of 0.0125 yr −1 per Milky Way-sized
galaxy.