DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

supernovae, white dwarf accretion Type
Ia supernovae are powered by the thermonuclear
explosion of accreting white dwarfs (see
supernovae, thermonuclear explosions). The favored
Type Ia supernova model requires that the
white dwarf accrete up to a Chandrasekhar mass.
However, accreting white dwarfs tend to lose
mass during nova eruptions. Only for accretion
rates greater than ∼ 10 −9 − 10 −8 M⊙ yr −1
are white dwarfs thought to gain mass from accretion.
Rapidly accreting ( 10 −6 M⊙ yr −1 )
C/O white dwarfs and most accreting OMgNe
white dwarfs, which gain enough matter to exceed
the Chandrasekhar mass, are thought to
collapse to neutron stars (Nomoto & Kondo)
before nuclear burning can drive a thermonuclear
explosion. Neutrino Urca processes prevent
these white dwarfs from getting hot enough
to burn efficiently. For more common accretion
rates, C/O white dwarfs are thought to ignite
their cores, initiating a thermonuclear explosion.
supernova rates The rate at which supernovae
of different types occur remains uncertain
by factors of ∼ 2. These rates can be indirectly
determined by observing metal abundances
(metals are produced almost entirely in
supernovae) of galaxies and using a theoretically
derived production rate of metals per supernova.
Alternatively, the rate of core-collapse
supernovae (Type II + Type Ib/c) can be calculated
by combining the theoretical estimates
of the mass range of stars with observations of
the initial mass function and the star formation
rate. However, the most direct and most accurate
technique to determine these rates is to
simply observe the supernovae that occur in the
universe and correct for the biases intrinsic to the
observed sample. Unfortunately, a large supernova
sample does not exist, and biases such as
luminosity differences and obscuration, make it
difficult to exactly determine the supernova rate.
Recent estimates of supernova rates are printed
in the following table. (Cappellaro et al., 1997.)
The units are per 10 10 solar luminosities (in the
blue) per century.
supernova remnant (SNR) The expanding
gas blown out during any type of supernova
SN explosion. Masses can range from a few
tenths to a few tens of solar masses. Expan-
© 2001 by CRC Press LLC
superposition principle
Galaxy Type
Supernova Rates
Supernova Type
Type Ia Type Ib/c Type II
E-SO 0.15 ± 0.06
SOa-Sb 0.20 ± 0.07 0.11 ± 0.06 0.40 ± 0.19
Sbc-Sd 0.24 ± 0.09 0.16 ± 0.08 0.88 ± 0.37
Early time spectra of supernovae. Type II supernovae
have hydrogen lines. Type Ia supernovae have strong
silicon lines whereas Type Ib/c supernovae do not.
Type Ib supernovae exhibit helium lines which are absent
in Type Ic supernovae. Figure courtesy of Alex
Filippenko.
sion velocities range from 2,000 to about 20,000
km/sec. Young SNRs sometimes have pulsars
in them (meaning that the SN was a core collapse
event). A few hundred SNRs are known
in our galaxy, from their emission line spectra,
radio, and X-ray radiation which occurs by
synchrotron and/or bremsstrahlung processes.
Their spectrum sometimes shows emission lines
characteristic of shocked and photoionized gas.
SNRs add kinetic energy and heat to the interstellar
medium and contribute to accelerating
cosmic rays.
superposition principle In linear systems
any collection of solutions to a physical problem
can be added to produce another solution.
An example is adding Fourier harmonics to describe
oscillations in the electromagnetic field in
a cavity. In particular situations a limited form
of superposition is possible in specific nonlinear
systems.
463