Fig. 1: Sketchy supernova classification scheme
We are calculating models of atmospheres and according spectra of supernovae. Atmospheres
of supernovae were among the first atmospheres that were modelled with PHOENIX.
A supernova is the most luminous event known. Its luminosity matches those of whole
galaxies. The name derives from the works of Walter Baade and Fritz Zwicky who studied
supernovae intensively in the early 1930s and used the term supernova therein.
Nowadays supernova is a collective term for different classes of objects, that exhibit a sudden
rise in luminosity that drops again on a timescale of weeks.
Those objects are subdivided into two classes, supernovae of type I or II (SNe I and SNe II). The distinguishing
feature is the absence or the presence of spectral lines of hydrogen. SNe I show no such lines as SNe II do.
The class of SNe I is further subdivided in the classes a, b and c. This time the distinguishing
feature are spectral features of helium and silicon. SN Ia show silicon features, SN Ib show helium but no silicon
features and SN Ic show both no silicon and no helium spectral features.
The class of SN II is further subdivided in two classes. Those are distinguished by the decline of the lightcurve.
Those SN II that show a linear decline are named SN II-L and those that pass through a plateau-phase are referred to as
SN II-P.
There seem to exist quite a few examples of supernova that don't fit perfectly into this scheme. For those cases there
exist other classes e.g. SN IIn and SN IIpec, but those are not in common use.
Supernova of type Ia are believed to be white dwarfs that accrete matter from a companion star and exceed the Chandrasekhar-mass in this process.
The resulting collaps kindels thermonuclear fusion processes and the star deflagrates. The resulting
hot gas expands rapidly and forms thereby the supernova.
Supernova of type II, Ib and Ic are believed to be the fate of massive stars (With a mass greater than 8 times that of the sun). Such a
star passes through numerous phases of core-contractions that ingnite new thermo-nuclear fusion processes. As soon as iron is synthesized
such a process is impossible. An iron nucleus has the lowest binding energy and therefore no selfsustained fusion process of iron is
possible. Hence iron starts to pile up in the center of the star. More and more iron rains down on this core as the ashes of silicon burning
is iron.
At some point this iron core exceeds the Chandrasekhar-mass and the core collapses. Hence this kind of supernova is also called core-collaps supernova.
During the collaps the iron nuclei are transformed to
neutrons. Most of the gravitational energy (about 99%) is transferred to neutrinos that leave the core.
The density in the inner part of the core (dark grey in fig. 2) is high enough that the local speed of sound is greater than the infall velocity. This
part collapses self similar. The speed of sound in the outer part of the core (light grey in fig. 2) is too small and therefore this part falls shocked
towards the center.
The degenerate neutrons (fermions) stop the infall at some point. Hence a huge pressure builds up that propagates outwards. The inner core is still
dense enough so that the speed of sound is not exceeded. When the pressure wave reaches the outer part of the core the wave becomes a shock. This shock propagates
outward. In the shockfront the gas is extremely heated and many kinds of nuclear processes occur. The propagation of the shock drains a lot of energy
from the shock but the densities in the shockfront are that high that the neutrinos interact with the shockfront and transfer energy and momentum. Hence the
shock is revived and can reach the surface of the star.
The hot gas rapidly expands and the shell around an inner core is ejected and leaves the gravitational potential well.
In the early stages of development this gas is optically thick and forms the visible supernova atmosphere. The core that is situated
in the center is composed of the dense matter that the pressure wave could transverse unshocked and the matter that was not ejected and accreted in another
shock back on the core. This core consists of degenerate neutrons and is appropriatly called a neutron star. If too much matter is accreted on the neutron star
the Chandrasekhar-mass is exeede again and the everlasting journey to a black hole begins.
The energy that is set free during the collaps is incredibly large. But just about a 1/1000th part of the energy is transmuted into light. 99% of the energy
are deposited in neutrinos and about 1% in the kinetic energy of the atmophere. So a supernova is the most luminous single object we know with just using
the 1/1000th part of the available energy.
In figure 2 the production of the shock is displayed.
All kinds of supernovae have a rapidly expanding atmosphere in common. The expansion velocity
can reach a few percent of the speed of light. Therefore the radiative transport not only has
to be calculated in moving media but even so relativistically.
Furthermore scattering is important in the atmosphere. Hence the description of radiative transport demands
to fully include the effects of NLTE (non-local thermodynamical equilibrium).
The structure of the supernova is assumed to be spherically symmetric. This may not be a very good assumption,
but it has to be made as long as the radiative transfer is treated onedimensionally.
The expansion of the atmosphere is not modelled with a hydrodynamical code. At the point when the supernova
is detectable the atmosphere is already in uniform motion and hydrodynamical effects exert no force on the atmosphere.
Hence hydrodynamical effects are negligible at the time when the spectrum of the supernova is calculated.
For the expansion of a shocked gas in spherical symmetry there exists an analytical solution from Sedov. This states that
the velocity is linearly dependent on the radius. Hence a linear velocity law is used in the modelling.
PHOENIX was sucessfully used in supernova related research. There are quite a few publications out there:
Last modified by Andreas Seelmann on Wednesday, 22-Jul-2009 17:52:57 CEST.
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