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The formation of nebular spectra in core-collapse supernovae [Elektronische Ressource] / Jakob Immanuel Maurer

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TU¨M¨M P  I  ¨ ATheFormationofNebularSpectrainCore CollapseSupernovaeJakobImmanuelMaurerVollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur Erlangung desakademischenGradeseinesDoktorsderNaturwissenschaftengenehmigtenDissertation.Vorsitzender: Univ. Prof. Dr. St. PaulPrüferderDissertation:1. Hon. Prof. Dr. W.Hillebrandt2. Univ. Prof. Dr. H.FriedrichDieDissertationwurdeam13.10.2010beiderTechnischenUniversitätMüncheneingereichtunddurchdieFakultätfürPhysikam26.11.2010angenommen.Contents1. Supernovae 11.1. HistoryofSupernovaAstronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2.1. ThermonuclearSupernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.2. Core CollapseSNe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.3. Pair InstabilitySNe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.3. Supernovae&Astrophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112. AtomicPhysics 152.1. RadiativeProcesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.veData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.

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TU¨M¨
M P  I  ¨ A
TheFormationofNebularSpectra
inCore CollapseSupernovae
JakobImmanuelMaurer
Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur Erlangung des
akademischenGradeseines
DoktorsderNaturwissenschaften
genehmigtenDissertation.
Vorsitzender: Univ. Prof. Dr. St. Paul
PrüferderDissertation:
1. Hon. Prof. Dr. W.Hillebrandt
2. Univ. Prof. Dr. H.Friedrich
DieDissertationwurdeam13.10.2010beiderTechnischenUniversitätMünchen
eingereichtunddurchdieFakultätfürPhysikam26.11.2010angenommen.Contents
1. Supernovae 1
1.1. HistoryofSupernovaAstronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1. ThermonuclearSupernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2. Core CollapseSNe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.3. Pair InstabilitySNe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3. Supernovae&Astrophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2. AtomicPhysics 15
2.1. RadiativeProcesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.veData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.2. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3. CollisionalProcesses&Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3. TheNebularPhase 27
3.1. NebularPhysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2. TheOne DimensionalNebularCode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3. TheThree DimensionalNebularCode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.1. Three dimensionalheattransport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.2.lineprofiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3.3. Thenewionisationtreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4. CharacteristicVelocitiesOfStripped EnvelopeCore CollapseSupernovae 37
4.1. DataSet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2. SpectralModelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3.1. Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3.2. DiscussionoftheMethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.4. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5. OxygenRecombinationinStripped EnvelopeCore CollapseSupernovae 55
5.1. OxygenlinesinthenebularphaseofCC SNe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.1. Effectiverecombinationratesforneutraloxygen . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.2. Recombinationlineformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1.3. ExcitationoftheO 7774Åline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.1.4. TestModel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.1.5. Emissionversusabsorptionlineshapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.2. AshellmodelofSN2002ap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.3. A2DmodelofSN1998bw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
iContents
6. HydrogenandHeliumInStripped EnvelopeCore CollapseSupernovae 77
6.1. HandHeinthenebularphase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.1.1. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.1.2. Helium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.1.3. MixedH/Helayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.2. SN2008ax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.3. OtherSNeofTypeIIb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.3.1. SN1993J . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.3.2. SNe2001ig&2003bg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.3.3. SNe2007Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.4. Analternativetoshockinteraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
6.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.5.1. LateHαemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.5.2. NebularlineprofilesofSNeIIb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.5.3. SN2008ax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
7. Supernova1987A 99
7.1. NebularModellingofSN1987A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7.2. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
8. Conclusions 105
Bibliography 109
ii1. Supernovae
1.1. HistoryofSupernovaAstronomy
The term ’nova’ stems from the Latin denomination for ’new star’ (stella nova) since to observers of yesteryear
novaeappearedtobemomentaryappearancesofstar likespotsinthenightskies. Theterm’supernova’wascoined
muchlaterinthe1930s,tohigh lighttheextraordinaryluminosityofthoseevents.
The earliest records of ’new stars’ reach back up to three thousand years and essentially hail from the Far East,
but also from the Near East and Europe. A clear identification of historical supernovae is often not possible, since
confusionwithcometsandclassicalnovaecannotbeexcluded. However,somehavebeenidentifiedunambiguously.
Thebestknownexamplesaredatedtotheyears1006,1054,1572and1604A.D.
Supernova 1006 is to this day the brightest (apparent magnitude) supernova recorded. Contemporary witnesses
compareditsluminancewiththatofthemooninthefirstquarter. Scoresofreportsfromallovertheworldareavail
able. Thankstotheserecordssupernova1006canbelinkedtotheremnantPKS1459 41(e.g. Clark&Stephenson
1977).
Only 48 years later supernova 1054 was observed. Its luminance was comparable to that of Venus at maximum
light. Thanks to numerous reports mainly from Asia and the Near East, today supernova 1054 can be associated
with the Crab Nebula (Mayall & Oort 1942, Clark & Stephenson 1977), discovered by John Bevis in 1731. The
Crab Nebula has played an important role for the development of modern astronomy. It is the strongest persistent
sourceof γ raysintheskyandhasprovidedastronomerswithobservationsofvariousradiationphenomena,which
were hardly observed before, like synchrotron radiation and pulsar emission. The centre of the Crab Nebula hosts
the Crab Pulsar, which was among the first neutrons stars discovered in the 1960s and has been studied intensively
sincethen.
Supernova 1572 occurred near the constellation Cassiopeia and was comparable in brilliance to supernova 1054.
In contrast to the Far and the Near East, astronomy in Europe had undergone a rapid development during the Re
naissanceinthe15thand16thcentury. ParticularlythankstotheworkoftheDanishastronomerTychoBrahethere
is more information about supernova 1572 than for any other recorded before. An amelioration of the Sextant al
lowed him to determine the position of this supernova relative to other stars accurately. For that reason its remnant
(3C10) can be unambiguously identified with the supernova (Argelander 1864, Hanbury Brown & Hazard 1952).
Furthermore it was possible to reconstruct the light curve of supernova 1572 (Baade 1945), since detailed compar-
isons between the apparent luminosity of the supernova and other planets and stars over a period of one year exist.
Supernova 1572 is a ’normal’ Type Ia (see below), as it was found from its light echos (Rest et al. 2008, Krause
etal.2008).
In1604,just32yearslater,anotherbrightsupernovaappearedatthefirmament. DetailedreportsfromFabricius,
KeplerandotherEuropeanastronomers,butalsofromKoreaallowtoreconstructthelightcurveofsupernova1604
(Baade 1943) and to identify its remnant (Schlier 1935, Baade 1943). Kepler’s Supernova was the last galactic one
observed to this day. Although the galactic remnants Cas A and Gl.9+03 suggest that some light from Milky Way
supernovaemighthavereachedearthinthelastcenturies,noneofthosehadbeenrecorded.
Beginning with the 17th century, the invention of the telescope heralded a new era of astronomy. Thanks to
steadily improving instrumentation a rising number of ’new stars’ has been observed since then. As it became
possibletodeterminethedistanceofnovaeatthebeginningofthe20thcentury(e.g.Lundmark1919)itwasrealised
that they can be separated into two groups differing in absolute brightness. The brighter class, which exceeds
other novae by several orders in magnitude was termed supernova (Baade & Zwicky 1934b), to emphasise their
extraordinary luminosity. This subdivision is relevant since it turned out that novae and supernovae are physically
distinct phenomenons. While novae results from the explosion of thin layers of material accreted onto white dwarf
surfaces (e.g. Bode 2010), supernovae mark the death of white dwarfs or more massive stars (e.g. Hillebrandt &
Niemeyer2000,Jankaetal.2007,Podsiadlowskietal.2008,Nomotoetal.2010).
The rapid improvement of observational methods and fundamental physical theories in the 20th century was
1Supernovae
Figure1.1.: Spectraofcore collapsesupernova1993Jat45(early),182(earlynebular),387(latenebular)and1766(rem
nant) days after explosion taken from Matheson et al. (2000). While the early time spectrum is dominated by absorption,
the nebular spectra show strong O [] λλ 6300, 6363 and some H  λ 6583 (Hα) emission, which becomes increasingly
strongerwithtime. TheremnantphaseisdominatedbyO[]andHαemission.
accompaniedbyagrowingtheoreticalunderstandingofsupernovae. Shortlyaftercoiningthetermsupernova,Baade
& Zwicky (1934a) proposed the idea that supernovae could be powered by the gravitational energy released by the
transition of ’ordinary’ to neutron stars. In the following years it was discovered that neutron stars may collapse to
even more compact objects (black holes) (Schwarzschild 1916, Tolman 1934, Oppenheimer & Volkoff 1939) and
theimportanceofneutrinosinthecontextofsupernovaewasfirstrealised(Gamow&Schoenberg1941). Untilthat
timespectralobservationsshowedthattherearedifferentclassesofsupernovae,someshowinghydrogenlines(Type
II), others not (Type I) (Minkowski 1940, 1941). The idea thatvae could be exploded by nuclear burning
processeswasdevelopedlater(Borst1950,Burbidgeetal.1956,Baadeetal.1956,Hoyle&Fowler1960,Colgate
&White1966). Today,itiscommonlyacceptedthatdifferenttypesofsupernovaeexistandthatsomearepowered
by gravitational energy and some by thermonuclear burning. Advanced calculations of silicon nucleosynthesis
56 56 56(Truran et al. 1967, Bodansky et al. 1968) led to the recognition that the decay chain Ni→ Co→ Fe is the
radioactive source of supernova light curves ( Colgate & McKee 1969). Today it is believed that both gravitational
56andthermonuclearvaeproduce Ni, whichisthecauseoftheiroutstandingbrightness. Inthelastdecades
the understanding of supernovae has further improved owing to advances of observational methods, particle and
atomic physics, supernova theory and computational facilities. The modern picture of supernovae is discussed in
Section1.2inmoredetail.
Althoughthereisconsenseaboutthefundamentalsofsupernovaphysics,manydetailsremainuncleartothisday.
A key role in the progress of astrophysics has always been played by the comparison of theory and observations.
In the case of supernova astronomy this also demands an accurate and reliable understanding of the formation of
supernovalightcurves,spectraandotherobservablessuchaspolarisation,neutrinosorgravitationalwaves.
In the 19th century the existence of electromagnetic waves was observed in interference experiments (Young
21.1 History of Supernova Astronomy
1802), predicted by Maxwell’s theory of electromagnetism and confirmed by experiments of Hertz and others.
Maxwell’stheorydescribeslightbytransversalwaves,whichmeansthattheyoscillateinadirectionperpendicular
tothedirectionofpropagation. Theorientationoftheseoscillationsiscalledpolarisation. Theconditionsgenerating
aradiationfieldhavedirectinfluenceonitspolarisationstate. Therefore,certainphysicalpropertiesoflightsources
can be inferred from polarisation measurements. Polarisation measurements of supernova light have been used to
detectsynchrotronradiationorejectaasymmetries,forexample.
The neutrino was proposed by Pauli in the 1930s to explain the energy spectrum of the β decay, but was not
experimentallydetectedbefore1956. Today,theneutrinoplaysanimportantroleinthestandardmodelofparticles
andvariousotherareasofphysics. Inmodernparticlephysicstheneutrinoisdescribedasweaklyinteracting,which
means that it is hard to detect but it can escape from high density regions which are obscured for electromagnetic
observations. It is predicted that more than 99% of the explosion energy of core collapse supernovae escapes as
neutrinos,whichinprinciplemakesthemattractivefordirectobservationsofthecentreoftheexplosion. However,
since they are hard to detect the only supernova where neutrinos could be verified unambiguously so far is SN
1987A(Biontaetal.1987,Hirataetal.1987,Agliettaetal.1987).
In1915AlbertEinsteinproposed’generalrelativity’,atheorywhichpredictstheexistenceofgravitationalwaves,
i.e. a deformation of space time, which travels detached from its source. Today, general relativity is one of the
best tested theories of physics and there is little doubt that gravitational waves exist. However, their detection is
difficult. Gravitational waves are expected if the quadrupole (or any higher) moment of a gravitational system is
changed. Therefore, compact binaries but also certain deformations of supernova cores during the explosion and
theformationofcompactobjectsareexpectedtogenerategravitationalwaves. Althoughgravitationalwaveseluded
directdetectiontothisday,observationsoftherotationalperiodofclosebinarysystemsconsistingoftwocompact
objects(Hulse&Taylor1974)allowedtoconfirmtheirexistenceindirectly.
Supernovalightcurvesdescribethetimeresolvedluminosityofasupernova. Abolometriclightcurveisobtained
by integrating the luminosity at all wavelengths, but also light curves of specific wavelength intervals are common
(e.g. V−,B−,R− band). If these wavelength intervals become very small, i.e. there are several bands observed
simultaneously one speaks of a spectrum. The characteristics of supernova spectra evolve with the epoch (the time
sinceexplosion). Althoughthesetransitionsaregradual,atleastthreedistinctphasescanbeidentified.
In the ’early’ phase, which describes about the first 100 days of a supernova, the inner parts of the ejecta are
optically thick at all observable wavelengths. Light emitted from this region is scattered in the outer parts. The
resulting spectra resemble a black body rugged by absorption valleys and emission bumps (see Figure 1.1). The
typical shape of these scattering lines is called a P Cygni profile, which is characteristic for expanding, centrally
illuminated atmospheres. Photons emitted in the optical thick centre are line scattered in the outer regions if they
come into resonance with any optical thick line. Since the expansion velocity increases from the inside to the
outside, the flux bluewards of the line’s rest wavelength is reduced by the absorption. The wavelength of the re
emitted photons in the frame of the observer depends on the relative velocity of the gas and the observer and is
centred around the line’s rest wavelength in the observer’s frame if the scattering region is approximately spherical
symmetric. Therefore,thelineisobservedbyablueshiftedabsorptionvalleyandarestwavelengthcentredemission
bump. Early phase observations can primarily be used to study the outer parts of supernovae, since the inner parts
are obscured. Beside possible ejecta inhomogeneities and the composition of the outer layers, the total mass and
kineticenergycanbebestestimatedfromthisphase.
Between 100 and 200 days after explosion supernovae change into the ’nebular’ phase. As the supernova ejecta
expand with time they become transparent to optical light, the continuum emission ceases and absorption becomes
a subordinate process. Instead, strong emission lines, powered by electron collisions and recombination form.
The spectra are dominated by blends of numerous weak and scattered strong emission lines (see Figure 1.1). In
principle all parts of the supernova can be observed in this phase. However, because of the lower density of the
outer regions, a clear interpretation of the data is often restricted to the core of the supernova. Early and nebular
phaseobservationsarethereforecomplementary. Whiletheouterpartscanbestudiedfromearlytimeobservations
more accurately, interesting observables of supernova cores, like asymmetries in the ejecta or the products of the
centralnucleosynthesis,whichlinkmostdirectlytotheexplosionmechanism,canbeaccessedbestduringthislate
phase.
The nebular phase is followed by the ’remnant’ phase but this transition is subtle. Like in the nebular phase
the spectra are dominated by emission lines, but the gas is more ionised and the characteristic emission lines of
both phases are different (see Figure 1.1). The gas falls out of ionisation equilibrium and the interaction with the
circumstellar medium becomes dominant causing compression, heating and mixing of the gas. Physical processes,
3Supernovae
Figure1.2.: ClassificationschemeofvarioustypesofsupernovaetakenfromTurattoetal.(2007).
whicheludedirectobservations,liketheformationofmagneticfields,thedecelerationandannihilationofpositrons
or shocks, become important. Since the supernova ejecta are decelerated considerably, it is more difficult to trace
back the original structure of the supernova ejecta from remnant phase observations. On the other hand, such
observations can be used to obtain information about compact remnants of the supernova (neutron stars, black
holes) or to study physical processes like synchrotron radiation. An important example is the Crab Nebula, which
hasextensivelybeenusedasanastrophysicallaboratory.
Thisworkisexclusivelydevotedtotheformationofspectrainthenebularphaseofsupernovae.
1.2. Classification
WiththepossibilityofspectralobservationsitbecameevidentthatSupernovae(SNe)canbeseparatedintodifferent
groups by their early characteristics. Initially, SNe had been separated into SNe I and II, depending on the
presence of hydrogen absorption lines in their early spectra (Minkowski 1940, 1941). However, later it turned out
thattheclassificationschemehadtoberefined. Today,supernovaearegroupedintoTypeIa,Ib,Ic,IIb,IIP,IILand
IIn (e.g. Barbon et al. 1979, Wheeler & Harkness 1986, Branch et al. 1991, Filippenko 1991, 1997). While SNe I
shownoobvioussignsofhydrogen, SNeIIdo. AmongtheSNeI,someshowstrongSi  λ6355absorption. These
are called SNe Ia (see Section 1.2.1). The remaining SNe I are divided into SN Ib and Ic depending on whether
thereisheliumabsorption(especiallyHe  λ5876)ornot,respectively. AmongtheSNeII,SNeshowingonlyweak
signs of hydrogen are classified as SNe IIb, while the others are distinguished by their light curves (II P ’plateau
like’ and II L ’linear decreasing’ light curves) and the width of their spectral lines (IIn ’narrow’ emission lines,
almost no absorption lines, slow decline of the light curve) (see Section 1.2.2). SNe Ib, Ic and IIb are often called
stripped envelopecore collapse(SECC)supernovae,sincetheylostallormostoftheirhydrogenbeforeexplosion.
Although the classification is sometimes not unambiguous (for example it can be difficult to distinguish between
SNeIbandIcorbetweenSNeIbandIIb),aconnectionbetweenphysicalpropertiesofSNeandtheirclassification
can be established (e.g. Nomoto et al. 1996, Filippenko 1997, Heger et al. 2003). While SNe Ia originate from
white dwarf explosions, SNe Ib, Ic and II are associated with the collapse of massive stars. While the progenitor
systemandthedetailsofthestellarmasslossandevolutiondecidewhichtypeofSNisproduced,thedetailsofthis
chainingarenotunderstoodcompletely.
In addition to the types of supernovae listed here there are others, which are however poorly established, as for
examplesupernovaeIbn(e.g.Pastorelloetal.2008). Aninterestingsupernovaspecies,whichhashowevernospec
tralclassificationyetsincetheseeventsarerarelyobservedandsincethereisstilldebateaboutthesedetections(e.g.
Woosley et al. 2007, Umeda & Nomoto 2008, Blinnikov 2010, Moriya et al. 2010), are pair instability supernovae
(seeSection1.2.3). Pair instabilitysupernovaehavebeenpredictedbytheory,butsofartheonlypossibledetections
areSNe2006gy(e.g.Smithetal.2007)and2007bi(e.g.Gal Yametal. 2009).
41.2.1 Thermonuclear Supernovae
1.2.1. ThermonuclearSupernovae
SNe Ia are classified by the absence of obvious hydrogen and by the presence of clear Si  features in their early
spectra. While the spectra are dominated by absorption lines of neutral and singly ionised intermediate mass el
ements (O, Mg, Si, S, Ca) directly after the explosion, the relative contribution of iron group elements quickly
increases as the photosphere recedes into the core of the SN (e.g. Filippenko 1997). About two weeks after maxi
mum light the spectra are dominated by Fe . In the nebular phase the spectra are dominated by blends of dozens
offorbiddenFe,butalsoColines.
Although there is growing evidence for diversity among SNe Ia, their physical properties are probably more
homogeneous than those of core collapse supernovae. It was noted by Branch et al. (1993), Filippenko (1997) that
about 80% of all SNe Ia belong to the ’normal’ type, which is defined by a sample of well observed events. More
recent studies suggest that about 60%− 70% of all SNe Ia belong to that ’normal’ type (e.g. Li et al. 2001, Foley
etal.2009),whichmayhoweverbeaffectedbyselectioneffects.
SupernovaeIa,whicharenotof’normal’typebytheirspectralcharacteristicscanbedividedintosub andsuper-
luminousevents. TheluminosityofsupernovaeIavariesbyroughlyafactoroftenbutonlybyaboutafactoroftwo
among’normal’supernovaeIa.
1.2.1.1. Progenitorsandexplosionmechanism
ItiscommonlyacceptedthatsupernovaeIaarethermonuclearexplosionsofwhitedwarfs(e.g.Nomotoetal.2009)
butthedetailsarestillunderdebate.
White dwarfs (WDs) can consist of various light and intermediate mass elements (e.g. He, C, O, Ne, Mg) (e.g.
Iben&Tutukov1985)butthemostpromisingcandidatesforSNeIaare’C+O’WDs(e.g.Hillebrandt&Niemeyer
2000),sinceHeWDsexplodewhenreachingabout0.7M (e.g.Woosleyetal.1986)inconsistentwiththeobserva
tions and O+Ne+Mg WDs tend to collapse to neutron stars instead of exploding in SNe Ia (e.g. Nomoto & Kondo
1991,Saio&Nomoto1998).
AllWDshaveincommonthattheirmaterialiscompletelyionisedandthatmostofthefreeelectrongasisdegen
erate to varying degrees (e.g. Hillebrandt & Niemeyer 2000, Hansen 2004). Because of the quantum mechanical
uncertainty principle, the momentum of a particle becomes increasingly indefinite the more localised in space it is.
If this quantum mechanical exceeds the thermal momentum or the rest mass energy of the particle it is
called degenerate or relativistically degenerate, respectively. Because of this quantum effect degenerate electrons
causeapressure,whichcanstabiliseawhitedwarfagainstgravitationalcollapseuptoacriticalmass,knownasthe
Chandrasekhar(CS)limit(e.g.Chandrasekhar1931).
Since WDs are inert systems (’C+ O’ WDs are born with typical masses of 0.6 M Homeier et al. 1998, well