T U¨ M¨ M-P-I ¨ A The Formation of Nebular Spectra in Core-Collapse Supernovae Jakob Immanuel Maurer Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. St. Paul Prüfer der Dissertation: 1. Hon.-Prof. Dr. W. Hillebrandt 2. Univ.-Prof. Dr. H. Friedrich Die Dissertation wurde am 13.10.2010 bei der Technischen Universität München eingereicht und durch die Fakultät für Physik am 26.11.2010 angenommen. Contents 1. Supernovae 1 1.1. History of Supernova Astronomy .................................... 1 1.2. Classification .............................................. 4 1.2.1. Thermonuclear Supernovae .................................. 5 1.2.2. Core-Collapse SNe ....................................... 8 1.2.3. Pair-Instability SNe ....................................... 11 1.3. Supernovae & Astrophysics ....................................... 11 2. Atomic Physics 15 2.1. Radiative Processes ........................................... 15 2.2. Radiative Data .............................................. 18 2.2.1. Hydrogen ............................................ 18 2.2.2. Oxygen ............................................. 20 2.3. Collisional Processes & Data ...................................... 23 3. The Nebular Phase 27 3.1. Nebular Physics ............................................. 27 3.2. The One-Dimensional Nebular Code .................................. 28 3.3. The Three-Dimensional Nebular Code ................................. 30 3.3.1. Three-dimensional heat transport ............................... 30 3.3.2. Three-dimensional line profiles ................................ 31 3.3.3. The new ionisation treatment .................................. 33 4. Characteristic Velocities Of Stripped-Envelope Core-Collapse Supernovae 37 4.1. Data Set ................................................. 38 4.2. Spectral Modelling ........................................... 40 4.3. Discussion ................................................ 45 4.3.1. Tests .............................................. 45 4.3.2. Discussion of the Method ................................... 45 4.4. Results .................................................. 49 4.5. Summary ................................................ 53 5. Oxygen Recombination in Stripped-Envelope Core-Collapse Supernovae 55 5.1. Oxygen lines in the nebular phase of CC-SNe ............................. 55 5.1.1. Effective recombination rates for neutral oxygen ....................... 55 5.1.2. Recombination line formation ................................. 59 5.1.3. Excitation of the O 7774 Å line ................................ 61 5.1.4. Test Model ........................................... 63 5.1.5. Emission versus absorption line shapes ............................ 65 5.2. A shell model of SN 2002ap ...................................... 66 5.3. A 2D model of SN 1998bw ....................................... 67 5.4. Discussion ................................................ 70 5.5. Summary ................................................ 74 i Contents 6. Hydrogen and Helium In Stripped-Envelope Core-Collapse Supernovae 77 6.1. H and He in the nebular phase ..................................... 78 6.1.1. Hydrogen ............................................ 78 6.1.2. Helium ............................................. 79 6.1.3. Mixed H/He layers ....................................... 79 6.2. SN 2008ax ............................................... 83 6.3. Other SNe of Type IIb ......................................... 88 6.3.1. SN 1993J ............................................ 89 6.3.2. SNe 2001ig & 2003bg ..................................... 89 6.3.3. SNe 2007Y ........................................... 90 6.4. An alternative to shock interaction ................................... 91 6.5. Discussion ................................................ 95 6.5.1. Late Hα emission ........................................ 95 6.5.2. Nebular line profiles of SNe IIb ................................ 96 6.5.3. SN 2008ax ........................................... 96 6.6. Summary ................................................ 97 7. Supernova 1987A 99 7.1. Nebular Modelling of SN 1987A .................................... 100 7.2. Summary ................................................ 104 8. Conclusions 105 Bibliography 109 ii 1. Supernovae 1.1. History of Supernova Astronomy The term ’nova’ stems from the Latin denomination for ’new star’ (stella nova) since to observers of yesteryear novae appeared to be momentary appearances of star-like spots in the night skies. The term ’supernova’ was coined much later in the 1930s, to high-light the extraordinary luminosity of those events. 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 confusion with comets and classical novae cannot be excluded. However, some have been identified unambiguously. The best known examples are dated to the years 1006, 1054, 1572 and 1604 A.D. Supernova 1006 is to this day the brightest (apparent magnitude) supernova recorded. Contemporary witnesses compared its luminance with that of the moon in the first quarter. Scores of reports from all over the world are avail- able. Thanks to these records supernova 1006 can be linked to the remnant PKS 1459-41 (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 source of γ-rays in the sky and has provided astronomers with observations of various radiation phenomena, 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 since then. 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- naissance in the 15th and 16th century. Particularly thanks to the work of the Danish astronomer Tycho Brahe there 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 et al. 2008). In 1604, just 32 years later, another bright supernova appeared at the firmament. Detailed reports from Fabricius, Kepler and other European astronomers, but also from Korea allow to reconstruct the light curve of supernova 1604 (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 supernovae might have reached earth in the last centuries, none of those had been recorded. 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 possible to determine the distance of novae at the beginning of the 20th century (e.g. Lundmark 1919) it was realised 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 & Niemeyer 2000, Janka et al. 2007, Podsiadlowski et al. 2008, Nomoto et al. 2010). The rapid improvement of observational methods and fundamental physical theories in the 20th century was 1 Supernovae Figure 1.1.: Spectra of core-collapse supernova 1993J at 45 (early), 182 (early nebular), 387 (late nebular) and 1766 (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 stronger with time. The remnant phase is dominated by O [] and Hα emission. accompanied by a growing theoretical understanding of supernovae. Shortly