Department of Astronomy Stockholm University LICENTIATE THESIS Bumpy light curves of interacting supernovae Author: Anders Nyholm Department of Astronomy and The Oskar Klein Centre Stockholm University AlbaNova 106 91 Stockholm Sweden Supervisor: Jesper Sollerman Co-Supervisor: Peter Lundqvist April 27, 2017 Abstract A supernova (SN) is the explosive destruction of a star. Via a luminous outpouring of radiation, the SN can rival the brightness of its SN host galaxy for months or years. In the past decade, astronomical surveys regularly observing the sky to deep limiting magnitudes have revealed that core collapse SNe (the demises of massive stars) are sometimes preceded by eruptive episodes by the progenitor stars dur- ing the years before the eventual SN explosion. Such SNe tend to show strong signatures of interaction between the SN ejecta and the circumstellar medium (CSM) deposited by the star before the SN explo- sion, likely by mass-loss episodes like the ones we have started to observe regularly. The complex CSM resolved around certain giant stars in our own galaxy and the eruptions of giant stars like η Car in the 19th century can be seen in this context. As the SN ejecta of an interacting SN sweep up the CSM of the progenitor, radiation from this process offers observers opportunity to scan the late mass loss history of the progenitor. In this thesis, interacting SNe and eruptive mass loss of their progenitors is discussed. The SN iPTF13z (discovered by the intermediate Palomar Transient Factory, iPTF) is presented. This transient was followed with optical photometry and spectroscopy during 1000 days and displayed a light curve with several conspicuous re-brigthenings ("bumps"), likely arising from SN ejecta interacting with denser regions in the CSM. Around 200 days before discovery, in archival data we found a clear precursor outburst lasting & 50 days. A well-observed (but not necessarily well understood) event like SN 2009ip, which showed both precursor outbursts and a light curve bump, makes an interesting comparison object. The embedding of the (possible) SN in a CSM makes it hard to tell if a destructive SN explosion actually happened. In this respect, iPTF13z is compared to e.g. SN 2009ip but also to long-lived interacting SNe like SN 1988Z. Some suggestions for future investigations are offered, to tie light curve bumps to precursor events and to clarify the question of core collapse in the ambiguous cases of some interacting SNe. Paper I: • Nyholm, A., Sollerman. J., Taddia. F., Fremling. C., Moriya, T. J., Ofek E. O., Gal-Yam A., De Cia, A., Roy, R., Kasliwal, M. M., Cao, Y., Nugent, P. E., Masci, F. J., 2017, The bumpy light curve of Type IIn supernova iPTF13z during 3 years, accepted for publication in Astronomy & Astrophysics (preprint arXiv:1703.09679v1) Contributions of the thesis author: I led the work on the paper and, from 2014 and onwards, organised the requests for new photometry and spectra of the SN. Sections Abstract, 1, 2, 3.1, 3.2.3, 3.5, 3.6.1 and 4 were written by me. Section 3.6.2 was written by Moriya, revised and expanded by me. The remaining sections were written in cooperation with Taddia. Figures 2, 5, 6 were drawn by me. The other figures were drawn by Taddia and revised by me. Figures 13 and 14 were based on input from Moriya. Conference appearances: During the progress of this work, I presented parts of it in Lund (poster at The Transient Universe for All, 2015 February), Uppsala (poster at Astronomdagarna, 2015 Oc- tober) and Athens (3 min talk at European Week of Astronomy and Space Science, 2016 July). After appearance of the preprint, I presented it in Lund (15 min talk at Big Questions in Astrophysics, 2017 April). Contents 1 Introduction 1 1.1 The origins of the field...................................1 1.2 Scope of the thesis.....................................3 1.3 Outline of the thesis.....................................3 1.4 Practical notes........................................4 2 Stars and supernovae5 2.1 Evolution and demise of a massive star...........................5 2.2 Mass loss mechanisms...................................7 2.2.1 Eruptive mass loss.................................8 2.3 Supernova classification and progenitor stars........................9 2.3.1 Supernovae Type I................................. 10 2.3.2 Supernovae Type II................................. 11 2.3.3 Interacting supernovae............................... 12 2.3.4 Recent classification developments........................ 13 3 Observing supernovae 15 3.1 Development of modern supernova searches........................ 15 3.2 The intermediate Palomar Transient Factory........................ 17 3.3 Supernova follow-up.................................... 18 3.3.1 Optical photometry................................. 18 3.3.2 Optical spectroscopy................................ 19 4 Interacting core collapse supernovae 23 4.1 Circumstellar interaction.................................. 23 4.1.1 Luminosity of the interaction............................ 27 4.2 Observational properties of Type IIn supernovae...................... 28 4.2.1 Outside visible wavelengths............................ 28 4.2.2 Optical light curves................................. 29 4.2.3 Optical spectra................................... 31 4.3 Type IIn fractions, hosts and environments......................... 32 4.4 Type IIn progenitor star candidate observations...................... 33 3 5 Supernova iPTF13z 36 5.1 Light curve with bumps................................... 36 5.1.1 Pre-discovery outburst............................... 38 5.2 Spectra........................................... 39 5.3 Comparison to other Type IIn supernovae......................... 39 5.3.1 Bumpy light curves................................. 39 5.3.2 Durable light curves................................ 41 5.3.3 Progenitor outbursts................................ 41 5.3.4 Dwarf host galaxies................................. 42 5.4 Analytical model...................................... 43 5.5 Discussion.......................................... 44 5.5.1 Progenitor scenarios and the SN nature of iPTF13z................ 44 5.5.2 Mechanisms generating bumpy light curves.................... 45 5.5.3 Old Type IIn supernovae as neutrino sources................... 46 6 Outlook 47 7 Populärvetenskaplig beskrivning 49 List of Figures 51 Bibliography 54 1 Introduction Skywatchers throughout human history have followed the regular motions of the Sun, the Moon and the planets against the backdrop of the seemingly fixed and steadily shining stars. Sometimes, comets appeared and moved across the sky, disturbing the orderliness. On rare occasions, bright and apparently "new" stars would show up in the sky. They could be visible in daylight and be seen at night for months before fading from visibility. The bright, rare and fleeting events of this kind are the topic of this thesis. 1.1 The origins of the field A bright "new" star appeared in the constellation Cassiopeia in 1572 November, with apparent magnitude ≈ −4 (comparable to Venus). This provoked the interest of many observers. At Herrevadskloster in Skåne, Tycho Brahe used a sextant of his own manufacturing to measure the position of the new star. From its lack of observable parallax during 6 months, Tycho concluded that the new star was located "among the other fixed stars" (Brahe 1573). Comparable "new", bright stars had been observed a few times before in history (Stephenson & Green 2002) but the careful and sustained observations by Tycho were pioneering in showing that changes could occur among the "fixed" stars. In Latin, the academic language in Europe at the time, ”nova stella” means ”new star”. Eventually, the word nova alone became used to designate the phenomenon of a new star suddenly appearing in the sky. Another bright, new star was seen in the constellation Ophiuchus in 1604 by Johannes Kepler and others. During the next three centuries ∼ 10 novae were seen (Pickering 1895) but none of them as bright as the ones in 1572 and 1604. Use of telescopes during the decades around 1900 brought discoveries of novae in some nebulae, e.g. one in 1885 in what was then called the Andromeda nebula. By the 1930s, it was known that some "nebulae" actually were galaxies, located at distances ∼ 105 pc and more. To be seen at such distances, the new stars seen appearing in other galaxies had to very luminous. This insight led to the distinction between a nova and a supernova1 as it became clear that supernovae are intrinsically ∼ 10 magnitudes more luminous than novae. The new stars of 1572 and 1604 were indeed supernovae. The mechanism which gave rise to such a luminous event as a supernova (SN) was unknown at the time, but pioneering work by Baade & Zwicky(1934) suggested that supernovae were stars blowing up, shedding most of their mass and leaving bodies of much smaller mass behind. 1Fritz Zwicky and Walter Baade ostensibly used the word supernova for the first time, in lectures given 1931 (Zwicky 1940). Knut Lundmark appears to have been the first to use the word in print (Lundmark 1932). 1 2 Figure 1.1: Galaxy M 101 before and during the thermonuclear SN 2011fe. The yellow arrow points to the SN position. The field of view in each image is ≈ 140 × 140, with north up and east to the left. Image courtesy of PTF. Understanding of the internal mechanisms of stars, which started developing during the 20th century, helped understanding SNe. A star is a body in hydrostatic equilibrium maintained by the