The direct identification of core-collapse supernova progenitors
Dr Schuyler Van Dyk, Infrared Processing and Analysis Center, California Institute of Technology, USA
To place core-collapse supernovae in context with the evolution of massive stars, it is necessary to determine their stellar origins. I describe the direct identification of supernova progenitors in existing pre-explosion images, particularly those obtained through serendipitous imaging of nearby galaxies by the Hubble Space Telescope. I comment on specific cases representing the various core-collapse supernova types. Establishing the astrometric coincidence of a supernova with its putative progenitor is relatively straightforward. One merely needs a comparably high-resolution image of the supernova itself and its stellar environment to perform this matching. The interpretation of these results, though, is far more complicated and fraught with larger uncertainties, including assumptions of the distance and the extinction to the supernova, as well as the metallicity of the supernova environment. Furthermore, existing theoretical stellar evolutionary tracks exhibit significant variations one from the next. Nonetheless, it appears fairly certain that Type II-Plateau supernovae arise from massive stars in the red supergiant phase. Many of the known cases are associated with subluminous Type II-Plateau events. The progenitors of Type II-Linear supernovae are less established. Among the stripped-envelope supernovae, there are now a number of examples of cool, but not red, supergiants as Type IIb progenitors. We appear now finally to have an identified progenitor of a Type Ib supernova, but no known example yet for a Type Ic. The connection has been made between Type IIn supernovae and progenitor stars in a luminous blue variable phase, but that link is still thin, based on direct identifications. Finally, I also describe the need to revisit the supernova site, long after the supernova has faded, to confirm the progenitor identification through the star's disappearance and potentially to detect a putative binary companion that may have survived the explosion.
The explosions mechanisms of core-collapse supernovae
Dr Sean Couch, Michigan State University, USA
Core-collapse supernovae are the luminous explosions that herald the death of massive stars. Stellar collapse and the violent explosions that follow give birth to neutron stars and black holes, and in the process synthesises most of the elements heavier than helium throughout the universe. While core-collapse supernovae are observed on a daily basis in nature, the details of the mechanism that reverses stellar collapse and drives these explosions remains unclear. I will review the major models for the core-core collapse supernova explosion mechanism, with particular emphasis on the delayed neutrino heating mechanism and recent insights that have been gained from three dimensional simulations. I will discuss recent developments in our understanding of the impact of multidimensional progenitor structures as well as magnetohydrodynamic effects.
Incidence of stellar rotation on the explosion mechanism of massive stars
Contributory talk: Mr Remi Kazeroni, PhD student, CEA Saclay, France
The impact of stellar rotation on the explosion of massive stars has been investigated in extreme cases so far, where the kinetic energy is large enough to contribute to power a bipolar explosion mediated by the growth magnetic fields. The explosion mechanism is likely to be sensitive to the profile of angular momentum in the stellar core even in more common situations where the centrifugal force is minor. In particular, differential rotation can affect the development of one-armed instabilities such as the Standing Accretion Shock Instability (SASI), and the corotation instability known as the low T/W instability. These non-axisymmetric instabilities are able to redistribute angular momentum radially.
Numerical simulations of a simplified model are performed to demonstrate that rotation affects both the degree of asymmetry and the mean radius of the unstable shock. The interplay of SASI and the low T/W instability is discussed. Surprisingly, both instabilities can be illustrated with a simple experiment based on a shallow water analogy. Results are analysed in view of the constraints on the angular momentum budget set by stellar evolution on the one hand and by the spin properties of pulsars on the other hand.
The supernova-progenitor connection
Dr Luc Dessart, Observatoire de la Cote d’Azur, France
In this presentation, I will focus on progenitor and explosion properties inferred from the analysis of SN light curves and spectra.The evolution of isolated massive stars has a profound impact on both the deep interior and the envelope, changing the density/temperature structure, the composition, as well as the total mass of the object. Binarity complicates this further, producing a wide range of progenitor characteristics at the onset of core collapse. Finally, the explosion itself may have different properties in different progenitors, reflecting different core structures (eg density, rotation). This expected diversity in progenitor and explosion properties is the main origin for the observed diversity of core-collapse supernovae. I will review the fate of massive stars from the low to the high mass end and how these explosions lead to the radiation properties of SNe II-P/II-pec, SNe IIb/Ib/Ic, and SNe IIn.
Understanding supernova ejecta from observations and modelling at late epochs
Professor Claes Fransson, Stockholm University, Sweden
I will discuss how observations and modelling of supernovae in the nebular stage give information about the structure and composition of the ejecta, including both core collapse and thermonuclear supernovae. At this stage the ejecta is transparent in the continuum, allowing observations of the processed material in the core, including different radioactive isotopes created in the explosion. During this phase the ejecta undergoes a thermal instability, the IR-catastrophe, evolving from a plasma dominated by thermal processes emitting in the optical, to one dominated by non-thermal emission, mainly in the far-IR. In addition to this, the connection of hydrodynamic instabilities arising in the explosion and the observed 3D morphology will be discussed, as well as the formation of molecules and dust. This will be illustrated by observations of SN 1987A, as observed with HST, VLT and ALMA, as well as the nearby Type Ia SN2011fe and other well observed supernovae.
Dead or alive? History of SN2015bh
Contributory talk: Nancy Elias-Rosa, Postdoctoral researcher, INAF - Osservatorio Astronomico di Padova, Italy
SN2015 was discovered in NGC 2770 on 2015 February 07.39 UT with an absolute magnitude of Mr ~ 13 mag, and classified as a SN impostor. Three months later, the object had a sudden bright increase of about 3 mag. Later, the transient seemed to recede to the pre-burst state.
Here I present the photometric and spectroscopic evolution of SN2015bh from an early- (from 16 yrs before its discovery) to late-time (1 year after) phase. Based on the detailed data analysis, I propose this transient was produced by a massive star, which had experienced several big outbursts from 2002 to late 2014. These outbursts were followed by a possible terminal core-collapse SN explosion and an interaction from the ejecta with the massive shells formed through the repeated mass loss events. Therefore, SN2015bh is a plausible example of connection between massive stars, SN impostors and interacting SNe. An interesting research field, which provides new clues to understand the last stages in the evolution of a star and its environment.
Observational constraints on failed supernovae
Contributory talk: Dr Morgan Fraser, Postdoctoral Researcher, Institute of Astronomy, Cambridge, UK
The apparent lack of high mass (>16 Msun) progenitors for nearby core-collapse supernovae has been proposed as a result of stars above this threshold preferentially collapsing to form a black hole without a bright optical display. I will review the observational evidence for the absence of high mass supernova progenitors, and present the results of an ongoing project to identify candidates for failed supernovae in deep imaging of nearby galaxies, and through searches for ultra-faint transients.