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Bridging the gap: from massive stars to supernovae









Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ


Theo Murphy scientific meeting organised by Dr Justyn Maund, Professor Paul Crowther, Dr Hans-Thomas Janka and Professor Norbert Langer.

Large-scale mixing asymmetries in a supernova 9,000 seconds after the onset of the explosion as obtained in a 3D computer simulation of a 15 solar-mass star. Credit: Simulation: N. Hammer, H.-T. Janka and E. Muller (MPI for Astrophysics); Visualisation: M. Rampp (Rechenzentrum Garching)

The few seconds it takes to explode a massive star separate millions of years of stellar evolution from thousands of years of supernova evolution. This meeting discussed massive stars both before and after the explosion, from theoretical and observational perspectives, and addressed how the pre-collapse life affects the explosion mechanism and supernova display.

Speaker biographies, abstracts and the schedule of talks are available below. 

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This event has taken place. Recorded audio of the presentations will be added below shortly, and papers from the meeting will be published in a future issue of Philosophical Transactions A.

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Schedule of talks

01 June


Session 1

6 talks Show detail Hide detail


Professor Norbert Langer, Argelander-Institut für Astronomie, Germany

09:05-09:35 The lives of massive stars

Dr Selma de Mink, Anton Pannekoek Institute for Astronomy, The Netherlands


Recent large spectroscopic observing campaigns have been providing stringent constraints on the key properties of young massive stars. These include their rotation rates, how frequently they are found in close binary systems and distribution their separations and mass ratios. I will discuss the new observational constraints and the results of simulations investigating the implications for the frequency and diversity core collapse supernova types.

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09:35-10:05 Late stages of massive star evolution: the calm before the storm?

Dr Raphael Hirschi, Keele University, UK


Even though massive stars appear from their surface to evolve quietly after helium burning, their carbon-oxygen core is all but peaceful. High density and temperature leads to strong neutrino production, which leads to high energy losses. These losses cause the evolution of the core to accelerate through the advanced burning stages from time scales of hundreds of years for carbon burning, to days for silicon burning. The large spread in time scales and an increasing number of important nuclear reactions make modelling these late phases a very challenging task. Three-dimensional (3D) processes like convection, rotation and magnetic field make the task even more difficult. It is therefore not a surprise that 1D stellar evolution models still have important uncertainties that affect their predictive power, in particular the predicted SN progenitor structures (eg extent of the many convective zones, possible convective shell mergers during the early collapse phase). Traditional observing methods are of little help for the advanced phases since the outer layers hide all the action. Fortunately, targeted 3D hydrodynamics simulations are able to provide independent guidance on these poorly constrained phases and several efforts are under way to improve 1D models with the help of 3D hydrodynamics simulations. In this talk, after an overview of the evolution and importance of the late stages of massive stars evolution, I will discuss the major uncertainties in their modelling and review ongoing efforts to overcome them.

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10:05-10:20 A 3D hydrodynamical perspective on shear instabilities in massive stars


Contributory talk: Dr Philipp Edelmann, Postdoctoral researcher, Heidelberg Institute for Theoretical Studies, Germany

Most massive stars exhibit fast rotation, which significantly influences their evolution. Many observed phenomena, like the production of nitrogen in massive stars early in the Universe, have been successfully explained by stellar evolution models which include the effect of rotation. Even though the problem is at least two-dimensional, several one-dimensional codes manage to factor in rotation using the approximation of shellular rotation. This comes at the price of using simplified prescriptions for rotational mixing processes in the star.

These involve parameters which cannot easily be constrained by observations because of their degeneracy with respect to other parameters, eg those governing convection. Modelling rotation in full 3D hydrodynamics poses a promising approach to address these deficiencies of 1D models. By comparing the 3D results to the predictions of the 1D model, their prescriptions for treating mixing can be improved. We study shear instabilities using 3D hydrodynamics in collaboration with the group of Raphael Hirschi. I will discuss our simulations of dynamical shear instabilities in massive stars and highlight challenges of mapping the 1D stellar models to 3D. To benchmark the treatment of dynamical shear in the stellar evolution code we simulate a region devoid of other instabilities, including convection. We simulate this at an evolutionary phase with time scales short enough to allow the comparison of time evolution in both codes.

10:45-11:15 Binary evolution and the final fate of massive stars

Professor Philipp Podsiadlowski, University of Oxford, UK


Binary interactions do not only affect the envelope structure of massive supernova progenitors, thereby determining the appearance of the resulting supernova, but also the final fate of the core, specifically whether the core collapses to a neutron star or black hole, or produces a gamma-ray burst or other exotic event. In this talk I will summarise how various binary interactions (mass loss, accretion, mergers, tidal interactions) affect the final fate of stars and its potential implications for a variety of ‘normal’ and exotic supernova events, including supernovae with a circumstellar medium (‘LBV supernovae’), superluminous supernovae, gamma-ray burst sources, pair-instability supernovae and aLIGO gravitational-waves sources.

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11:15-11:45 SN progenitor predictions

Professor José Groh, Trinity College Dublin, Ireland


Stars more massive than about 8 solar masses end their lives as a supernova (SN), an event of fundamental importance universe-wide. The physical properties of massive stars before the SN event is very uncertain, both from theoretical and observational perspectives. In this talk, I will review recent efforts to couple stellar evolution and atmosphere modelling of stars in the pre-SN stage. These models are able to predict the high-resolution spectrum and broadband photometry, which can then be directly compared to the observations of core-collapse SN progenitors. I will discuss the surprising predictions of spectral types of massive stars before death. Depending on the initial mass and rotation, single star models indicate that massive stars die as red supergiants, yellow hypergiants, luminous blue variables, and Wolf-Rayet stars of the WN and WO subtypes. The presence of a close companion profoundly affects the fate of massive stars, and I will review the latest predictions of SN progenitors based on binary star evolution. I will finish by assessing the detectability of the different types of SN progenitors.

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11:45-12:00 The lowest mass supernova progenitors


Contributory talk: Dr Carolyn Doherty, Postdoctoral Research Fellow, Konkoly Observatory, Budapest, Hungary

We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero-age main sequence until near the end of the thermally pulsing phase. These super-asymptotic giant branch (super-AGB) and massive star models are in the mass range between 6.5 and 10.0 M⊙. We probe the mass limits M_up, M_n and M_mass, the minimum masses for the onset of carbon burning, the formation of a neutron star and the iron core-collapse supernovae, respectively, to constrain the white dwarf/electron-capture supernova (EC-SN) boundary. We predict EC-SN rates for lower metallicities which are significantly lower than existing values from parametric studies in the literature. We conclude that the EC-SN channel (for single stars and with the critical assumption being the choice of mass-loss rate) is very narrow in initial mass, at most ≈0.2 M⊙, which implies EC-SNe are expected to contribute only a small fraction of all gravitational collapse supernova. We examine heavy element nucleosynthesis within the pre-SN phase of the most massive super-AGB stars and discuss the potential observable signatures that may help constrain this crucial evolutionary phase. Lastly we provide a mass limit for the lowest mass supernova over a broad range of metallicities from the earliest time (Z=0) right through until today (Z~0.04).




Session 2

7 talks Show detail Hide detail


Professor Paul Crowther, University of Sheffield, UK

13:30-14:00 The evolution of massive stars: bridging the gap in the Local Group

Dr Philip Massey, Lowell Observatory, USA


Observations of massive stars in the galaxies of the Local Group provide the means for furthering our understanding of massive star evolution, and for testing (and improving) the current generation of evolutionary models. Getting the evolutionary models RIGHT, and knowing their limitations, is important not only for understanding the evolution of massive stars per se, but also for a host of other problems of astrophysical interest, such as the interpretation of the integrated spectral energy distributions of distant galaxies. Additionally, most popular theories of the origins of GRBs invoke rapidly spinning WC-type Wolf-Rayet stars as the progenitors due to the association of nearby GRBs with broad-lined Type Ic supernovae. However, observations now show that long-duration GRBs occur preferentially (though not exclusively) in low-metallicity host environments, where stellar evolutionary theory says that WCs are rare.

The star-forming galaxies of the Local Group span a range of a factor of 10 in metallicity, allowing us to test massive star evolution models as a function of metallicity. Recent studies have identified relatively complete samples of yellow and red supergiants and Wolf-Rayet stars throughout the Local Group. This work has provided some exacting tests of current evolutionary models. I will also discuss the significant progress we have made in finding luminous blue variable candidates, and work on the UNEVOLVED massive star populations. What have these studies taught us, and what might we expect to learn in the near future?

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14:00-14:30 Very massive stars and LBVs

Dr Nathan Smith, University of Arizona, USA


Luminous Blue Variables (LBVs) had long been considered massive stars in transition to the Wolf-Rayet (WR) phase, so their identification as possible progenitors of some peculiar supernovae was surprising.  The link between LBVs and Type IIn supernovae comes from two lines of evidence: (i) the properties of the CSM around SNe IIn, requiring very large masses (20 Msun or more in some cases), in H-rich shells moving at 100s of km/s, and (ii) direct detection of luminous progenitors and precursor outbursts that are consistent with LBVs. More recently, the environment statistics of LBVs show that most of them cannot be in transition to the WR phase after all. The high mass H shells around luminous SNe IIn require that some very massive stars above 40 Msun die without shedding their H envelopes, and the precursor outbursts are a challenge for understanding the final burning sequences leading to core collapse.  Besides LBVs, massive RSGs are also good candidates for progenitors of SNe IIn. In fact, recent evidence suggests a clear continuum in pre-SN mass loss from super-luminous SNe IIn, to regular SNe IIn, to SNe II-L and II-P, whereas most stripped-envelope SNe (excluding perhaps broad-lined Type Ic) seem to arise from a separate channel from lower-mass stars rather than massive WR stars.

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14:30-14:45 Constraining binary effects on Wolf-Rayet stars in the Small Magellanic Cloud


Contributory talk: Mr Tomer Shenar, PhD student, University of Potsdam, Germany

Wolf-Rayet (WR) stars are evolved stars characterised by powerful, radiation-driven stellar winds. Massive stars reach the Wolf-Rayet phase after having shed enough material to approach the Eddington limit, either via stellar winds or via mass-transfer in binary systems. About 40% of the known Wolf-Rayet stars are found in short period binary systems, raising the question as to the impact of binarity on the WR population. Using the PoWR code, we perform a non-LTE spectral analysis of the five known multiple WR systems in the SMC with the goal of testing mass-luminosity relations against orbital masses and constraining evolutionary channels for each system using the BPASS and BONNSAI tools.

We find that, in contrast to the single WR stars in the SMC, the WR components in the systems analysed are generally not compatible with quasi homogeneous evolution, and show that current models imply that mass-transfer has occurred in at least 4 systems before the primary entered the WR phase. Using binary evolution tracks, we derive initial conditions and ages for each system. Surprisingly, the implied initial masses ($\gtrsim 60\,M_\odot$) are large enough for the primaries to have entered the WR phase as single stars. Our results suggest that, contrary to predictions, mass transfer in binaries is not directly responsible for the existing number of WR stars in the SMC.

15:15-15:45 Mass loss and stellar winds

Dr Jorick Vink, Armagh Observatory, Ireland


Mass loss bridges the gap between massive stars and supernovae (SNe) in two major ways: (i) theoretically it is the amount of mass lost that determines the mass of the star prior to explosion, and (ii) observations of the circumstellar material around SNe may teach us the type of progenitor that produced the SN event.

Here, I present the latest models and observations of mass loss from massive stars, both for canonical massive O stars, as well as very massive stars (VMS) that show Wolf-Rayet type features.

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15:45-16:15 Red supergiants as supernova progenitors

Dr Ben Davies, Liverpool John Moores University, UK


It is now well-established from pre-explosion imaging that red supergiants (RSGs) are the direct progenitors of Type-IIP supernovae. These images have been used to infer the physical properties of the exploding stars, yielding some surprising results. In particular, the differences between the observed and predicted mass spectrum has provided a challenge to our view of stellar evolutionary theory. However, turning what is typically a small number of pre-explosion photometric points into the physical quantities of stellar luminosity and mass requires a number of assumptions about the spectral appearance of RSGs, as well as their evolution in the last few years of life. Here I will review what we know about RSGs, with a few recent updates on how they look and how their appearance changes as they approach SN.

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16:15-16:30 Are supernova progenitors more massive than we think? The evolution of red supergiants


Contributory talk: Ms Emma Beasor, PhD student, Astrophysics Research Institute, Liverpool John Moores University, UK

The mass loss rates of red supergiants (RSGs) govern their evolution towards supernova and dictate the appearance of the resulting explosion. To study how mass-loss rates change with evolution we measure the mass-loss rates (M ̇) and extinctions of 19 red supergiants in the young massive cluster NGC2100 in the Large Magellanic Cloud. We find that the M ̇ in red supergiants increases as the star evolves, and is well described by M ̇ prescription of de Jager, used widely in stellar evolution calculations. We find the extinction caused by the warm dust is negligible, meaning the warm circumstellar material of the inner wind cannot explain the higher levels of extinction found in the RSGs compared to other cluster stars. We argue there is little justification for substantially increasing the M ̇ during the RSG phase, as has been argued recently in order to explain the absence of high mass Type IIP supernova progenitors. We also argue that enhanced extinction we observe for the two most evolved stars in the cluster may provide a solution to the red supergiant problem.

16:30-17:30 Panel discussion 1

02 June


Session 3

7 talks Show detail Hide detail


Dr Hans-Thomas Janka, Max Planck Institute for Astrophysics, Germany

09:00-09:30 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.

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09:30-10:00 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.

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10:00-10:15 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.

10:45-11:15 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.

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11:15-11:45 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. 

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11:45-12:00 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.

12:00-12:15 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.




Session 4

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Dr Justyn Maund, University of Sheffield, UK

13:45-14:15 Three-dimensional geometry of core-collapse supernovae: observations and modelling

Dr Masaomi Tanaka, National Astronomical Observatory, Japan


Multi-dimensional geometry is a key to understanding the explosion mechanism of core-collapse supernovae (SNe). Spectropolarimetry is one of the most powerful methods used to study the multi-dimensional geometry of extragalactic SNe. We have performed spectropolarimetric observations of Type Ib/c SNe. Spectropolarimetric data commonly shows a ‘loop’ in the Stokes Q-U plane. Similar signals are also observed in Type Ia SNe. We show simple 3D radiative transfer simulations and demonstrate that the loop implies 3D, non-axisymmetric or clumpy element distribution in the SN ejecta. We discuss the size and number of the clumps, and possible connection to the explosion mechanism.

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14:15-14:45 Superluminous supernovae

Professor Avishay Gal-Yam, Weizmann Institute of Science, Israel


Only about a decade ago, wide-field sky surveys revealed the existence of a new class of stellar explosions. These events, about 10 times more luminous than bright regular supernovae, are commonly referred to as superluminous supernovae (SLSNe). I will present a brief historical review of this class of events, and their sub-classification to two main spectroscopic sub-classes: hydrogen-rich SLSN-II and hydrogen-poor SLSN-I. I will review their physical properties and the proposed progenitors and explosion mechanisms, and focus on new insights arising from recent observations of specific events and accumulated samples, as well as their environments and rates.

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14:45-15:15 Statistical studies of CCSN observations

Dr Maryam Modjaz, New York University, USA


I will review the observational status of Core-collapse SNe (CCSNe), the most common type of explosions. I will focus on Stripped SNe (ie SNe of types IIb, Ib, Ic and broad-lined Ic) and their connection with GRBs, and demonstrate how statistical analysis of the largest datasets of photometry and spectroscopy constrain their progenitor and explosion models. Finally, I will present recent work on whether the observed spectral diversity of SNe IIb can be explained solely by explosion asphericity, as observed in Cas A via its light echoes.

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15:15-15:30 The progenitor masses of ~200 core-collapse SNe


Contributory talk: Dr Jeremiah Murphy, Assistant Professor, Florida State University, USA

By age-dating the stellar populations in the vicinity of supernova remnants (SNRs), we derive the progenitor masses for more than 200 core-collapse SNe. With this large statistical sample, we are able to characterise the distribution of progenitor masses. Using Bayesian statistical inference, we find that the minimum mass of SNR progenitors is 7.2 +/- 0.3 solar masses, the maximum mass is 33 +17/-6 solar masses, and the power law slope in between is 2.8 +/- 0.5, consistent with the Saltpeter IMF. The accuracy of the minimum mass may provide interesting constraints on stellar evolution. With regard to the maximum mass, either the most massive of massive stars are not exploding, or there is severe bias against forming SNRs by the explosions of the most massive stars.

16:00-17:00 Panel discussion 2

Bridging the gap: from massive stars to supernovae Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ