How to blow up a massive star Müller , B., & Smartt, S. (2017). How to blow up a massive star. Astronomy & Geophysics, 58(2), 2.32-2.37. https://doi.org/10.1093/astrogeo/atx061 Published in: Astronomy & Geophysics Document Version: Publisher's PDF, also known as Version of record Queen's University Belfast - Research Portal: Link to publication record in Queen's University Belfast Research Portal General rights Copyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made to ensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in the Research Portal that you believe breaches copyright or violates any law, please contact [email protected]. Download date:29. Sep. 2021 SUPERNOVAE How to blow up a massive star Bernhard Müller and Stephen Smartt explain how simulations combined with astronomical observations are shaping a more coherent picture of the explosions of massive stars. 1 Supernova remnant SN 1987a in false colour. ALMA data (red) show newly formed dust; Hubble (green) and Chandra (blue) data show where the expanding shock wave meets an outer ring of dust and gas. (ALMA [ESO/NAOJ/NRAO]/A Angelich. Visible: NASA/ESA Hubble. X-ray: NASA Chandra) upernovae have a peculiar place in an explosion energy of ~1051 erg. In view the history of astronomy. They are of this enormous energy, Baade & Zwicky Sthe only transient phenomena in (1934a) suggested that some supernovae are the sky outside the solar system with a associated with gravitational collapse of a documented history of observations by massive star to a neutron star – only two the naked eye for almost two millennia: years after the discovery of the neutron and prominent examples include the supernova more than 30 years before the first neutron of 1054 that made the Crab Nebula, and star was observed. those of 1572 and 1604 – now named after As stellar evolution and nuclear astro­ Tycho Brahe and Johannes Kepler – which physics matured, the details of Baade and shook the medieval belief in an immutable Zwicky’s far­sighted idea – what we now celestial sphere. Yet it was not until the call “core­collapse supernovae”– were 20th century that we began to grasp their worked out. They occur in stars that begin true nature and realized – starting with the their lives with a mass of more than 8 M ⊙ work of Baade & Zwicky (1934b) – that they (solar masses). In this mass range, stars can are violent stellar explosions. ignite advanced burning stages (carbon, The energy budget of supernovae is neon, oxygen and silicon burning) after impressive indeed: they can outshine their processing hydrogen and then helium in host galaxy for weeks and the ejected mat­ their cores. Eventually, these stars form an erial reaches several 1000 km s–1, implying onion shell structure with a core composed 2.32 A&G • April 2017 • Vol. 58 • aandg.org SUPERNOVAE 2 Sketch of the neutrino-driven supernova explosion mechanism. After collapse, the iron core has formed a proto-neutron star (orange). The proto-neutron star cools by the emission of neutrinos that diffuse out on a timescale of about 10 seconds from the proto-neutron star interior. Infalling matter that settles on the proto-neutron star surface as an “atmosphere” (grey) also readily emits neutrinos. Outside the “gain radius” (dotted circle), neutrino heating through reabsorption dominates over cooling and drives convective overturn. The shock can also develop an oscillatory instability (“SASI”). Neutrino cooling drives convection inside the proto-neutron star as well. of iron group nuclei – the ashes of silicon core of the progenitor reaches then over­ (radius, neutrino energy and one direction burning – and various outer shells con­ shoots nuclear density, it becomes highly angle). Moreover, there are further techni­ sisting of lighter elements. The iron core incompressible and “rebounds” almost cal complications, for example one needs to is supported against gravity mainly by elastically, launching a shock wave into model the transport of neutrinos in curved the pressure of degenerate relativistic the surrounding shells. But this “bounce” spacetime because of the strong gravity of electrons, which implies an upper mass does not deliver enough energy to eject the the proto­neutron star. limit: the Chandrasekhar mass. As the envelope. Instead, the shock quickly “stalls” It therefore took several decades after core grows towards this limit by shell as it uses up about 1.7 × 1051 erg per 0.1 M as the original idea of Colgate & White (1966) ⊙ burning, it contracts. Eventually, a new it propagates through the infalling shells. and the birth of general relativistic kinetic process becomes possible, electron capture Hydro dynamic simulations show that the theory (Lindquist 1966) for spherically reactions on heavy nuclei and free protons, shock winds up hovering some 100–200 km symmetric (1D) simulations to become which reduces the electron above the newly formed sufficiently accurate to model the hydro­ degeneracy pressure. At “There are several “proto­neutron star”, kept dynamics and neutrino transport in the densities of 109–1010 g cm–3, direct detections of in place by the ram pressure core of a supernova. Modern 1D simulations the iron core eventually col­ the progenitors of low- of the infalling outer shells. solve the Boltzmann equation for neutrinos lapses on a free­fall time­ energy supernovae” For the star to explode, there and have now convincingly established that scale. The iron core is about must be some mechanism the “neutrino­driven mechanism” does not the size of the Earth (radius of 6000 km), for depositing energy to “revive” the shock work under the assumption of spherical and has a dynamical free­fall time of about front and power the flow of matter out of the symmetry (Liebendoerfer et al. 2001, Rampp 0.5 seconds. The outer envelope of the gravitational potential well of the proto­ & Janka 2000) for most massive stars. Only star, with a size of up to a thousand solar neutron star. the lightest supernova progenitors form radii (7 × 108 km) remains oblivious to its The most popular idea for shock revival an exception. For stars with initial mass impending doom as the core implodes. dates back – in a very crude form – to Col­ 8–10 M , the density declines so rapidly ⊙ The next step turns the collapse into an gate & White (1966), and is illustrated in its outside the iron core that the pre­shock ram explosion, and is not fully understood. The modern guise in figure 2. The gravitational pressure of the infalling shells plummets collapse of what is effectively an iron white binding energy released during collapse is very quickly and allows neutrino heating dwarf to form a neutron star (of order 10 km initially stored as thermal energy in the hot to drive a low­energy explosion of ~1050 erg radius) supplies a gravitational potential proto­neutron star, whose surface tempera­ (Kitaura et al. 2006). There are several direct energy reservoir of several 1053 erg. This ture is about 5 × 1010 K. The thermal energy detections of the progenitor stars of low­ appears sufficient to explode a star with is then radiated away over timescales energy supernovae and they are consistent ~10 51 erg of kinetic energy. We require just of seconds in the form of neutrinos. If a with being red supergiants in the 8–12 M ⊙ a small fraction of the liberated binding sufficient fraction of the neutrinos are reab­ mass range, such as those for SN 2005cs, energy of the neutron star to be transferred sorbed behind the shock, the concomitant SN 2009md and SN 2008bk (Fraser et al. 2011, to the remaining stellar envelope. But increase of thermal energy and pressure Maund et al. 2005, 2014). how? And does collapse always lead to a may be enough to drive the shock outward. successful explosion, or do some progeni­ To verify that this idea works in Nature, we Multidimensional models tors collapse to black holes instead? If so, need to model the emission, absorption, The solution for the supernova explo­ what type of explosion (if any) ensues when scattering and propagation of neutrinos in sion problem for more massive stars lies a black hole is formed? And what decides the core of the supernova using numeri­ in breaking spherical symmetry. As was whether massive stars make neutron stars cal simulations. Because the neutrinos fall pointed out by several authors in the 1990s or black holes? out of thermal equilibrium with the stellar (e.g. Herant et al. 1992, Burrows et al. 1995), matter in the proto­neutron star surface, multidimensional effects facilitate neu­ Simulating the mechanism this is a problem in kinetic theory. Even if trino­powered explosions in various ways. Tapping the gravitational energy directly we assume the collapse and explosion as Convective overturn, driven by strong after collapse might appear to be the sim­ perfectly spherical, we are still left with a neutrino heating close to the proto­neutron plest solution. Once the inner part of the iron three­dimensional problem in phase space star, transports hot material outwards, A&G • April 2017 • Vol. 58 • aandg.org 2.33 SUPERNOVAE 3 (Left) Convection during oxygen shell burning in a 3D progenitor model of an 18 M star. The snapshot shows plumes of upwelling silicon-rich ashes (red) in ⊙ the 2 o’clock and 9 o’clock directions and a weaker plume in the 5 o’clock direction.