Tuesday, 27 March 2012

Core Collapse Supernovae


Core collapse supernovae are the spectacular explosions that mark the violent deaths of massive stars. These events are the most energetic explosions in the cosmos, releasing energy of order 10^{53} erg at the staggering rate of 10^{45-46} watts. Core collapse supernovae produce and disseminate most of the nuclei found in the Universe. Elements heavier than helium, through the iron group, are synthesized during the course of stellar evolution, and via supernovae, are disseminated into the interstellar medium to be reprocessed later in new stars, solar systems, and other astrophysical systems. Supernovae play a key role in the synthesis of heavy elements. In the neutron-rich ``wind'' that emanates from the hot remnant protoneutron star left behind after the star explodes, trans-iron elements are synthesized by a rapid neutron capture process. Additional nucleosynthesis may occur via the $\nu$-process, during which neutrinos emanating from the cooling protoneutron star cause spallations of light particles, particularly neutrons and protons, from abundant heavier nuclei, producing certain rare isotopes. Supernovae also signal the birth of neutron stars and black holes. These important and enigmatic astrophysical objects form from the cooling postsupernova remnant and are the basic building blocks of other astrophysical systems such as pulsars and x-ray binaries.
Core collapse supernovae occur when the iron core of a massive star collapses due to the force of gravity. Once the density in the core exceeds that of nuclear matter, the core rebounds generating pressure waves that propagate outward. At the sonic point, the point at which the velocity of the infalling material exceeds the velocity of sound, the pressure waves become a shock wave that propagates toward the surface of the iron core. A shock generated by this process, collapse and bounce, lacks the energy needed to overcome dissipation due to iron dissociation and neutrino losses and will stall before reaching this surface. What is needed for the star to explode is a shock reheating/reenergizing mechanism.
Contemporary approaches to the reheating mechanism are based on the idea that neutrinos produced in the core transfer gravitational energy released by core collapse to the cooler outer regions of the star. During the reheating process, core electron neutrinos and antineutrinos radiate from their respective neutrinospheres (the sphere within the core of the star defined by the radius above which the stellar material becomes optically thin to neutrinos), and a fraction of these neutrinos are absorbed by the material immediately behind the shock, thereby adding energy to the shock. This process is believed to be enhanced by convection, which may increase the neutrino luminosities and/or the neutrino heating efficiencies, both of which would enhance shock reheating. If the reheating is successful, the shock gains enough energy to reach the surface of the star, and as a result, the star explodes.
My research into the role of convection in the core collapse supernova process resulted in the development of the computer code EVH-1, a new hydrodynamics code for multidimensional flows that couples Piecewise Parabolic Method hydrodynamics to general (not just polytropic/constant $\gamma$) equations of state, and to radiation (in this case, neutrino) transport. Currently, when the matter in our simulations is in nuclear statistical equilibrium we describe its thermodynamic state using the Lattimer-Swesty equation of state, a well-known nuclear equation of state. The code makes use of a nuclear equation of state, and it has a module that calculates local neutrino heating, cooling, and deleptonization, using, at present, neutrino data from other simulations that implement multigroup flux-limited diffusion (MGFLD) and one-dimensional Lagrangian hydrodynamics. MGFLD simulates with sufficient realism the transport of neutrinos in opaque, semitransparent (neutrinosphere), and transparent regions, obviating the need to patch together transport schemes for optically thick and thin regions or the need to oversimplify the neutrino transport and matter coupling above the neutrinospheres. This research was the first implementation of multigroup transport in the context of multidimensional supernova simulations.
My research focused on two modes of convection, proto-neutron star convection and neutrino-driven convection. Proto-neutron star convection is a type of convection that may occur deep in the core of the star immediately after the formation of the shock wave. This type of convection occurs deep enough in the star that the material is optically thick to neutrinos. Convection in this region may enhance the neutrino luminosity by acting as a dredge of hot, neutrino-rich material, which upon rising and cooling will emit the neutrinos thereby boosting the luminosity. By boosting the luminosity of neutrinos emitted from the core, proto-neutron star convection could have the effect adding additional energy (deposited by the neutrinos) to the shock and thereby increase the likelihood of obtaining an explosion. This type of convection is also thought to act as a seed for other types of convection occuring later and farther out of the star. Neutrino-driven convection occurs farther out in the core of the star. It is large-scale convection that occurs just below the shock. Neutrino-driven convection is thought to boost the neutrino heating efficiency (in the material just below the shock). The effect of this would be to increase the neutrino heating of the shock for a given neutrino luminosity and, again, increase the likelihood of an explosion. Also, the large scale nature of this type of convection is thought to affect the dynamics of the shock front leading an aspherical shock. In the case of proto-neutron star convection, the results of my research were rather dramatic: we discovered that proto-neutron star convection is essentially wiped out by neutrino transport. The full details of our work may be found in the proto-neutron star convection paper that is in the Astrophysical Journal (vol. 493 p. 848, 1998).  In the case of neutrino-driven convection, vigorous ---even supersonic--- convection is evident in our simulations, but despite this, we do not obtain explosions. The full details of our work are in the neutrino-driven convection paper that is also in the Astrophysical Journal (vol. 495 p. 911, 1998).
Our implementation of realistic one-dimensional multigroup neutrino transport renders these results particularly troublesome for supernova modelers, and indicates that the final verdict on core collapse supernovae must await simulations that implement multidimensional multigroup neutrino transport and hydrodynamics. These simulations must (1) implement detailed neutrino transport that accurately computes the neutrino luminosities, spectra, and angular distributions emerging from the semi-transparent region in which the radiating neutrinospheres are embedded, (2) implement a numerically nondiffusive, high-order hydrodynamics method able to resolve and evolve turbulent convection, strong shocks, and rotation, and (3) implement (1)--(2) self-consistently to compute the feedback between transport, convection, and rotation.

Key Ideas

End of the Life of a Massive Star
Burn H through Si in successive cores
Finally build a massive Iron core.
Iron core collapse & core bounce
Supernova Explosion:
Explosive envelope ejection
Nucleosynthesis
Creation of elements heavier than Hydrogen & Helium in stars.

Last Days of a Massive Star

Star burns through a succession of nuclear fusion fuels:
  1. Hydrogen burning: 10 Myr
  2. Helium burning: 1 Myr
  3. Carbon burning: 1000 years
  4. Neon burning: ~10 years
  5. Oxygen burning: ~1 year
  6. Silicon burning: ~1 day
Finally builds up an inert Iron core in the center.

Iron Core Collapse

Iron core grows until its mas is about 1.2-1.4 Msun
Collapses & begins to heat up
Core temperature reaches T>10 Billion K & density ~1010 g/cc
At these temperatures, two important energy consuming processes kick in:
Photodisintegration:
High-energy photons hit heavy the nuclei, which disintegrate into He, protons & neutrons
This effectively "reverses" the previous fusion, draining energy (in the form of high-energy photons) out of the system.
Neutronization:
Free protons & electrons fuse into neutrons & neutrinos.
A neutron has more mass than a proton+electron, so this takes energy.
The neutrinos escape, carrying even more energy away from the star.
Both processes rob the core of energy, hastening its collapse.

Catastrophic Collapse

At the start of Iron Core collapse, the core properties are:
  • Radius ~ 6000 km (~Rearth)
  • Density ~ 108 g/cc
A second later, the properties are:
  • Radius ~50 km
  • Density ~1014 g/cc
  • Collapse Speed ~0.25 c !

Core Bounce

Core collapses until its density hits ~2.4x1014 g/cc, which is about density of an atomic nucleus! At this point, the strong nuclear force comes into play!
Inner ~0.7Msun of the core:
  • comes to a screeching halt
  • overshoots & springs back a little ("bounces")
Infalling gas hits the bouncing core head-on!

Post-Bounce Shockwave

Core bounce creates a supersonic shockwave that blasts out into the star:
Kinetic Energy is ~1051 ergs!
About 24-40 milliseconds later, the amount of matter swept up by the shockwave equals the amount of matter in the shock itself.
Traffic jam between in falling & outflowing gas.
Shock Stalls out
Meanwhile, neutrinos are pouring out of the host central core:
  • neutrinos get trapped by the dense surrounding gas.
  • this leads to rapid heating of the gas.
  • this in turn leads to violent convection above the core.

New, Improved Shockwave

The violent convection from trapped neutrinos breaks the traffic jam. The shockwave is regenerated after ~300 milliseconds.
A blastwave smashes outwards through the star:
  • Explosive nuclear fusion in wake of shock produces more heavy elements.
  • Shock violently heats and accelerates the stellar envelope.
In a few hours, the shock breaks out of the surface moving at a speed of about 1/10th the speed of light.
Seen from a distance, the star explodes...

Supernova!

At shock breakout:
  • Star brightens to ~10 Billion Lsun in minutes.
  • Can outshine an entire galaxy of stars!
Outer envelope is blasted off:
  • accelerated to a few x 10,000 km/sec
  • gas expands & cools off
Only the core remains behind.

Echoes

After its initial brilliance, the Supernova fades out after a few months. The fade-out is slower than it could be because of extra energy from gamma rays released by the decay of radioactive elements (primarily Nickel and Cobalt) created in the final wave of explosive nuclear burning before breakout.
How fast a supernova fades depends on how much Nickel was created by the explosion.
  • More nickel created = slower fade out

Historical Supernovae

1054 AD:"Guest Star" in Taurus
  • Observed by Chinese astronomers (late Song dynasty)
  • Visible in daylight for 23 days
  • Visible at night for ~6 months
  • Left behind the Crab Nebula
1572: Tycho Brahe's Supernova
1604: Johannes Kepler's Supernova
  • Important supernovae that were influential at the beginnings of modern astronomy.
6000-8000BC: Vela supernova
  • Observed by the Sumerians; appears in legends about the god Ea.

Supernova 1987a

Nearest naked-eye visible supernova seen since 1604. Explosion occured on February 23, 1987:
  • 15 Msun Blue Supergiant Star named SK-69o202 exploded in the Large Magellanic Cloud (a satellite galaxy of our Milky Way located some 50,000 pc away).
  • Particle experiments on Earth recorded a pulse of neutrinos arriving just before the burst of light from shock breakout.
  • Astronomers have continued to follow its development over the last 15 years.
SN1987a has provided us with a great wealth of information about supernova physics, and help to largely experimentally confirm the basic predictions of the core-bounce picture (although with good data, many details still remain murky).

Nucleosynthesis

Start with Hydrogen & Helium:
  • Fuse Hydrogen into the light elements up to Iron/Nickel
  • These accumulate in the core layers of stars.
Supernova Explosion:
  • "explosive" nuclear fusion builds more light elements up to Iron & Nickel.
  • fast & slow neutron reactions build Iron & Nickel into heavy elements up to 254Cf
Supernova explosions are responsible for creating nearly all of the heavy elements seen in nature, with a few important exceptions. The universe starts out with only Hydrogen (75%), Helium (~25%), and a smattering of light metals like Lithium, Boron, and Beryllium. Most other elements are forged by nuclear reactions occurring inside of stars or in the final moments of supernova explosions.

Top Ten Most Abundant Elements

10) Sulphur
9) Magnesium
8) Iron
7) Silicon
6) Nitrogen
5) Neon
4) Carbon
3) Oxygen
2) Helium
1) Hydrogen

Supernova Remnants

What happens to the envelope?
  • Fusion-enriched with metals in the explosion
  • Expands at a few x10,000 km/sec
Supernova Blast Wave:
  • Plows up the surrounding interstellar gas
  • Heats & stirs up the interstellar medium
  • Hot enough to shine as ionized nebulae up to a few thousand years after the explosion

Stardust

Metal-enriched supernova ejecta mixes with interstellar gas.
  • Next generation of stars includes these metals.
  • Successive generations are more metal rich.
Sun & planets (& us):
  • Contain many metals (iron, silicon, etc.)
  • Only ~5 Gyr old
The solar system formed from gas enriched by a previous generation of massive stars.

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