Outburst Mechanism

The Set-up

The interior structure of SK -69 202 just before the outburst is essentially the same as that of a red supergiant. The core of the star is around the mass of the Sun and the size of the Earth and is composed of iron. The core is surrounded by the shell sources ignited after each phase of core burning ran to completion. The outer envelope of the star is 90 % hydrogen and roughly 10 % helium (as is the rest of the Universe). Because iron is the last element which can be profitably used as fuel for the fusion process, something catastrophic must happen next.

Instability

Let's look at the core of iron. It is roughly like an iron white dwarf. It is made out of hot iron nuclei and degenerate electrons. It is supported by degenerate electron pressure and the hot iron nuclei.

Due to energy losses from the star and the fact that the iron core is inert, the iron core cools and so slowly contracts increasing its temperature and density (as any star would after nuclear reactions in its core cease).
When its temperature reaches around 5,000,000,000 Kelvins, the core becomes so hot that the radiation in the core is energetic enough to start destroying the iron nuclei in a process known as photo-disintegration. It takes a lot of energy to break apart an iron nucleus and this process steals a lot of energy from the core.
At around the same time another interesting thing happens. The electrons (-) can combine with the protons (+) to make neutrons (o)!. This electron capture process also does nasty things to the core. Since degenerate electrons are a large part of the core pressure support, by stealing electrons from the core the pressure in the core decreases. In addition note that the electron capture causes the protons in the star to be converted into neutrons.

Both of the above processes cause the pressure in the core to decrease further. Also note that as the core contracts in response, the above problems are only exacerbated and the collapse accelerates ===> instability.

Core Collapse

Time Scale

The collapse once initiated is very quick. The time scale can be estimated using the expression we deduced for the free-fall time;

free-fall time ~ 1 / sqrt(G x density)

where G = 6.7 x 10**(-8) is the gravitational constant in c.g.s. units. For the iron core, the density is roughly 10**9 grams per c.c. The free-fall time is thus roughly

free-fall time ~ 1 / sqrt( 67 ) seconds ~ tenth of a second!

The collapse will take less than 1 second.

Some Details

The collapse is inside-out in that the denser regions collapse more quickly than the less dense regions. For the star this means that the core collapses in less than a second but that the outer burning shells and the envelope of the star (being less dense) not realizing that the core is gone are suspended above the collapsing core. They do not take part in things until the explosion begins in earnest.

So, what happens?

The inner core collapses unimpeded until another threshold is reached (a new way to generate pressure -- MS stars; normal gas and radiation pressure -- RG's and AGB's and SG's can use degenerate electron pressure).
- when the density of the core becomes roughly around the density of the nucleus of an atom (the protons and neutrons in the star are, in a sense, touching) ===> pressure increases greatly.
- at high compression the nuclear force which is normally attractive (==> fusion and finding together of nuclei) becomes repulsive! This causes the pressure of the core to increase strongly and the collapse halts.
The core is now like a steel ball. The higher lying layers have a chance to catch-up and they fall onto the hard core of the star.
Because the outer layers accelerate to high speeds as they fall onto the core (~ tenth the speed of light or so), they have high kinetic energies.
When they are forced to come to a halt at the core, this energy is converted into heat which then drives a shock wave out into the star -- it is this shock wave which causes the explosion.
- Up to here, the scenario has been modeled quite nicely on the computer. However, the details of how the shock wave is generated and fed and then causes the ejection of the outer layers of the star have never been convincingly demonstrated. This is a very active field of current research.
We discuss the passage of the shock wave through the star in the unit on Nucleosynthesis.

Neutrinos

A strong prediction of all models of Type II SN outbursts is that, initially, the dominant source of energy loss is neutrino emission. Recall that in order to make a star from a large ISM cloud requires that a large amount of energy be lost. The same is true for a SN outburst -- in order to make a small remnant from a large star, a large amount of energy must be lost.

The star must shed an amount of energy roughly equal to the increase in its gravitational potential energy. If it didn't, its temperature would increase wildly and the collapse would be halted rather quickly. The star must radiate roughly

Energy ~ [ G M(remnant) / R(remnant) ] x M(remnant)
The first term is the gravitational potential energy per gram of the remnant and the second term is the mass of the remnant. The remnant has a mass of roughly 1.4 M(Sun) and a radius of 10 - 15 kilometers and so
Energy ~ 5 x 10**53 ergs !
must be radiated in neutrinos!! The entire optical outburst amounted to only 10**49 ergs or so.
This is a strong prediction of the model and must be true if the model is to be taken seriously.
Neutrinos observed from SN1987A:

The Kamioka and IMB experiments saw around 20 events. Using the information on how far away the LMC is and the difficulty with which neutrinos can be detected ===> that these 20 events are what are expected for a star which collapses and radiates ~ 5 x 10**53 ergs in neutrinos -- this is around 5 x 10**58 neutrinos at the source (in the LMC) so that ~ 2.4 x 10**11 neutrinos passed through every square centimeter of the Earth.

The detection of the neutrinos is strong corroboration that our basic model for Type II SN is correct.

The neutrinos are produced by the core collapse (the initial phase of the SN outburst) and so are expected to lead the optical fireworks by anywhere from hours to days (depending upon the type of progenitor star). The neutrinos lead the optical outburst of SN1987A by several hours.

The neutrinos were detected on 23.316 February 1987
The optical SN was not detected on 23.39 February but was seen on 23.443 February ===> the optical brightening lagged the neutrino outburst by 0.074 to 0.127 days or 1.8 to 3 hours
Implications?

Typical Type II SN are expected to show a lag of several days between the neutrino outburst and the optical display. The short lag is entirely consistent with the fact that SK -69 202 was a smallish blue supergiant and not a humongous red supergiant.