Pair Instability Supernovae & Electron Capture Supernovae

[tovisonnenberg]

Hi! I've been browsing the internet for information about supernovae and I came across this chart describing 4 types of core collapse causes (the chart may have copied weirdly because not all the information fits into this text box):

Cause of collapse	Progenitor star approximate initial mass (solar masses)	Supernova type	Remnant
Electron capture in a degenerate O+Ne+Mg core	9–10	Faint II-P	Neutron star
Iron core collapse	10–25	Faint II-P	Neutron star
25–40 with low or solar metallicity	Normal II-P	Black hole after fallback of material onto an initial neutron star
25–40 with very high metallicity	II-L or II-b	Neutron star
40–90 with low metallicity	None	Black hole
≥40 with near-solar metallicity	Faint Ib/c, or hypernova with gamma-ray burst (GRB)	Black hole after fallback of material onto an initial neutron star
≥40 with very high metallicity	Ib/c	Neutron star
≥90 with low metallicity	None, possible GRB	Black hole
Pair instability	140–250 with low metallicity	II-P, sometimes a hypernova, possible GRB	No remnant
Photodisintegration	≥250 with low metallicity	None (or luminous supernova?), possible GRB	Massive black hole

I am familiar with iron core collapse supernovae and photodisintegration supernovae, but I have some questions about the other two types:
1. Why are electrons captured in an O+Ne+Mg core?
2. If the electron-positron pairs annihilate and return their energy to gamma rays (in the progenitors of pair instability supernovae), why am I reading that the radiation pressure is being reduced? Also, why am I reading that a pair instability supernova leaves behind no remnant?

 

[Drakkith]

1. Why are electrons captured in an O+Ne+Mg core?

I'm not certain, but it may be because the degenerate core simply gains enough mass to undergo collapse into a neutron star. The immense pressure makes it energetically favorable for protons and electrons to combine into neutrons.

2. If the electron-positron pairs annihilate and return their energy to gamma rays (in the progenitors of pair instability supernovae), why am I reading that the radiation pressure is being reduced? Also, why am I reading that a pair instability supernova leaves behind no remnant?

If I understand things correctly, fluctuations in the core's density and temperature cause an increase in the energy of the generated gamma rays, which increases the amount of electron-positron pairs created. In stars not massive enough, this temporarily reduces the radiation pressure until the electron-positron pairs annihilate with each other to produce gamma rays, since many gamma rays are used up in creating the particles and can't support the star. The increase is only temporarily in star not massive enough for pair-instability supernova.

In stars massive enough, the following occurs (from wiki):

As temperatures and gamma ray energies increase, more and more gamma ray energy is absorbed in creating electron–positron pairs. This reduction in gamma ray energy density reduces the radiation pressure that resists gravitational collapse and supports the outer layers of the star. The star contracts, compressing and heating the core, thereby increasing the rate of energy production. This increases the energy of the gamma rays that are produced making them more likely to interact and so increases the rate at which energy is absorbed in further pair production. As a result, the stellar core loses its support in a runaway process, in which gamma rays are created at an increasing rate, but more and more of the gamma rays are absorbed to produce electron–positron pairs, and the annihilation of the electron–positron pairs is insufficient to halt further contraction of the core, resulting in a supernova.

No remnant is left behind because the contraction and subsequent increase in temperature and density drastically increases the rate of fusion in the core, which ends up blowing the star apart.

 

[Drakkith]

Interesting. But why doesn't the mass gain ignite magnesium and neon fusion instead?

That's a good question, and I'm afraid I don't know the answer. I'll look around some and see if I can find more information.