What happens to the Core of a main sequence star as additional mass is added?

[Drakkith]

If one ran a simulation where they “gently” filled the core with oxygen on a relatively short time scale would it supernova? Is a star that size hot enough to burn oxygen or would you get gravitational collapse?

Short of magic, I can't think of any way to add mass to the core of the star in a short timescale (let's say a million years or less) that doesn't substantially heat up the core.

 

[Devin-M]

So it wouldn’t be essentially a white dwarf way over the Chandrasekhar limit at the start of the sim?

 

[Drakkith]

So it wouldn’t be essentially a white dwarf way over the Chandrasekhar limit at the start of the sim?

A main sequence star? No, not at all.

 

[Devin-M]

I thought you need over 10 solar masses to burn oxygen so why wouldn’t you get a gravitational collapse?

 

[Drakkith]

Main sequence stars don't have cores made out of degenerate matter. The core's not as dense as a white dwarf and there's still plenty of normal gas pressure holding the core up, so no collapse.

 

[PeterDonis]

I thought you need over 10 solar masses to burn oxygen so why wouldn’t you get a gravitational collapse?

Adding water is not just adding oxygen. It's also adding hydrogen. A solar mass of water contains a lot of hydrogen. All of which the star can and will burn.

My guess (and it's just a guess, I haven't done any math) is that the end result of this (after a Rayleigh-Taylor instability had put the densest atoms at the bottom and the least dense at the top) would be something like an oxygen core with a shell of helium around it, and hydrogen around that. The hydrogen would burn, quite possibly the helium would burn; the oxygen probably would not.

Eventually, when the hydrogen and helium burning was exhausted, since the total mass of this thing is over the Chandrasekhar limit but still (probably) less than the Tolman-Oppenheimer-Volkov limit (AFAIK the lower bound on that limit now is at least 2 Suns), then it would collapse to a neutron star.

 

[Devin-M]

Would that final collapse into a neutron star likely involve explosive oxygen burning?

 

[PeterDonis]

Would that final collapse into a neutron star likely involve explosive oxygen burning?

It could, since it would most likely be a supernova.