Hey, I was wondering what would happen if a neutron star in a contact binary accreted enough mass for it to exceed 3.0 solar masses, the upper limit of mass of a neutron star before it collapses into a black hole.
When a white dwarf in the same contact binary configuration exceeds the 1.4 solar mass limit for a white dwarf (the Chandrasekhar limit), it becomes a type 1a supernova.
I expect that since once a white dwarf is formed at the end stage of lower mass stars, it cannot be made into a neutron star by gobbling up mass - it explodes if it reaches that 1.4 solar mass limit rather than “transitioning” into a neutron star. So a neutron star can be formed only due to a Type II supernova that occurs only with high mass stars - but not too high.
I was wondering whether a neutron star can become a black hole if it exceeds 3.0 solar masses due to accretion of mass, or whether there will be an explosion analogous to the Type Ia supernova that occurs when the 1.4 solar mass limit of white dwarfs is reached.
Probably both. The formation of a black hole is generally a very violent process, so you can have some material blasted away, while some forms a black hole.
Also note, by the way, that the actual upper limit of neutron star mass isn’t known: It’s more than 2 solar masses (since one has been observed at that mass), but probably not very much more. It depends on the equation of state (relationship between pressure and density) of neutron-star matter, which in turn depends on fine details of how the strong nuclear force works, which aren’t at present known.
Interesting. Could this lead to a gamma-ray burst, specifically a short gamma ray burst? Scientists are fairly certain that long gamma ray bursts are caused by Type II supernovas that (directly) form black holes rather than neutron stars, but they’re not sure how the short-period ones work or what their causes are.
It has been hypothesized that the collision of two neutron stars is responsible for short-period gamma-ray bursts. I’m thinking that one neutron star + accreted matter from a neighboring star that makes the total mass of the neutron star exceed the neutron star mass limit (if it exists) and the collision of two neutron stars which (also) makes their total combined mass exceed the same mass limit are two phenomena which are very similar, especially because if matter (that wasn’t initially neutron star matter) falls on a neutron star, it becomes compressed so much that its properties become pretty much indistinguishable from the star itself.