The Alchemy of Neutron Star Collisions

Glenn Holland

Joined Dec 26, 2014
705
Since an ordinary neutron that's "free" (not bound to the nucleus of an atom) will decay in an average of 15 minutes, then why doesn't a neutron star decay? The answer seems to be that the intense gravity counteracts any forces that would cause decay. However, if two neutron stars get close enough that the gravity is neutralized, it seems that they could decay - and possibly explosively.
 

Thread Starter

nsaspook

Joined Aug 27, 2009
8,759
Since an ordinary neutron that's "free" (not bound to the nucleus of an atom) will decay in an average of 15 minutes, then why doesn't a neutron star decay? The answer seems to be that the intense gravity counteracts any forces that would cause decay. However, if two neutron stars get close enough that the gravity is neutralized, it seems that they could decay - and possibly explosively.
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Good question but I don't think you mean to use the word neutralized. How could the gravity well of either neutron star be neutralized? The fast-moving mass ejecta from a binary neutron merger is from gravitational energy being transferred in the matter as KE tidal forces rips mass into the space around the pair. As this ripped away mass moves away from the spinning pair mass it naturally moves into a area of less gravity to become unbound to the star in the right conditions. The bulk star mass doesn't escape or neutralize gravity. It falls into a deeper gravity well.
Our simulations also make two predictions that could be tested. First, they predict that some of the ejecta expand so rapidly that some of the neutrons might escape capture by seed nuclei during the decompression. The neutrons would then decay on a timescale of minutes and produce a bump in the UV bands on a time scale of an hour. in the first hour of the merger. Second, the radio fluency from GW170817 might reveal the presence of a substantial amount of fast-moving ejecta in the next few months or years.
Visualization of the electron fraction in a binary neutron star merger simulation. The blue color denotes neutron rich material, while the red color denotes material with electron fraction 0.5 (i.e., equal number of neutrons and protons).
Neutron star mergers generically result in the ejection of a small fraction (0.1% - 1%) of a solar mass of neutron rich material. As these ejecta expands and cools, they may undergo the so-called r-process and produce heavy nuclei, like gold. The radioactive decay of by-products of the r-process can power an UV/optical/infrared transient known as kilonova.

During the merger, material is ejected from the neutron stars because of tidal torques and shocks. Most of this material is bound and forms an accretion disk around the merger remnant, a massive neutron star or a black hole. A small fraction achieves velocities in excess of the escape velocity and is unbound. Additionally, a substantial fraction of the disk (10% - 40%) becomes unbound over a timescale of a second due to magnetic and neutrino processes in the disk.
 
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