These nuclear reactions, electron captures, neutron emissions, and pycnonuclear reactions, are of utmost importance because they set both the heating and compositional profile.
In the outer crust, electrons can capture into excited states of nuclei; the subsequent radiative de-excitation can release ~4 times more energy than previous models predicted, which assumed electron capture into the ground state. Interestingly, Daligault & Gupta (2009) find that the outer crust is amorphous, i.e. it does not form a lattice! This result is exciting, but conflicts with previous calculations (like Chuck Horowitz's simulations). Does it conflict with observations? Unclear right now, but it would be useful to find out (hint, hint...).
A multi-component inner crust may form a lattice, as some leaked, free neutrons redistribute themselves among the other nuclei and thereby "homogenize" the plasma. This neutron rearrangement can affect pycnonuclear heating as well.
Yes, it seems to be a contradiction in the results of Gupta vs Horowitz. As far as I understand, these two MD simulations start with the same ion composition of Gupta et al. Since both authors use the same composition and numerical method I would expect agreement in the structure of the solid phase. During the coffee break I talked about this concern with Ed Brown. He suggested a possible difference in the thermalization of the ions.
ReplyDeleteSo far, cooling curves from accreting NS agree with a crystal phase.
New observations (see Nathalie Degenaar talk) could support a crystal.
Stay tuned for these results and Brown model fit for them.
OK, after some discussions with Horowitz and Daligault here are my interpretations of the situation. Indeed both simulations used the same ion composition. Both simulations have some density and temperature values in common. Horowitz results focus on finding the phase and thermal conductivity of the NS crust. Therefore, this simulation decrease temperature gradually till a solid is found. This solid is a crystal. After the crystal is found the temperature is increased again to find the thermal conductivity at higher temperatures. Gupta and Daligault simulation focuses on the composition. This simulation does not have a gradual decrease of T till a crystal is found. If the the system is not allow to reach the crystal phase and temperature is decrease it simply forms an amorphous solid.
ReplyDeleteChuck, Jerome, Sanjib: would you add something else?