Sanjib S. Gupta (NSCL/MSU)

Nonequilibrium reactions in the crust of accreting NS.

• Ashes of rp-process moved deeper – increase in electron chemical potential which at zero temperature is the Fermi Energy:

• Blocks decay because available phase space is restricted and thus allows n-rich material to exist at depth.

• Allows EC on n-rich material with thresholds:

1110511.02/13/24

eMeVF YE

FEC EQw

Earlier calculations of Heat Deposition HZ 90 , HZ03

• Find single species which minimizes Gibbs free energy

• Follow the changes starting from this species as subsequent thresholds are attained

• No T9-dependent neutron rates• No excited states in EC• No pre-threshold captures • Pycnonuclear heating is the main source of crust

heat and occurs very deep g/cc 1012

Electron Captures heat the crust.

• A fraction of the energy difference is lost as freely-streaming neutrinos.

• However, if the capture is to an excited state in the daughter nucleus then the de-excitation heats the crust in rays.

• EC rates are temperature-dependent.• Use B(GT) values from QRPA calculations from g.s. of parent to

excited states of the daughter (work done in collaboration with Peter Moller, K. Kratz)

• The n-rich matter starts to lose neutrons with the rise in temperature: neutrons lost by less bound nuclei are captured by more bound nuclei : the photodisintegration rearrangement heats the crust until equilibrium is established. This equilibrium shifts slowly on a timescale set by the electron captures that are occurring.

ECF QE

nn ,,

example : 86As

MeV 5.13FE

As86

Ge86

MeV 9.378ECQ

MeV 08.4EXCITEDQ

9.4

HZ

EXC

Heat

Heat

Astrophysical Modeling

• 1-zone models (self-heating using nuclear energy feedback + heat

capacity). Good enough if thermal diffusion timescale >> local heating rate. Need correct initial composition – can start with stable at depth.

• Multizone models – use actual composition-dependent conductivity to model heat flow through zones, neutrino cooling rates that takes into account actual state of matter in crust and core (work done in collaboration with Ed Brown). Need to move processed material from one zone to next deeper zone to model accretion correctly.

• Steady-state model of accreted fluid element during its travels through the crust – use to get the composition at depth (work done in collaboration with Ed Brown).

Conclusions

• Nuclear reactions in the upper crust can significantly contribute to observed heat flux.

• New nuclear processes are observed by using realistic Electron Capture, neutron-capture and photodisintegration rates and not assuming a cold crust or a single species at a depth.

• We get a new view into the interior of NS by developing models where the interplay of the new nuclear processes with the accurately computed conductivity and neutrino losses can be studied.