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Reaction Rates Calculations in Dense Stellar Matter. Mary Beard University of Notre Dame. [email protected] Frontiers 2005. Aim: To establish a general reaction rate expression for all stellar burning regimes To establish uncertainties in existing reaction rate expressions.

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Reaction Rates Calculations in Dense

Stellar Matter

Mary Beard

University of Notre Dame

[email protected]

Frontiers 2005

  • Aim:
  • To establish a general reaction rate expression for all stellar burning regimes
  • To establish uncertainties in existing reaction rate expressions
dense stellar environments
Dense Stellar Environments

Accreting neutron stars: The ashes of the rp process are compressed and undergo electron captures producing extremely neutron rich, light nuclei


Barrier Penetration Model

(fusion cross sections)

(Hill-Wheeler formula)

(WKB approximation)


Density dependence

M. Stoitsov et al., Phys. Rev. C58 (1998) 2086.

A. V. Afanasjev et al., Phys. Rev. C60 (1999) 051303.

Phys. Rev. Lett. 78 (1997) 3270

Phys. Rev. Lett. 79 (1997) 5218

Phys. Rev. C58 (1998) 576

Phys. Rev C66 (2002) 014610

São Paulo potential

The Nuclear Potential

- Densities obtained through theoretical calculations (RMF, for example).



Pycnonuclear Reaction Rates

In a neutron star crust, ions form a Coulomb lattice structure surrounded by a degenerate electron gas.

Electron screening effects become so strong that rates of nuclear reactionsincrease considerably even at low energies;

Pycnonuclear reactions take place under very high density conditions and are more sensitive to density than to temperature – from the Greek, pyknos means compact, dense;

Pycnonuclear reactions between neutron-rich isotopes can provide a new heat source in accreting neutron star crust.

pycnonuclear formalism
Pycnonuclear Formalism

There are a couple of models available for pycnonuclear calculations,

(eg Salpeter and Van Horn Astrophys. J. 155, 183 1969)

All can be written in one general (user-friendly) way, with

dimensionless parameters representing model differences

Where length parameter λ is defined by:

CPYC, Cexp and CL are dimensionless model parameters


Burning Regimes

Boltzmann Gas

Coupled quantum Coulomb system

Strongly coupled quantum system

Coupled classical Coulomb system

single analytical approximation in all regimes
Single analytical approximation in all regimes

Thermonuclear reaction rate is defined by:

Where Pth and Fth are given by


Reaction rate approximation is then given by:

This reduces to appropriate expression in all burning

regimes; when T>>Tp ΔR  Rth >> Rpyc retrieve Rth T  0 ΔR 0 retrieve Rpyc

By Analogy, the thermally enhanced pycnonuclear rate can be written as:

Where P and F are given by:


The rates involving isotopes with identical charge number show only minor differences which are entirely due to the difference in S-factor;

For higher Z-values the rates decrease steeply at density values less than 1012 g/cm3 because of the strong Z-dependence in the pycno equation.



Nuclear Physics:

We are using the São Paulo potential to describe the fusion process.

Nuclear Astrophysics:

We are proposing a single analytical expression for the fusion rate, which is valid in all regimes. The parameters reflect theoretical uncertanties of the reaction rates.

An exact calculation should take into account many effects as lattice impurities and imperfections, classical motion of plasma ions, related structure of Coulomb plasma fields, etc.

The next step is extend the treatment presented for one-component-plasma case towards a general formalism for the fusion rate between different isotopes.