Temperatures Of Astrophysical Interest Research Articles

A statistical investigation of the rate constants for the H+ + HD reaction at temperatures of astrophysical interest.

The H+ + HD(v, j) reaction has been investigated in detail by means of a statistical quantum method. State-to-state cross sections and rate constants for transitions between reactants and rovibrational states HD(v', j') of the product arrangement with energies below 0.9eV collision energy [that is, HD(v = 0, j = 0-11) and HD(v = 1, j = 0-6)] have been calculated. For the other product channel, D+ + H2(v', j'), rovibrational states up to (v' = 0, j' = 9) have been considered for the calculation of the corresponding thermal rate. Present predictions are compared with previously reported theoretical and experimental rates. Finally, cooling functions for HD due to proton and atomic hydrogen collisions are computed in the low-density regime. We find that the much larger HD-H+ cooling function, as compared with that of HD-H, does not compensate for the low H+/H abundance ratio in astrophysical media so that HD cooling is dominated by HD-H (or HD-H2) collisions.

Weak-interaction rates in stellar conditions

Weak-interaction rates, including β-decay and electron captures, are studied in several mass regions at various densities and temperatures of astrophysical interest. In particular, we study odd-A nuclei in the pf-shell region, which are involved in presupernova formations. Weak rates are relevant to understand the late stages of the stellar evolution, as well as the nucleosynthesis of heavy nuclei. The nuclear structure involved in the weak processes is studied within a quasiparticle proton-neutron random-phase approximation with residual interactions in both particle-hole and particle-particle channels on top of a deformed Skyrme Hartree-Fock mean field with pairing correlations. First, the energy distributions of the Gamow-Teller strength are discussed and compared with the available experimental information, measured under terrestrial conditions from charge-exchange reactions. Then, the sensitivity of the weak-interaction rates to both astrophysical densities and temperatures is studied. Special attention is paid to the relative contribution to these rates of thermally populated excited states in the decaying nucleus and to the electron captures from the degenerate electron plasma.

Rotational Energy Transfer Non-Reactive Hydrogen-Hydrogen Collisions at Low Temperatures of Astrophysical Interest

We perform a pure quantum-mechanical calculation for the non-reactive scattering in atomic and molecular hydrogen-hydrogen collisions, i.e. H2+H2 and H+H2. Different potential energy surfaces (PESs) for the H3 and H4 atomic systems have been used. The rigid rotor model of the diatomic molecules, i.e. when the distance between hydrogen atoms is fixed at some average equilibrium value, has been applied in our calculation. After this preparatory stage the astrophysical H2-cooling function can be estimated at different astrophysical conditions and temperatures.

Contribution of excited states to stellar weak-interaction rates in odd-Anuclei

Weak-interaction rates, including beta-decay and electron capture, are studied in several odd-A nuclei in the pf-shell region at various densities and temperatures of astrophysical interest. Special attention is paid to the relative contribution to these rates of thermally populated excited states in the decaying nucleus. The nuclear structure involved in the weak processes is studied within a quasiparticle random-phase approximation with residual interactions in both particle-hole and particle-particle channels on top of a deformed Skyrme Hartree-Fock mean field with pairing correlations. In the range of densities and temperatures considered, it is found that the total rates do not differ much from the rates of the ground state fully populated. In any case, the changes are not larger than the uncertainties due to the nuclear model dependence of the rates.

HD +ortho-/para-H2rotational energy transfer non-reactive scattering at low astrophysical temperatures: 2 K ≲ T ≲ 300 K

State-resolved integral cross sections and corresponding thermal rate coefficients are computed for the HD + ortho -/ para -H 2 rotational energy transfer collision at low temperatures of astrophysical interest: 2 K ≲ T ≲ 300 K. A recent original full-dimensional H 2 -H2 potential energy surface (PES) was adopted and appropriately modified for the current non-symmetrical four-atomic system.

Low-energy resonances in14N(α,γ)18Fand their astrophysical implications

The strengths of low-energy resonances in ${}^{14}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{18}\mathrm{F}$ at 573 keV and 1136 keV have been measured using an activation method. In addition, their relative strength and the energy of the lower resonance have been determined in a prompt $\ensuremath{\gamma}$-ray experiment. The results of these measurements are used to reevaluate the stellar reaction rate of ${}^{14}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{18}\mathrm{F}.$ The present reaction rate at temperatures of astrophysical interest is a factor of 2 smaller than previously reported.

Dissociation equilibrium constants of molecules of astrophysical interest

Dissociation equilibrium constants are computed for 53 molecules containing H, C, N, O, Mg, Si, S, and Fe in a range of temperatures of astrophysical interest. The results are obtained using recent molecular constants and are compared with previously published data. The equilibrium constants are approximated by an analytical expression and the fitting coefficients are supplied.

Collisional excitation of the O III auroral line

The cross section for electron impact excitation of the auroral transition O2+(2p2 1D to 2p2 1S) at 4363 AA has been computed by means of the non-relativistic Coulomb-Born approximation. The contribution from high angular momentum partial waves (L>or=4) is used to supplement the recent close-coupling calculation of Baluja et al. (1981) in order to obtain a total collision strength at impact energies above the region of resonances. A simple graphical method based on the high-energy Born limit is given for interpolating collision strengths over the entire energy range. From the present results the authors conclude that at temperatures of astrophysical interest the previously published rate coefficient is underestimated by only a few per cent.

Fine-structure excitation of ground-state C+ions by hydrogen atoms

Cross sections are computed for the low-energy fine-structure excitation of singly ionized carbon (2P electronic ground state) by atomic hydrogen (2S electronic ground state) in a close-coupling formulation; recent accurate potential energy curves are employed. The effects of rotational coupling and of the fine-structure energy defect are investigated. The excitation cross section is found to be larger than previously computed. The cooling function is calculated for temperatures of astrophysical interest.

Determination of Van Der Waals broadening at temperatures of astrophysical interest

This experiment analyzes the widths of shock-excited emission lines at temperatures of about 5000°K. Impurity compounds were added to a conventional pressure driven shock tube in a powdered form as an aerosol and the emission line profiles were analyzed with a rapid scanning Fabry-Perot interferometer. Line widths due to the broadening of argon were measured for Si I λ4102 and λ5948 and for Cs I λ4593. The latter line was also measured as broadened by neon and the shift of Si I λ4102 was measured for broadening by argon. These data are compared with other experimental data to determine the temperature dependence of the broadening. Significant disagreements with simple theory are found, the experimental values of the widths being larger than the theoretical values by factors of 1.5–2. These results emphasize the deficiency of the assumption of a simple 1/ r 6 interaction potential.