8 research outputs found
Low energy H+CO scattering revisited - CO rotational excitation with new potential surfaces
Context. A recent modeling study of brightness ratios for CO rotational transitions in gas typical of the diffuse ISM by Liszt found the role of H collisions to be more important than previously assumed. This conclusion was based on recent quantum scattering calculations using the so-called WKS potential energy surface (PES) which reported a large cross section for the important 0 → 1 rotational transition. This result is in contradiction to one obtained using the earlier BBH PES for which the cross section is quite small and which is consistent with an expected homonuclear-like propensity for even ∆J transitions.
Aims. We revisit this contradiction with new scattering calculations using two new ab initio PESs that focus on the important long- range behavior and explore the validity of the apparent departure from the expected even ∆J propensity in H-CO rotational excitation obtained with the WKS PES.
Methods. Close-coupling (CC) rigid-rotor calculations for CO(v = 0, J = 0) excitation by H are performed on four different PESs. Two of the PESs are obtained in this work using state-of-the-art quantum chemistry techniques at the CCSD(T) and MRCI levels of theory.
Results. Cross sections for the J = 0 → 1, as well as other odd ∆J, transitions are significantly suppressed compared to even ∆J transitions in thermal energy CC calculations using the CCSD(T) and MRCI surfaces. This is consistent with the expected even ∆J propensity and in contrast to CC calculations using the WKS PES which predict a dominating 0 → 1 transition.
Conclusions. Inelastic collision cross section calculations are sensitive to fine details in the anisotropic components of the PES and its long-range behavior. The current results obtained with new surfaces for H-CO scattering suggest that the original astrophysical assumption that excitation of CO by H2 dominates the kinetics of CO in diffuse ISM gas is likely to remain valid
The First Stars
The first stars to form in the Universe -- the so-called Population III stars
-- bring an end to the cosmological Dark Ages, and exert an important influence
on the formation of subsequent generations of stars and on the assembly of the
first galaxies. Developing an understanding of how and when the first
Population III stars formed and what their properties were is an important goal
of modern astrophysical research. In this review, I discuss our current
understanding of the physical processes involved in the formation of Population
III stars. I show how we can identify the mass scale of the first dark matter
halos to host Population III star formation, and discuss how gas undergoes
gravitational collapse within these halos, eventually reaching protostellar
densities. I highlight some of the most important physical processes occurring
during this collapse, and indicate the areas where our current understanding
remains incomplete. Finally, I discuss in some detail the behaviour of the gas
after the formation of the first Population III protostar. I discuss both the
conventional picture, where the gas does not undergo further fragmentation and
the final stellar mass is set by the interplay between protostellar accretion
and protostellar feedback, and also the recently advanced picture in which the
gas does fragment and where dynamical interactions between fragments have an
important influence on the final distribution of stellar masses.Comment: 72 pages, 4 figures. Book chapter to appear in "The First Galaxies -
Theoretical Predictions and Observational Clues", 2012 by Springer, eds. V.
Bromm, B. Mobasher, T. Wiklin
Proton-Induced Collisions on Potential Prebiotic Species
International audienc