Molecular evolution in astrophysical environments.

Abstract

Molecular formation and destruction processes are explored in rapidly evolving, non-equilibrium astrophysical environments. First, a semi-classical calculation is made for the rate coefficients of excited atom radiative association to form molecular hydrogen and of the process C⁺ + O → CO⁺ + hν. The latter process may be important to the formation of CO in the core of Supernova 1987A. It is shown that the excited atom process may have been important to the formation of H$\sb2$ during the early part of the epoch of recombination in the early Universe. The equations of ionization balance and molecular formation and destruction have been integrated through the epoch of recombination. Other processes are examined in detail. These include heating and cooling of the primordial plasma, damping of fluctuations prior to decoupling, and the possibility of a radiation-driven instability at the onset of recombination. A calculation is presented of the time-dependent chemical evolution in the rapidly expanding outer envelope of SN 1987A. Various cooling rates and hydrogen abundances in the envelope have been examined. It is found that large molecular abundances, in particular CO, form rapidly, while hydrogen remains mostly in its atomic forms. Near-infrared observations of the proto-planetary nebula CRL 618 are presented and discussed. Images acquired in the H and K bandpasses are consistent with a bipolar axis highly inclined to the plane of the sky. From the spectrum, a visual extinction of Aᵥ = 5.3 magnitudes to the molecular hydrogen emitting lobes is found. It is shown by comparison with spectral models that the near-infrared H₂ spectrum exhibits emission from thermally excited molecules at Tₑₓ ∼ 2000 K. A component of fluorescent emission may also be present. The suggestion is explored that large molecules, in particular polycyclic aromatic hydrocarbons (PAHs), are formed in stellar winds. It is asserted that the primary source of interstellar PAHs is mass-losing asymptotic giant branch carbon stars. It is apparent that the known numbers of the most extreme mass-losing carbon stars are able to produce PAHs in sufficient quantities to maintain an interstellar medium well mixed in such molecules at the inferred abundance.