The Molecular Astrophysics of Stars and Galaxies
Abstract
This book provides a comprehensive survey of modern molecular astrophysics. It includes an introduction to molecular spectroscopy and then addresses the main areas of current molecular astrophysics, including galaxy formation, star forming regions, mass loss from young as well as highly evolved stars and supernovae, starburst galaxies plus the tori and discs near the central engines of active galactic nuclei. All chapters have been written by invited authors who are acknowledged experts in their fields. The thorough editorial process has ensured a uniformly high standard of exposition and a coherent style. The book is unique in giving a detailed view of its wide-ranging subject. It will provide the standard introduction for research students in molecular astrophysics. The book will be read by research astronomers and astrophysicists who wish to broaden the basis of their knowledge or are moving their activities into this burgeoning field. It will enable chemists to learn the astrophysics most related to chemistry as well as instruct physicists about the molecular processes most important in astronomy.
... the lower state (Thomas and David, 1999). In the region of the interstellar space with little dust, the number of excited states of atoms is negligible. ...
The potential energy curves (PECs) and transition dipole moments (TDMs) of PH + and PO are computed with the multireference configuration interaction method, and the cross-sections for the radiative association (RA) of PH + and PO, which is the most efficient way to form the ground states, are presented via the quantum mechanical (QM) theory and computed using ab initio molecular data. The thermal rate coefficients are also expressed and fitted with the standard formula k(T) = A(T 300) α e − β T in the range of 10 K-15,000 K. Meanwhile, the photodissociation, that is the inverse process of RA for PH + , is also studied, including eight photodissociation channels for the computation of state-resolved cross-sections. Careful comparisons with the Leiden Observatory database are made. Considering the cross-sections mentioned above, the local thermodynamic equilibrium cross-sections at the temperatures of 0, 500, 1,000, and 2,000 K are also shown. We expect the results to be helpful for studies of phosphorus chemistry in the interstellar medium and planetary atmospheres.
The formation of CN and its isotopologue C¹⁵N through radiative association has been investigated. We considered two processes for the collisions of ground state C(³P) and N(⁴S) atoms (A²Π → X²Σ⁺ and b⁴Π → a⁴Σ⁺), as well as, for the first time, we investigated the radiative association of ground state C(³P) and excited N(²D) atoms (B²Σ⁺ → X²Σ⁺). The cross-sections for the three processes have been calculated using semi-classical, quantum mechanical, and Breit–Wigner theories. The rate constants, derived from the combination of semi-classical and Breit–Wigner results, have been fitted to the Kooij formula to be used in astrochemical modeling. The rate constant for the B²Σ⁺ → X²Σ⁺ process dominates in the investigated temperature range (10–10 000 K), when its own asymptotic energy is used as a reference level. Moreover, the B²Σ⁺ → X²Σ⁺ process presents the most pronounced isotope effect on the rate constant. We suggest considering these newly investigated radiative association processes for the formation of CN and C¹⁵N in the interstellar medium.
We have developed the polyatomic extension of the established [M. Gustafsson, J. Chem. Phys. 138, 074308 (2013)] classical theory of radiative association in the absence of electronic transitions. The cross section and the emission spectrum of the process is calculated by a quasiclassical trajectory method combined with the classical Larmor formula which can provide the radiated power in collisions. We have also proposed a Monte Carlo scheme for efficient computation of ro-vibrationally quantum state resolved cross sections for radiative association. Besides the method development, the global potential energy and dipole surfaces for H + CN collisions have been calculated and fitted to test our polyatomic semiclassical method.
Molecular ions have been ubiquitous in a variety of environments in the interstellar medium, from Circumstellar Envelopes to Dark Molecular Clouds and to Diffuse Clouds. Their role in the multitude of molecular processes which have been found to occur in those environments has been the subject of many studies over the years, so that we have acquired by now a complex body of data on their chemical structures, their possible function within chemical reactions and their most likely paths to formation. In the present work we review a broad range of such molecular ions, focusing exclusively on positive ions involving the smallest and simplest cations which have been either detected or conjectured as present in the interstellar medium (ISM). We therefore consider mainly molecular cations formed with components like H, H+, He and He+, atomic species which are by far the most abundant baryons in the ISM in general. Their likely structures and their roles in a variety of chemical energy flow paths are discussed and presented within the context of their interstellar environments.
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