Molecular Excitation in the Interstellar Medium: Recent Advances in Collisional, Radiative, and Chemical Processes

Roueff, E.; Lique, François

doi:10.1021/cr400145a

Cited by 224 publications

(192 citation statements)

References 445 publications

(811 reference statements)

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“…Roueff & Lique 2013, and references therein), including collisional excitation, spontaneous radiative decay, radiative excitation, and formation pumping. At sufficiently high densities, the first of these processes drives the molecular excitation to local thermodynamic equilibrium (LTE), in which the relative level populations are given by Boltzmann factors determined by the gas kinetic temperature.…”

Section: Excitation Of Interstellar Hydridesmentioning

confidence: 99%

Interstellar Hydrides

Gérin

Neufeld

Goicoechea

2016

Annu. Rev. Astron. Astrophys.

129

171

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Interstellar hydrides -that is, molecules containing a single heavy element atom with one or more hydrogen atoms -were among the first molecules detected outside the solar system. They lie at the root of interstellar chemistry, being among the first species to form in initially-atomic gas, along with molecular hydrogen and its associated ions. Because the chemical pathways leading to the formation of interstellar hydrides are relatively simple, the analysis of the observed abundances is relatively straightforward and provides key information about the environments where hydrides are found. Recent years have seen rapid progress in our understanding of interstellar hydrides, thanks largely to far-IR and submillimeter observations performed with the Herschel Space Observatory. In this review, we will discuss observations of interstellar hydrides, along with the advanced modeling approaches that have been used to interpret them, and the unique information that has thereby been obtained.

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Section: Excitation Of Interstellar Hydridesmentioning

confidence: 99%

Interstellar Hydrides

Gérin

Neufeld

Goicoechea

2016

Annu. Rev. Astron. Astrophys.

129

171

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“…The total time-dependent wave function for the quantum part of the system is expressed in MQCT as follows (in atomic units): (1) where amn are time-dependent expansion coefficients, Ψmn is basis set of rotational eigenstates of the system, and En are their corresponding energy eigenvalues. Index n is a composite index that labels states and its meaning depends on the system.…”

Section: Ii1 Quantum and Classical Degrees Of Freedommentioning

confidence: 99%

Recent Advances in Development and Applications of the Mixed Quantum/Classical Theory for Inelastic Scattering

Babikov

Семенов

2015

J. Phys. Chem. A

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A mixed quantum/classical approach to inelastic scattering (MQCT) is developed in which the relative motion of two collision partners is treated classically, and the rotational and vibrational motion of each molecule is treated quantum mechanically. The cases of molecule + atom and molecule + molecule are considered including diatomics, symmetric-top rotors, and asymmetric-top rotor molecules. Phase information is taken into consideration, permitting calculations of elastic and inelastic, total and differential cross sections for excitation and quenching. The method is numerically efficient and intrinsically parallel. The scaling law of MQCT is favorable, which enables calculations at high collision energies and for complicated molecules. Benchmark studies are carried out for several quite different molecular systems (N2 + Na, H2 + He, CO + He, CH3 + He, H2O + He, HCOOCH3 + He, and H2 + N2) in a broad range of collision energies, which demonstrates that MQCT is a viable approach to inelastic scattering. At higher collision energies it can confidently replace the computationally expensive full-quantum calculations. At low collision energies and for low-mass systems results of MQCT are less accurate but are still reasonable. A proposal is made for blending MQCT calculations at higher energies with full-quantum calculations at low energies.

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“…Schöier et al (2005) and van der Tak (2011) describe in detail the accuracy of molecular data needed for radiative transfer analysis, such as collisional rate coefficients, spontaneous emission probabilities, transition frequencies, statistical weights, and energy levels. The uncertainties involved in determining collisional rate coefficients are summarized by Flower (1990), Roueff (1990), and Roueff & Lique (2013). The determination of collisional rate coefficients includes uncertainties that vary for different species.…”

Section: Line Modelingmentioning

confidence: 99%

The HIFI spectral survey of AFGL 2591 (CHESS)

Kaźmierczak-Barthel

Semenov

Tak

et al. 2015

A&A

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Aims. The aim of this work is to understand the richness of chemical species observed in the isolated high-mass envelope of AFGL 2591, a prototypical object for studying massive star formation. Methods. Based on HIFI and JCMT data, the molecular abundances of species found in the protostellar envelope of AFGL 2591 were derived with a Monte Carlo radiative transfer code (Ratran), assuming a mixture of constant and 1D stepwise radial profiles for abundance distributions. The reconstructed 1D abundances were compared with the results of the time-dependent gas-grain chemical modeling, using the best-fit 1D power-law density structure. The chemical simulations were performed considering ages of 1−5 × 10 4 years, cosmic ray ionization rates of 5−500 × 10 −17 s −1 , uniformly-sized 0.1−1 μm dust grains, a dust/gas ratio of 1%, and several sets of initial molecular abundances with C/O < 1 and >1. The most important model parameters varied one by one in the simulations are age, cosmic ray ionization rate, external UV intensity, and grain size. Results. Constant abundance models give good fits to the data for CO, CN, CS, HCO + , H 2 CO, N 2 H + , CCH, NO, OCS, OH, H 2 CS, O, C, C + , and CH. Models with an abundance jump at 100 K give good fits to the data for NH 3 , SO, SO 2 , H 2 S, H 2 O, HCl, and CH 3 OH. For HCN and HNC, the best models have an abundance jump at 230 K. The time-dependent chemical model can accurately explain abundance profiles of 15 out of these 24 species. The jump-like radial profiles for key species like HCO + , NH 3 , and H 2 O are consistent with the outcome of the time-dependent chemical modeling. The best-fit model has a chemical age of ∼10−50 kyr, a solar C/O ratio of 0.44, and a cosmic-ray ionization rate of ∼5 × 10 −17 s −1 . The grain properties and the intensity of the external UV field do not strongly affect the chemical structure of the AFGL 2591 envelope, whereas its chemical age, the cosmic-ray ionization rate, and the initial abundances play an important role. Conclusions. We demonstrate that simple constant or jump-like abundance profiles constrained with 1D Ratran line radiative transfer simulations are in agreement with time-dependent chemical modeling for most key C-, O-, N-, and S-bearing molecules. The main exceptions are species with very few observed transitions (C, O, C + , and CH) or with a poorly established chemical network (HCl, H 2 S) or whose chemistry is strongly affected by surface processes (CH 3 OH).

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Molecular Excitation in the Interstellar Medium: Recent Advances in Collisional, Radiative, and Chemical Processes

Cited by 224 publications

References 445 publications

Interstellar Hydrides

Interstellar Hydrides

Recent Advances in Development and Applications of the Mixed Quantum/Classical Theory for Inelastic Scattering

The HIFI spectral survey of AFGL 2591 (CHESS)

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