Revised study of the collisional excitation of HCO+ by H2(j = 0)

Yazidi, O.; Abdallah, D. Ben; Lique, François

doi:10.1093/mnras/stu580

Cited by 33 publications

(33 citation statements)

References 43 publications

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“…This well depth is almost an order of magnitude larger than in the case of neutral molecules interacting with H2. Such effect has been already observed for the HCO + cation (Yazidi et al 2014) or for the the CN − anion (K los & Lique 2011) but is absent when using He as a model for H2. Hence, considering specifically H2 will be of crucial importance for interstellar ions by comparison to neutral species.…”

Section: Hyperfine Rate Coefficientsmentioning

confidence: 55%

Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

Lique

Daniel

Pagani

et al. 2014

Monthly Notices of the Royal Astronomical Society

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The modelling of emission spectra of molecules seen in interstellar clouds requires the knowledge of collisional rate coefficients. Among the commonly observed species, N 2 H + is of particular interest since it was shown to be a good probe of the physical conditions of cold molecular clouds. Thus, we have calculated hyperfine-structure resolved excitation rate coefficients of N 2 H + (X 1 Σ + ) by H 2 (j = 0), the most abundant collisional partner in the cold interstellar medium. The calculations are based on a new potential energy surface, obtained from highly correlated ab initio calculations. State-to-state rate coefficients between the first hyperfine levels were calculated, for temperatures ranging from 5 K to 70 K. By comparison with previously published N 2 H + -He rate coefficients, we found significant differences which cannot be reproduced by a simple scaling relationship. As a first application, we also performed radiative transfer calculations, for physical conditions typical of cold molecular clouds. We found that the simulated line intensities significantly increase when using the new H 2 rate coefficients, by comparison with the predictions based on the He rate coefficients. In particular, we revisited the modelling of the N 2 H + emission in the LDN 183 core, using the new collisional data, and found that all three of the density, gas kinetic temperature and N 2 H + abundance had to be revised.

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Section: Hyperfine Rate Coefficientsmentioning

confidence: 55%

Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

Lique

Daniel

Pagani

et al. 2014

Monthly Notices of the Royal Astronomical Society

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“…21 The Very recently, a PES describing the interaction of HCO + and isotropic H 2 has been computed. 11 This PES depends thus on two coordinates only, R and θ in our conventions, since it averages the influence of the H 2 orientation, angles θ , φ. The minimum of the PES was found on the H + side of the molecule.…”

Section: A Potential Energy Surfacementioning

confidence: 99%

“…While some of these coefficients have been computed (e − , 7 He, 8 ), only partial results have been computed for collisions with H 2 , restricted to para-H 2 , in its isotropic ground rotational state J 2 = 0. [9][10][11] A full picture of the energy transfer between HCO + and H 2 is necessary to progress towards a full understanding of the HCO + excitation schemes.…”

Section: Introductionmentioning

confidence: 99%

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The HCO+–H2 van der Waals interaction: Potential energy and scattering

Massó

Wiesenfeld

2014

The Journal of Chemical Physics

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We compute the rigid-body, four-dimensional interaction potential between HCO(+) and H2. The ab initio energies are obtained at the coupled-cluster single double triple level of theory, corrected for Basis Set Superposition Errors. The ab initio points are fit onto the spherical basis relevant for quantum scattering. We present elastic and rotationally inelastic coupled channels scattering between low lying rotational levels of HCO(+) and para-/ortho-H2. Results are compared with similar earlier computations with He or isotropic para-H2 as the projectile. Computations agree with earlier pressure broadening measurements.

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“…This implies a scaling with f H 2 such that n(H)(0.125+f H 2 )/1.125 ≈ constant. Yazidi et al (2014) 2 See also Buffa et al (2009) …”

Section: Scaling With the Electron And H 2 Fractionsmentioning

confidence: 99%

The Detectability of Millimeter-Wave Molecular Rotational Transitions

Liszt

Pety

2016

ApJ

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Elaborating on a formalism that was first expressed some 40 years ago, we consider the brightness of low-lying mm-wave rotational lines of strongly polar molecules at the threshold of detectability. We derive a simple expression relating the brightness to the line of sight integral of the product of the total gas and molecular number densities and a suitably-defined temperature-dependent excitation rate into the upper level of the transition. Detectability of a line is contingent only on the ability of a molecule to channel enough of the ambient thermal energy into the line and the excitation can be computed in bulk by summing over rates without solving the multi-level rate equations or computing optical depths and excitation temperatures. Results for HCO + , HNC and CS are compared with escape-probability solutions of the rate equations using closed-form expressions for the expected range of validity of our ansatz, with the result that gas number densities as high as 10 4 cm −3 or optical depths as high as 100 can be accommodated in some cases. For densities below a well-defined upper bound, the range of validity of the discussion can be cast as an upper bound on the line brightness which is 0.3 K for the J=1-0 lines and 0.8 -1.7 K for the J=2-1 lines of these species. The discussion casts new light on interpretation of line brightnesses under conditions of weak excitation, simplifies derivation of physical parameters and eliminates the need to construct grids of numerical solutions of the rate equations.

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Revised study of the collisional excitation of HCO+ by H2(j = 0)

Cited by 33 publications

References 43 publications

Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

The HCO+–H2 van der Waals interaction: Potential energy and scattering

The Detectability of Millimeter-Wave Molecular Rotational Transitions

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