Rotational excitation and de-excitation of C2(X Σ1g+) by para-H2(j=)

Najar, Faouzi; Abdallah, D. Ben; Jaı̈dane, N.; Lakhdar, Z. Ben; Chambaud, Gilberte; Hochlaf, M.

doi:10.1063/1.3137583

Cited by 33 publications

(30 citation statements)

References 24 publications

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“…Second, the cross section for excitation via collisions is larger than the value suggested by van Dischoek & Black (1982). More recent quantum mechanical calculations (Lavendy et al 1991;Robbe et al 1992;Najar et al 2008Najar et al , 2009 indicate a cross section of 4 × 10 −16 cm 2 , rather than 2 × 10 −16 cm 2 .…”

Section: Interstellar Cmentioning

confidence: 98%

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C₂

Hupe

Sheffer

Federman

2012

ApJ

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We analyzed archival spectra acquired with the Hubble Space Telescope for a study of interstellar C 2 . Absorption from the electronic transitions, D 1 Σ + u -X 1 Σ + g (0,0) as well as F 1 Π u -X 1 Σ + g (0,0) and (1,0), was the focus of the study. Our profile syntheses revealed that the lines of the F − X bands were broadened as a result of a perturbation involving the upper levels. Further evidence for the perturbation came from anomalies in line strength and position for the F − X (0,0) band. The perturbation likely arises from a combination of triplet-singlet interactions involving spin-orbit mixing between 3 Π u states and F 1 Π u and an avoided crossing between the 3 Π u states. Tunneling through a potential barrier caused by the 3 and 4 1 Π u states and spin-orbit mixing with other close-lying triplet states of ungerade symmetry are less likely. Except for the broadening, lines in the F − X (1,0) band appear free from anomalies and can be used to study interstellar C 2 ; new results for 10 sight lines are presented.

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Section: Interstellar Cmentioning

confidence: 98%

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C₂

Hupe

Sheffer

Federman

2012

ApJ

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“…Collisional rates for excitation of C 2 with H 2 were computed by Lavendy et al (1991) in IOS approximation. Recently, collisional rates with He have been computed by Najar et al (2009). Both approaches are consistent at higher kinetic temperatures.…”

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confidence: 91%

The excitation of circumstellar C₂in carbon-rich post-AGB object IRAS 22272+5435

Schmidt

Začs

Pułecka

et al. 2013

A&A

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Context. Observations of the circumstellar envelopes of post-asymptotic giant branch (post-AGB) stars provide information about their mass-loss history during the AGB phase and about the chemistry inside envelopes after that phase. Aims. We analyze visual observations of C 2 molecule in absorption and millimeter and sub-millimeter observations of CO molecule in emission with the aim of determining the physical and chemical conditions in the circumstellar shell of the carbon-rich post-AGB star IRAS 22272+5435. Methods. We determined the column densities and excitation structure of C 2 from equivalent width analysis of the molecular absorptions. The thermal and density structure of post-AGB shell (AGB remnant) is constrained by multilevel radiative transfer modeling of CO emission lines. The chemical structure of the envelope was computed and then used in the multilevel radiative transfer in C 2 for comparison with observed column densities. Results. We estimate the column density of C 2 to be 3.2 × 10 15 cm −2 . From the chemical model we estimate peak abundance of C 2 as 6.8 × 10 −6 relative to nucleon density. The absorption of molecular lines originate in the ring between 5 and 10 × 10 16 cm. The excitation temperature of the lowest levels of 58 K is consistent with the gas kinetic temperature derived from the CO modeling. The initial abundance of the parent molecule C 2 H 2 inferred from the analysis is found to be 1.27 × 10 −5 relative to nucleon density. Conclusions. C 2 molecule is a promising tool for probing the temperature structure of the envelopes of post-AGB objects and indirectly for determining the chemical abundance of acetylene.

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“…More recently, Lique and coworkers (Dumouchel, Faure & Lique 2010; Sarrasin et al 2010) provided accurate HCN and HNC rate coefficients for a large number of rotational levels ( j = 0–25) and a wide range of temperature (5 to 500 K). However, the collisional partner remained to be the He atom and recent results on rotational excitation of CO (Wernli et al 2006), SO (Lique et al 2007), SiS (Kłos & Lique 2008; Lique et al 2008) and C 2 (Najar et al 2009) have pointed out that rate coefficients for collisions with para‐H 2 ( j = 0) can be up to a factor of 3 larger or smaller than those for collisions with He, depending on the selected transition. Hence, it seems important to provide rate coefficients for the HCN–H 2 collisional system.…”

Section: Introductionmentioning

confidence: 99%

Hyperfine excitation of HCN by H2 at low temperature

Abdallah¹,

Najar²,

Jaı̈dane³

et al. 2011

Monthly Notices of the Royal Astronomical Society

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Modelling of molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for (de‐)excitation by collisions with the most abundant species. We calculate rate coefficients for the rotational and hyperfine (de‐)excitation of the hydrogen cyanide (HCN) by collisions with H2 (j= 0), the most abundant collisional partner in cold molecular clouds. The scattering calculations are based on a new ab initio potential energy surface for the HCN–H2 collisional system, averaged over the H2 orientations. Close‐coupling calculations of pure rotational cross‐sections are performed for levels up to j= 10 and for total energies up to 1000 cm−1. The hyperfine cross‐sections are then obtained using a recoupling technique. The rotational and hyperfine cross‐sections are used to determine collisional rate coefficients for temperatures ranging from 5 to 100 K. A clear propensity rule in favour of even Δj rotational transitions is observed. The usual Δj=ΔF propensity rules are observed for the hyperfine transitions. The new rate coefficients are compared with the previous results obtained for the HCN molecule. Significant differences are found, mainly due to the use of H2 as a collisional partner instead of He. The new rate coefficients will significantly help in interpreting HCN emission lines observed with current and future telescopes.

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Rotational excitation and de-excitation of C2(X Σ1g+) by para-H2(j=)

Cited by 33 publications

References 24 publications

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C₂

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C₂

The excitation of circumstellar C₂in carbon-rich post-AGB object IRAS 22272+5435

Hyperfine excitation of HCN by H2 at low temperature

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Rotational excitation and de-excitation of C2(X Σ1g+) by para-H2(j=)

Cited by 33 publications

References 24 publications

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C2

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C2

The excitation of circumstellar C2in carbon-rich post-AGB object IRAS 22272+5435

Hyperfine excitation of HCN by H2 at low temperature

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ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C₂

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C₂

The excitation of circumstellar C₂in carbon-rich post-AGB object IRAS 22272+5435