Rotational excitation of HCN by para- and ortho-H2

Vera, Mario Hernández; Kalugina, Yulia N.; Denis-Alpizar, Otoniel; Stoecklin, Thierry; Lique, François

doi:10.1063/1.4880499

Cited by 30 publications

(30 citation statements)

References 41 publications

(40 reference statements)

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“…In Ben Abdallah et al (2012) the calculations considered H2 as a structureless collisional partner. The results of this approximation differ from those obtained by Vera et al (2014), who use a more advanced HCN-H2 PES, taking fully into account the orientation of the H2 molecule, performing scattering calculations which consider the rotational structure of both species, but the computational burden of the more precise calculations did not allow for the calculation of the individual F-level rate coefficients. Work is ongoing and hyperfine rates from the new PES are expected in the future.…”

Section: Collisional Coefficient Formalismcontrasting

confidence: 61%

“…Thus the collisional rates derived using this approximate PES, including the individual hyperfines or F-level rates, may not be as accurate as rates derived from more accurate calculations of the PES, even if the F-level rates are derived with further approximate methods rather than calculated explicitly. For example, we can scale the J-level rate coefficients derived in Vera et al (2014) into F-level rates by assuming that the J-level rates are scaled into the same proportions as the F-level rates calculated explicitly by Ben Abdallah et al (2012). For comparison, we also scale the J-level rate coefficients of Green & Thaddeus (1974) according to the 'proportional method', as initially suggested by Guilloteau & Baudry (1981), and further demonstrated in modelling non-LTE hyperfine line emission for N2H + in Keto & Rybicki (2010).…”

Section: Introductionmentioning

confidence: 99%

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Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Loughnane

Redman

Wiles

et al. 2016

Mon. Not. R. Astron. Soc.

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Molecules with hyperfine splitting of their rotational line spectra are useful probes of optical depth, via the relative line strengths of their hyperfine components.The hyperfine splitting is particularly advantageous in interpreting the physical conditions of the emitting gas because with a second rotational transition, both gas density and temperature can be derived. For HCN however, the relative strengths of the hyperfine lines are anomalous. They appear in ratios which can vary significantly from source to source, and are inconsistent with local thermodynamic equilibrium. This is the HCN hyperfine anomaly, and it prevents the use of simple LTE models of HCN emission to derive reliable optical depths. In this paper we demonstrate how to model HCN hyperfine line emission, and derive accurate line ratios, spectral line shapes and optical depths. We show that by carrying out radiative transfer calculations over each hyperfine level individually, as opposed to summing them over each rotational level, the anomalous hyperfine emission emerges naturally. To do this requires not only accurate radiative rates between hyperfine states, but also accurate collisional rates. We investigate the effects of different sets of hyperfine collisional rates, derived via theÒproportional methodÓ and through direct recoupling calculations. Through an extensive parameter sweep over typical low mass star forming conditions, we show the HCN line ratios to be highly variable to optical depth. We also reproduce an observed effect whereby the red-blue asymmetry of the hyperfine lines (an infall signature) switches sense within a single rotational transition.

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Section: Collisional Coefficient Formalismcontrasting

confidence: 61%

Section: Introductionmentioning

confidence: 99%

Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Loughnane

Redman

Wiles

et al. 2016

Mon. Not. R. Astron. Soc.

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“…It was previously observed for many other systems where the target has a large dipole, e.g. for HCN-H2 (Vera et al 2014). In the case of formaldehyde (H2CO), it was even used to indirectly constrain the orthoto-para ratio of H2 (Troscompt et al 2009).…”

Section: Collisional Propensity Rulessupporting

confidence: 59%

Collisional excitation of HC₃N by para- and ortho-H₂

Fauré

Lique

Wiesenfeld

2016

Mon. Not. R. Astron. Soc.

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New calculations for rotational excitation of cyanoacetylene by collisions with hydrogen molecules are performed to include the lowest 38 rotational levels of HC 3 N and kinetic temperatures to 300 K. Calculations are based on the interaction potential of Wernli et al. A&A, 464, 1147 (2007) whose accuracy is checked against spectroscopic measurements of the HC 3 N-H 2 complex. The quantum coupled-channel approach is employed and complemented by quasi-classical trajectory calculations. Rate coefficients for ortho-H 2 are provided for the first time. Hyperfine resolved rate coefficients are also deduced. Collisional propensity rules are discussed and comparisons between quantum and classical rate coefficients are presented. This collisional data should prove useful in interpreting HC 3 N observations in the cold and warm ISM, as well as in protoplanetary disks.

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“…We again adopt HCN as a representative high-dipole moment molecule. We adopt the rate coefficients for collisions with H 2 from Dumouchel et al (2010), understanding that while the result of Ben Abdallah et al (2012) are quite similar, the situation could be quite different if a large fraction of the H 2 were in states having j ≥ 1 and the results of Vera et al (2014) discussed above obtain.…”

Section: Simplified Models Of Excitationmentioning

confidence: 99%

“…Ben Abdallah et al (2012) treat the colliding H 2 molecule as having internal structure, but average over H 2 orientations, considering effectively only molecular hydrogen in the j = 0 level (we employ lower case j to denote the rotational level of H 2 in order to avoid confusion with the rotational level of HCN). Vera et al (2014) have recently calculated collisions between HCN and H 2 molecules, considering for the first time the latter in individual rotational states. They find that there is a significant difference between collisions with the H 2 in the j = 0 level, compared to being in higher rotational levels.…”

Section: Hcn Excitation By H and Hmentioning

confidence: 99%

Electron Excitation of High Dipole Moment Molecules Re-examined

Goldsmith

Kauffmann

2017

ApJ

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Emission from high-dipole moment molecules such as HCN allows determination of the density in molecular clouds, and is often considered to trace the "dense" gas available for star formation. We assess the importance of electron excitation in various environments. The ratio of the rate coefficients for electrons and H 2 molecules, 10 5 for HCN, yields the requirements for electron excitation to be of practical importance if n(H 2 ) ≤ 10 5.5 cm −3 and X(e − ) ≥ 10 −5 , where the numerical factors reflect critical values n c (H 2 ) and X * (e − ). This indicates that in regions where a large fraction of carbon is ionized, X(e − ) will be large enough to make electron excitation significant. The situation is in general similar for other "high density tracers", including HCO + , CN, and CS. But there are significant differences in the critical electron fractional abundance, X * (e − ), defined by the value required for equal effect from collisions with H 2 and e − . Electron excitation is, for example, unimportant for CO and C + . Electron excitation may be responsible for the surprisingly large spatial extent of the emission from dense gas tracers in some molecular clouds (Pety et al. 2017;Kauffmann, Goldsmith et al. 2017). The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO + ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers. The importance of electron excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-static.

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Rotational excitation of HCN by para- and ortho-H2

Cited by 30 publications

References 41 publications

Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Collisional excitation of HC₃N by para- and ortho-H₂

Electron Excitation of High Dipole Moment Molecules Re-examined

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Rotational excitation of HCN by para- and ortho-H2

Cited by 30 publications

References 41 publications

Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Collisional excitation of HC3N by para- and ortho-H2

Electron Excitation of High Dipole Moment Molecules Re-examined

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Collisional excitation of HC₃N by para- and ortho-H₂