The rotational excitation by helium of methanol in its ground and first excited torsional states

Pottage, J T; Flower, D. R.; Davis, Stephen L.

doi:10.1088/0953-4075/35/11/312

Cited by 28 publications

(35 citation statements)

References 12 publications

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“…Only recently (see Pottage et al 2001Pottage et al , 2002, rate coefficients for collisions of methanol with helium, for both CH 3 OH-A and CH 3 OH-E, have been computed for levels up to (J, k 1 ) = 9 at kinetic temperatures up to 200 K. Calculations of rate coefficients for collisions with para-H 2 have been recently carried out and comparisons of our results with this new dataset will be analysed in a forthcoming paper. With this new set of collisional rates, we have carried out statistical equilibrium calculations on both A-and E-types for the torsional ground state, using the Large Velocity Gradient (LVG) method with spherical geometry in the derivation of de Jong et al (1975).…”

Section: Ch 3 Oh-he Collisional Ratesmentioning

confidence: 94%

“…Our calculations are extended to the first 100 levels for each state, for which the collisional rates from Pottage et al (2001Pottage et al ( , 2002 are available; the coverage in energy is thus incomplete above 100 K, (see Figs. 2a,b), and does not allow a reliable estimation of the partition function at high temperatures.…”

Section: Ch 3 Oh-he Collisional Ratesmentioning

confidence: 99%

“…Menten et al 1988), but up to now an extremely poor knowledge of the CH 3 OH collisional rates and of their propensity rules has prevented realistic systematic studies exploiting methanol's full potential as an interstellar tracer. Recently, this situation has changed with the calculation of collisional rate coefficients by Pottage et al (2001Pottage et al ( , 2002, which we have integrated in a "standard" Large Velocity Gradient program aimed at modelling methanol excitation. Here, we present an application of our model calculations to observations of clouds without IR radiation, while extensive modelling of a variety of evolved star-forming regions as well as Class I methanol masers will be discussed in accompanying papers.…”

Section: Introductionmentioning

confidence: 99%

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Methanol as a diagnostic tool of interstellar clouds

Leurini

Schilke

Menten

et al. 2004

A&A

106

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Abstract. We present a detailed analysis of the diagnostic properties of methanol, (CH 3 OH), in dense molecular clouds, made possible by the availability of new (CH 3 OH-He) collisional rate coefficients. Using a spherical Large Velocity Gradient (LVG) model, the dependence on kinetic temperature and spatial density of various millimeter and submillimeter line bands is investigated over a range of physical parameters typical of high-and low-mass star-forming regions. We find CH 3 OH to be a good tracer of high-density environments and sensitive to the kinetic temperature. Using our LVG model, we have also developed an innovative technique to handle the problem of deriving physical parameters from observed multi-line spectra of a molecule, based on the simultaneous fit of all the lines with a synthetic spectrum, finding the best physical parameters using numerical methods.

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Section: Ch 3 Oh-he Collisional Ratesmentioning

confidence: 94%

Section: Ch 3 Oh-he Collisional Ratesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Methanol as a diagnostic tool of interstellar clouds

Leurini

Schilke

Menten

et al. 2004

A&A

106

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“…For the analysis, we used the technique described by Leurini et al (2004) for the study of multi-line CH 3 OH observations, which consists of modelling all the lines simultaneously with a synthetic spectrum computed using the large velocity gradient approximation, and comparing it to the observations. Rest frequencies are taken from Xu & Lovas (1997), while the collisional rates were computed by Pottage et al (2002Pottage et al ( , 2004. The parameters defining the synthetic spectrum are: source size, kinetic temperature, column density, velocity width, and velocity offset (from the systematic velocity of the object).…”

Section: Physical Conditions In the Clumps : Ch 3 Oh Analysismentioning

confidence: 99%

Deuterium chemistry in the Orion Bar PDR

Parise¹,

Leurini

Schilke³

et al. 2009

A&A

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Context. High levels of deuterium fractionation in gas-phase molecules are usually associated with cold regions, such as prestellar cores. Significant fractionation ratios are also observed in hot environments such as hot cores or hot corinos, where they are believed to be produced by the evaporation of the icy mantles surrounding dust grains, and are thus remnants of a previous cold (either gasphase or grain surface) chemistry. The recent detection of DCN towards the Orion Bar, in a clump at a characteristic temperature of 70 K, has shown that high deuterium fractionation can also be detected in PDRs. The Orion Bar clumps thus appear to be a good environment for the observational study of deuterium fractionation in luke warm gas, allowing us to validate chemistry models for a different temperature range, where dominating fractionation processes are predicted to differ from those in cold gas (<20 K). Aims. We aimed to study observationally in detail the chemistry at work in the Orion Bar PDR, to understand whether DCN is either produced by ice mantle evaporation or is the result of warm gas-phase chemistry, involving the CH 2 D + precursor ion (which survives higher temperatures than the usual H 2 D + precursor). Methods. Using the APEX and the IRAM 30 m telescopes, we targeted selected deuterated species towards two clumps in the Orion Bar. Results. We confirmed the detection of DCN and detected two new deuterated molecules (DCO + and HDCO) towards one clump in the Orion Bar PDR. Significant deuterium fractionations are found for HCN and H 2 CO, but we measured a low fractionation in HCO + . We also provide upper limits to other molecules relevant to deuterium chemistry. Conclusions. We argue that grain evaporation in the clumps is unlikely to be a dominant process, and we find that the observed deuterium fractionation ratios are consistent with predictions of pure gas-phase chemistry models at warm temperatures (T ∼ 50 K). We show evidence that warm deuterium chemistry driven by CH 2 D + is at work in the clumps.

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“…An LVG source code for methanol (CH 3 OH) was kindly provided by S. Leurini. Einstein coefficients were taken from Cragg et al (1993), energy levels were adopted from Xu & Lovas (1997), and collision rates are from Pottage et al (2001Pottage et al ( , 2002 for T kin ∼ 20 K and from D. Flower (priv. comm.)…”

Section: Ch 3 Ohmentioning

confidence: 99%

Dense gas in nearby galaxies

Wang

Henkel²,

Chin

et al. 2004

A&A

129

184

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Abstract.A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945 has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are also presented. An analysis of CO and 1.3 mm continuum fluxes indicates that the conversion factor between H 2 column density and CO J = 1−0 integrated intensity is smaller than in the galactic disk by factors of 5−10. A large number of molecular species indicate the presence of a prominent high density interstellar gas component characterized by n H 2 ∼ 10 5 cm −3 . Some spectra show Gaussian profiles. Others exhibit two main velocity components, one at ∼450 km s −1 , the other at ∼710 km s −1 . While the gas in the former component has a higher linewidth, the latter component arises from gas that is more highly excited as is indicated by HCN, HCO + and CN spectra. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an "overabundance" of HNC in the nuclear environment of NGC 4945. While the HNC/HCN J = 1−0 line intensity ratio is ∼0.5, the HNC/HCN abundance ratio is ∼1. From a comparison of K a = 0 and 1 HNCO line intensities, an upper limit to the background radiation of 30 K is derived. While HCN is subthermally excited (T ex ∼ 8 K), CN is even less excited (T ex ∼ 3−4 K), indicating that it arises from a less dense gas component and that its N = 2−1 line can be optically thin even though its N = 1−0 emission is moderately optically thick. Overall, fractional abundances of NGC 4945 suggest that the starburst has reached a stage of evolution that is intermediate between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. Within the limits of uncertainty, carbon and oxygen isotope ratios appear to be the same in the nuclear regions of NGC 4945 and NGC 253. S ratios (6.4 ± 0.3, 195 ± 45, 105 ± 25 and 13.5 ± 2.5 in NGC 4945, respectively) appear to be characteristic properties of a starburst environment in which massive stars have had sufficient time to affect the isotopic composition of the surrounding interstellar medium.

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The rotational excitation by helium of methanol in its ground and first excited torsional states

Cited by 28 publications

References 12 publications

Methanol as a diagnostic tool of interstellar clouds

Methanol as a diagnostic tool of interstellar clouds

Deuterium chemistry in the Orion Bar PDR

Dense gas in nearby galaxies

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