Methanol as a diagnostic tool of interstellar clouds

Leurini, S.; Schilke, P.; Menten, K. M.; Flower, D. R.; Pottage, J T; Xu, Li‐Hong

doi:10.1051/0004-6361:20047046

Cited by 89 publications

(110 citation statements)

References 26 publications

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“…It may, however, suffer from opacity problems, and its excitation is often more complex than LTE (see e.g. Menten et al 1988;Leurini et al 2004). Methyl cyanide is a well-known probe of kinetic temperature (Boucher et al 1980).…”

Section: Comparison With Previous Studiesmentioning

confidence: 99%

HCOOCH₃as a probe of temperature and structure in Orion-KL

Favre

Despois

Brouillet

et al. 2011

A&A

100

238

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Context. The Orion Kleinmann-Low nebula (Orion-KL) is a complex region of star formation. Whereas its proximity allows studies on a scale of a few hundred AU, spectral confusion makes it difficult to identify molecules with low abundances. Aims. We studied an important oxygenated molecule, HCOOCH 3 , to characterize the physical conditions, temperature, and density of the different molecular source components. Methyl formate presents strong close rotational transitions covering a wide range of energy, and its emission in Orion-KL is not contaminated by the emission of N-bearing molecules. This study will help in the future 1) to constrain chemical models for the formation of methyl formate in gas phase or on grain mantles and 2) to search for more complex or prebiotic molecules. Methods. We used high-resolution observations from the IRAM Plateau de Bure Interferometer to reduce spectral confusion and to better isolate the molecular emission regions. We used twelve data sets with a spatial resolution down to 1.8 × 0.8 . Continuum emission was subtracted by selecting apparently line-free channels. Results. We identify 28 methyl formate emission peaks throughout the 50 field of observations. The two strongest peaks, named MF1 and MF2, are in the Compact Ridge and in the southwest of the Hot Core, respectively. From a comparison with single-dish observations, we estimate that we miss less than 15% of the flux and that spectral confusion is still prevailing as half of the expected transitions are blended over the region. Assuming that the transitions are thermalized, we derive the temperature at the five main emission peaks. At the MF1 position in the Compact Ridge we find a temperature of 80 K in a 1.8 × 0.8 beam size and 120 K on a larger scale (3.6 × 2.2 ), suggesting an external source of heating, whereas the temperature is about 130 K at the MF2 position on both scales. Transitions of methyl formate in its first torsionally excited state are detected as well, and the good agreement of the positions on the rotational diagrams between the ground state and the v t = 1 transitions suggests a similar temperature. The LSR velocity of the gas is between 7.5 and 8.0 km s −1 depending on the positions and column density peaks vary from 1.6 × 10 16 to 1.6 × 10 17 cm −2 . A second velocity component is observed around 9−10 km s −1 in a north-south structure stretching from the Compact Ridge up to the BN object, and this component is warmer at the MF1 peak. The two other C 2 H 4 O 2 isomers are not detected, and the derived upper limit for the column density is ≤3 × 10 14 cm −2 for glycolaldehyde and ≤2 × 10 15 cm −2 for acetic acid. From the 223 GHz continuum map, we identify several dust clumps with associated gas masses in the range 0.8 to 5.8 M . Assuming that the methyl formate is spatially distributed as the dust is, we find relative abundances of methyl formate in the range ≤0.1 × 10 −8 to 5.2 × 10 −8 . We suggest a relation between the methyl formate distribution and shocks as traced by 2.12 μm H 2 emission.

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Section: Comparison With Previous Studiesmentioning

confidence: 99%

HCOOCH₃as a probe of temperature and structure in Orion-KL

Favre

Despois

Brouillet

et al. 2011

A&A

100

238

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“…The advantage is that column density, kinetic temperature and volume density can be constrained, if accurate collision rates are known. As with rotation diagrams, this method can be used to compute synthetic spectra to be compared with data with a χ 2 statistic Leurini et al 2004).…”

Section: Introductionmentioning

confidence: 99%

A computer program for fast non-LTE analysis of interstellar line spectra

Tak¹,

Black²,

Schöier³

et al. 2007

A&A

1,383

836

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Aims. The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. Methods. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/∼vdtak/radex/index.shtml. The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. Results. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. Conclusions. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes.

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“…Therefore, multi-line observation on methanol can be used to determine these two parameters in an interstellar cloud with one molecule only, thereby avoiding the problem of different spatial distributions of different tracers. 11 Despite the ample occurrence of methanol and the abundance of information gained by observations of this species, its interstellar formation has so far remained elusive. Although processes on grain surfaces and in grain mantles have been discussed, 12-14 a two step gas phase mechanism involving the radiative association of CH 3 + and H 2 O with subsequent dissociative recombination (DR) of the resulting CH 3 OH 2 + ion has been thought to be responsible for interstellar methanol production CH Radiative association reactions normally have rate coefficients that are too low to successfully compete with other interstellar ion processes.…”

Section: Introductionmentioning

confidence: 99%

Dissociative recombination of protonated methanol

et al. 2006

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The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH 3 OH 2 + and CD 3 OD 2 + have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH 3 OH 2 + and CD 3 OD 2 + , respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C-O bond, the major process being the three-body break-up forming CH 3 , OH and H (CD 3 , OD and D). The overall cross sections are best fitted by s = 1.2 AE 0.1 Â 10 À15 E À1.15AE0.02 cm 2 and s = 9.6 AE 0.9 Â 10 À16 E À1.20AE0.02 cm 2 for CH 3 OH 2 + and CD 3 OD 2 + , respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 AE 0.9 Â 10 À7 (T/300) À0.59AE0.02 cm 3 s À1 (CH 3 OH 2 + ) and k(T) = 9.1 AE 0.9 Â 10 À7 (T/300) À0.63AE0.02 cm 3 s À1 (CD 3 OD 2 + ) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH 3 + and H 2 O and subsequent dissociative recombination of the resulting CH 3 OH 2 + ion to yield methanol and hydrogen atoms is therefore very unlikely.

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Methanol as a diagnostic tool of interstellar clouds

Cited by 89 publications

References 26 publications

HCOOCH₃as a probe of temperature and structure in Orion-KL

HCOOCH₃as a probe of temperature and structure in Orion-KL

A computer program for fast non-LTE analysis of interstellar line spectra

Dissociative recombination of protonated methanol

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Methanol as a diagnostic tool of interstellar clouds

Cited by 89 publications

References 26 publications

HCOOCH3as a probe of temperature and structure in Orion-KL

HCOOCH3as a probe of temperature and structure in Orion-KL

A computer program for fast non-LTE analysis of interstellar line spectra

Dissociative recombination of protonated methanol

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HCOOCH₃as a probe of temperature and structure in Orion-KL

HCOOCH₃as a probe of temperature and structure in Orion-KL