The importance of non-LTE models for the interpretation of observations of interstellar NO

Lique, François; Tak, van der Floris; Kłos, Jacek; Bulthuis, J.; Alexander, Millard H.

doi:10.1051/0004-6361:200810453

Cited by 50 publications

(40 citation statements)

References 30 publications

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“…The collisional rate coefficients for NO with H 2 were adopted from the lamda database, where they were scaled from the rates for NO with He by Lique et al (2009). We obtained abundances of [NO] = 5 × 10 −8 and a very similar value was estimated by Stäuber et al (2007), [NO] = 3.1 × 10 −8 .…”

Section: Constant Abundance Model With a Few Transitionsmentioning

confidence: 99%

The HIFI spectral survey of AFGL 2591 (CHESS)

Kaźmierczak-Barthel

Semenov

Tak

et al. 2015

A&A

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Aims. The aim of this work is to understand the richness of chemical species observed in the isolated high-mass envelope of AFGL 2591, a prototypical object for studying massive star formation. Methods. Based on HIFI and JCMT data, the molecular abundances of species found in the protostellar envelope of AFGL 2591 were derived with a Monte Carlo radiative transfer code (Ratran), assuming a mixture of constant and 1D stepwise radial profiles for abundance distributions. The reconstructed 1D abundances were compared with the results of the time-dependent gas-grain chemical modeling, using the best-fit 1D power-law density structure. The chemical simulations were performed considering ages of 1−5 × 10 4 years, cosmic ray ionization rates of 5−500 × 10 −17 s −1 , uniformly-sized 0.1−1 μm dust grains, a dust/gas ratio of 1%, and several sets of initial molecular abundances with C/O < 1 and >1. The most important model parameters varied one by one in the simulations are age, cosmic ray ionization rate, external UV intensity, and grain size. Results. Constant abundance models give good fits to the data for CO, CN, CS, HCO + , H 2 CO, N 2 H + , CCH, NO, OCS, OH, H 2 CS, O, C, C + , and CH. Models with an abundance jump at 100 K give good fits to the data for NH 3 , SO, SO 2 , H 2 S, H 2 O, HCl, and CH 3 OH. For HCN and HNC, the best models have an abundance jump at 230 K. The time-dependent chemical model can accurately explain abundance profiles of 15 out of these 24 species. The jump-like radial profiles for key species like HCO + , NH 3 , and H 2 O are consistent with the outcome of the time-dependent chemical modeling. The best-fit model has a chemical age of ∼10−50 kyr, a solar C/O ratio of 0.44, and a cosmic-ray ionization rate of ∼5 × 10 −17 s −1 . The grain properties and the intensity of the external UV field do not strongly affect the chemical structure of the AFGL 2591 envelope, whereas its chemical age, the cosmic-ray ionization rate, and the initial abundances play an important role. Conclusions. We demonstrate that simple constant or jump-like abundance profiles constrained with 1D Ratran line radiative transfer simulations are in agreement with time-dependent chemical modeling for most key C-, O-, N-, and S-bearing molecules. The main exceptions are species with very few observed transitions (C, O, C + , and CH) or with a poorly established chemical network (HCl, H 2 S) or whose chemistry is strongly affected by surface processes (CH 3 OH).

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Section: Constant Abundance Model With a Few Transitionsmentioning

confidence: 99%

The HIFI spectral survey of AFGL 2591 (CHESS)

Kaźmierczak-Barthel

Semenov

Tak

et al. 2015

A&A

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“…Non-LTE models of NO were carried out using the RADEX code and collision rates from Lique et al (2009b for n(H 2 ) = 5 × 10 5 cm −3 , T k = 77.5 K, and 10 4 cm −3 , 100 K, respectively. The corresponding column densities of O 2 in a non-LTE treatment are 6.5 and 7.5 ×10 16 cm −2 , respectively.…”

Section: O 2 Line Velocities and Source Identificationmentioning

confidence: 99%

HERSCHELMEASUREMENTS OF MOLECULAR OXYGEN IN ORION

Goldsmith

Liseau

Bell

et al. 2011

ApJ

150

146

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“…Following Turner et al (1999), we approximate the collision rates of C 2 H using those for HCN calculated by Green & Thaddeus (1974). From these rates, which do not include hyperfine structure, we derive a new set of rates with hyperfine structure assuming that the new rates are simply proportional to the degeneracy of the final state (Guilloteau & Baudry 1981;Lique et al 2009). Additional ΔN = 0 rates were included by following the recipe from Turner et al (1999), and a total of 30 levels with 37 transitions (up to N = 7 and an energy equivalent to 120 K) were used in the calculation.…”

Section: Monte Carlo Treatment Of Radiative Transfer In the C 2 H Rotmentioning

confidence: 99%

C₂H in prestellar cores

Padovani

Walmsley

Tafalla³

et al. 2009

A&A

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Aims. We study the abundance of C 2 H in prestellar cores both because of its role in the chemistry and because it is a potential probe of the magnetic field. We also consider the non-LTE behaviour of the N = 1−0 and N = 2−1 transitions of C 2 H and improve current estimates of the spectroscopic constants of C 2 H. Methods. We used the IRAM 30 m radiotelescope to map the N = 1−0 and N = 2−1 transitions of C 2 H towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J = 1−0 transition of N 2 H + and the J = 2−1 transition of C 18 O. We used a Monte Carlo radiative transfer program to analyse the C 2 H observations of L1498. We derived the distribution of C 2 H column densities and compared with the H 2 column densities inferred from dust emission. Results. We find that while non-LTE intensity ratios of different components of the N = 1−0 and N = 2−1 lines are present, they are of minor importance and do not impede C 2 H column density determinations based upon LTE analysis. Moreover, the comparison of our Monte-Carlo calculations with observations suggest that the non-LTE deviations can be qualitatively understood. For extinctions less than 20 visual magnitudes, we derive toward these two cores (assuming LTE) a relative abundance [C 2 H]/[H 2 ] of (1.0± 0.3)× 10 −8 in L1498 and (0.9 ± 0.3) × 10 −8 in CB246 in reasonable agreement with our Monte-Carlo estimates. For L1498, our observations in conjunction with the Monte Carlo code imply a C 2 H depletion hole of radius 9 × 10 16 cm similar to that found for other C-containing species. We briefly discuss the significance of the observed C 2 H abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the C 2 H (1−0) line and seven components of C 2 H (2−1). Based on these results, we compute improved spectroscopic constants for C 2 H. We also give a brief discussion of the prospects for measuring magnetic field strengths using C 2 H.

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The importance of non-LTE models for the interpretation of observations of interstellar NO

Cited by 50 publications

References 30 publications

The HIFI spectral survey of AFGL 2591 (CHESS)

The HIFI spectral survey of AFGL 2591 (CHESS)

HERSCHELMEASUREMENTS OF MOLECULAR OXYGEN IN ORION

C₂H in prestellar cores

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The importance of non-LTE models for the interpretation of observations of interstellar NO

Cited by 50 publications

References 30 publications

The HIFI spectral survey of AFGL 2591 (CHESS)

The HIFI spectral survey of AFGL 2591 (CHESS)

HERSCHELMEASUREMENTS OF MOLECULAR OXYGEN IN ORION

C2H in prestellar cores

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Resources

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C₂H in prestellar cores