Methanol line formation in outflow sources

The understanding of organic content of protoplanetary discs is one of the main goals of the planet formation studies. As an attempt to guide the observational searches for weak lines of complex species in discs, we modelled the (sub-)millimetre spectrum of gaseous methanol (CH 3 OH), one of the simplest organic molecules, in the representative T Tauri system. We used 1+1D disc physical model coupled to the gas-grain ALCHEMIC chemical model with and without 2D-turbulent mixing. The computed CH 3 OH abundances along with the CH 3 OH scheme of energy levels of ground and excited torsional states were used to produce model spectra obtained with the non-local thermodynamic equilibrium (non-LTE) 3D line radiative transfer code LIME. We found that the modelled non-LTE intensities of the CH 3 OH lines can be lower by factor of > 10-100 than those calculated under assumption of LTE. Though population inversion occurs in the model calculations for many (sub-)millimetre transitions, it does not lead to the strong maser amplification and noticeably high line intensities. We identify the strongest CH 3 OH (sub-)millimetre lines that could be searched for with the Atacama Large Millimeter Array (ALMA) in nearby discs. The two best candidates are the CH 3 OH 5 0 − 4 0 A + (241.791 GHz) and 5 −1 − 4 −1 E (241.767 GHz) lines, which could possibly be detected with the ∼ 5σ signal-to-noise ratio after ∼ 3 hours of integration with the full ALMA array.

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“…For our LRT calculations we assumed that the A and E species of methanol are equally abundant (see e.g. Flower, Pineau Des Forêts, Rabli 2010).…”

Section: Turbulent Chemistry Model and Methanol Abundancesmentioning

confidence: 99%

Towards detecting methanol emission in low-mass protoplanetary discs with ALMA: the role of non-LTE excitation

Parfenov

Semenov

Sobolev

et al. 2016

Mon. Not. R. Astron. Soc.

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“…Recent applications of MHD shock wave models to the study of outflow sources are to be found in Gusdorf et al (2008a,b), Flower, Pineau des Forêts & Rabli (2010) and Viti et al (2011). In all of these studies, the treatment of the shock wave dynamics derives from the work of Flower & Pineau des Forêts (2003), who considered the electrical charging of the grains and their influence on the dynamical and chemical profiles of the shock wave.…”

Section: Introductionmentioning

confidence: 99%

“…Given the dynamical age of the blue lobe of the L1157 outflow, which has been estimated to be only 2000–3000 yr (Gueth, Guilloteau & Bachiller 1998), the shock wave is unlikely to have reached steady state. The C‐ and CJ‐type shock wave models that we have used to simulate the spectrum of L1157 B1 incorporate a number of significant improvements on the work of Gusdorf et al (2008b) and the more recent study of methanol by Flower et al (2010). The emission‐line spectra of CO, SiO, ortho‐ and para‐H 2 O, ortho‐ and para‐NH 3 , and A‐ and E‐type CH 3 OH are all computed in a completely self‐consistent manner within the shock wave model , applying a large‐velocity‐gradient (LVG) approximation to the line transfer problem.…”

Section: Introductionmentioning

confidence: 99%

Time-dependent modelling of the molecular line emission from shock waves in outflow sources

Flower

Forêts

2012

Monthly Notices of the Royal Astronomical Society

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We have developed further the technique of time‐dependent modelling of magnetohydrodynamic shock waves, with a view to interpreting the molecular line emission from outflow sources. The extensively observed source L1157 B1 was chosen as an exemplar of the application of this technique. The dynamical age of the shock wave model was varied in the range 500 ≤t≤ 5000 yr, with the best fit to the observed line intensities being obtained for t= 1000 yr; this is of the same order as the dynamical age derived by Gueth, Guilloteau & Bachiller from their observations of L1157 B1. The emission line spectra of H2, CO, SiO, ortho‐ and para‐H2O, ortho‐ and para‐NH3, and A‐ and E‐type CH3OH were calculated in parallel with the dynamical and chemical parameters of the model, using the ‘large velocity gradient’ (LVG) approximation to the line transfer problem. We compared the predictions of the models with the observed intensities of emission lines of H2, CO, SiO, ortho‐H2O, ortho‐NH3 and CH3OH, which include recent Herschel satellite measurements. In the case of SiO, we show (in Appendix A) that extrapolations of the collisional rate coefficients beyond the range of kinetic temperature for which they were originally calculated lead to spurious rotational line intensities and profiles. The computed emission‐line spectra of SiO, NH3 and CH3OH are shown to depend on the assumed initial composition of the grain mantles, from whence they are released, by sputtering in the shock wave, into the gas phase. The dependence of the model predictions on the adopted form of the grain‐size distribution is investigated in Appendix B; the corresponding integral line intensities are given in tabular form, for a range of C‐type shock speeds, in the online Supporting Information.

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“…Methanol has been incorporated into our MHD shock wave code, and its predictions were compared with the Herschel HIFI spectrum of L1157 B1 [5]. The equations for the escape probabilities and level populations were integrated in parallel with the dynamical and chemical rate equations.…”

Section: Shock-wave Modelmentioning

confidence: 99%

Quantum mechanical and astrophysical studies of methanol

Flower

Rabli

Forêts

2012

EPJ Web of Conferences

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Abstract. Interstellar methanol has been observed extensively at radio frequencies from the ground, and, very recently, it has been observed at higher frequencies, by means of the Herschel satellite. Being a complex molecule, methanol has a rich spectrum exhibiting rotational and internal torsional motions. However, by the same token, the determination of the cross sections and rate coefficients for the excitation of methanol by the principal perturbers, helium and molecular hydrogen, is a far from trivial task. We have recently extended and considerably improved previous calculations of these data. In the case of molecular hydrogen, results are now available for the excitation of both A-and E-type methanol, not only by para-but also by ortho-H 2 . These data have been used to model the HIFI observations of the outflow source L1157 B1. The methanol emission is computed self-consistently, in parallel with the dynamics and the chemistry, allowing for the optical depths in the emission lines by means of the LVG approximation. The results of these calculations are summarized.

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Methanol line formation in outflow sources

Cited by 33 publications

References 20 publications

Towards detecting methanol emission in low-mass protoplanetary discs with ALMA: the role of non-LTE excitation

Towards detecting methanol emission in low-mass protoplanetary discs with ALMA: the role of non-LTE excitation

Time-dependent modelling of the molecular line emission from shock waves in outflow sources

Quantum mechanical and astrophysical studies of methanol

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