C-type shock modelling – the effect of new H2–H collisional rate coefficients

Abstract

We consider collisional excitation of H 2 molecules in C-type shocks propagating in dense molecular clouds. New data on collisional rate coefficients for (de-)excitation of H 2 molecule in collisions with H atoms and new H 2 dissociation rates are used. The new H 2 -H collisional data are state of the art and are based on the most accurate H 3 potential energy surface. We re-examine the excitation of rotational levels of H 2 molecule, the para-to-ortho-H 2 conversion, and H 2 dissociation by H 2 -H collisions. At cosmic ray ionization rates ζ 10 −16 s −1 and at moderate shock speeds, the H/H 2 ratio at the shock front is mainly determined by the cosmic ray ionization rate. The H 2 -H collisions play the main role in the para-to-ortho-H 2 conversion and, at ζ 10 −15 s −1 , in the excitation of vibrationally excited states of H 2 molecule in the shock. The H 2 ortho-to-para ratio (OPR) of the shocked gas and column densities of rotational levels of vibrationally excited states of H 2 are found to depend strongly on the cosmic ray ionization rate. We discuss the applicability of the presented results to interpretation of observations of H 2 emission in supernova remnants.

Modelling cosmic masers in C-type shock waves -- the coexistence of Class I CH3OH and 1720 MHz OH masers

“…In our model, the dust is assumed to be composed of spherical silicate particles of 0.05 µm in radius, and the dust-gas mass ratio is taken to be equal to 0.01. The specific surface area of dust is 10 −21 cm 2 per H. These dust parameters are reconciled with the mean H 2 production rate observed in molecular clouds (Nesterenok et al 2019).…”

“…As an estimate of the magnetic field component in the pre-shock gas transverse to the gas flow direction, we use equation ( 4) with the parameter 𝛽 = 1 (maximal magnetic field strength). Note, we used the parametrization of the magnetic field strength by Crutcher (1999) in our previous studies (Nesterenok et al 2019;Nesterenok 2020a). The parametrizations by Crutcher (1999) and Crutcher et al (2010) give similar estimates of the magnetic field strength in molecular gas at 10 3 𝑛 H,tot 10 5 cm −3 .…”

“…In a hot molecular gas behind interstellar shock, the reactive collisions (which include exchange of protons) of H 2 with H is the main mechanism of ortho-/para-H 2 interconversion. The para-to-ortho-H 2 conversion in the shock wave was considered in detail in our recent paper (Nesterenok et al 2019). In simulations, we take into account the following processes of the ortho-/para-H 2 interconversion: reactive collisions H 2 -H (Lique 2015;Bossion et al 2018), collisions H 2 -H + (González-Lezana & Honvault 2017), and H 2 formation on dust grains.…”

“…Moreover, the gas temperature, velocity gradient and other parameters are not constant and vary with the coordinates. In our calculations we use the magnetohydrodynamic model of C-type shock coupled to a full gas-grain chemical network (Nesterenok 2018;Nesterenok et al 2019). Flower et al (2010) reported the first calculations of the spectrum of methanol emerging from the shock waves in molecular outflows.…”

The candidates for Class I methanol masers

“…The interstellar shocks compress and heat up the gas, which is a conducive condition for the collisional pumping of Class I methanol masers. Here, we use the magnetohydrodynamic model of C-type shock wave published by [16,17] to study the collisional excitation of methanol and emergence of maser radiation. In this work we provide the list of methanol transitions that have population inversion in the post-shock region and may be observed as masers.…”