Effects of turbulent dust grain motion to interstellar chemistry

Abstract

Theoretical studies have revealed that dust grains are usually moving fast through the turbulent interstellar gas, which could have significant effects upon interstellar chemistry by modifying grain accretion. This effect is investigated in this work on the basis of numerical gas-grain chemical modeling. Major features of the grain motion effect in the typical environment of dark clouds (DC) can be summarised as follows: 1) decrease of gas-phase (both neutral and ionic) abundances and increase of surface abundances by up to 2-3 orders of magnitude; 2) shifts of the existing chemical jumps to earlier evolution ages for gas-phase species and to later ages for surface species by factors of about ten; 3) a few exceptional cases in which some species turn out to be insensitive to this effect and some other species can show opposite behaviors too. These effects usually begin to emerge from a typical DC model age of about 10 5 yr. The grain motion in a typical cold neutral medium (CNM) can help overcome the Coulomb repulsive barrier to enable effective accretion of cations onto positively charged grains. As a result, the grain motion greatly enhances the abundances of some gas-phase and surface species by factors up to 2-6 or more orders of magnitude in the CNM model. The grain motion effect in a typical molecular cloud (MC) is intermediate between that of the DC and CNM models, but with weaker strength. The grain motion is found to be important to consider in chemical simulations of typical interstellar medium.

Three-dimensional Projection Effects on Chemistry in a Planck Galactic Cold Clump

“…The reaction network was taken from Semenov et al (2010) that has included most of the chemical species and reactions relevant to dark clouds. The photo-electron ejection from dust grains and ion accretion were added self-consistently by Ge et al (2016b) with extended range of grain charges from -5e to +99e.…”

“…We use the FORTRAN chemistry code "ggchem" that was successfully benchmarked against the standard models of Semenov et al (2010) and was updated to investigate the chemistry effects of turbulent dust motion (Ge et al 2016b) and the chemical differentiation of different dust grain size distributions (Ge et al 2016a). In this code, we use typical dust grain radius of 0.1 µm and bulk density of 3.0 g cm −3 .…”

Chemical Properties of Two Dense Cores in a Planck Galactic Cold Clump G168.72-15.48

“…For the initial abundances, the low-metal set is used (as listed in Table 5), which has been widely used for dark cloud models (Acharyya & Herbst 2017;Wakelam, & Herbst 2008). In addition, the GGCHEM Fortran code was used, which has been successfully benchmarked with various models in Semenov et al (2010), and has been used to study a number of astrochemical subjects such as the effects of turbulent dust motion on interstellar chemistry (Ge, He & Yan 2016a) and the chemical differentiation across dust grain sizes (Ge, He & Li 2016b).…”

Magnetic field measurement from the Davis–Chandrasekhar–Fermi method employed with atomic alignment

“…In addition, the magnetic fields impact several processes, including but not limited to plasma turbulence (Goldreich & Sridhar 1995;Cho & Vishniac 2000;Cho & Lazarian 2003), star formation (Crutcher 2012;McKee & Ostriker 2007;Fissel et al 2016), stellar feedback, cosmic-ray transport and acceleration (Schlickeiser 2002;Yan & Lazarian 2002, 2004, accretion disk dynamics, astrophysical jets, and the chemical evolution of the galaxy (see, e.g. Ge et al 2016). Therefore, accurately measuring the interstellar magnetic fields and their contributions to these processes is crucial in developing consistent theories.…”