Chemical solver to compute molecule and grain abundances and non-ideal MHD resistivities in prestellar core-collapse calculations

Abstract

We develop a detailed chemical network relevant to calculate the conditions that are characteristic of prestellar core collapse. We solve the system of time-dependent differential equations to calculate the equilibrium abundances of molecules and dust grains, with a size distribution given by size-bins for these latter. These abundances are used to compute the different non-ideal magneto-hydrodynamics resistivities (ambipolar, Ohmic and Hall), needed to carry out simulations of protostellar collapse. For the first time in this context, we take into account the evaporation of the grains, the thermal ionisation of Potassium, Sodium and Hydrogen at high temperature, and the thermionic emission of grains in the chemical network, and we explore the impact of various cosmic ray ionisation rates. All these processes significantly affect the non-ideal magneto-hydrodynamics resistivities, which will modify the dynamics of the collapse. Ambipolar diffusion and Hall effect dominate at low densities, up to n H = 10 12 cm −3 , after which Ohmic diffusion takes over. We find that the time-scale needed to reach chemical equilibrium is always shorter than the typical dynamical (free fall) one. This allows us to build a large, multi-dimensional multi-species equilibrium abundance table over a large temperature, density and ionisation rate ranges. This table, which we make accessible to the community, is used during first and second prestellar core collapse calculations to compute the non-ideal magneto-hydrodynamics resistivities, yielding a consistent dynamical-chemical description of this process.

Ambipolar diffusion in low-mass star formation

“…We agree with these conclusions and note that the resistivity above which they formed disks corresponds to or is slightly stronger than the typical values of ambipolar resistivity we used (see Marchand et al 2015) that lead to the formation of disks. However, these authors assumed a constant enhanced resistivity in the induction equation in the Laplace operator.…”

“…We followed Kunz & Mouschovias (2009) to compute the relevant charged species abundances including grains sizes in a classical MRN distribution (Mathis et al 1977), which were sampled using 50 bins. For an exhaustive description of the chemical model used and its application in the context of star formation, we refer to Marchand et al (2015). We computed a three-dimensional table of density, temperature, and magnetic field dependent resistivities covering the ranges 10 −24 < ρ < 10 −10 g cm −3 , 5 < T < 2000 K, and 10 −6 < B < 10 2 G, respectively.…”

Magnetically Self-Regulated Formation of Early Protoplanetary Disks

“…Because of the complex dependence of the resistivity η AD upon magnetic intensity B and density n (Marchand et al 2016), eqn. (1) cannot be solved analytically.…”

“…Note that, in principle, the magnetic resistivity η AD depends on density (see Fig. 5 of Marchand et al (2016)), but this dependence is very shallow. We find a more pronounced, although still moderate dependence of the radius upon B as ∼ B −0.5 z .…”

Ionization and Dust Charging in Protoplanetary Disks

“…When considering recombination on grains, we have so far also implicitly assumed that only free electrons and ions contribute to this process. Such an approach is natural since the thermal velocities of plasma species are much larger than typical thermal velocities of grains, and therefore the terms describing recombination due to mutual dust collisions (Umebayashi 1983;Umebayashi & Nakano 1990;Marchand et al 2016) are omitted in Equation (11). On the other hand, in a DD plasma grains become the most abundant charged ) recombination regimes, is marked by the grey shading.…”