Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: II. Ionization and Magnetic Field

Abstract

We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the line-of-sight magnetic field, have an abundance that decreases toward the center of the core much faster than the density increases. As a result, Zeeman observations tend to sample the outer layers of the core and consistently underestimate the core magnetic field. The degree of ionization follows a complicated dependence on the number density at central densities up to 10^5 cm^{-3} for magnetic models and 10^6 cm^{-3} in non-magnetic models. At higher central densities the scaling approaches a power-law with a slope of -0.6 and a normalization which depends on the cosmic-ray ionization rate ζ and the temperature T as (ζT)^1/2. The mean molecular weight of ions is systematically lower than the usually assumed value of 20 - 30, and, at high densities, approaches a value of 3 due to the asymptotic dominance of the H3+ ion. This significantly lower value implies that ambipolar diffusion operates faster.

7 pages, 5 figures, accepted for publication in ApJ

References in corpus (5)

• Theory of Star Formation
• Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations
• Dynamics of Dense Cores in the Perseus Molecular Cloud
• Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to Physical Units
• Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)

Cited by in corpus (14)

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• Non-ideal MHD simulations of subcritical prestellar cores with non-equilibrium chemistry
• Comparison of Prestellar Core Elongations and Large-Scale Molecular Cloud Structures in the Lupus I Region
• The widths of magnetised filaments in molecular clouds
• Investigating the role of magnetic fields in star formation using molecular line profiles
• Chemistry as a diagnostic of prestellar core geometry
• Can we observe the ion-neutral drift velocity in prestellar cores?
• Effect of OH depletion on measurements of the mass-to-flux ratio in molecular cloud cores
• The initial magnetic criticality of prestellar cores
• GMC Collisions As Triggers of Star Formation. IV. The Role of Ambipolar Diffusion
• Unveiling the substructure of the massive clump AGAL G035.1330$-$00.7450