Deuterium chemodynamics of massive pre-stellar cores

Abstract

ABSTRACT High levels of deuterium fractionation of N2H+ (i.e. $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$≳ 0.1) are often observed in pre-stellar cores (PSCs) and detection of N2D+ is a promising method to identify elusive massive PSCs. However, the physical and chemical conditions required to reach such high levels of deuteration are still uncertain, as is the diagnostic utility of N2H+ and N2D+ observations of PSCs. We perform 3D magnetohydrodynamics simulations of a massive, turbulent, magnetized PSC, coupled with a sophisticated deuteration astrochemical network. Although the core has some magnetic/turbulent support, it collapses under gravity in about one free-fall time, which marks the end of the simulations. Our fiducial model achieves relatively low $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$∼0.002 during this time. We then investigate effects of initial ortho-para ratio of H2 ($\mathrm{OPR^{H_2}}$), temperature, cosmic ray (CR) ionization rate, CO and N-species depletion factors, and prior PSC chemical evolution. We find that high CR ionization rates and high depletion factors allow the simulated $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$ and absolute abundances to match observational values within one free-fall time. For $\mathrm{OPR^{H_2}}$, while a lower initial value helps the growth of $\mathrm{D^{{\mathrm{N_2H^+}}}_{\text{frac}}}$, the spatial structure of deuteration is too widespread compared to observed systems. For an example model with elevated CR ionization rates and significant heavy element depletion, we then study the kinematic and dynamic properties of the core as traced by its N2D+ emission. The core, undergoing quite rapid collapse, exhibits disturbed kinematics in its average velocity map. Still, because of magnetic support, the core often appears kinematically subvirial based on its N2D+ velocity dispersion.