01whole.pdf (11.47 MB)
Multifluid shock waves in molecular clouds
thesisposted on 2022-03-28, 15:02 authored by Andrew Lehmann
Nonthermal linewidths in molecular clouds reveal the presence of highly supersonic turbulence, which inevitably dissipates by a network of shock waves. A multifluid treatment of these shocks is necessitated by low ionization fractions and strong magnetic field gradients. In this thesis, a two-fluid model of magnetised radiative shocks is developed in which neutrals are heated by ion-neutral friction and cooled by ro-vibrational molecular lines. The structure of fast and slow magnetohydrodynamic shocks are compared at velocities of the order of the Alfvén velocity, appropriate for shocks driven by turbulence. Slow shocks are hotter than fast shocks at the same velocity, and their radiative signatures fit observations of infrared dark clouds in the Milky Way and giant molecular clouds near the Galactic Centre. An algorithm is developed to characterise the shocks in simulations of molecular cloud turbulence. Both fast and slow shocks are present, and the distributions of shock speeds,Alfvénic Mach numbers and preshock conditions are used to produce synthetic emission maps of CO and to predict the volume of shock-heated gas. Finally, two-fluid dusty gas shocks in protoplanetary discs are considered. Two distinct shock solutions analogous to C- and J-type magnetised shocks are identified and these shocks are ideal benchmarking problems for numerical codes seeking to simulate dusty gas in protoplanetary discs. In addition, a J-type dusty shock is used to model the accretion shock above protoplanetary discs.Two-fluid effects are most important for grains larger than 1 ɥm,and dust emission from the shock is sensitive to the dust-to-gas ratio of the infalling material.