Abstract
When a fast moving star or a protostellar jet hits an interstellar cloud, the surrounding gas gets heated and illuminated: a bow shock is born which delineates the wake of the impact. In such a process, the new molecules that are formed and excited in the gas phase become accessible to observations. In this article, we revisit models of H2 emission in these bow shocks. We approximate the bow shock by a statistical distribution of planar shocks computed with a magnetized shock model. We improve on previous works by considering arbitrary bow shapes, a finite irradiation field, and by including the age effect of non-stationary C-type shocks on the excitation diagram and line profiles of H2. We also examine the dependence of the line profiles on the shock velocity and on the viewing angle: we suggest that spectrally resolved observations may greatly help to probe the dynamics inside the bow shock. For reasonable bow shapes, our analysis shows that low velocity shocks largely contribute to H2 excitation diagram. This can result in an observational bias towards low velocities when planar shocks are used to interpret H2 emission from an unresolved bow. We also report a large magnetization bias when the velocity of the planar model is set independently. Our 3D models reproduce excitation diagrams in BHR71 and Orion bow shocks better than previous 1D models. Our 3D model is also able to reproduce the shape and width of the broad H2 1-0S(1) line profile in an Orion bow shock.
Highlights
Jets or winds are generated in the early stages and the late phases of stellar evolution
We extend Gustafsson et al (2010)’s works on H2 emission by computing the excitation diagram and line profiles integrated over the bow, and by considering the effect of short ages where C-shocks have not yet reached steady-state
We examine the observable H2 excitation diagram and the potential biases which arise when 1D models are fit to intrinsically 3D models
Summary
Jets or winds are generated in the early stages and the late phases of stellar evolution. The local effective entrance velocity and the transverse magnetic field change along the shock working surface This leads to differences in the local physical and chemical conditions, which cause varying emission properties throughout the bow shock. In addition the Paris-Durham code (Flower et al 2003; Flower & Pineau des Forets 2015), recently improved by Lesaffre et al (2013), allows to consider finite UV irradiation conditions and we use a standard interstellar irradiation field of G0=1 (Draine 1978) throughout the paper This lowers slighlty further the magnetosonic speed as the ionisation degree/fraction increases but we checked it does not introduce critical changes for the H2 emission properties.
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