Shock structures of astrospheres

TL;DR

This paper reviews shock structures in astrospheres, focusing on hydrodynamic models, and explores how heating, cooling, and analytical relations influence the observable shock features.

Contribution

It provides a detailed analysis of shock structures in astrospheres using single fluid hydrodynamic models, including effects of heating and cooling, and compares analytical and numerical approaches.

Findings

Shock structures are complex and observable in some objects.

Heating and cooling significantly affect the shock region.

The termination shock distance is the only length scale obtainable.

Abstract

The interaction between a supersonic stellar wind and a (super-)sonic interstellar wind has recently been viewed with new interest. We here first give an overview of the modeling, which includes the heliosphere as an example of a special astrosphere. Then we concentrate on the shock structures of fluid models, especially of hydrodynamic (HD) models. More involved models taking into account radiation transfer and magnetic fields are briefly sketched. Even the relatively simple HD models show a rich shock structure, which might be observable in some objects. We employ a single fluid model to study these complex shock structures, and compare the results obtained including heating and cooling with results obtained without these effects. Furthermore, we show that in the hypersonic case valuable information of the shock structure can be obtained from the Rankine-Hugoniot equations. We solved the Euler equations for the single fluid case and also for a case including cooling and heating. We also discuss the analytical Rankine-Hugoniot relations and their relevance to observations. We show that the only obtainable length scale is the termination shock distance. Moreover, the so-called thin shell approximation is usually not valid. We present the shock structure in the model that includes heating and cooling, which differs remarkably from that of a single fluid scenario in the region of the shocked interstellar medium. We find that the heating and cooling is mainly important in this region and is negligible in the regions dominated by the stellar wind beyond an inner boundary.