Implicit Changes of Model Uses in Astrophysics, Illustrated on the Paris-Durham Shock Model

This paper explores the epistemic status of models and simulations between theory, on the one hand, and observations, on the other. In particular, I will argue that the interpretation of an essentially invariant astrophysical model structure can change substantially over time. I will illustrate this claim using as an example the first 20 years (1985-2004) of development of the Paris-Durham shock code-a numerical model of slow interstellar shock waves (i.e. a disturbance of the medium that travel faster than the local speed of sound). I will show that the model's interpretation and, in particular, its underlying representational ideal-the modeler's (often implicit) goal governing the development and the use of the model-changed notably during this period. Whereas the code was originally used in a purely exploratory fashion, it was later taken to represent and encompass the target phenomenon as completely as possible. It is noteworthy that during this transition the model's change of epistemic status was never explicitly acknowledged or in any way articulated. However, the impetus for the change can, I claim, be found in the role that observational data came to play in the later publications.