Time scale on which the inclination is expected to change and spiral arms are expected to form for the disc parameters, and corresponding typical radii (see Sect. 4.2), in our simulations as a function of their ambient density and relative velocity (Eq. (8)). The spacing of the isochrones depends on the influence of ram pressure stripping. The circled regions provide an order of magnitude estimate for different environments: winds from massive stars and giants (Owocki 2013; Van der Helm et al., in prep.), supernova ejecta (SN, Ouellette et al. 2007) and embedded open and young massive clusters (EC, Lada & Lada 2003; Portegies Zwart et al. 2010).