The timesteps with which the positions of the particles are traced have to be decreased in the dense regions to guarantee the stability and accuracy of the integration. This ultimately leads to the decrease of the timesteps with very small values and the computation becomes inefficient and must be stopped. No boundary conditions are applied to the integration volume, the hot, ionized gas is allowed to stream away freely. This corresponds to the situation which is observed often in H II -regions which are located at the border of a dense molecular cloud and from which the ionized gas can flow freely into the interstellar medium with densities orders of magnitude lower than those in the molecular cloud. Ionizing radiation impinging from the bottom.
Density is plotted logarithmically coded in colour, ionization fraction as contour lines.
Arrows indicate velocity in the plane relative to the center of mass. Maximum velocity for black arrows indicated in the plot title as , for yellow arrows as . In this study has been fixed to represent typical average densities in molecular clouds and the parameters and R have been varied. Table 5.1 shows the parameters for the different calculations discussed below.