SIMBAD references

We investigate the stability of D-type ionization fronts (IF). We find the dispersion relation for surface modes of weak-D IF illuminated by a plane-parallel radiation field, including both the inclination of the IF to the ionizing radiation field and the effects of recombination in the ionized gas. It is well known that IF are unstable in the absence of recombination in the ionized gas, but that recombination tends to stabilize them. For finite inclination, however, IF are unstable at essentially all wavelengths in the limit of very cold upstream gas.

We present numerical hydrodynamic simulations of the development of the instabilities. These confirm the presence of long-wavelength instabilities of IF when the sound speed in the neutral gas is a finite fraction of that in the ionized gas and the ionization cross section is finite. They also show the development of the instabilities to non-linear amplitude.