Naked singularities in dilatonic domain wall space-times
We investigate gravitational effects of extreme nonextreme and ultraextreme domain walls in the presence of a dilaton field $\phi$. The dilaton is a scalar field without self-interaction that couples to the matter potential of the wall. Motivated by superstring and supergravity theories, we consider both an exponential dilaton coupling (parametrized with the coupling constant $alpha$) and the case where the coupling is self-dual, i.e. it has an extremum for a finite value of $\phi$. For an exponential dilaton coupling ($e^{2 \sqrt{\alpha} \phi}$), extreme walls (which are static planar configurations with surface energy density $\sigma=\sigma_{ext}$ saturating the corresponding Bogonol'nyi bound) have a naked (planar) singularity outside the wall for $\alpha>1$, while for $\alpha \leq 1$ the singularity is null. On the other hand, nonextreme walls (bubbles with two insides and $\sigma=\sigma_{non} > \sigma_{ext}$) and ultraextreme walls (bubbles of false vacuum decay and $\sigma=\sigma_{ultra} < \sigma_{ext}$) always have naked singularities. There are solutions with self-dual couplings, which reduce to singularity free vacuum domain wall space-times. However, only non- and ultraextreme walls of such a type are dynamically stable.