On the turbulence driving mode of expanding HII regions
  We investigate the turbulence driving mode of ionizing radiation from massive
stars on the surrounding interstellar medium (ISM). We run hydrodynamical
simulations of a turbulent cloud impinged by a plane-parallel ionization front.
We find that the ionizing radiation forms pillars of neutral gas reminiscent of
those seen in observations. We quantify the driving mode of the turbulence in
the neutral gas by calculating the driving parameter $b$, which is
characterised by the relation $\sigma_s^2 = \ln({1+b^2\mathcal{M}^2})$ between
the variance of the logarithmic density contrast $\sigma_s^2$ (where $s =
\ln({\rho/\rho_0})$ with the gas density $\rho$ and its average $\rho_0$), and
the turbulent Mach number $\mathcal{M}$. Previous works have shown that
$b\sim1/3$ indicates solenoidal (divergence-free) driving and $b\sim1$
indicates compressive (curl-free) driving, with $b\sim1$ producing up to ten
times higher star formation rates than $b\sim1/3$. The time variation of $b$ in
our study allows us to infer that ionizing radiation is inherently a
compressive turbulence driving source, with a time-averaged $b\sim 0.76 \pm
0.08$. We also investigate the value of $b$ of the pillars, where star
formation is expected to occur, and find that the pillars are characterised by
a natural mixture of both solenoidal and compressive turbulent modes
($b\sim0.4$) when they form, and later evolve into a more compressive turbulent
state with $b\sim0.5$--$0.6$. A virial parameter analysis of the pillar regions
supports this conclusion. This indicates that ionizing radiation from massive
stars may be able to trigger star formation by producing predominately
compressive turbulent gas in the pillars.
