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. 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 characterized by the relation sigma(2)(s) = ln(1 + b(2)M(2)) between the variance of the logarithmic density contrast sigma(2)(s) [where s = ln (rho/rho(0)) with the gas density rho and its average rho(0)], and the turbulent Mach number M. Previous works have shown that b similar to 1/3 indicates solenoidal (divergence-free) driving and b similar to 1 indicates compressive (curl-free) driving, with b similar to 1 producing up to ten times higher star formation rates than b similar to 1/3. 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 similar to 0.76 +/- 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 characterized by a natural mixture of both solenoidal and compressive turbulent modes (b similar to 0.4) when they form, and later evolve into a more compressive turbulent state with b similar to 0.5-0.6. 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.