On the turbulence driving mode of expanding HII regions

Abstract

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 σs2 = (1+b2M2) between the variance of the logarithmic density contrast σs2 (where s = (/0) with the gas density and its average 0), and the turbulent Mach number M. Previous works have shown that b1/3 indicates solenoidal (divergence-free) driving and b1 indicates compressive (curl-free) driving, with b1 producing up to ten times higher star formation rates than b1/3. 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 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 characterised by a natural mixture of both solenoidal and compressive turbulent modes (b0.4) when they form, and later evolve into a more compressive turbulent state with b0.5--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.