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 ${\sigma }_s^2=\ln (1+b^2{\mathcal{M}}^2)$ 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 $b\sim 1/3$ indicates solenoidal (divergence-free) driving and $b\sim 1$ indicates compressive (curl-free) driving, with $b\sim 1$ producing up to ten times higher star formation rates than $b\sim 1/3$. 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 $b\sim 0.76\pm 0.08$. 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 ($b\sim 0.4$) when they form, and later evolve into a more compressive turbulent state with $b\sim 0.5$--$0.6$. 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.

10 pages, 6 figures. Accepted for publication in MNRAS

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• Makemake + Sedna: A Continuum Radiation Transport and Photoionization Framework for Astrophysical Newtonian Fluid Dynamics
• Infrared Radiation Feedback Does Not Regulate Star Cluster Formation
• First extragalactic measurement of the turbulence driving parameter: ALMA observations of the star-forming region N159E in the Large Magellanic Cloud
• SH$\alpha$DE: Survey description and mass-kinematics scaling relations for dwarf galaxies
• Gas Column Density Distribution of Molecular Clouds in the Third Quadrant of the Milky Way
• The density distribution and physical origins of intermittency in supersonic, highly magnetised turbulence with diverse modes of driving
• Lyman-$\alpha$ feedback prevails at Cosmic Dawn: Implications for the first galaxies, stars, and star clusters
• Gas kinematics around filamentary structures in the Orion B cloud
• Kinematics of Galactic Centre clouds shaped by shear-seeded solenoidal turbulence
• Population III star formation in the presence of turbulence, magnetic fields and ionizing radiation feedback
• Galatic HII region IRAS 17149$-$3916 -- A multiwavelength study
• The turbulence driving parameter of molecular clouds in disc galaxies
• The ionization structure and chemical history in isolated H II regions of dwarf galaxies with IFU I. The Sagittarius Dwarf Irregular Galaxy