Contributed Talk - Splinter ISM

Thursday, 24 September 2020, 12:23   (virtual room F)

Compressive nature of turbulence driven by ionising radiation

Shyam Harimohan Menon, Christoph Federrath, Rolf Kuiper, Pamela Klaassen, Megan Reiter
Australian National University, University of Tübingen, UK Astronomy Technology Centre Edinburgh

The ionising radiation of massive stars are known to sculpt the surrounding molecular gas onto pillar-like structures that are found to host sites of star formation, especially at the tips, with suggestions that this star formation could have been triggered locally by the ionising radiation. To consider this possibility, it is necessary to investigate the local physical conditions for star formation in the pillars that are dynamically altered by the ionising radiation. The turbulence driving parameter (b), which encapsulates the mode of turbulence that dominates in the gas, i.e. solenoidal (b~0.3) or compressive modes (b~1.0), is one such physical parameter, with compressive turbulence leading to star formation rates higher than solenoidal turbulence. We study the turbulence driving parameter in the pillars through numerical simulations, and find that the ionising radiation changes the dominant mode of turbulence from that of an equipartition mixture of compressive and solenoidal modes, to a compressive dominated turbulence regime with b~0.8. We then test this hypothesis with high-resolution observations of pillars in the Carina Nebula imaged with ALMA, and derive values of turbulence driving parameters for them. We find that the observationally derived values of this parameter agree with the numerical simulations, and lie in the range of predominantly compressive dominated turbulence. We suggest that through this mechanism of driving compressive turbulence, star formation rates are boosted by a factor ~ 3-5 in the pillars, and thus potentially triggered by the ionising radiation.