Turbulence modes and star formation efficiency in the Milky Way

Abstract: The nature of turbulence in molecular clouds is one of the driving factors that influence star formation. It is speculated that the high star formation efficiency observed in spiral-arm clouds is linked to the prevalence of compressive (curl- free) turbulent modes, while the shear-driven solenoidal (divergence-free) modes appear to be the main cause of the low star formation rate that characterises clouds in the Central Molecular Zone (CMZ). Similarly, the analysis of the Orion B molecular cloud confirmed that, although turbulent modes vary locally and at different scales within the cloud, the dominant solenoidal turbulence is compatible with its low star formation rate. This evidence points to inter-and intra-cloud fluctuations of the solenoidal modes being an agent for the variability of star formation efficiency. We present a quantitative estimation of the fraction of momentum density (ρv) power contained in the solenoidal modes of the turbulence in a large sam- ple of plane molecular clouds in the 13CO/C18O (J = 3 → 2) Heterodyne Inner Milky Way Plane Survey (CHIMPS). Our goal is to investigate how the fraction of solenoidal modes may probe the variation of the star formation efficiency in different Galactic molecular environments by comparing the “solenoidal fraction” across clouds with varying features to the clouds’ star-formation efficiency, derived independently.