Solenoidal turbulent modes and star formation efficiency in Galactic-plane molecular clouds

TL;DR

This study investigates how the dominance of solenoidal turbulent modes in molecular clouds correlates with star formation efficiency, revealing that higher solenoidal fractions tend to suppress star formation across the Galactic plane.

Contribution

It provides a quantitative analysis of solenoidal turbulence fractions in a large sample of molecular clouds and links these to star formation efficiency variations.

Findings

Negative correlation between solenoidal fraction and star formation efficiency.

Higher solenoidal fractions in the Inner Galaxy decrease with Galactocentric distance.

Solenoidal turbulence appears unaffected by spiral arms outside the Inner Galaxy.

Abstract

It is speculated that the high star-formation efficiency observed in spiral-arm molecular 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 efficiency that characterises clouds in the Central Molecular Zone. Similarly, analysis of the Orion B molecular cloud has 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 relative fractions of momentum density in the solenoidal modes of turbulence in a large sample of plane molecular clouds in the \ce{^{13}CO}/\ce{C^{18}O} ($J=3\rightarrow 2$) Heterodyne Inner Milky Way Plane Survey (CHIMPS). We find a negative correlation between the solenoidal fraction and star-formation efficiency. This feature is consistent with the hypothesis that solenoidal modes prevent or slow down the collapse of dense cores. In addition, the relative power in the solenoidal modes of turbulence (solenoidal fraction) appears to be higher in the Inner Galaxy declining with a shallow gradient with increasing Galactocentric distance. Outside the Inner Galaxy, the slowly, monotonically declining values suggest that the solenoidal fraction is unaffected by the spiral arms.