Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B

TL;DR

This study uses a novel observational method to analyze turbulence modes in Orion B, revealing that solenoidal motions dominate overall but compressive motions are prevalent in star-forming regions, influencing star formation efficiency.

Contribution

It applies a statistical technique to observational data to distinguish turbulence modes in molecular clouds, linking these modes to star formation activity for the first time.

Findings

Orion B's turbulence is mostly solenoidal, correlating with low star formation rate.

Star-forming regions exhibit strongly compressive turbulence.

Intra-cloud variability in turbulence modes affects star formation efficiency.

Abstract

The nature of turbulence in molecular clouds is one of the key parameters that control star formation efficiency: compressive motions, as opposed to solenoidal motions, can trigger the collapse of cores, or mark the expansion of Hii regions. We try to observationally derive the fractions of momentum density ($\rho v$) contained in the solenoidal and compressive modes of turbulence in the Orion B molecular cloud and relate these fractions to the star formation efficiency in the cloud. The implementation of a statistical method developed by Brunt & Federrath (2014), applied to a $^{13}$CO(J=1-0) datacube obtained with the IRAM-30m telescope, allows us to retrieve 3-dimensional quantities from the projected quantities provided by the observations, yielding an estimate of the compressive versus solenoidal ratio in various regions of the cloud. Despite the Orion B molecular cloud being highly supersonic (mean Mach number $\sim$ 6), the fractions of motion in each mode diverge significantly from equipartition. The cloud's motions are on average mostly solenoidal (excess > 8 % with respect to equipartition), which is consistent with its low star formation rate. On the other hand, the motions around the main star-forming regions (NGC 2023 and NGC 2024) prove to be strongly compressive. We have successfully applied to observational data a method that was so far only tested on simulations, and have shown that there can be a strong intra-cloud variability of the compressive and solenoidal fractions, these fractions being in turn related to the star formation efficiency. This opens a new possibility for star-formation diagnostics in galactic molecular clouds.