title: Turbulence and star formation efficiency in giant molecular clouds
Creators: name: Rani, R
Publisher: name: Liverpool John Moores University
Year: 2021

description: The nature of turbulence in molecular clouds is one of the driving factors that influence star formation efficiency. 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 efficiency that characterises clouds in the Central Molecular Zone (CMZ). Similarly, the analysis of the Orion B molecular cloud confirmed that the dominant solenoidal turbulence is compatible with its low star formation rate. However, turbulent modes vary locally and at different scales within the cloud, and turbulent motions surrounding the main star-forming regions display a strongly compressive nature. This evidence points to inter-and intra-cloud fluctuations of the solenoidal modes being an agent for the variability of star formation efficiency and cloud collision being a facilitator of stars’ formation through the production of highly compressive gas flows. This thesis presents a quantitative estimation of the relative fractions of momentum density in the solenoidal and compressible modes of turbulence in the plane molecular clouds found in the 3CO/C18O (J = 3 → 2) Heterodyne Inner Milky Way Plane Survey (CHIMPS). This calculation is achieved through a statistical method that allows us to reconstruct the 3-dimensional distribution of the density momentum from its line-of-sight projected counterparts (zeroth, first, and second velocity moments) provided by the observations, producing an estimate of the power contained in the solenoidal and compressive turbulent modes within each cloud. The project investigates how different fractions of compressive and solenoidal modes in CHIMPS clouds probe the variation of the star formation efficiency across clouds with varying environments. A negative correlation between the solenoidal fraction and star formation efficiency is found. 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 of the solenoidal fraction suggest that the solenoidal fraction is unaffected by the spiral arms. The sample of clouds considered is extracted via the dendrogram-based Spectral Analysis for Interstellar Molecular Emission Segmentation (SCIMES). The comparison of the geometrical and physical properties of the SCIMES extracted 13CO (3-2) clouds in CHIMPS with the results originally obtained with the FellWalker method show that the SCIMES segmentation includes a wider range of cloud sizes. In crowded fields, SCIMES produces more detailed maps of the structure of molecular cloud, by identifying and tracing out more rarefied features and avoiding “clump localisation” with artificial boundaries arising in the FellWalker extraction. The physical properties defined by the volume and mass of individual clouds mirror this feature. The survey-wide distributions of physical properties of the 13CO emission however are similar in the two segmentations. To compare the properties of the extracted clouds to those identified using a different tracer, a SCIMES segmentation of the 12CO(3 − 2) emission from the CO High Resolution Survey (COHRS) through SCIMES is considered (where the data are available).