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The structure of molecular clouds

Rowles, Jonathan Henry (2011) The structure of molecular clouds. Doctor of Philosophy (PhD) thesis, University of Kent. (doi:10.22024/UniKent/01.02.94624) (KAR id:94624)

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The physics and turbulent properties responsible for shaping the structure in molecular clouds are analysed. Data from the Two-Micron All-Sky Survey are used to determine all-sky median near-infrared extinction maps. Two types of map are made: maps with a near-constant noise in each pixel and maps with a constant resolution. The standard map has a resolution as high as l' along the Galactic plane. The resolution and depth of optical extinction are significantly better than those achieved in previous large-scale studies. The maps are used to examine the column density and mass distributions of 30 nearby molecular clouds. The low column density (i.e. turbulence dominated) parts of the clouds could be well fit by a log-normal distribution. A universal extinction threshold of AV = 5.6 ± 1.2 mag is found, above which there is an excess of material when compared to a log-normal distribution. Gravity is implied as the dominant factor in structure shap­ing above this threshold, as the column density and mass distributions are similar for all clouds. Below this threshold, the structure is determined by turbulence and the environ­ment of the cloud. Two groups of clouds with distinct column density/mass distributions are identified, that correspond to ‘star forming’ and ‘non-star forming’ groups. Structure function and Δ-variance analyses are performed on the same nearby molec­ular clouds. Average values are calculated for the structure function parameters A, intermittency, codimension and fractal dimension. The results imply a mixture of solenoidal and compressive forcing in the turbulence dominated regions of the clouds. An average value for the mass index scaling exponent is calculated which is close to the value expected for clouds where turbulence is driven/governed by solenoidal forc­ing. For the ‘star forming’ clouds the average value implies a mixture of solenoidal and compressive forcing, while the value for the ‘non-star forming’ group implies exclusively solenoidal forcing. It is concluded from this that compressive forcing is associated with the presence of clustered star formation.

Item Type: Thesis (Doctor of Philosophy (PhD))
Thesis advisor: Froebrich, Dirk
DOI/Identification number: 10.22024/UniKent/01.02.94624
Additional information: This thesis has been digitised by EThOS, the British Library digitisation service, for purposes of preservation and dissemination. It was uploaded to KAR on 25 April 2022 in order to hold its content and record within University of Kent systems. It is available Open Access using a Creative Commons Attribution, Non-commercial, No Derivatives ( licence so that the thesis and its author, can benefit from opportunities for increased readership and citation. This was done in line with University of Kent policies ( If you feel that your rights are compromised by open access to this thesis, or if you would like more information about its availability, please contact us at and we will seriously consider your claim under the terms of our Take-Down Policy (
Subjects: Q Science > QB Astronomy
Divisions: Divisions > Division of Natural Sciences > Physics and Astronomy
SWORD Depositor: SWORD Copy
Depositing User: SWORD Copy
Date Deposited: 18 Nov 2022 16:37 UTC
Last Modified: 18 Nov 2022 16:38 UTC
Resource URI: (The current URI for this page, for reference purposes)

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