Infall, Outflows and Substructure in Massive Star Forming Regions

  • Catherine Mcguire

Student thesis: Phd


A central question in theories of massive star formation is how massive stars gain their mass. Two of the ways in which this can be investigated are via studies of the infall activity and substructure in massive star forming regions. Blue asymmetric spectral line profiles are frequently used as an indicator of infall in star forming regions. In the first part of this thesis we investigate infall in 11 high mass protostellar infall candidates. We assess the spectral asymmetry in IRAM 30m observations of 9 point maps in the optically thick HCO^{+}(1-0) transition. Where infall is detected, we calculate infall velocities using the two-layer infall model. We find 6 sources with predominantly blue asymmetry in their HCO^{+}spectra, providing the strongest evidence for infall. The 5 remaining sources show red asymmetry in 11-38% cases. Where infall is detected we calculate infall velocities in the range 0.12–8.38 km/s, corresponding to mass infall rates between 1x10^{-4}– 9x10^{-3} M/yr, typical of the higher accretion rates seen in massive star forming regions. We conclude that a single central observation of blue asymmetry towards the centre of a source does not necessarily translate in to blue asymmetry at all offset positions around the source. Spectral line maps, showing blue asymmetry at multiple off set positions around a source, provide stronger evidence for infall and can also tell us how infall velocity varies with position. Based on comparisons between the HCO+ maps and JCMT ^{12}CO(3-2) and ^{13}CO(3-2) observations, showing the outflow activity in each source, we strengthen the case for infall in 5 sources, finding blue asymmetry across the full extent of their HCO+ maps, which may also indicate that infall in occurring on a large scale. Further work is required to calculate outflow properties, and determine the variation of infall velocity with molecular tracer, allowing the properties of the infall to be further constrained. In the second part of this thesis we study the substructure in the two largest fragments, MM1 and MM2, in the IRDC SDC13. As MM1 appears to be in a later stage of evolution than MM2, comparing their substructure provides an insight into massive clump evolution. Investigations into the substructure of massive star forming regions are essential for understanding relationships between the CMF and the IMF. We report the results of high resolution SMA dust continuum observations towards MM1 and MM2, and carry out RADMC-3D radiative transfer modelling to characterise the observed substructure. The continuum results indicate the presence of 4 sub-fragments in the SDC13 region. The nature of the second brightest sub-fragment (B) is uncertain as it does not have a corresponding feature at the lower resolution or at other wavelengths. MM1, which is actively forming stars, consists of two sub-fragments A and C. This is confirmed by 70 micron Herschel data. While MM1 and MM2 appear quite similar in previous low resolution observations, at high resolution, the sub-fragment at the centre of MM2 (D) is much fainter than the sub-fragment at the centre of MM1 (A). RADMC-3D models of MM1 and MM2 are able to reproduce these results by modelling MM2 with a steeper density profile and higher mass than is required for MM1. The proximity of MM1 and MM2 suggests they were formed at the same time, indicating a shorter evolutionary time scale for MM1, which has already formed stars. The different evolutionary state of the two fragments hints at the significance of the evolution of the density structure of clumps in regulating the star formation process. The low level of fragmentation and estimated turbulent support in MM2 suggest it may be a strong candidate for a massive turbulent core.
Date of Award31 Dec 2017
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorGary Fuller (Supervisor)


  • Massive Star Formation

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