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Four dimensional analysis of submillimetre observations to untangle dust properties and structure in nearby starforming regions

Howard, Alexander 2020. Four dimensional analysis of submillimetre observations to untangle dust properties and structure in nearby starforming regions. PhD Thesis, Cardiff University.
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The analysis of far infrared and sub-millimetre observations of filamentary structures within star forming regions has relied upon single temperature modified black body fitting techniques. While these procedures are simple to execute, they require a number of unphysical assumptions regarding the properties of the dust in the target molecular cloud, such as assuming that the dust temperature is uniform along the line of sight, and that the dust opacity index does not vary across the cloud. Furthermore, the observations are commonly degraded to the coarsest common resolution, leading to data loss. The Bayesian fitting procedure, PPMAP, is able to overcome these limitations by utilising multi-band dust continuum observations at their native resolutions to produce high accuracy models of H2 column density distributed into multiple discrete dust temperature and dust opacity index bands along the line of sight. In this thesis I present the results of a number of tests of PPMAP with synthetic data, and use the algorithm to analyse local star forming regions. By constructing model filaments, I produce synthetic far infrared and sub-millimetre observations which I use to test the capabilities of the PPMAP algorithm. I show that PPMAP is able to more accurately estimate the total mass of a filament than a conventional modified black body fitting technique. I also determine that PPMAP is capable of lifting the dust temperature and dust opacity index degeneracy commonly seen in modified black body results. This allows PPMAP to probe line of sight variations in the dust properties. I also determine that PPMAP is able to estimate the contribution to the final model of each of the input observations equally well or better than conventional modified black body fitting techniques. I apply PPMAP to Herschel and SCUBA-2 dust continuum observations of the main filament in the Taurus L1495/B213 star forming complex. I produce a length averaged profile of the B211/B213 filament and fit this profile with a Plummer-like function. I find it has a FWHM approximately 0.087pc and a Plummer-like exponent, p approximately 1.88. The FWHM is nearly half the characteristic width reported in Arzoumanian et al. (2011) and other studies when adjusted for differences in fitting technique, while p is consistent with previous studies. By instead fitting multiple Plummer-like functions at different positions along the length of the filament, I determine that the filament is better approximated by a p=4 Plummer-like exponent, though I still recover a narrow filament width. The shallower function attributed to the length averaged profile is likely to be a consequence of smoothing over areas of resolved sub-structure along the filament. Therefore, I conclude length averaged fitting should be avoided. The ability of PPMAP to distinguish dust emitting at different temperatures, and thereby to discriminate between the warm outer layers of the filament and the cold inner layers near the spine, leads to a significant reduction in the column density through the filament, and hence in the line-density, mu. If we assume a gas temperature of 10K, and thus a critical line density, critical mu approximately 16 solar masses per parsec, then the B211/B213 is, on average, trans-critical, with mu = 17.8 solar masses per pc. The locations of pre-stellar cores agree well with segments of the filament that are locally super-critical. I also apply PPMAP to Herschel and SCUBA-2 dust continuum observations of the Ophiuchus L1688 and L1689 sub-regions. I identify a network of filamentary structures in both regions. As with the B211/B213 filament in Taurus, I determine that the widths of the dense filaments in L1688 and L1689 are also significantly narrower than the canonical characteristic filament width. I find that the filaments in L1688 and L1689 have a median line density of (43 +/- 13) solar masses per pc, and hence are super-critical assuming a gas temperature of 10K. However, both regions are strongly heated by feedback from the Upper Scorpius OB association, which may raise the gas temperature enough that the filaments are globally trans-critical. By examining the distribution of mass, I determine that while L1688 contains twice as much material with Av > 7 as L1689, the proportion of that material which is associated with starless cores and dense, compact clumps is similar in the two sub-regions. In addition, I present an analysis of the dust properties as revealed by PPMAP. The variations in dust opacity index in both the Taurus and Ophiuchus regions exhibit broadly similar properties. The more diffuse medium surrounding the networks of filaments and cores is generally composed of dust with opacity indices >1.5. In contrast, the dense structures harbour reservoirs dust with opacity indices approximately 1.0. This may be indicative of grain growth in the densest regions of star forming clouds.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Uncontrolled Keywords: star-formation, filaments, ISM, dust, PPMAP
Funders: STFC
Date of First Compliant Deposit: 5 November 2020
Last Modified: 10 Nov 2021 02:09

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