Owen, PJ; (2016) Herschel studies of core collapse supernova remnants at infrared wavelengths. Doctoral thesis , UCL (University College London).

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Abstract

Core collapse supernovae play a vital role in the evolution of the universe. They a major producers of metals and the only place where elements heavier than iron can be made. These metals can then combine to form molecules and dust, enriching the interstellar medium to provide cooling to form new stars. These molecules and dust, as well as metal cooling lines can all be observed in the infrared, making it a very interesting wavelength range in which to study supernovae and their remnants. This investigation of supernova remnants in the infrared has been made possible by the European Space Agency’s Herschel Space Observatory. Herschel studied the universe between 60 and 670 µm, covering the peak of dust emission, a large range of atomic fine structure cooling lines, a large portion of the carbon monoxide rotation ladder and other more exotic molecular line transitions. This thesis presents spectroscopic observations of the Cassiopeia A and Crab supernova remnants using two of the Herschel instruments to investigate their structure, physics and chemistry. It also presents the results of radiative transfer models based on photometric Herschel data to investigate more physical measurements of the mass of dust in supernova remnants. These models, along with others are then used to investigate the nature of dust formation in supernovae and the composition of this dust. Observations of the Crab Nebula with Herschel SPIRE made the first ever detection of a molecule containing a noble gas in space. Within the dense filaments of the nebula, two rotational lines of 36ArH+ were observed. This discovery was followed up with the VLT to try to find vibrational lines in the near infrared as well as an investigation of the potential presence of HeH+. Unfortunately these were unsuccessful. PACS and LWS observations were used to diagnose the conditions in the remnant, density, oxygen to nitrogen ratio and the ionic abundances of Nitrogen. Velocity information was also investigated and emitting regions compared to optical emission. Cassiopeia A was observed with Herschel PACS-IFU and SPIRE-FTS spectrometers. The PACS observations were used to look at the structure of the bright ring of the remnant. The 63 µm [O i] detected line emission along with upper limits on 146 µm [O i] emission were used to diagnose the temperature in the remnant assuming neutral hydrogen as a collision partner, found to be within a range of 100-1000K . Along with the SPIRE-FTS observations they were also used to investigate the formation of CO in the remnant. These CO observations were also used to diagnose conditions in the remnant. The temperature determined from CO in the neutral regions of the remnant was 300-500 K which is in agreement with the temperatures determined using the [O i] line ratios. By using photoionisation models of the Crab Nebula we were able to determine the mass of dust in the remnant. This modelling takes into account realistic heating of the dust, density distribution of the dust, the gas in the nebula and the size and species of the dust grains. We found that the dust mass is larger when these physical conditions are taken in to account. With our favoured geometry, distribution and grain species optical constants, we find a mass of 0.18 ± 0.03 M of dust, as opposed to the 0.1 M determined empirically, and a gas mass of 7.0 ± 0.5 M for the remnant.

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