The origins of cosmic dust : a study of dust in galactic supernova remnants

This thesis investigates the question of whether supernovae (SNe) are important contributors of dust to the interstellar medium (ISM). Using far-infrared (FIR) observations of 190 supernova remnants (SNRs) covered by the Herschel Infrared Galactic Plane Survey (HiGal), we augment the current sample of dusty SNRs with an additional 38 from which dust emission is newly detected. This gives a 21 per cent detection rate of dust within SNRs in the Galactic Plane, although this is expected to be a lower limit given confusion with the ISM in this region. The majority of detected SNRs are young (less than 5 kyr), as expected since these objects tend to be more compact and have higher surface brightness, making them more easily detected above the level of the ISM. Nevertheless, we expand the age range of SNRs containing dust to roughly 100 kyr. Of our detections, 13 are from core collapse (CC) and 2 from Type Ia SNe (24 are of unknown type). Consistent with previous observations and theoretical models, we find that dust detected within the Type Ia SNRs is not expected to be of ejecta origin, but is more likely to have been swept up from the ISM. It is likely that many of our other detections are also dominated by ISM, although there are several objects within which we expect that the FIR emission originates from freshly formed ejecta material, including G350.1-0.3. We make new discoveries of ejecta dust within pulsar wind nebulae (PWNe) associated with three SNRs: G11.2-0.3, G21.5-0.9, and Kes 75. Using point process mapping (PPMAP) to analyse the FIR emission indicates the presence of dust within each object with temperatures between 20 and 40 K, and a PWN dust mass of 0.34 +/- 0.14 solar mass, 0.29 +/- 0.08 solar mass, 0.51 +/- 0.13 solar mass for G11, G21, and Kes 75 respectively. This suggests that SNe are important contributors of dust to the ISM, and PWNe in particular may play an important role in this. Although some of this dust may yet to be destroyed, there is evidence that the reverse shock has already reached the centre of G11, leaving behind a considerable mass of surviving dust. We use PPMAP further in an attempt to examine the dust properties, by studying the emissivity index, beta. The signal-to-noise in the longer wavebands is too low to constrain b for G11 or Kes 75; however, we find some evidence of a variation in the dust properties of ejecta dust within G21 (beta = 1.4 +/- 0.5), compared with that of the surrounding ISM (beta = 1.8 +/- 0.1). We study the dust within 11 of the other detected SNRs which are in regions with limited ISM variation, making background subtraction and thus dust analysis possible. By producing temperature maps, using their 24 - 70 micron flux ratios, we reveal the presence of warm dust (25 - 40 K) within all of these objects. For the objects with associated distance estimates, this analysis gives dust masses of between 0.2 and 340 solar mass, some of which are extremely large compared to the typical combined mass of freshly formed and swept up material. It is most likely that, despite careful background subtraction, the largest of these masses are overestimated due to contaminating ISM, which reduces the average dust temperature. As the dust mass is highly sensitive to even small changes in the estimated temperature, ISM confusion is the largest barrier to overcome when analysing the dust content of SNRs within the Galactic Plane. We detect a region of dust coincident with a strange radio source at the centre of G351.2-0.1. Previous studies have suggested that this could a PWN, or simply an unrelated HII region; we therefore attempt to determine its nature using FIR observations. Its spectral energy distribution (SED) suggests that the region consists of two dust components: (3.9 +/- 0.4) x 10^(-6) solar mass of warm dust with a temperature of 242 +/- 7 K, and 0.18 solar mass of cool dust with a temperature of 45.8 K. The temperature of the dust is higher than typical for a HII region, although not unheard of, and when considering the IR colours the emission from the core is consistent with either a HII region or SNR. We therefore cannot rule out either option based only on the IR emission, and the G351.2 core could be either an unrelated HII region, or a PWN with a considerable dust mass. A large region of dust was detected within a very unusual object known as the Tornado, G357.7-0.1. This is an object with a very uncertain past, which, it has been suggested, could have been formed by a SN or an X-ray binary. We detect FIR emission from multiple components of the Tornado: a filamentary structure within the tail region, peaks within the head, and an unrelated HII region dubbed the 'eye'. Using PPMAP we reveal complex dust structures within the Tornado head and tail with temperatures between 15 and 61 K, and a total dust mass for the head of 16.7 solar mass. Within the head there is clear variation in the IR emission: an IR peak is detected to the north-west where the head is thought to be expanding into dense molecular material, and there is IR emission along radio filaments found towards the east. Comparing the IR and IR - radio colours highlights a variation in the emission from the two regions, suggesting that the dust properties or heat processes are different. When considering the nature of the Tornado, the head can be explained by a SNR aged between 2 and 8 kyr, with a large mass of dust swept up from the dense surrounding material. However, the tail is bizarre and more difficult to explain with a SN origin. We find it unlikely that it was formed by a progenitor prior to exploding. The structures in the tail could have been formed by jets from an X-ray binary, and we propose that the Tornado is similar to the W50 - SS 433 system, in which the jets of an X-ray binary plough into the SNR shell. This thesis has found a considerable dust mass contained within numerous Galactic SNRs, with several examples of newly detected ejecta dust. In particular, PWNe seem to be significant in the search for dusty SNRs. These findings suggest that SNe are important contributors of dust to the ISM.