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Title: The gaseous component to planetary debris discs at white dwarfs
Author: Manser, Christopher J.
ISNI:       0000 0004 7972 4203
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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Via the spectroscopic detection of metal contamination of white dwarf photospheres, it has been well established that 25 - 50% of these stars host remnant planetary systems. This pollution arises from the accretion of disrupted planetesimals, and the majority of metal-enhanced white dwarfs are actively accreting from a debris disc. These discs are detected in the form of an infrared excess at 1 - 3% of white dwarfs, and a subset host a co-orbiting gaseous component. In this Thesis, I analyse the morphological evolution of the gaseous emission from debris discs around two white dwarfs, including the prototypical gas disc host SDSS J122859.93+104032.9 (SDSS J1228+1040) which shows variability on short (hourly) and long (yearly) timescales. Long-term monitoring of the emission profiles from gaseous debris discs reveals that the majority of them share this seemingly-periodic, morphological evolution. For SDSS J1228+1040, I could model the variable emission profiles remarkably well by the precession of a fixed, asymmetric intensity pattern in the disc, and I produced the first image of a gaseous debris disc using the method of Doppler tomography. I suggest that the variability of the other gas discs is also generated by fixed intensity patterns in the discs that precess. Motivated by the detection of the long-term variability of gaseous debris discs, I collected short-cadence spectroscopy of the emission from the debris disc around SDSS J1228+1040 to probe for orbital timescale (' hours) variability. I detected clear, periodic variability in the Ca ii emission lines on a ' 2 hr period, which I interpret as the signature of a planetesimal orbiting within the debris disc. I ruled out other likely scenarios, and I hypothesise that the planetesimal generates the gas we observe, as well as inducing the long- and short-term variability. Finally, using a spectroscopic sample of white dwarfs from the Sloan Digital Sky Survey, I calculated the fraction of white dwarfs that host a detectable gaseous debris disc as 0.06 _ 0:03 0:02 per cent. This occurrence rate can be combined with the fraction of white dwarfs that host a dusty disc (1 - 3 %) to find that only 1 - 10% of these systems have an observable gaseous component. Determining an occurrence rate using the number of known gas (7) and dust (' 38) discs results in a value up to an order of magnitude larger (' 18 %) than the one I have calculated, and is due to observational bias. My research has shown that while variability of gaseous debris discs is common, appearing on time-scales of decades, months and hours, their prevalence is not. From the results of my work, I hypothesise that these discs are tracers for the presence of close-in planetesimals. Future observations to identify additional gaseous debris discs, as well as characterising their long- and short-term variability will allow this hypothesis to be tested.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics