Accretion is one of the dominant sources of radiation from a low mass young stellar object for the first few million years. This process regulates the flow of material and angular momentum from the surroundings to the central object, and is thought to play an important role in the definition of the long term stellar properties. Variability is a well documented attribute of accretion, and has been observed on time-scales ranging from days to years. However it is not clear where these variations come. The current model for accretion is magnetospheric accretion, where the stellar magnetic field truncates the disc, allowing the matter to flow from the disc onto the surface of the star. This model allows for variations in the accretion rate to come from many different sources, such as the magnetic field, the circumstellar disc or the interaction of the different parts of the system. This thesis sets out to use the intrinsic accretion variability to probe the inner regions of these systems. Two spectroscopic surveys were utilised that concentrated on the Hα emission line, which is known to be closely connected to the accretion process. Together, these surveys covered 24 object including low mass T Tauri, intermediate mass T Tauri stars and Herbig Ae stars, on time-scales of minutes, days, weeks, months to years. These two studies found the accretion variations to be less than 1 Mo/yr and dominated by time-scales close to the rotation period. A further photometric monitoring campaign was undertaken to confirm the short term variations found in the low mass sample. The results from all three of these works are in agreement with each other, and they suggest that the majority of the variations in typical accreting objects are the result of an asymmetric accretion flow.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:676277 |
| Date | January 2014 |
| Creators | Costigan, Gráinne |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0031 seconds