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Title: A photographic survey of very low mass stars and brown dwarfs
Author: Thackrah, Andrew David
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1997
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This thesis presents a study of the very low mass stars and hypothesised brown dwarf population that straddle the end of the stellar main sequence. The current status of the physical and observational arguments for the existence of sub-stellar objects is reviewed and the most recent numerical models are assessed. The suitability of wide-field photographic plates as detectors of this type of object is investigated, a 150 square degree photographic survey is described and subsequent candidate observations are presented. It is found that for conventional survey-grade U.K. Schmidt plates, the detection probability is sensitive to a range of assumptions concerning the initial mass function and density distribution function. For objects of spectral class later than M5, brown dwarfs may constitute up to one third of the objects detected, though this value is extremely sensitive to the photographic flux limit and is also found to decrease toward the plate limit if a time-dependent density function is used to describe the space distribution. The height of the Sun above the Galactic plane is considered and it is calculated that given the present estimates, an upper limit to the likely variation in number density is around 50%, corresponding to less than ten objects per square degree. Variations of the assumed initial mass function strongly affect the absolute values. The predicted age distribution of detectable brown dwarfs suggest that the vast majority of them will be less than 1 gigayear in age. The predicted spectral distribution begins around M4 to 5, peaking around M6 to M7. A photographic brown dwarf survey is feasible provided suitable follow-up measurements are carried-out to assess the ages of candidate objects. In mid-to-high latitude Galactic fields photographic plates are more suited to such a survey than current CCD detectors. A photographic survey is described making use of R and I band photographic plates from the U.K. and ESO Schmidt telescopes. The ROE's COSMOS and SUPERCOSMOS densitometers were used to measure the plates with candidates being selected by photometric colour. The techniques devised to match the image catalogues, photometrically calibrate the magnitude scale, automatically classify stellar and galaxian images and measure image displacements are described. The luminosity function derived from the survey is found to conform to previous determinations, showing no sign of increasing space density in the faintest bins.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available