A photographic survey of very low mass stars and brown dwarfs

Abstract

This thesis has taken the detection o f very low mass stars and brow n dwarfs by photographic methods as its theme. The case for the existence of brown dwarfs has been reviewed and it is concluded that current theories of the formation of low mass bodies in the inter-stellar medium are not sufficiently developed yet to able to place a lower limit to the minimum stellar fragmentation mass. Existing models consider the formation of binary and planetary systems as being more likely than the formation of free-floating objects. Models of stellar structure are more fully developed and suggest a continuation of the properties of low mass stars through a transitionary region near the minimum hydrogen burning mass down to objects with masses at least as small as 0.01M⨀. The weakest facet of the numerical models are the atmospheres which are essential for the relation of physical values to observable values. A seemingly systematic offset of 100-200K exists between model atmospheres and observations of the coolest stars although progress is being made in modelling the atmospheres of bluer, low metallicity stars where the effects of TiO and VO are less dominant.

To conclude, recent technical advances in measuring machine speed and accuracy and in plate and film material provide new opportunities to the stately art of photographic survey science. With recently reported detections of very convincing brown dwarf candidates in the Pleiades, the goals of this field of study have moved away from the singular issue of detecting sub-stellar objects and towards a quantification of their place as a population in the Milky Way and this is a task to which modern wide-field photographic techniques are well suited.