1 to 4 mum photometry and the near infrared classification of L and T dwarfs

A complete classification of the L dwarfs will require the use of both far optical and near infrared spectral features so that the classification transitions smoothly from the latest M dwarfs into the earliest T dwarfs. However very little work has been done to understand the near infrared spectral features and colors of the L dwarfs and how these features relate to the far optical spectral classification and effective temperature of the L dwarfs. Therefore the main goal of this dissertation is to find near infrared broad and narrow band colors that can be used to create a spectral classification for the L dwarfs and estimate their effective temperature scale by comparing their observed near infrared photometry with model results. From the near infrared observations, we found that it is almost impossible to create a spectral classification that is a function of temperature using spectral indices and filters in either the near infrared J or H windows because the flux emitted in these windows is controlled by the dust opacity and not the temperature. We also found that L dwarfs display a very large range of cloud opacities for objects with the same far optical spectral type, therefore the trends observed in the near infrared colors are actually due to differences in the cloud opacity and not the temperature. In other words, two objects with similar spectral features in the far optical will have very different spectral features in the near infrared due their different cloud opacities. Consequently it is impossible to create one classification for the L dwarfs using a combination of far optical and near infrared spectral features because the features in the far optical are a function of effective temperature and the features in the near infrared are a function of the cloud opacity. Therefore to classify the L dwarfs will require the creation of two spectral indices, one that characterized the effective temperature of the L dwarf using its far optical spectral features, and a second index that characterizes the cloud opacity of the L dwarf using its near infrared spectral features.