| In Chapter 2, we use the results of I, J, H, and Ks imaging of portions of the Chamaeleon II, Lupus I, and Ophiuchus molecular clouds with with 3.6 to 24 mum imaging from the Spitzer Legacy Program, "From Molecular Cores to Planet Forming Disks", to identify a sample of 19 young stars, brown dwarfs and sub-brown dwarfs showing mid-infrared excess in the Chamaeleon II, Lupus I, and Ophiuchus star-forming clouds. The resulting sample includes sources with luminosities of 0.56 > */L logL*/L ⊙ > -3.11. Our sample includes the lowest luminosity young brown dwarfs with mid-IR excesses observed to date, with masses possibly as low as 6 MJ. Five of the sources in our sample have nominal masses at or below the deuterium burning limit (12 MJ); a declining IMF for sub-brown dwarfs would not be able to explain the mass distribution of our sample.; In Chapter 3, we compare photometry and spectra of the objects found to have circum-object disks with predictions of evolutionary and atmospheric models of young brown dwarfs. We discuss spectra obtained of 5 objects from our sample of brown dwarfs with disks which confirm their previous identification as young brown dwarfs. The spectrum of one of our objects, cha1305-7739, indicates that its spectral type is later than M9.5, making it the latest spectral type young brown dwarf with a circum-object disk reported to date. Comparing spectra of young brown dwarfs, field brown dwarfs and giants, we find an H2O index capable of determining spectral type to +/-1 sub-type, independent of gravity.; In Chapter 4, we discuss photodissociation regions, where UV radiation dominates the energetics and chemistry of the neutral gas, and which contain most of the mass in the dense interstellar medium of our galaxy. Observations of H2 rotational and ro-vibrational lines reveal that PDRs contain unexpectedly large amounts of very warm (400--700 K) molecular gas. Theoretical models have difficulty explaining the existence of so much warm gas. Possible problems include errors in the heating and cooling functions or in the formation rate for H2. To date, observations of H2 rotational lines smear out the structure of the PDR. Only by resolving the hottest layers of H2 can one test the predictions and assumptions of current models. Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission in the H2 v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2" resolution. (Abstract shortened by UMI.)... |
