Disks and dissociation regions: the interaction of young stellar objects with their environments
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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 m 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 > log(L*/L❁. 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. Over three decades in luminosity, our sources have an approximately constant ratio of excess to stellar luminosity. We compare our observed SEDs to theoretical models of a central source with a passive irradiated circum-object disk and test the effects of disk inclination, disk flaring, and the size of the inner disk hole on the strength/shape of the excess. The observed SEDs of all but one of our sources are well fitted by models of flared and/or flat disks. 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. We also created an index based on the 1.14 m sodium feature that is sensitive to gravity, but only weakly dependent on spectral type for field brown dwarfs. Using Teff ’s determined from the spectral types of our objects along with luminosity derived from near and mid-IR photometry, we place our objects on the H-R diagram and overlay evolutionary models. The implied ages of three of our sources, found by placing them on the H-R diagram, are substantially older than typical ages of young stars with disks. Comparison of the model colors with our spectra and photometry show that the models predict much bluer colors than observed, and warn against relying on atmospheric models to derive the Av ’s, Teff ’s, and gravities of objects. 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 H 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 H . To date, observations of H2 rotational lines smear out the structure of the PDR. Only by resolving the hottest layers of H can one test the predictions and assumptions of current models. Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission in the H v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2 resolution. We also observed H v = 0-0 S(4) at selected points toward the front of the PDR. Our maps cover a 12 by 40 region of the bar where H ro-vibrational lines are bright. The distributions of H 0-0 S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail. The high spatial resolution (0.002 pc) of our observations allows us to probe the distribution of warm gas in the Orion Bar to a distance approaching the scale length for FUV photon absorption. We use these new observational results to set parameters for the PDR models described in a companion paper (Draine et al. 2005, in prep). The best-fit model can account for the separation of the H emission from the ionization front and the intensities of the ground state rotational lines as well as the 1-0 S(1) and 2-1 S(1) lines. This model requires significant adjustments to the commonly used values for the dust UV attenuation cross section and the photoelectric heating rate.