Browsing by Subject "circumstellar disk"

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CO Rovibrational Emission As A Probe Of Inner Disk Structure
(2011-12) Salyk, Colette; Blake, Geoffrey A.; Boogert, Adwin C. A.; Brown, Joanna M.; Salyk, Colette
We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width similar to 0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust.
A Comparative Astrochemical Study Of The High-Mass Protostellar Objects NGC 7538 IRS 9 And IRS 1
(2012-10) Barentine, John C.; Lacy, John H.; Barentine, John C.; Lacy, John H.
We report the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 and compare our observations to published data on the nearby object NGC 7538 IRS 1. Both objects originated in the same molecular cloud and appear to be at different points in their evolutionary histories, offering an unusual opportunity to study the temporal evolution of envelope chemistry in objects sharing a presumably identical starting composition. Observations were made with the Texas Echelon Cross Echelle Spectrograph, a sensitive, high spectral resolution (R = lambda/Delta lambda similar or equal to 100,000) mid-infrared grating spectrometer. Forty-six individual lines in vibrational modes of the molecules C2H2, CH4, HCN, NH3, and CO were detected, including two isotopologues ((CO)-C-13, (CO)-C-12-O-18) and one combination mode (nu(4) + nu(5) C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density, and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (similar to 2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9.
High-Resolution Mid-Infrared Spectroscopy Of NGC 7538 IRS 1: Probing Chemistry In A Massive Young Stellar Object
(2009-05) Knez, Claudia; Lacy, John H.; Evans, Neal J.; van Dishoeck, Ewine F.; Richter, Matthew J.; Knez, Claudia; Lacy, John H.; Evans, Neal J.
We present high-resolution (R = 75,000-100,000) mid-infrared spectra of the high-mass embedded young star IRS 1 in the NGC 7538 star-forming region. Absorption lines from many rotational states of C(2)H(2), (13)C(12)CH(2), CH(3), CH(4), NH(3), HCN, HNCO, and CS are seen. The gas temperature, column density, covering factor, line width, and Doppler shift for each molecule are derived. All molecules were fit with two velocity components between -54 and -63 km s(-1). We find high column densities (similar to 10(16) cm(-2)) for all the observed molecules compared to values previously reported and present new results for CH3 and HNCO. Several physical and chemical models are considered. The favored model involves a nearly edge-on disk around a massive star. Radiation from dust in the inner disk passes through the disk atmosphere, where large molecular column densities can produce the observed absorption line spectrum.