Browsing by Subject "grain mantles"

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The c2d Spitzer Spectroscopic Survey Of Ices Around Low-Mass Young Stellar Objects. I. H2O And The 5-8 Mu M Bands
(2008-05) Boogert, Adwin C. A.; Pontoppidan, Klaus M.; Knez, Claudia; Lahuis, Fred; Kessler-Silacci, J.; van Dishoeck, Ewine F.; Blake, Geoffrey A.; Augereau, Jean-Charles; Bisschop, S. E.; Bottinelli, Sandrine; Brooke, Tyler Y.; Brown, Justin; Crapsi, Antonio; Evans, Neal J.; Fraser, Helen J.; Geers, V.; Huard, Tracy L.; Jorgensen, Jes K.; Oberg, Karin I.; Allen, Lori E.; Harvey, Paul M.; Koerner, David W.; Mundy, Lee G.; Padgett, Deborah L.; Sargent, Anneila I.; Stapelfeldt, Karl R.; Evans, Neal J.; Harvey, Paul M.; Kessler-Silacci, J.
To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L similar to 0.1-10 L-circle dot) using 3-38 mu m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L similar to 10(5) L-circle dot). The long-known 6.0 and 6.85 mu m bands are detected toward all sources, with the Class 0-type YSOs showing the deepest bands ever observed. The 6.0 mu m band is often deeper than expected from the bending mode of pure solid H2O. The additional 5-7 mu m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%-30% for CH3OH, 3%-8% for NH3, 1%-5% for HCOOH, similar to 6% for H2CO, and similar to 0.3% for HCOO- relative to solid H2O. The 6.85 mu m band has one or two carriers, of which one may be less volatile than H2O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH4+- containing salt is the carrier, its abundance relative to solid H2O is similar to 7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-ray-induced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 mu m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.
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.