Browsing by Subject "features"

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Are C-60 Molecules Detectable In Circumstellar Shells Of R Coronae Borealis Stars?
(2011-03) Garcia-Hernandez, D. Anibal; Rao, N. Kameswara; Lambert, David L.; Rao, N. Kameswara; Lambert, David L.
The hydrogen-poor, helium-rich, and carbon-rich character of the gas around R Coronae Borealis (RCB) stars has been suggested to be a site for formation of C-60 molecules. This suggestion is not supported by observations reported here showing that infrared transitions of C-60 are not seen in a large sample of RCB stars observed with the Infrared Spectrograph on the Spitzer Space Telescope. The infrared C-60 transitions are seen, however, in emission and blended with polycyclic aromatic hydrocarbon (PAH) features in spectra of DY Cen and possibly also of V854 Cen, the two least hydrogen-deficient (hydrogen deficiency of only similar to 10-100) RCB stars. The speculation is offered that C-60 (and the PAHs) in the moderately H-deficient circumstellar envelopes may be formed by the decomposition of hydrogenated amorphous carbon but fullerene formation is inefficient in the highly H-deficient environments of most RCBs.
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.
High-Velocity Line Forming Regions In The Type Ia Supernova 2009ig
(2013-11) Marion, G. H.; Vinko, Jozsef; Wheeler, J. Craig; Foley, Ryan J.; Hsiao, Eric Y.; Brown, Peter J.; Challis, Peter; Filippenko, Alexei V.; Garnavich, Peter; Kirshner, Robert P.; Landsman, Wayne B.; Parrent, Jerod T.; Pritchard, Tyler A.; Roming, Peter W. A.; Silverman, Jeffrey M.; Wang, Xiaofeng; Marion, G. H.; Vinko, Jozsef; Wheeler, J. Craig; Silverman, Jeffrey M.
We report measurements and analysis of high-velocity (HVF) (>20,000 km s(-1)) and photospheric absorption features in a series of spectra of the Type Ia supernova (SN) 2009ig obtained between -14 days and +13 days with respect to the time of maximum B-band luminosity (B-max). We identify lines of Si II, Si III, S II, Ca II, and Fe II that produce both HVF and photospheric-velocity (PVF) absorption features. SN 2009ig is unusual for the large number of lines with detectable HVF in the spectra, but the light-curve parameters correspond to a slightly overluminous but unexceptional SN Ia (M-B = -19.46 mag and Delta m(15)(B) = 0.90 mag). Similarly, the Si II lambda 6355 velocity at the time of B-max is greater than "normal" for an SN Ia, but it is not extreme (upsilon(Si) = 13,400 km s(-1)). The -14 days and -13 days spectra clearly resolve HVF from Si II lambda 6355 as separate absorptions from a detached line forming region. At these very early phases, detached HVF are prevalent in all lines. From -12 days to -6 days, HVF and PVF are detected simultaneously, and the two line forming regions maintain a constant separation of about 8000 km s(-1). After -6 days all absorption features are PVF. The observations of SN 2009ig provide a complete picture of the transition from HVF to PVF. Most SNe Ia show evidence for HVF from multiple lines in spectra obtained before -10 days, and we compare the spectra of SN 2009ig to observations of other SNe. We show that each of the unusual line profiles for Si II lambda 6355 found in early-time spectra of SNe Ia correlate to a specific phase in a common development sequence from HVF to PVF.