Browsing by Subject "polycyclic aromatic-hydrocarbons"
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Item 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.Item Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission(2008-09) Iglesias-Groth, S.; Manchado, A.; Garcia-Hernandez, D. A.; Hernandez, J. I. G.; Lambert, David L.; Lambert, D. L.We report high-resolution spectroscopy of the moderately reddened (A(v) = 3) early-type star Cernis 52 located in a region of the Perseus molecular cloud complex with anomalous microwave emission. In addition to the presence of the most common diffuse interstellar bands (DIBs) we detect two new interstellar or circumstellar bands coincident to within 0.01% in wavelength with the two strongest bands of the naphthalene cation (C(10)H(8)(+)) as measured in gas-phase laboratory spectroscopy at low temperatures and find marginal evidence for the third strongest band. Assuming these features are caused by the naphthalene cation, from the measured intensity and available oscillator strengths we find that 0.008% of the carbon in the cloud could be in the form of this molecule. We expect hydrogen additions to cause hydronaphthalene cations to be abundant in the cloud and to contribute via electric dipole radiation to the anomalous microwave emission. The identification of new interstellar features consistent with transitions of the simplest polycyclic aromatic hydrocarbon adds support to the hypothesis that this type of molecules are the carriers of both diffuse interstellar bands and anomalous microwave emission.Item Mapping The Interstellar Medium With Near-Infrared Diffuse Interstellar Bands(2015-01) Zasowski, Gail; Menard, B.; Bizyaev, Dmitry; Garcia-Hernandez, D. Anibal; Perez, A. E. G.; Hayden, M. R.; Holtzman, Jon; Johnson, John A.; Kinemuchi, K.; Majewski, Steven R.; Nidever, David L.; Shetrone, Matthew; Wilson, J. C.; Shetrone, MatthewWe map the distribution and properties of the Milky Way's interstellar medium as traced by diffuse interstellar bands (DIBs) detected in near-infrared stellar spectra from the SDSS-III/APOGEE survey. Focusing exclusively on the strongest DIB in the H band, at lambda similar to 1.527 mu m, we present a projected map of the DIB absorption field in the Galactic plane, using a set of about 60,000 sightlines that reach up to 15 kpc from the Sun and probe up to 30 mag of visual extinction. The strength of this DIB is linearly correlated with dust reddening over three orders of magnitude in both DIB equivalent width (Wpm) and extinction, with a power law index of 1.01 +/- 0.01, a mean relationship of W-DIB/A(v) = 0.1 angstrom mag(-1) and a dispersion of similar to 0.05 angstrom mag(-1) at extinctions characteristic of the Galactic midplane. These properties establish this DIB as a powerful, independent probe of dust extinction over a wide range of Av values. The subset of about 14,000 robustly detected DIB features have a W-DIB distribution that follows an exponential trend. We empirically determine the intrinsic rest wavelength of this transition to be lambda(0) = 15 272.42 angstrom and use it to calculate absolute radial velocities of the carrier, which display the kinematical signature of the rotating Galactic disk. We probe the DIB carrier distribution in three dimensions and show that it can be characterized by an exponential disk model with a scale height of about 100 pc and a scale length of about 5 kpc. Finally, we show that the DIB distribution also traces large-scale Galactic structures, including the Galactic long bar and the warp of the outer disk.Item Review: A Search For Diffuse Bands In The Circumstellar Envelopes Of Post-AGB Stars(2008-03) Luna, R.; Cox, N. L. J.; Satorre, M. A.; Hernández, D. A. García.; Suarez, O.; Lario, P. G.; García Hernández, D. A.In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on analyzing 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high-resolution optical spectroscopy, and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening, we conclude that the carrier(s)of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. This conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.Item A Spitzer High-Resolution Mid-Infrared Spectral Atlas of Starburst Galaxies(2009-10) Bernard-Salas, J.; Spoon, H. W. W.; Charmandaris, V.; Lebouteiller, V.; Farrah, D.; Devost, D.; Brandl, B. R.; Wu, Y. L.; Armus, L.; Hao, L.; Sloan, G. C.; Weedman, D.; Houck, J. R.; Hao, L.We present an atlas of Spitzer/IRS high-resolution (R similar to 600) 10-37 mu m spectra for 24 well known starburst galaxies. The spectra are dominated by fine-structure lines, molecular hydrogen lines, and emission bands of polycyclic aromatic hydrocarbons (PAHs). Six out of the eight objects with a known active galactic nucleus (AGN) component show emission of the high excitation [Ne v] line. This line is also seen in one other object (NGC 4194) with, a priori, no known AGN component. In addition to strong PAH emission features in this wavelength range (11.3, 12.7, 16.4 mu m), the spectra reveal other weak hydrocarbon features at 10.6, 13.5, 14.2 mu m, and a previously unreported emission feature at 10.75 mu m. An unidentified absorption feature at 13.7 mu m is detected in many of the starbursts. We use the fine-structure lines to derive the abundance of neon and sulfur for 14 objects where the Hi 7-6 line is detected. We further use the molecular hydrogen lines to sample the properties of the warm molecular gas. Several basic diagrams characterizing the properties of the sample are also shown. We have combined the spectra of all the pure starburst objects to create a high signal-to-noise ratio template, which is available to the community.Item Two Pseudobulges In The "Boxy Bulge" Galaxy NGC 5746(2012-08) Barentine, John C.; Kormendy, John; Barentine, John C.; Kormendy, JohnGalaxy formation and growth under the Lambda CDM paradigm is expected to proceed in a hierarchical, bottom-up fashion by which small galaxies grow into large galaxies; this mechanism leaves behind large "classical bulges" kinematically distinct from "pseudobulges" grown by internal, secular processes. We use archival data (Spitzer Space Telescope 3.6 mu m wavelength, Hubble Space Telescope H-band, Two Micron All Sky Survey K-s-band, and Sloan Digital Sky Survey gri-band) to measure composite minor-and major-axis surface brightness profiles of the almost-edge-on spiral galaxy NGC 5746. These light profiles span a large range of radii and surface brightnesses to reveal an inner, high surface brightness stellar component that is distinct from the well-known boxy bulge. It is well fitted by Sersic functions with indices n = 0.99 +/- 0.08 and 1.17 +/- 0.24 along the minor and major axes, respectively. Since n < 2, we conclude that this innermost component is a secularly evolved pseudobulge that is distinct from the boxy pseudobulge. This inner pseudobulge makes up 0.136 +/- 0.019 of the total light of the galaxy. It is therefore considerably less luminous than the boxy structure, which is now understood to be a bar seen nearly end-on. The infrared imagery shows further evidence for secular evolution in the form of a bright inner ring of inner radius 9.1 kpc and width 1.6 kpc. NGC 5746 is therefore a giant, pure-disk SB(r) bc galaxy with no sign of a merger-built bulge. We do not understand how such galaxies form in a Lambda DM universe.