Browsing by Subject "absorption"
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Item 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.Item Characterization of two distinct, simultaneous hot electron beams in intense laser-solid interactions(2009-11) Cho, B. I.; Osterholz, J.; Bernstein, A. C.; Dyer, G. M.; Karmakar, A.; Pukhov, A.; Ditmire, T.; Cho, B. I.; Osterholz, J.; Bernstein, A. C.; Dyer, G. M.; Ditmire, T.The transport of energetic electron beams generated from aluminum foils irradiated by ultraintense laser pulses has been studied by imaging coherent transition radiation from the rear side of the target. Two distinct beams of MeV electrons are emitted from the target rear side at the same time. This measurement indicates that two different mechanisms, namely resonance absorption and jxB heating, accelerate the electrons at the targets front side and drive them to different directions, with different temperatures. This interpretation is consistent with 3D-particle-in-cell simulations.Item Characterizing Transition Temperature Gas In The Galactic Corona(2012-04) Wakker, Bart P.; Savage, Blair D.; Fox, Andrew J.; Benjamin, Robert A.; Shapiro, Paul R.; Shapiro, Paul R.We present a study of the properties of the transition temperature (T similar to 10(5) K) gas in the Milky Way corona, based on the measurements of O VI, N V, C IV, Si IV, and Fe III absorption lines seen in the far-ultraviolet spectra of 58 sight lines to extragalactic targets, obtained with the Far-Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph. In many sight lines the Galactic absorption profiles show multiple components, which are analyzed separately. We find that the highly ionized atoms are distributed irregularly in a layer with a scale height of about 3 kpc, which rotates along with the gas in the disk, without an obvious gradient in the rotation velocity away from the Galactic plane. Within this layer the gas has randomly oriented velocities with a dispersion of 40-60 km s(-1). On average the integrated column densities are log N(O VI) = 14.3, log N(N V) = 13.5, log N(C IV) = 14.2, log N(Si IV) = 13.6, and log N(Fe III) = 14.2, with a dispersion of just 0.2 dex in each case. In sight lines around the Galactic center and Galactic north pole, all column densities are enhanced by a factor similar to 2, while at intermediate latitudes in the southern sky there is a deficit in N(O VI) of about a factor of two, but no deficit for the other ions. We compare the column densities and ionic ratios to a series of theoretical predictions: collisional ionization equilibrium, shock ionization, conductive interfaces, turbulent mixing, thick disk supernovae, static non-equilibrium ionization (NIE) radiative cooling, and an NIE radiative cooling model in which the gas flows through the cooling zone. None of these models can fully reproduce the data, but it is clear that NIE radiative cooling is important in generating the transition temperature gas.Item Discovery, Progenitor and Early Evolution of A Stripped Envelope Supernova iPTF13bvn(2013-09) Cao, Yi; Kasliwal, Mansi M.; Arcavi, Iair; Horesh, Assaf; Hancock, Paul; Valenti, Stefano; Cenko, S. Bradley; Kulkarni, S. R.; Gal-Yam, Avishay; Gorbikov, Evgeny; Ofek, Eran O.; Sand, David; Yaron, Ofer; Graham, Melissa; Silverman, Jeffrey M.; Wheeler, J. Craig; Marion, G. H.; Walker, Emma S.; Mazzali, Paolo; Howell, D. Andrew; Li, K. L.; Kong, A. K. H.; Bloom, Joshua S.; Nugent, Peter E.; Surace, Jason; Masci, Frank; Carpenter, John; Degenaar, Nathalie; Gelino, Christopher R.; Silverman, Jeffrey M.; Wheeler, J. Craig; Marion, G. H.The intermediate Palomar Transient Factory reports our discovery of a young supernova, iPTF13bvn, in the nearby galaxy, NGC 5806 (22.5 Mpc). Our spectral sequence in the optical and infrared suggests a Type Ib classification. We identify a blue progenitor candidate in deep pre-explosion imaging within a 2 sigma error circle of 80 mas (8.7 pc). The candidate has an M-B luminosity of -5.52 +/- 0.39 mag and a B-I color of 0.25 +/- 0.25 mag. If confirmed by future observations, this would be the first direct detection for a progenitor of a Type Ib. Fitting a power law to the early light curve, we find an extrapolated explosion date around 0.6 days before our first detection. We see no evidence of shock cooling. The pre-explosion detection limits constrain the radius of the progenitor to be smaller than a few solar radii. iPTF13bvn is also detected in centimeter and millimeter wavelengths. Fitting a synchrotron self-absorption model to our radio data, we find a mass-loading parameter of 1.3x10(12) g cm(-1). Assuming a wind velocity of 10(3) km s(-1), we derive a progenitor mass-loss rate of 3 x 10(-5) M-circle dot yr(-1). Our observations, taken as a whole, are consistent with a Wolf-Rayet progenitor of the supernova iPTF13bvn.Item Doping Nanocrystals And The Role Of Quantum Confinement(2007-09) Chan, T. L.; Tiago, M. L.; Chelikowsky, J. R.; Chan, Tzu‐Liang; Tiago, Murillo L.; Chelikowsky, James R.Recent progress in developing algorithms for solving the electronic structure problem for nanostructures is illustrated. Key ingredients in this approach include pseudopotentials implemented on a real space grid and the use of density functional theory. This procedure allows one to predict electronic properties for many materials across the nano-regime, i.e., from atoms to nanocrystals of sufficient size to replicate bulk properties. We will illustrate this method for doping silicon nanocrystals with phosphorous.Item The First Mid-Infrared Spectra Of Cool White Dwarfs(2008-05) Kilic, Mukremin; Kowalski, Piotr M.; Mullally, Fergal; Reach, William T.; von Hippel, Ted; von Hippel, TedWe present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3: 5-8 mu m photometry for 19 cool white dwarfs with 5000 K <= T-eff <= 9000 K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-infrared photometry and 7.5-14.5 mu m spectrum of WD0018-267 are consistent with a T-eff 5720 K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar, with significant mid-infrared flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 mu m range. Even though this deficit is attributed to collision-induced absorption ( CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of the LHS 1126 spectral energy distribution is best fit with a power-law index of -1.99, identical to a Rayleigh-Jeans spectrum. This argues that the deficit may be due to an unrecognized graylike opacity source in the infrared.Item Five-Year Wilkinson Microwave Anisotropy Probe Observations: Galactic Foreground Emission(2009-02) Gold, B.; Bennett, C. L.; Hill, R. S.; Hinshaw, G.; Odegard, N.; Page, L.; Spergel, D. N.; Weiland, J. L.; Dunkley, J.; Halpern, M.; Jarosik, N.; Kogut, A.; Komatsu, Eiichiro; Larson, D.; Meyer, S. S.; Nolta, M. R.; Wollack, E.; Wright, E. L.; Komatsu, EiichiroWe present a new estimate of foreground emission in the Wilkinson Microwave Anisotropy Probe (WMAP) data, using a Markov chain Monte Carlo method. The new technique delivers maps of each foreground component for a variety of foreground models with estimates of the uncertainty of each foreground component, and it provides an overall goodness-of-fit estimate. The resulting foreground maps are in broad agreement with those from previous techniques used both within the collaboration and by other authors. We find that for WMAP data, a simple model with power-law synchrotron, free-free, and thermal dust components fits 90% of the sky with a reduced chi(2)(v) of 1.14. However, the model does not work well inside the Galactic plane. The addition of either synchrotron steepening or a modified spinning dust model improves the fit. This component may account for up to 14% of the total flux at the Ka band (33 GHz). We find no evidence for foreground contamination of the cosmic microwave background temperature map in the 85% of the sky used for cosmological analysis.Item Giant Photoresponsivity of Midinfrared Hyperbolic Metamaterials in the Photon-Assisted-Tunneling Regime(2016-04) Chen, Pai-Yen; Hajizadegan, Mehdi; Sakhdari, Maryam; Alu, Andrea; Alu, AndreaWe explore broadband and omnidirectional midinfrared rectification based on nanopatterned hyperbolic metamaterials, composed of two dissimilar metals separated by an ultrathin dielectric layer. The exotic slow-light modes supported by such periodically trenched hyperbolic metamaterials efficiently trap incident radiation in massively parallel metal-insulator-metal tunnel junctions producing ultrafast infrared rectification via photon-assisted tunneling. This leads to a highly efficient photon-to-electron transduction with obtained photocurrent orders of magnitude larger than conventional lumped-element devices, such as nanorectennas and point-contact rectifiers. Our results promise an impact on infrared energy harvesters and plasmonic photodetectors.Item High-Resolution Optical Spectroscopy of DY Cen: Diffuse Interstellar Bands in a Proto-Fullerene Circumstellar Environment?(2012-11) Garcia-Hernandez, D. A.; Rao, N. Kameswara; Lambert, David L.; Rao, N. Kameswara; Lambert, David L.We search high-resolution and high-quality VLT/UVES optical spectra of the hot R Coronae Borealis star DY Cen for electronic transitions of the C-60 molecule and diffuse interstellar bands (DIBs). We report the non-detection of the strongest C-60 electronic transitions (e. g., those at similar to 3760, 3980, and 4024 angstrom). The absence of C-60 absorption bands may support recent laboratory results, which show that the similar to 7.0, 8.5, 17.4, and 18.8 mu m emission features seen in DY Cen-and other similar objects with polycyclic-aromatic-hydrocarbon-like dominated IR spectra-are attributable to proto-fullerenes or fullerene precursors rather than to C-60. DIBs toward DY Cen are normal for its reddening; the only exception is the DIB at 6284 angstrom (possibly also the 7223 angstrom DIB) which is found to be unusually strong. We also report the detection of a new broad (FWHM similar to 2 angstrom) and unidentified feature centered at similar to 4000 angstrom. We suggest that this new band may be related to the circumstellar proto-fullerenes seen at infrared wavelengths.Item the Li-7/Li-6 Isotope Ratio Near the Supernova Remnant IC 443(2012-05) Taylor, C. J.; Ritchey, A. M.; Federman, S. R.; Lambert, David L.; Lambert, David L.We present an analysis of Li-7/Li-6 isotope ratios along four sight lines that probe diffuse molecular gas near the supernova remnant IC 443. Recent gamma-ray observations have revealed the presence of shock-accelerated cosmic rays interacting with the molecular cloud surrounding the remnant. Our results indicate that the Li-7/Li-6 ratio is lower in regions more strongly affected by these interactions, a sign of recent Li production by cosmic rays. We find that Li-7/Li-6 approximate to 7 toward HD 254755, which is located just outside the visible edge of IC 443, while Li-7/Li-6 approximate to 3 along the line of sight to HD 43582, which probes the interior region of the supernova remnant. No evidence of Li-7 synthesis by neutrino-induced spallation is found in material presumably contaminated by the ejecta of a core-collapse supernova. The lack of a neutrino signature in the Li-7/Li-6 ratios near IC 443 is consistent with recent models of Galactic chemical evolution, which suggest that the nu-process plays only a minor role in Li production.Item Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser(2013-02) Bang, W.; Dyer, G.; Quevedo, H. J.; Bernstein, A. C.; Gaul, E.; Donovan, M.; Ditmire, T.; Bang, W.; Dyer, G.; Quevedo, H. J.; Bernstein, A. C.; Gaul, E.; Donovan, M.; Ditmire, T.The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm(3)) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6x10(7) neutrons in a single shot with a 120 J, 170 fs laser pulse. Possible effects of prepulses are discussed. DOI: 10.1103/PhysRevE.87.023106Item Single-shot optical conductivity measurement of dense aluminum plasmas(2009-07) Churina, I. V.; Cho, B. I.; Bernstein, A.; Stoker, D. S.; Dalton, A.; Symes, D. R.; Ditmire, T.; Churina, I. V.; Cho, B. I.; Bernstein, A.; Stoker, D. S.; Dalton, A.; Symes, D. R.; Ditmire, T.The optical conductivity of a dense femtosecond laser-heated aluminum plasma heated to 0.1-1.5 eV was measured using frequency-domain interferometry with chirped pulses, permitting simultaneous observation of optical probe reflectivity and probe pulse phase shift. Coupled with published models of bound-electron contributions to the conductivity, these two independent experimental data yielded a direct measurement of both real and imaginary components of the plasma conductivity.