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Series

2008

University of Kentucky

Articles 1 - 10 of 10

Full-Text Articles in Physics

Rotational Quenching Rate Coefficients For H2 In Collisions With H2 From 2 To 10,000 K, T.-G. Lee, N. Balakrishnan, R. C. Forrey, P. C. Stancil, G. Shaw, D. R. Schultz, Gary J. Ferland Dec 2008

Rotational Quenching Rate Coefficients For H2 In Collisions With H2 From 2 To 10,000 K, T.-G. Lee, N. Balakrishnan, R. C. Forrey, P. C. Stancil, G. Shaw, D. R. Schultz, Gary J. Ferland

Physics and Astronomy Faculty Publications

Rate coefficients for rotational transitions in H2 induced by H2 impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H2)2 potential energy surface were performed. The potential energy surface used here has been demonstrated to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels of J≤8 were computed for collision energies ranging between 10-4 and 2.5 eV, and the corresponding rate coefficients were calculated for the temperature range 2≤T≤10,000 K. In general, agreement with earlier calculations, which were limited to 100-6000 K, …

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Sensitivity Of Pdr Calculations To Microphysical Details, N. P. Abel, P. A. M. Van Hoof, G. Shaw, Gary J. Ferland, T. Elwert Oct 2008

Sensitivity Of Pdr Calculations To Microphysical Details, N. P. Abel, P. A. M. Van Hoof, G. Shaw, Gary J. Ferland, T. Elwert

Physics and Astronomy Faculty Publications

Our understanding of physical processes in photodissociation regions or photon-dominated regions (PDRs) largely depends on the ability of spectral synthesis codes to reproduce the observed infrared emission-line spectrum. In this paper, we explore the sensitivity of a single PDR model to microphysical details. Our calculations use the Cloudy spectral synthesis code, recently modified to include a wealth of PDR physical processes. We show how the chemical/thermal structure of a PDR, along with the calculated spectrum, changes when the treatment of physical processes such as grain physics and atomic/molecular rates are varied. We find a significant variation in the intensities of …

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Agn Dusty Tori. I. Handling Of Clumpy Media, Maia Nenkova, Matthew M. Sirocky, Željko Ivezić, Moshe Elitzur Sep 2008

Agn Dusty Tori. I. Handling Of Clumpy Media, Maia Nenkova, Matthew M. Sirocky, Željko Ivezić, Moshe Elitzur

Physics and Astronomy Faculty Publications

According to unified schemes of active galactic nuclei (AGNs), the central engine is surrounded by dusty, optically thick clouds in a toroidal structure. We have recently developed a formalism that for the first time takes proper account of the clumpy nature of the AGN torus. We now provide a detailed report of our findings in a two-paper series. Here we present our general formalism for radiative transfer in clumpy media and construct its building blocks for the AGN problem-the source functions of individual dusty clouds heated by the AGN radiation field. We show that a fundamental difference from smooth density …

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Agn Dusty Tori. Ii. Observational Implications Of Clumpiness, Maia Nenkova, Matthew M. Sirocky, Robert Nikutta, Željko Ivezić, Moshe Elitzur May 2008

Agn Dusty Tori. Ii. Observational Implications Of Clumpiness, Maia Nenkova, Matthew M. Sirocky, Robert Nikutta, Željko Ivezić, Moshe Elitzur

Physics and Astronomy Faculty Publications

Clumpy torus models with N0 ~ 5–15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations. The dust has standard Galactic composition, with individual cloud optical depth τV ~ 30–100 at visual. The models naturally explain the observed behavior of the 10 μm silicate feature, in particular the lack of deep absorption features in AGNs of any type, and can reproduce the weak emission feature tentatively detected in type 2 QSOs. The clouds' angular distribution must have a soft edge, e.g., Gaussian, and the radial distribution should decrease as 1/r or 1/r2 …

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Silicates In Ultraluminous Infrared Galaxies, M. M. Sirocky, N. A. Levenson, Moshe Elitzur, H. W. W. Spoon, L. Armus May 2008

Silicates In Ultraluminous Infrared Galaxies, M. M. Sirocky, N. A. Levenson, Moshe Elitzur, H. W. W. Spoon, L. Armus

Physics and Astronomy Faculty Publications

We analyze the mid-infrared (MIR) spectra of ultraluminous infrared galaxies (ULIRGs) observed with the Spitzer Space Telescope's Infrared Spectrograph. Dust emission dominates the MIR spectra of ULIRGs, and the reprocessed radiation that emerges is independent of the underlying heating spectrum. Instead, the resulting emission depends sensitively on the geometric distribution of the dust, which we diagnose with comparisons of numerical simulations of radiative transfer. Quantifying the silicate emission and absorption features that appear near 10 and 18 μm requires a reliable determination of the continuum, and we demonstrate that including …

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The Effect Of 53 ΜM Ir Radiation On 18 Cm Oh Megamaser Emission, Philip Lockett, Moshe Elitzur Apr 2008

The Effect Of 53 ΜM Ir Radiation On 18 Cm Oh Megamaser Emission, Philip Lockett, Moshe Elitzur

Physics and Astronomy Faculty Publications

OH megamasers (OHMs) emit primarily in the main lines at 1667 and 1665 MHz and differ from their Galactic counterparts due to their immense luminosities, large line widths, and 1667/1665 MHz flux ratios, which are always greater than 1. We find that these maser properties result from strong 53 μm radiative pumping combined with line overlap effects caused by turbulent line widths ~20 km s-1 pumping calculations that do not include line overlap are unreliable. A minimum dust temperature of ~45 K is needed for inversion, and maximum maser efficiency occurs for dust temperatures ~80-140 K. We find …

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Excited-State Oh Masers And Supernova Remnants, Ylva M. Pihlström, Vincent L. Fish, Loránt O. Sjouwerman, Laura K. Zschaechner, Philip B. Lockett, Moshe Elitzur Mar 2008

Excited-State Oh Masers And Supernova Remnants, Ylva M. Pihlström, Vincent L. Fish, Loránt O. Sjouwerman, Laura K. Zschaechner, Philip B. Lockett, Moshe Elitzur

Physics and Astronomy Faculty Publications

The collisionally pumped, ground-state 1720 MHz maser line of OH is widely recognized as a tracer for shocked regions and observed in star-forming regions and supernova remnants. Whereas some lines of excited states of OH have been detected and studied in star-forming regions, the subject of excited-state OH in supernova remnants-where high collision rates are to be expected-is only recently being addressed. Modeling of collisional excitation of OH demonstrates that 1720, 4765, and 6049 MHz masers can occur under similar conditions in regions of shocked gas. In particular, the 6049 and 4765 MHz masers become more significant at increased OH …

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On The Enhanced Cosmic-Ray Ionization Rate In The Diffuse Cloud Toward Ζ Persei, Gargi Shaw, Gary J. Ferland, R. Srianand, N. P. Abel, P. A. M. Van Hoof, P. C. Stancil Mar 2008

On The Enhanced Cosmic-Ray Ionization Rate In The Diffuse Cloud Toward Ζ Persei, Gargi Shaw, Gary J. Ferland, R. Srianand, N. P. Abel, P. A. M. Van Hoof, P. C. Stancil

Physics and Astronomy Faculty Publications

The spatial distribution of the cosmic-ray flux is important in understanding the interstellar medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ=χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5×10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR=20 toward HD 185418. McCall et al. have reported χCR=48 toward ζ …

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Dielectronic Recombination And Stability Of Warm Gas In Active Galactic Nuclei, Susmita Chakravorty, Ajit K. Kembhavi, Martin Elvis, Gary J. Ferland, N. R. Badnell Jan 2008

Dielectronic Recombination And Stability Of Warm Gas In Active Galactic Nuclei, Susmita Chakravorty, Ajit K. Kembhavi, Martin Elvis, Gary J. Ferland, N. R. Badnell

Physics and Astronomy Faculty Publications

Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We find that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give significantly different results, especially in the regions corresponding to warm absorbers, leading to different physical predictions. Using the current rates we find a larger probability of having a thermally stable warm absorber at …

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The Origin Of Molecular Hydrogen Emission In Cooling-Flow Filaments, Gary J. Ferland, A. C. Fabian, N. A. Hatch, R. M. Johnstone, R. L. Porter, P. A. M. Vanhoof, R. J. R. Williams Jan 2008

The Origin Of Molecular Hydrogen Emission In Cooling-Flow Filaments, Gary J. Ferland, A. C. Fabian, N. A. Hatch, R. M. Johnstone, R. L. Porter, P. A. M. Vanhoof, R. J. R. Williams

Physics and Astronomy Faculty Publications

The optical filaments found in many cooling flows in galaxy clusters consist of low-density (∼103 cm−3) cool (∼103 K) gas surrounded by significant amounts of cosmic-ray and magnetic field energy. Their spectra show anomalously strong low-ionization and molecular emission lines when compared with Galactic molecular clouds exposed to ionizing radiation such as the Orion complex. Previous studies have shown that the spectra cannot be produced by O-star photoionization. Here, we calculate the physical conditions in dusty gas that is well shielded from external sources of ionizing photons and is energized either by cosmic rays or dissipative …

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