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Physics and Astronomy Faculty Publications

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Articles 571 - 600 of 679

Full-Text Articles in Astrophysics and Astronomy

Dissipative Heating And Quasar Emission Lines, Mark Bottorff, Gary J. Ferland Apr 2002

Dissipative Heating And Quasar Emission Lines, Mark Bottorff, Gary J. Ferland

Physics and Astronomy Faculty Publications

Recent observations reveal that the profiles of emission lines of active galactic nuclei are too smooth to be produced by discrete thermal (T~104 K) clouds. The lines may also be too bright to be powered by the continuum unless a large covering factor or additional heating mechanisms are present. We have been investigating one possible explanation of these observations, namely, that the clouds are turbulent. This paper focuses on observational effects caused by dissipation of turbulent energy into cloud heating. We find that internal heating can explain these observations. Clouds energized by both the ionizing continuum and …

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Metallicities And Abundance Ratios From Quasar Broad Emission Lines, Fred Hamann, K. T. Korista, Gary J. Ferland, Craig Warner, Jack Baldwin Jan 2002

Metallicities And Abundance Ratios From Quasar Broad Emission Lines, Fred Hamann, K. T. Korista, Gary J. Ferland, Craig Warner, Jack Baldwin

Physics and Astronomy Faculty Publications

The broad emission lines (BELs) of quasars and active galactic nuclei (AGNs) are important diagnostics of the relative abundances and overall metallicity in the gas. Here we present new theoretical predictions for several UV BELs. We focus specifically on the relative nitrogen abundance as a metallicity indicator, based on the expected secondary enrichment of nitrogen at metallicities Z≳0.2 Z☉. Among the lines we consider, NIII]λ1750/OIII]λ1664, NVλ1240/(CIVλ1549+OVIλ1034), AND NV/HeIIλ1640 are the most robust diagnostics. We argue, in particular, that the average N V BEL is not dominated by scattered Lyα photons from a broad absorption-line wind. We then …

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The Planetary Nebula A39: An Observational Benchmark For Numerical Modeling Of Photoionized Plasmas, George H. Jacoby, Gary J. Ferland, Kirk T. Korista Oct 2001

The Planetary Nebula A39: An Observational Benchmark For Numerical Modeling Of Photoionized Plasmas, George H. Jacoby, Gary J. Ferland, Kirk T. Korista

Physics and Astronomy Faculty Publications

Galactic nebulae are the main probes for the chemical evolution of the interstellar medium. Yet, recent observations have shown that chemical abundances determined from recombination and collisionally excited emission lines can differ by as much as an order of magnitude in some planetary nebulae (PNs). Many PNs have complex geometries and morphological evidence for interactions from stellar winds, and it is not clear to what extent winds, inhomogeneities, or shocked gas affect the observed spectrum. There currently is no full explanation for this discrepancy, which brings into question whether we understand the physical state of these low-density plasmas at all. …

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Measurement And Interpretation Of Deuterium-Line Emission In The Orion Nebula, C. R. O'Dell, Gary J. Ferland, W. J. Henney Jul 2001

Measurement And Interpretation Of Deuterium-Line Emission In The Orion Nebula, C. R. O'Dell, Gary J. Ferland, W. J. Henney

Physics and Astronomy Faculty Publications

We present new observations of the deuterium and hydrogen Balmer lines in the Orion Nebula. There is a real variation in the deuterium-to-hydrogen line ratios across the nebula, being greatest in the emission from the largest proplyd (Orion 244-440). We also present the results of a detailed model for the emission of these lines, the hydrogen lines being the result of photoionization and recombination while the deuterium lines are produced by fluorescent excitation of the upper energy states by the far-UV radiation from θ1 Ori C. Comparison of the observations and predictions of the line intensities shows good agreement, …

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Fractal Quasar Clouds, Mark Bottorff, Gary J. Ferland Mar 2001

Fractal Quasar Clouds, Mark Bottorff, Gary J. Ferland

Physics and Astronomy Faculty Publications

This paper examines whether a fractal cloud geometry can reproduce the emission-line spectra of active galactic nuclei (AGNs). The nature of the emitting clouds is unknown, but many current models invoke various types of magnetohydrodynamic confinement. Recent studies have argued that a fractal distribution of clouds, in which subsets of clouds occur in self-similar hierarchies, is a consequence of such confinement. Whatever the confinement mechanism, fractal cloud geometries are found in nature and may be present in AGNs too. We first outline how a fractal geometry can apply at the center of a luminous quasar. Scaling laws are derived that …

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A Desktop Universe For The Introductory Astronomy Laboratory, Laurence A. Marschall, Glenn A. Snyder, Paul Richard Cooper Dec 2000

A Desktop Universe For The Introductory Astronomy Laboratory, Laurence A. Marschall, Glenn A. Snyder, Paul Richard Cooper

Physics and Astronomy Faculty Publications

What is a well-intentioned astronomy instructor to do? There is no argument that experience with the real world is desirable in any astronomy course, especially the introductory classes that fulfill the science distribution requirements at many colleges and universities. Though it is a simple matter to take students out of doors, show them the motions of the Sun, Moon, and stars, and have them squint for a few seconds at Saturn's rings through a telescope, these activities represent only a small portion of the subject matter of modern astronomy. It is simply not possible, given the constraints of time, weather, …

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Continuum Pumping Of [Fe Ii] In The Orion Nebula, E. M. Verner, D. A. Verner, J. A. Baldwin, Gary J. Ferland, P. G. Martin Nov 2000

Continuum Pumping Of [Fe Ii] In The Orion Nebula, E. M. Verner, D. A. Verner, J. A. Baldwin, Gary J. Ferland, P. G. Martin

Physics and Astronomy Faculty Publications

This paper presents detailed comparisons between numerical simulations of Fe II emission spectra and recent high-resolution and signal-to-noise spectra of the Orion Nebula. We have identified 40 [Fe II] lines in the spectrum, allowing extensive comparisons between theory and observations. The identifications are based on predictions of a realistic model of the Fe II atom, which includes the lowest 371 levels (all levels up to 11.6 eV). We investigate the dependence of the spectrum on electron density and on pumping by the stellar continuum. Orion is important because it provides a relatively simple environment in which to test complex simulations. …

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Observational Constraints On The Internal Velocity Field Of Quasar Emission-Line Clouds, Mark Bottorff, Gary J. Ferland, Jack Baldwin, Kirk Korista Oct 2000

Observational Constraints On The Internal Velocity Field Of Quasar Emission-Line Clouds, Mark Bottorff, Gary J. Ferland, Jack Baldwin, Kirk Korista

Physics and Astronomy Faculty Publications

This paper addresses the question, what does the spectrum of a typical quasar reveal about the velocity structure within its broad emission line region clouds? Turbulent (i.e., nonthermal) broadening of spectral lines can be due to macroturbulence or microturbulence. Microturbulence affects line formation and the emitted spectrum and may be required to account for the observed smoothness of the line profiles. The velocity field is crucial since it addresses the fundamental nature of the individual clouds and the global structure of the active galactic nuclei (AGNs) environment. For example, stellar winds or magnetically confined blobs might be highly microturbulent, requiring …

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The Narrow-Line Region In The Seyfert 2 Galaxy Ngc 3393, Andrew J. Cooke, J. A. Baldwin, Gary J. Ferland, Hagai Netzer, Andrew S. Wilson Aug 2000

The Narrow-Line Region In The Seyfert 2 Galaxy Ngc 3393, Andrew J. Cooke, J. A. Baldwin, Gary J. Ferland, Hagai Netzer, Andrew S. Wilson

Physics and Astronomy Faculty Publications

The narrow-line region (NLR) of the Seyfert 2 galaxy NGC 3393 is dominated by a symmetric structure which appears as S-shaped arms in Hubble Space Telescope (HST) images. These arms, which occupy the central few arcseconds of the nucleus, border a linear, triple-lobed radio source. We use HST imaging and spectra, ground-based optical images, long-slit spectra, Fabry-Perot imaging spectroscopy, and VLA radio data to perform a detailed investigation of the kinematics and ionization of the line-emitting gas in NGC 3393 and of its relationship with the relativistic gas responsible for the radio emission. The excitation map [O III] …

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The Primordial Helium Abundance: Toward Understanding And Removing The Cosmic Scatter In The Dy/Dz Relation, D. R. Ballantyne, Gary J. Ferland, P. G. Martin Jun 2000

The Primordial Helium Abundance: Toward Understanding And Removing The Cosmic Scatter In The Dy/Dz Relation, D. R. Ballantyne, Gary J. Ferland, P. G. Martin

Physics and Astronomy Faculty Publications

We present results from photoionization models of low-metallicity H II regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be nonnegligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective temperature rises to above 45,000 K, the ICF can be significantly negative. This result is independent of the choice of stellar atmosphere. However, if an H II region has an [O III] λ5007/[O I] λ6300 ratio greater than 300, then our models show that, regardless of …

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Infrared Classification Of Galactic Objects, Željko Ivezić, Moshe Elitzur May 2000

Infrared Classification Of Galactic Objects, Željko Ivezić, Moshe Elitzur

Physics and Astronomy Faculty Publications

Unbiased analysis shows that IRAS data reliably differentiate between the early and late stages of stellar evolution because objects at these stages clearly segregate in infrared color-color diagrams. Structure in these diagrams is primarily controlled by the density distribution of circumstellar dust. The density profile around older objects is the steepest, declining as r-2, while young objects have profiles that vary as r-3/2 and flatter. The different density profiles reflect the different dynamics that govern the different environments. Our analysis also shows that high-mass star formation is strongly concentrated within ~5 kpc around the Galactic center, in …

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Properties Of Dust Grains In Planetary Nebulae. I. The Ionized Region Of Ngc 6445, Peter A. M. Van Hoof, Griet C. Van De Steene, Douwe A. Beintema, P. G. Martin, Stuart R. Pottasch, Gary J. Ferland Mar 2000

Properties Of Dust Grains In Planetary Nebulae. I. The Ionized Region Of Ngc 6445, Peter A. M. Van Hoof, Griet C. Van De Steene, Douwe A. Beintema, P. G. Martin, Stuart R. Pottasch, Gary J. Ferland

Physics and Astronomy Faculty Publications

One of the factors influencing the spectral evolution of a planetary nebula is the fate of the dust grains that are emitting the infrared continuum. Several processes have been proposed that either destroy the grains or remove them from the ionized region. To test whether these processes are effective, we study new infrared spectra of the evolved nebula NGC 6445. These data show that the thermal emission from the grains is very cool and has a low flux compared to Hβ. A model of the ionized region is constructed, using the photoionization code CLOUDY 90.05. Based on this model, we …

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The Universe On A Desktop: Observational Astronomy Simulations In The Instructional Laboratory, Laurence A. Marschall Jan 2000

The Universe On A Desktop: Observational Astronomy Simulations In The Instructional Laboratory, Laurence A. Marschall

Physics and Astronomy Faculty Publications

Though the value of hands-on learning has long been recognised by educators, it is difficult to design laboratories in astronomy classes that present realistic astrophysical techniques to undergraduate students. Unlike most other sciences, astronomy is largely observational, not experimental, and making useful observations involves expensive equipment over time scales inconvenient for pedagogy. In recent years, however, astronomy has gone almost completely digital, and the advent of large on-line databases and fast personal computers has made it possible to realistically simulate the experience of research astrophysics in the laboratory. Since 1992, Project CLEA (Contemporary Laboratory Experiences in Astronomy) has been developing …

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Magnetic Confinement, Magnetohydrodynamic Waves And Smooth Line Profiles In Active Galactic Nuclei, M. C. Bottorff, Gary J. Ferland Jan 2000

Magnetic Confinement, Magnetohydrodynamic Waves And Smooth Line Profiles In Active Galactic Nuclei, M. C. Bottorff, Gary J. Ferland

Physics and Astronomy Faculty Publications

In this paper, we show that if the broad-line region clouds are in approximate energy equipartition between the magnetic field and gravity, as hypothesized by Rees, there will be a significant effect on the shape and smoothness of broad emission-line profiles in active galactic nuclei. Linewidths of contributing clouds or flow elements are much wider than their thermal widths, because of the presence of non-dissipative magnetohydrodynamic waves and their collective contribution produce emission-line profiles broader and smoother than would be expected if a magnetic field were not present. As an illustration, a simple model of isotropically emitting clouds, normally distributed …

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The Type Ia Supernova 1998bu In M96 And The Hubble Constant, Saurabh Jha, Peter M. Garnavich, Robert P. Kirshner, Peter Challis, Alicia M. Soderberg, Lucas M. Macri, John P. Huchra, Pauline Barmby, Elizabeth J. Barton, Perry Berlind, Warren R. Brown, Nelson Caldwell, Michael L. Calkins, Sheila J. Kannappan, Daniel M. Koranyi, Michael A. Pahre, Kenneth J. Rines, Krzysztof Z. Stanek, Robert P. Stefanik, Andrew H. Szentgyorgyi, Petri Vaisanen, Zhong Wang, Joseph M. Zajac, Adam G. Riess, Alexei V. Filippenko, Weidong Li, Maryam Modjaz, Richard R. Treffers, Carl W. Hergenrother, Eva K. Grebel, Patrick Seitzer, George H. Jacoby, Priscilla J. Benson, Akbar H. Rizvi '99, Laurence A. Marschall, Jeffrey D. Goldader, Matthew Beasley, William D. Vacca, Bruno Leibundgut, Jason Spyromilio, Brian P. Schmidt, Peter R. Wood Nov 1999

The Type Ia Supernova 1998bu In M96 And The Hubble Constant, Saurabh Jha, Peter M. Garnavich, Robert P. Kirshner, Peter Challis, Alicia M. Soderberg, Lucas M. Macri, John P. Huchra, Pauline Barmby, Elizabeth J. Barton, Perry Berlind, Warren R. Brown, Nelson Caldwell, Michael L. Calkins, Sheila J. Kannappan, Daniel M. Koranyi, Michael A. Pahre, Kenneth J. Rines, Krzysztof Z. Stanek, Robert P. Stefanik, Andrew H. Szentgyorgyi, Petri Vaisanen, Zhong Wang, Joseph M. Zajac, Adam G. Riess, Alexei V. Filippenko, Weidong Li, Maryam Modjaz, Richard R. Treffers, Carl W. Hergenrother, Eva K. Grebel, Patrick Seitzer, George H. Jacoby, Priscilla J. Benson, Akbar H. Rizvi '99, Laurence A. Marschall, Jeffrey D. Goldader, Matthew Beasley, William D. Vacca, Bruno Leibundgut, Jason Spyromilio, Brian P. Schmidt, Peter R. Wood

Physics and Astronomy Faculty Publications

We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8^0.8) with B\12.22^0.03 and V \11.88^0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of AV\0.94^0.15 mag, and indicates the supernova was of average luminosity compared …

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Hubble Space Telescope/Faint Object Spectrograph Spectroscopy Of Spatially Resolved Narrow-Line Regions In The Seyfert 2 Galaxies Ngc 2110 And Ngc 5929, Pierre Ferruit, Andrew S. Wilson, Mark Whittle, Chris Simpson, John S. Mulchaey, Gary J. Ferland Sep 1999

Hubble Space Telescope/Faint Object Spectrograph Spectroscopy Of Spatially Resolved Narrow-Line Regions In The Seyfert 2 Galaxies Ngc 2110 And Ngc 5929, Pierre Ferruit, Andrew S. Wilson, Mark Whittle, Chris Simpson, John S. Mulchaey, Gary J. Ferland

Physics and Astronomy Faculty Publications

We present the results of UV and optical Hubble Space Telescope/Faint Object Spectrograph spectroscopy of bright, extranuclear regions of line emission in the Seyfert galaxies NGC 2110 and NGC 5929. We have obtained spectra of the brightest region of the ``nuclear jet'' of NGC 2110 (75 pc from the nucleus) and of the southwest emission-line cloud of NGC 5929 (90 pc from the nucleus), in the G130H (1090-1605 Å), G190H (1570-2310 Å), G400H (3235-4780 Å), and G570H (4570-6820 Å) configurations. The observed line ratios are compared with the predictions of the two component (matter- and ionization-bounded, MB-IB), central source …

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Dust Emission From Herbig Ae/Be Stars: Evidence For Disks And Envelopes, Anatoly Miroshnichenko, Željko Ivezić, Dejan Vinković, Moshe Elitzur Aug 1999

Dust Emission From Herbig Ae/Be Stars: Evidence For Disks And Envelopes, Anatoly Miroshnichenko, Željko Ivezić, Dejan Vinković, Moshe Elitzur

Physics and Astronomy Faculty Publications

Infrared and millimeter-wave emission from Herbig Ae/Be stars has produced conflicting conclusions regarding the dust geometry in these objects. We show that the compact dimensions of the millimeter-wave-emitting regions are a decisive indication for disks. But a disk cannot explain the spectral energy distribution unless it is embedded in an extended envelope that (1) dominates the IR emission and (2) provides additional disk heating on top of the direct stellar radiation. Detailed radiative transfer calculations based on the simplest model for envelope-embedded disks successfully fit the data from UV to millimeter wavelengths and show that the disks have central holes. …

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Evidence Of A Solar Origin For Pressure Balance Structures In The High-Latitude Solar Wind, Daniel B. Reisenfeld, D. J. Mccomas, J. T. Steinberg Jul 1999

Evidence Of A Solar Origin For Pressure Balance Structures In The High-Latitude Solar Wind, Daniel B. Reisenfeld, D. J. Mccomas, J. T. Steinberg

Physics and Astronomy Faculty Publications

Ulysses observations of the high-latitude solar wind have shown that on time scales of < 1 day, the polar wind is dominated by pressure balance structures (PBSs). Fluctuations of the plasma beta within PBSs appear to be strongly correlated with fluctuations in the helium abundance. The correlation occurs in both the northern and southern hemispheres. In addition, a mechanism is apparently at work in the high-latitude solar wind that dissipates the beta/He correlation over a distance of a few AU. Solar wind composition is established at the base of the corona; thus, the He abundance signature strongly suggests the observed solar wind PBSs are associated with structures low in the solar atmosphere. In particular, high-beta structures appear to originate in locations of enhanced He abundance. We suggest an interpretation of the high-beta portion of PBSs as the solar wind extensions of polar plumes.

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The Effects Of Charge Transfer On The Thermal Equilibrium Of Photoionized Nebulae, J. B. Kingdon, Gary J. Ferland May 1999

The Effects Of Charge Transfer On The Thermal Equilibrium Of Photoionized Nebulae, J. B. Kingdon, Gary J. Ferland

Physics and Astronomy Faculty Publications

Charge transfer can affect both the ionization and thermal balance of astrophysical plasmas. Using the most recent rate coefficients and energy defects, we calculate the heating/cooling rates for charge transfer reactions between hydrogen and elements up to Z=30. We incorporate these values into the photoionization code CLOUDY. Results from models approximating a wide range of astrophysical objects and conditions suggest that charge transfer can make a significant contribution to the heating near the H ionization front, particularly in objects with a hard ionizing continuum or enhanced abundances. Charge transfer heating can also be important in regimes in which the usual …

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The Pg X-Ray Qso Sample: Links Between The Ultraviolet-X-Ray Continuum And Emission Lines, Beverley J. Wills, A. Laor, M. S. Brotherton, D. Wills, B. J. Wilkes, Gary J. Ferland, Zhaohui Shang Apr 1999

The Pg X-Ray Qso Sample: Links Between The Ultraviolet-X-Ray Continuum And Emission Lines, Beverley J. Wills, A. Laor, M. S. Brotherton, D. Wills, B. J. Wilkes, Gary J. Ferland, Zhaohui Shang

Physics and Astronomy Faculty Publications

Two sets of relationships relate QSO UV to soft X-ray continua with the broad-line region. These are (i) the Baldwin relationships, which are inverse relationships between the broad-line equivalent width and the continuum luminosity, and (ii) Boroson & Green's optical "Principal Component 1'' relationships, linking steeper soft X-ray spectra with narrower Hβ emission, stronger Hβ blue wings, stronger optical Fe II emission, and weaker [O III] λ5007 lines. In order to understand these relationships, we extended the spectra into the UV for 22 QSOs with high-quality soft X-ray spectra. These are from the complete sample of QSOs from the Bright …

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Megamaser Disks In Active Galactic Nuclei, John F. Kartje, Arieh Königl, Moshe Elitzur Mar 1999

Megamaser Disks In Active Galactic Nuclei, John F. Kartje, Arieh Königl, Moshe Elitzur

Physics and Astronomy Faculty Publications

Recent spectroscopic and VLBI-imaging observations of bright extragalactic H2O maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show …

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He I 2.06 Micron Emission From Nebulae, Gary J. Ferland Feb 1999

He I 2.06 Micron Emission From Nebulae, Gary J. Ferland

Physics and Astronomy Faculty Publications

The spectrum emitted by any astronomical plasma is sensitive to a variety of details, some of which may not be obviously important. This paper describes the sensitivity of the He I 2.06 μm line to the gas opacity at ionizing energies. The intensity of the line relative to a hydrogen line depends on the He+/H+ ratio, but also on the ratio of continuous to He I Lyα line opacity, since this determines whether the Lyα line can scatter often enough to be converted to the 2.06 μm line. The intensity of the infrared line relative to Hβ …

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Oh 1720 Megahertz Masers In Supernova Remnants: C-Shock Indicators, Phil Lockett, Eric Gauthier, Moshe Elitzur Jan 1999

Oh 1720 Megahertz Masers In Supernova Remnants: C-Shock Indicators, Phil Lockett, Eric Gauthier, Moshe Elitzur

Physics and Astronomy Faculty Publications

Recent observations show that the OH 1720 MHz maser is a powerful probe of the shocked region where a supernova remnant strikes a molecular cloud. We perform a thorough study of the pumping of this maser and find tight constraints on the physical conditions needed for its production. The presence of the maser implies moderate temperatures (50-125 K) and densities (~105 cm-3) and OH column densities of order 1016 cm-2. We show that these conditions can exist only if the shocks are of C-type. J-shocks fail by such a wide margin that the presence …

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Numerical Simulations Of Fe Ii Emission Spectra, E. M. Verner, D. A. Verner, K. T. Korista, Jason W. Ferguson, F. Hamann, Gary J. Ferland Jan 1999

Numerical Simulations Of Fe Ii Emission Spectra, E. M. Verner, D. A. Verner, K. T. Korista, Jason W. Ferguson, F. Hamann, Gary J. Ferland

Physics and Astronomy Faculty Publications

This paper describes the techniques that we have used to incorporate a large-scale model of the Fe+ ion and resulting Fe IIemission into CLOUDY, a spectral synthesis code designed to simulate conditions within a plasma and model the resulting spectrum. We describe the numerical methods we use to determine the level populations, mutual line overlap fluorescence, collisional effects, and the heating-cooling effects of the atom on its environment. As currently implemented, the atom includes the lowest 371 levels (up to 11.6 eV) and predicts intensities of 68,635 lines. We describe our data sources, which include the most recent transition …

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Quasars As Cosmological Probes: The Ionizing Continuum, Gas Metallicity, And The WΛ-L Relation, Kirk Korista, Jack Baldwin, Gary J. Ferland Nov 1998

Quasars As Cosmological Probes: The Ionizing Continuum, Gas Metallicity, And The WΛ-L Relation, Kirk Korista, Jack Baldwin, Gary J. Ferland

Physics and Astronomy Faculty Publications

Using a realistic model for line emission from the broad emission line regions of quasars, we are able to reproduce the previously observed correlations of emission-line ratios with the shape of the spectral energy distribution (SED). In agreement with previous studies, we find that the primary driving force behind the Baldwin effect (Wλ ~ Lβ, β < 0) is a global change in the SED with quasar luminosity, in that more luminous quasars must have characteristically softer ionizing continua. This is completely consistent with observations that show (1) a correlation between Luv, αox, and αuvx (2) correlations of SED shape-sensitive line ratios with αox, αuvx, and Luv; and (3) correlations between line equivalent widths and αox, αuvx …

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Temperature Fluctuations In Photoionized Nebulae. Ii. The Effect Of Inhomogeneous Abundances, J. B. Kingdon, Gary J. Ferland Oct 1998

Temperature Fluctuations In Photoionized Nebulae. Ii. The Effect Of Inhomogeneous Abundances, J. B. Kingdon, Gary J. Ferland

Physics and Astronomy Faculty Publications

Recent abundance determinations based on recombination lines in several emission-line nebulae yield ionic abundances several times larger than those derived from forbidden lines. These results cast uncertainty over all abundance determinations in such objects. One possible explanation for these discrepancies frequently cited in the literature is the presence of chemical inhomogeneities. We have run a series of photoionization models to examine what effect such inhomogeneities will have on the resulting temperature structure of nebulae. We then derive abundances from these models, utilizing Peimbert's t2 formalism. Our results suggest that, although chemical inhomogeneities may produce nonnegligible biases in abundance determinations …

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Polarization Of Astronomical Maser Radiation. Iv. Circular Polarization Profiles, Moshe Elitzur Sep 1998

Polarization Of Astronomical Maser Radiation. Iv. Circular Polarization Profiles, Moshe Elitzur

Physics and Astronomy Faculty Publications

Profile comparison of the Stokes parameters V and I is a powerful tool for maser data analysis, which provides the first direct methods for unambiguous determination of (1) the maser saturation stage, (2) the amplification optical depth and intrinsic Doppler width of unsaturated masers, and (3) the comparative magnitudes of Zeeman splitting and Doppler line width. Circular polarization recently detected in OH 1720 MHz emission from the Galactic center appears to provide the first direct evidence for maser saturation.

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The Ultraviolet-Optical Albedo Of Broad Emission Line Clouds, Kirk Korista, Gary J. Ferland Mar 1998

The Ultraviolet-Optical Albedo Of Broad Emission Line Clouds, Kirk Korista, Gary J. Ferland

Physics and Astronomy Faculty Publications

We explore the effective UV-optical albedos of a variety of types of broad emission line clouds, as well as their possible effects on the observed spectra of active galactic nuclei (AGNs). An important albedo source in moderately ionized ionization-bounded clouds is that which is due to neutral hydrogen: Rayleigh scattering of continuum photons off the extreme damping wings of Lyα. The photons resulting from this scattering mechanism may contribute significantly to the Lyα emission line, especially in the very broad wings. In addition, line photons emitted near 1200 Å (e.g., N V λ1240) that stream toward the neutral portion of …

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Temperature Variations And N/O In The Orion Nebula From Hst Observations, R. H. Rubin, P. G. Martin, R. J. Dufour, Gary J. Ferland, J. A. Baldwin, J. J. Hester, D. K. Walter Mar 1998

Temperature Variations And N/O In The Orion Nebula From Hst Observations, R. H. Rubin, P. G. Martin, R. J. Dufour, Gary J. Ferland, J. A. Baldwin, J. J. Hester, D. K. Walter

Physics and Astronomy Faculty Publications

Using the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) on the Hubble Space Telescope, we measured the flux of the N II] (2s2p3 5S2 --> 2s22p2 3P2,1) lines at λvac = 2143.45, 2139.68 Å in the Orion Nebula--the first detection of these lines in an H II region. In order to assess the N+/O+ ratio, we also measured the flux of the [O II] (2p32Po1/2,3/2-->2p34So3/2) lines at λvac …

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New Rotation Periods In The Pleiades: Interpreting Activity Indicators, Anita Krishnamurthi, D. M. Terndrup, M. H. Pinsonneault, K. Sellgren, John R. Stauffer, Rudolph Schild, D. E. Backman, K. B. Beisser, D. B. Dahari, Amil Dasgupta, J. T. Hagelgans, M. A. Seeds, Rajan Anand '98, Bentley D. Laaksonen '95, Laurence A. Marschall, T. Ramseyer Jan 1998

New Rotation Periods In The Pleiades: Interpreting Activity Indicators, Anita Krishnamurthi, D. M. Terndrup, M. H. Pinsonneault, K. Sellgren, John R. Stauffer, Rudolph Schild, D. E. Backman, K. B. Beisser, D. B. Dahari, Amil Dasgupta, J. T. Hagelgans, M. A. Seeds, Rajan Anand '98, Bentley D. Laaksonen '95, Laurence A. Marschall, T. Ramseyer

Physics and Astronomy Faculty Publications

We present results of photometric monitoring campaigns of G, K and M dwarfs in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation periods for 18 stars in this cluster. In this paper, we examine the validity of using observables such as X-ray activity and amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be …

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