Results for "G. J. Ferland"

total 248244took 0.15s
Atomic data for astrophysics. I. Radiative recombination rates for H-like, He-like, Li-like and Na-like ions over a broad range of temperatureSep 16 1995We present new calculations and analytic fits to the rates of radiative recombination towards H-like, He-like, Li-like and Na-like ions of all elements from H through Zn (Z=30). The fits are valid over a wide range of temperature, from 3 K to 10**9 K. ... More
The validity of 21 cm spin temperature as a kinetic temperature indicator in atomic and molecular gasJun 05 2017The gas kinetic temperature ($T_K$) of various interstellar environments is often inferred from observations that can deduce level populations of atoms, ions, or molecules using spectral line observations; H I 21 cm is perhaps the most widely used with ... More
Magnetic Confinement, MHD Waves, and Smooth Line Profiles in AGNJan 31 2000In 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 ... More
Revisiting He-like X-ray Emission Line Plasma DiagnosticsApr 20 2007A complete model of helium-like line and continuum emission has been incorporated into the plasma simulation code Cloudy. All elements between He and Zn are treated, any number of levels can be considered, and radiative and collisional processes are included. ... More
Determining the H+ Region/PDR Equation of State in Star Forming RegionsApr 10 2006The emission line regions of starburst galaxies and active nuclei reveal a wealth of spectroscopic information. A unified picture of the relationship between ionized, atomic, and molecular gas makes it possible to better understand these observations. ... More
Prospects for the Search for a Doubly-Charged Higgs in the Left-Right Symmetric Model with ATLASMar 10 2005We estimate the potential for observation at the LHC of a doubly charged Higgs boson, as predicted in Left-Right symmetric models. Single production by vector boson fusion, $W^+W^+ \to \Delta_{L,R}^{++}$ and pair production by the Drell-Yan process $q ... More
The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ RelationFeb 02 2000We present results from photoionization models of low-metallicity HII regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be non-negligible for nebulae ... More
The Effects of Low-Temperature Dielectronic Recombination on the Relative Populations of the Fe M-Shell StatesNov 25 2003We examine the effects of low-temperature, or $\Delta$$n = 0$, dielectronic recombination (DR) on the ionization balance of the Fe M-Shell (Fe IX through Fe XVI). Since $\Delta$$n = 0$ rates are not available for these ions, we have derived estimates ... More
A Multi-Instrument Study of the Helix Nebula Knots with the Hubble Space TelescopeApr 08 2005We have conducted a combined observational and theoretical investigation of the ubiquitous knots in the Helix Nebula (NGC 7293). We have constructed a combined hydrodynamic+radiation model for the ionized portion of these knots and have accurately calculated ... More
Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared ObservationsJan 23 2007[Abridged] We use new HST and archived images to clarify the nature of the knots in the Helix Nebula. We employ published far infrared spectrophotometry and existing 2.12 micron images to establish that the population distribution of the lowest ro-vibrational ... More
A Bright Molecular Core in a Crab Nebula FilamentMay 05 2010May 24 2010In a sub-arcsec near-infrared survey of the Crab Nebula using the new Spartan Infrared Camera, we have found several knots with high surface brightness in the H_2 2.12 micron line and a very large H_2 2.12 micron to Br-gamma ratio. The brightest of these ... More
Structure and Physical Conditions in the Huygens Region of the Orion NebulaOct 20 2016HST images, MUSE maps of emission-lines, and an atlas of high velocity resolution emission-line spectra have been used to establish for the firrst time correlations of the electron temperature, electron density, radial velocity, turbulence, and orientation ... More
He~I Emission in the Orion Nebula and Implications for Primordial Helium AbundanceNov 17 2006We apply a recently developed theoretical model of helium emission to observations of both the Orion Nebula and a sample of extragalactic H II regions. In the Orion analysis, we eliminate some weak and blended lines and compare theory and observation ... More
Atomic Data for Permitted Resonance Lines of Atoms and Ions from H to Si, and S, Ar, Ca and FeApr 10 1996We list vacuum wavelengths, energy levels, statistical weights, transition probabilities and oscillator strengths for permitted resonance spectral lines of all ions of 18 astrophysically important elements (H through Si, S, Ar, Ca, Fe). Using a compilation ... More
The physical conditions within dense cold clouds in cooling flows IIMar 05 2002May 13 2002This is a progress report on our numerical simulations of conditions in the cold cores of cooling flow condensations. The physical conditions in any non-equilibrium plasma are the result of a host of microphysical processes, many involving reactions that ... More
Which Stars are Ionizing the Orion Nebula ?Mar 17 2017The common assumption that Theta-1-Ori C is the dominant ionizing source for the Orion Nebula is critically examined. This assumption underlies much of the existing analysis of the nebula. In this paper we establish through comparison of the relative ... More
Physical conditions in the ISM towards HD185418Oct 24 2005We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, ... More
Understanding strong molecular hydrogen emission in astronomical environmentsJul 22 2011Here I describe recent studies of objects with molecular hydrogen emission that is strong relative to other spectral lines. Large telescopes and fast spectrometers have made the 2 {\mu}m window accessible even for relatively faint objects. I summarize ... More
Magnetic fields and the location of the PDRNov 12 2008I review recent studies of the emission-line regions in Orion and M17. Both have similar geometries, a bubble of hot shocked gas surrounding the central star cluster, with H^+, H^0, and H_2 regions, often referred to as H II regions, PDRs, and molecular ... More
Physical Conditions in the Orion HII RegionAug 11 1998The Orion Nebula is the defining galactic HII Region, and has been well studied over a broad range of wavelengths. The gas-phase metallicity is 2/3 of solar, close to the star cluster. Grains survive throughout. The Trapezium cluster's radiation field ... More
The spectrum of a single photoionized cloudJul 25 2003The emission-line spectrum of a quasar is most likely emitted by an ensemble of photoionized clouds moving with a variety of velocities, with a range of densities and distances from the central object. The state of the art in this field is to first compute ... More
Powering The Intra-cluster Filaments in Cool-Core Clusters of GalaxiesSep 15 2011The first radio surveys of the sky discovered that some large clusters of galaxies contained powerful sources of synchrotron emission. Optical images showed that long linear filaments with bizarre emission-line spectra permeated the intra-cluster medium. ... More
Magnetic fields, stellar feedback, and the geometry of H II RegionsMar 02 2009Magnetic pressure has long been known to dominate over gas pressure in atomic and molecular regions of the interstellar medium. Here I review several recent observational studies of the relationships between the H^+, H^0 and H_2 regions in M42 (the Orion ... More
Grain Size Distributions and Photo-Electric Heating in Ionized RegionsOct 08 2003In this paper we present results obtained with the new grain code in Cloudy which underline the strong effect of photo-electric heating by grains in photo-ionized regions. We study the effect that the distribution of grain sizes has on the magnitude of ... More
Star formation in self-gravitating disks in active galactic nuclei. II. Episodic formation of broad line regionsFeb 01 2012(abridged) We study the consequence of star formation (SF) in an self-gravity dominated accretion disk in quasars. The warm skins of the SF disk are governed by the radiation from the inner part of the accretion disk to form Compton atmosphere (CAS). ... More
Grain size distributions and photo-electric heating in ionized mediaFeb 16 2004Mar 24 2004Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. A detailed code is necessary to model grains in a photo-ionized medium. In this paper ... More
Grains in Photo-Ionized EnvironmentsJul 10 2001Jul 12 2001Ever since the pioneering study of Spitzer, it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. This includes the diffuse ISM, as well as H II regions, planetary nebulae, and photo-dissociation ... More
Improved He I Emissivities in the Case B ApproximationJun 19 2012We update our prior work on the case B collisional-recombination spectrum of He I to incorporate \textit{ab initio} photoionisation cross-sections. This large set of accurate, self-consistent cross-sections represents a significant improvement in He I ... More
The three dimensional dynamic structure of the inner Orion NebulaOct 24 2008The three dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and new data. We find that the widely accepted model of a concave blister of ionized material needs to be altered in the southwest direction ... More
The location and kinematics of the coronal-line emitting regions in AGNNov 19 2008We use the photoionisation code Cloudy to determine both the location and the kinematics of the optical forbidden, high ionisation line (hereafter, FHIL) emitting gas in the narrow line Seyfert 1 galaxy Ark 564. The results of our models are compared ... More
The PG X-Ray QSO Sample: Links among X-ray, UV & Optical SpectraAug 07 1998A unique, essentially complete sample of 22 QSOs, with high quality soft X-ray spectra from ROSAT, as well as HST and optical spectrophotometry from below Ly-alpha to above H-alpha, is being used to investigate the relationships among the ionizing continuum ... More
Corrigendum: Improved He~I Emissivities in the Case B ApproximationMar 20 2013Errors were discovered in our recent calculations of HeI emissivities (Porter et al. 2012; arXiv:1206.4115). This corrigendum discusses the errors and reports corrected emissivities.
Ion-by-ion Cooling efficienciesNov 29 2011We present ion-by-ion cooling efficiencies for low-density gas. We use Cloudy (ver. 08.00) to estimate the cooling efficiencies for each ion of the first 30 elements (H-Zn) individually. We present results for gas temperatures between 1e4 and 1e8K, assuming ... More
The Structure of the Homunculus. II. Modeling the physical conditions in Eta Car's molecular shellOct 17 2006We present models that reproduce the observed double-shell structure of the Homunculus Nebula around eta Carinae, including the stratification of infrared H2 and [FeII] emission seen in data obtained with the Phoenix spectrograph on Gemini South, as well ... More
Molecular Hydrogen in Star-forming regions: implementation of its micro-physics in CloudyJan 22 2005Much of the baryonic matter in the Universe is in the form of H2 which includes most of the gas in galactic and extragalactic interstellar clouds. Molecular hydrogen plays a significant role in establishing the thermal balance in many astrophysical environments ... More
The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming RegionsJun 21 2005We have performed a series of calculations designed to reproduce infrared diagnostics used to determine physical conditions in star forming regions. We self-consistently calculate the thermal and chemical structure of an H II region and photodissociation ... More
Bootstrapping dielectronic recombination from second-row elements and the Orion NebulaMar 19 2015Dielectronic recombination (DR) is the dominant recombination process for most heavy elements in photoionized clouds. Accurate DR rates for a species can be predicted when the positions of autoionizing states are known. Unfortunately such data are not ... More
Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and IonsJan 03 1996Jan 16 1996We present a complete set of analytic fits to the non-relativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity ... More
Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the TrapeziumDec 21 2005Orion's Veil is an absorbing screen that lies along the line of sight to the Orion H II region. It consists of two or more layers of gas that must lie within a few parsecs of the Trapezium cluster. Our previous work considered the Veil as a whole and ... More
Current Development of the Photo-Ionization Code CloudyJan 11 2000This paper describes the most important differences between Cloudy 90 and Cloudy 94, the most recent upgrade of the code which has now been posted on the WWW. It also describes the upgrade of the grain model in Cloudy which is currently underway.
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--II:Physical ConditionsJan 28 2013We have performed a detailed analysis of the electron temperature and density in the the Ring Nebula using the calibrated HST WFC3 images described in the preceding paper. The electron temperature (Te) determined from [N II] and [O III] rises slightly ... More
Chemical Abundances in Broad Emission Line Regions: The "Nitrogen-Loud" QSO 0353-383Oct 07 2002The intensity of the strong N V 1240 line relative to C IV 1549 or to He II 1640 has been proposed as an indicator of the metallicity of QSO broad emission line regions, allowing abundance measurements in a large number of QSOs out to the highest redshifts. ... More
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--III:Tangential Motions using AstroDrizzle ImagesApr 18 2013We have been able to compare with astrometric precision AstroDrizzle processed images of NGC 6720 (the Ring Nebula) made using two cameras on the Hubble Space Telescope. The time difference of the observations was 12.925 yrs. This large time-base allowed ... More
Electron impact excitation of OII fine-structure levelsJul 24 2009Effective collision strengths for forbidden transitions among the 5 energetically lowest finestructure levels of O II are calculated in the Breit-Pauli approximation using the R-matrix method. Results are presented for the electron temperature range 100 ... More
The abundance discrepancy factor and t^2 in nebulae: are non-thermal electrons the culprits?May 11 2016Photoionization produces supra-thermal electrons, electrons with much more energy than is found in a thermalized gas at electron temperatures characteristic of nebulae. The presence of these high energy electrons may solve the long-standing t^2/ADF puzzle, ... More
Orion's Bar: Physical Conditions across the Definitive H+ / H0 / H2 InterfaceNov 07 2008Nov 14 2008Previous work has shown the Orion Bar to be an interface between ionized and molecular gas, viewed roughly edge on, which is excited by the light from the Trapezium cluster. Much of the emission from any star-forming region will originate from such interfaces, ... More
Sensitivity of PDR Calculations to Microphysical DetailsAug 19 2008Our 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 ... More
Detecting the rapidly expanding outer shell of the Crab Nebula: where to lookJul 03 2013We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrained ... More
The Origin of Fe II Emission in AGNJul 19 2004We used a very large set of models of broad emission line (BEL) clouds in AGN to investigate the formation of the observed Fe II emission lines. We show that photoionized BEL clouds cannot produce both the observed shape and observed equivalent width ... More
The Mass of Quasar Broad Emission Line RegionsSep 17 2002We show that the mass of ionized gas in the Broad Line Regions (BLRs) of luminous QSOs is at least several hundred Msun, and probably of order 10^3-10^4 M_sun. BLR mass estimates in several existing textbooks suggest lower values, but pertain to much ... More
Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--I: Structure and EvolutionJan 28 2013We have performed a detailed analysis of the Ring Nebula (NGC 6720) using HST WFC3 images and derived a new 3-D model. Existing high spectral resolution spectra played an important supplementary role in our modeling. It is shown that the Main Ring of ... More
Uncertainties in Theoretical HeI Emissivities: HII Regions, Primordial Abundance, and Cosmological RecombinationNov 07 2008A number of recent works in astronomy and cosmology have relied upon theoretical He I emissivities, but we know of no effort to quantify the uncertainties in the atomic data. We analyze and assign uncertainties to all relevant atomic data, perform Monte ... More
Optically Thick [O I] and [C II] Emission Towards NGC 6334 AFeb 09 2007This work focuses on [O I] and [C II] emission towards NGC 6334 A, an embedded H+ region/PDR only observable at infrared or longer wavelengths. A geometry where nearly all the emission escapes out the side of the cloud facing the stars, such as Orion, ... More
Fluorescent excitation of Balmer lines in gaseous nebulae: case DOct 30 2008(abridged) Non-ionizing stellar continua are a source of photons for continuum pumping in the hydrogen Lyman transitions. In the environments where these transitions are optically thick, deexcitation occurs through higher series lines, so that the flux ... More
Metallicities and Abundance Ratios from Quasar Broad Emission LinesSep 02 2001Nov 20 2001The 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 ... More
The intermediate line region in active galactic nucleiJun 01 2016Aug 16 2016We show that the recently observed suppression of the gap between the broad line region (BLR) and the narrow line region (NLR) in some AGN can be fully explained by an increase of the gas density in the emitting region. Our model predicts the formation ... More
Suppression of Dielectronic Recombination Due to Finite Density EffectsMar 10 2013We have developed a general model for determining density-dependent effective dielectronic recombination (DR) rate coefficients in order to explore finite-density effects on the ionization balance of plasmas. Our model consists of multiplying by a suppression ... More
The collision strength of the [Ne V] infrared fine-structure linesDec 13 1999The calculation of accurate collision strengths for atomic transitions has been a long standing problem in quantitative spectroscopy. Most modern calculations are based on the R-matrix method and problems pertaining to the use of this method have led ... More
Theoretical He I Emissivities in the Case B ApproximationFeb 10 2005We calculate the He I case B recombination cascade spectrum using improved radiative and collisional data. We present new emissivities over a range of electron temperatures and densities. The differences between our results and the current standard are ... More
$^{12}$C/$^{13}$C ratio in planetary nebulae from the IUE archivesJan 06 2004We investigated the abundance ratio of $^{12}$C/$^{13}$C in planetary nebulae by examining emission lines arising from \ion{C}{3} $2s2p ^3P_{2,1,0} \to 2s^2 ^1S_0$. Spectra were retrieved from the International Ultraviolet Explorer archives, and multiple ... More
Charge Transfer Between Neutral Atoms and Highly Ionized Species: Implications for ISO ObservationsMar 17 1997We estimate rate coefficients for charge transfer between neutral hydrogen and helium and moderate to highly ionized heavy elements. Although charge transfer does not have much influence on hot collisionally ionized plasmas, its effects on photoionized ... More
The Curious Conundrum Regarding Sulfur Abundances In Planetary NebulaeFeb 07 2012Sulfur abundances derived from optical emission line measurements and ionization correction factors in planetary nebulae are systematically lower than expected for the objects' metallicities. We have carefully considered a large range of explanations ... More
Implications of IR continua for x-ray emission/reflection in AGNNov 22 2003Observations of infrared emission from AGN show that grains exist over a broad range of distances from the central object, extending to the point where they are destroyed by sublimation. These ~1000 K grains produce much of the 1 micron continuum. In ... More
Emission-line Helium Abundances in Highly Obscured NebulaeJul 27 1999This paper outlines a way to determine the ICF using only infrared data. We identify four line pairs, [NeIII] 36$\micron$/[NeII] 12.8$\micron$, [NeIII]~15.6$\micron$ /[NeII] 12.8$\micron$, [ArIII] 9$\micron$/[ArII] 6.9$\micron$, and [ArIII] 21$\micron$/[ArII] ... More
J-resolved He I emission predictions in the low-density limitApr 04 2005Determinations of the primordial helium abundance are used in precision cosmological tests. These require highly accurate He I recombination rate coefficients. Here we reconsider the formation of He I recombination lines in the low-density limit. This ... More
Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the WIM Component of the ISMMar 14 2011We have supplemented existing spectra of Barnard's Loop with high accuracy spectrophotometry of one new position. Cloudy photoionization models were calculated for a variety of ionization parameters and stellar temperatures and compared with the observations. ... More
The curious conundrum regarding sulfur and oxygen abundances in planetary nebulaeSep 12 2011We carefully consider numerous explanations for the sulfur abundance anomaly in planetary nebulae. No one rationale appears to be satisfactory, and we suggest that the ultimate explanation is likely to be a heretofore unidentified feature of the nebular ... More
Rotational quenching rate coefficients for H_2 in collisions with H_2 from 2 to 10,000 KMay 12 2008Rate coefficients for rotational transitions in H_2 induced by H_2 impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H_2)_2 potential energy surface were performed. The potential energy surface ... More
The PG X-ray QSO sample: Links between the UV-X-ray Continuum and Emission LinesMay 07 1999The UV to soft X-rays of luminous AGNs dominate their bolometric luminosity, driven by an accretion-powered dynamo at the center. These photons ionize the surrounding gas, thereby providing clues to fueling and exhaust. Two sets of important relationships ... More
The energy source of the filaments around the giant galaxy NGC1275May 09 2011The brightest galaxy in the nearby Perseus cluster, NGC1275, is surrounded by a network of filaments. These were first observed through their Halpha emission but are now known to have a large molecular component with a total mass approaching 10^11Msun ... More
H2 Temperatures in the Crab NebulaDec 07 2011We used K-band spectra to measure the H2 excitation temperatures in six molecular knots associated with the filaments in the Crab Nebula. The temperatures are quite high - in the range T ~ 2000-3000K, just below the H2 dissociation temperature. This is ... More
Physical Conditions in Orion's VeilApr 20 2004Orion's veil consists of several layers of largely neutral gas lying between us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I absorption and in optical and UV absorption lines of H I and other species. Toward the Trapezium, ... More
The Nature and Frequency of Outflows from Stars in the Central Orion Nebula ClusterAug 14 2015Recent Hubble Space Telescope images have allowed the determination with unprecedented accuracy of motions and changes of shocks within the inner Orion Nebula. These originate from collimated outflows from very young stars, some within the ionized portion ... More
H, He-like recombination spectra II: $l$-changing collisions for He Rydberg statesAug 31 2016Cosmological models can be constrained by determining primordial abundances. Accurate predictions of the He I spectrum are needed to determine the primordial helium abundance to a precision of $< 1$% in order to constrain Big Bang Nucleosynthesis models. ... More
A Cloudy/Xspec InterfaceJun 01 2006We discuss new functionality of the spectral simulation code CLOUDY which allows the user to calculate grids with one or more initial parameters varied and formats the predicted spectra in the standard FITS format. These files can then be imported into ... More
H, He-like recombination spectra I: $l$-changing collisions for hydrogenApr 13 2016Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He I and H I emission lines can be used to measure the He$^+$/H$^+$ abundance ... More
Radiative cooling in collisionally and photo ionized plasmasDec 06 2012We discuss recent improvements in the calculation of the radiative cooling in both collisionally and photo ionized plasmas. We are extending the spectral simulation code Cloudy so that as much as possible of the underlying atomic data is taken from external ... More
The Nature of the H2-Emitting Gas in the Crab NebulaDec 22 2012Jan 01 2013Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers, we found that the H2 emission ... More
Accurate Hydrogen Spectral Simulations with a Compact Model AtomOct 18 1996Many large scale numerical simulations of astrophysical plasmas must also reproduce the hydrogen ionization and the resulting emission spectrum, in some cases quite accurately. We describe a compact model hydrogen atom that can be readily incorporated ... More
N II 5668-5712, a New Class of Spectral Features in Eta CarinaeApr 11 2011May 20 2011We report on the N II 5668-5712 emission and absorption lines in the spectrum of Eta Carinae. Spectral lines of the stellar wind regions can be classified into four physically distinct categories: 1) low-excitation emission such as H I and Fe II, 2) higher ... More
Radiative Cooling II: Effects of Density and MetallicityMar 12 2014May 08 2014This work follows Lykins et al. discussion of classic plasma cooling function at low density and solar metallicity. Here we focus on how the cooling function changes over a wide range of density (n_H<10^12 cm^(-3)) and metallicity (Z<30Z _sun ). We find ... More
H-, He-like recombination spectra III: $n$-changing collisions in highly-excited Rydberg states and their impact on the radio, IR and optical recombination linesMar 13 2019At intermediate to high densities, electron (de-)excitation collisions are the dominant process for populating or depopulating high Rydberg states. In particular, the accurate knowledge of the energy changing ($n$-changing) collisional rates is determinant ... More
On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta PerseiNov 02 2007The 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 ... More
H-, He-like recombination spectra III: $n$-changing collisions in highly-excited Rydberg states and their impact on the radio, IR and optical recombination linesMar 13 2019Mar 18 2019At intermediate to high densities, electron (de-)excitation collisions are the dominant process for populating or depopulating high Rydberg states. In particular, the accurate knowledge of the energy changing ($n$-changing) collisional rates is determinant ... More
Conference Summary: The Cosmic Agitator - Magnetic Fields in the GalaxyApr 21 2008We present a summary of the conference "The Cosmic Agitator: Magnetic Fields in the Galaxy" held in Lexington KY in 2008 Mar 26-29. The presentation draws primarily from material in the slides prepared for the Conference Summary by one of us (Carl Heiles). ... More
Temperature Variations from HST Spectroscopy of the Orion NebulaDec 10 2002We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_e variation (t_A^2) across the nebula. We also ... More
Star formation in self-gravitating disks in active galactic nuclei. I. Metallicity gradients in broad line regionsJul 19 2011It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around the supermassive black holes. We designate this ... More
Properties of Dust Grains in Planetary Nebulae - I. The Ionized Region of NGC 6445Oct 21 1999In this paper we study new infrared spectra of the evolved planetary nebula NGC 6445 obtained with ISO. These data show that the thermal emission from the grains is very cool and has a low flux compared to H beta. A model of the ionized region is constructed, ... More
The Lexington Benchmarks for Numerical Simulations of NebulaeMar 29 2016We present the results of a meeting on numerical simulations of ionized nebulae held at the University of Kentucky in conjunction with the celebration of the 70th birthdays of Profs. Donald Osterbrock and Michael Seaton.
A Magnetically-Supported PDR in M17Nov 27 2006The SW part of the bright Galactic H II region M17 contains an obscured ionization front that is most easily seen at infrared and radio wavelengths. This "SW bar" has received considerable attention because the ionization front is seen nearly edge-on, ... More
A Magnetically-Supported Photodissociation Region in M17Nov 27 2006Aug 01 2017The southwestern (SW) part of the Galactic H II region M17 contains an obscured ionization front that is most easily seen at infrared and radio wavelengths. It is nearly edge-on, thus offering an excellent opportunity to study the way in which the gas ... More
Pumping up the [N I] nebular linesAug 31 2012The optical [N I] doublet near 5200 {\AA} is anomalously strong in a variety of emission-line objects. We compute a detailed photoionization model and use it to show that pumping by far-ultraviolet (FUV) stellar radiation previously posited as a general ... More
Rotationally Warm Molecular Hydrogen in the Orion BarJun 12 2009The Orion Bar is one of the nearest and best-studied photodissociation or photon-dominated regions (PDRs). Observations reveal the presence of H2 lines from vibrationally or rotationally excited upper levels that suggest warm gas temperatures (400 to ... More
Suppression of Dielectronic Recombination Due to Finite Density Effects II: Analytical Refinement and Application to Density-dependent Ionization Balances and AGN Broad-line EmissionAug 22 2018We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination ... More
Detection of optical coronal emission from 10^6 K gas in the core of the Centaurus clusterSep 14 2010We report a detection (3.5x10^37 \pm 5.6x10^36 ergps) of the optical coronal emission line [Fe X]6374 and upper limits of four other coronal lines using high resolution VIMOS spectra centred on NGC 4696, the brightest cluster galaxy in the Centaurus cluster. ... More
Accurate determination of the free-free Gaunt factor; I - non-relativistic Gaunt factorsJul 18 2014Aug 17 2014Modern spectral synthesis codes need the thermally averaged free-free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum emitted by an ionized plasma. Until now no set of data exists ... More
Collisional heating as the origin of filament emission in galaxy clustersOct 29 2008[Abridged] Photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. Our first paper examined whether heating by energetic ... More
Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275Jun 27 2012Far ultraviolet emission has been detected from a knot of Halpha emission in the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected relative to the brightness of the Halpha line in the same spatial region is very close to that expected ... More
A deep spectroscopic study of the filamentary nebulosity in NGC4696, the brightest cluster galaxy in the Centaurus clusterJul 20 2011We present results of deep integral field spectroscopy observations using high resolution optical (4150-7200 A) VIMOS VLT spectra, of NGC 4696, the dominant galaxy in the Centaurus cluster (Abell 3526). After the Virgo cluster, this is the second nearest ... More
Close-coupling calculations of low-energy inelastic and elastic processes in $^4$He collisions with H$_2$: A comparative study of two potential energy surfacesDec 09 2004The two most recently published potential energy surfaces (PESs) for the HeH$_2$ complex, the so-called MR (Muchnick and Russek) and BMP (Boothroyd, Martin, and Peterson) surfaces, are quantitatively evaluated and compared through the investigation of ... More