arXiv subjects

G. J. Ferland

Publications and source records attributed to G. J. Ferland.

At least 127 records · Page 7Linked to original sources

arXiv2008

Rotational quenching rate coefficients for H_2 in collisions with H_2 from 2 to 10,000 K

Rate 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 used here is presumed to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels J <= 8 were computed for collision energies ranging between 0.0001 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, is good though discrepancies are found at the lowest and highest temperatures. Low-density-limit cooling functions due to para- and ortho-H_2 collisions are obtained from the collisional rate coefficients. Implications of the new results for non-thermal H_2 rotational distributions in molecular regions are also investigated.

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arXiv2008

Conference Summary: The Cosmic Agitator - Magnetic Fields in the Galaxy

We 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). Interested readers may navigate to the conference web site given in the paper to view the posted presentations in detail.

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arXiv2008

The origin of molecular hydrogen emission in cooling-flow filaments

The optical filaments found in many cooling flows in galaxy clusters consist of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ 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 MHD waves. Strong molecular hydrogen lines, with relative intensities similar to those observed, are produced. Selection effects introduced by the microphysics produce a correlation between the \htwo line upper level energy and the population temperature. These selection effects allow a purely collisional gas to produce \htwo emission that masquerades as starlight-pumped \htwo but with intensities that are far stronger. This physics may find application to any environment where a broad range of gas densities or heating rates occur.

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arXiv2007

On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei

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 in diffuse clouds compared to the standard value, zeta(average)=2.5e-17 s^-1, measured toward dense molecular clouds. In an earlier work we reported an enhancement in the cosmic ray rate of 20 towards HD185418. McCall et al. have reported enhancements of 48 towards zeta Persei based on the observed abundance of H$_{3}^+ while Le Petit et al. found a cosmic ray enhancement of ~10 to be consistent with their models for this same sight line. Here we revisit zeta Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of zeta deduced from the H$_{3}^+$ column density in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with a cosmic ray enhancement of 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.

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arXiv2007

Revisiting He-like X-ray Emission Line Plasma Diagnostics

A 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. This includes photoionization from all levels, line transfer including continuum pumping and destruction by background opacities, scattering, and collisional processes. The model is calculated self-consistently along with the ionization and thermal structure of the surrounding nebula. The result is a complete line and continuum spectrum of the plasma. Here we focus on the ions of the He I sequence and reconsider the standard helium-like X-ray diagnostics. We first consider semi-analytical predictions and compare these with previous work in the low-density, optically-thin limit. We then perform numerical calculations of helium-like X-ray emission (such as is observed in some regions of Seyferts) and predict line ratios as a function of ionizing flux, hydrogen density, and column density. In particular, we demonstrate that, in photoionized plasmas, the $R$-ratio, a density indicator in a collisional plasma, depends on the ionization fraction and is strongly affected by optical depth for large column densities. We also introduce the notion that the $R$-ratio is a measure of the incident continuum at UV wavelengths. The $G$-ratio, which is temperature-sensitive in a collisional plasma, is also discussed, and shown to be strongly affected by continuum pumping and optical depth as well. These distinguish a photoionized plasma from the more commonly studied collisional case.

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arXiv2007

Optically Thick [O I] and [C II] Emission Towards NGC 6334 A

This 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, is not applicable to this region. Instead, we find the geometry to be one where the H+ region and associated PDR is embedded in the molecular cloud. Constant density PDR calculations are presented which predict line intensities as a function of AV (or N(H)), hydrogen density (nH), and incident UV radiation field (G0). We find that a single component model with AV ~650 mag, nH = 5x10^5 cm-3, and G0 = 7x10^4 reproduces the observed [O I] and [C II] intensities, and that the low [O I] 63 to 146 micron ratio is due to line optical depth effects in the [O I] lines, produced by a large column density of atomic/molecular gas. We find that the effects of a density-law would increase our derived AV, while the effects of an asymmetric geometry would decrease AV, with the two effects largely canceling. We conclude that optically selected H+ regions adjacent to PDRs, such as Orion, likely have a different viewing angle or geometry than similar regions detected through IR observations. Overall, the theoretical calculations presented in this work have utility for any PDR embedded in a molecular cloud.

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arXiv2007

A PDR-Code Comparison Study

We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. A number of benchmark models have been created, covering low and high gas densities and far ultraviolet intensities. The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature selfconsistently. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly.

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arXiv2007

Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations

[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 states of H2 is close to the distribution of a gas in LTE at 988 +- 119 K. We derive a total flux from the nebula in H2 lines and compare this with the power available from the central star for producing this radiation. We establish that neither soft X-rays nor FUV radiation has enough energy to power the H2 radiation, only the stellar EUV radiation shortward of 912 Angstrom does. Advection of material from the cold regions of the knots produces an extensive zone where both atomic and molecular hydrogen are found, allowing the H2 to directly be heated by Lyman continuum radiation, thus providing a mechanism that can explain the excitation temperature and surface brightness of the cusps and tails. New images of the knot 378-801 reveal that the 2.12 micron cusp and tail lie immediately inside the ionized atomic gas zone. This firmly establishes that the "tail" structure is an ionization bounded radiation shadow behind the optically thick core of the knot. A unique new image in the HeII 4686 Angstrom line fails to show any emission from knots that might have been found in the He++ core of the nebula. We also re-examined high signal-to-noise ratio ground-based telescope images of this same inner region and found no evidence of structures that could be related to knots.

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arXiv2006

He~I Emission in the Orion Nebula and Implications for Primordial Helium Abundance

We 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 for our reduced line list. With our best theoretical model we find an average difference between theoretical and observed intensities $ = 6.5%$. We argue that both the red and blue ends of the spectrum may have been inadequately corrected for reddening. For the 22 highest quality lines, with $3499 Å\leλ\le6678 Å$, our best model predicts observations to an average of 3.8%. We also perform an analysis of the reported observational errors and conclude they have been underestimated. In the extragalactic analysis, we demonstrate the likelihood of a large systematic error in the reported data and discuss possible causes. This systematic error is at least as large as the errors associated with nearly all attempts to calculate the primordial helium abundance from such observations. Our Orion analysis suggests that the problem does not lie in the theoretical models. We demonstrate a correlation between equivalent width and apparent helium abundance of lines from extragalactic sources that is most likely due to underlying stellar absorption. Finally, we present fits to collisionless case-B He I emissivities as well as the relative contributions due to collisional excitations out of the metastable $2s {}^{3}S$ term.

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arXiv2006

A Cloudy/Xspec Interface

We 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 the x-ray spectral analysis software XSPEC and used as theoretical models for observations. We present and verify a test case. Finally, we consider a few observations and discuss our results.

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arXiv2006

Determining the H+ Region/PDR Equation of State in Star Forming Regions

The 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. We performed a series of calculations designed to determine the equation of state, the relationship between density, temperature, and pressure, through emission-line diagnostic ratios that form in the H+ region and PDR. We consider a wide range of physical conditions in the H+ region. We connect the H+ region to the PDR by considering two constant pressure cases, one with no magnetic field and one where the magnetic field overwhelms the thermal pressure. We show that diagnostic ratios can yield the equation of state for single H+ regions adjacent to single PDRs, with the results being more ambiguous when considering observations of entire galaxies. As a test, we apply our calculations to the Orion H+/PDR region behind the Trapezium. We find the ratio of thermal to magnetic pressure in the PDR to be \~1.2. If magnetic and turbulent energy are in equipartition, our results mean the magnetic field is not the cause of the unexplained broadening in M 42, but may significantly affect line broadening in the PDR. Since Orion is often used to understand physical processes in extragalactic environments, our calculations suggest magnetic pressure should be considered in modeling such regions.

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arXiv2005

Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium

Orion'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 found that the magnetic field dominates the energetics of the gas in at least one component. Here we use high-resolution STIS UV spectra that resolve the two velocity components in absorption and determine the conditions in each. We derive a volume hydrogen density, 21 cm spin temperature, turbulent velocity, and kinetic temperature, for each. We combine these estimates with magnetic field measurements to find that magnetic energy significantly dominates turbulent and thermal energies in one component, while the other component is close to equipartition between turbulent and magnetic energies. We observe molecular hydrogen absorption for highly excited v, J levels that are photoexcited by the stellar continuum, and detect blueshifted S III and P III. These ions must arise from ionized gas between the mostly neutral portions of the Veil and the Trapezium and shields the Veil from ionizing radiation. We find that this layer of ionized gas is also responsible for He I absorption in the Veil, which resolves a 40-year-old debate on the origin of He I absorption towards the Trapezium. Finally, we determine that the ionized and mostly atomic layers of the Veil will collide in less than 85,000 years.

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arXiv2005

Physical conditions in the ISM towards HD185418

We 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, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a line-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements predicted by MHD simulations.

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arXiv2005

The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming Regions

We 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 region (PDR) that are in pressure equilibrium. This differs from previous work, which used separate calculations for each gas phase. Our calculations span a wide range of stellar temperatures, gas densities, and ionization parameters. We describe improvements made to the spectral synthesis code Cloudy that made these calculations possible. These include the addition of a molecular network with ~1000 reactions involving 68 molecular species and improved treatment of the grain physics. Data from the Spitzer First Look Survey, along with other archives, are used to derive important physical characteristics of the H II region and PDR. These include stellar temperatures, electron densities, ionization parameters, UV radiation flux, and PDR density. Finally, we calculate the contribution of the H II region to PDR emission line diagnostics, which allows for a more accurate determination of physical conditions in the PDR.

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arXiv2005

A Multi-Instrument Study of the Helix Nebula Knots with the Hubble Space Telescope

We 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 a static model for their molecular regions. Imaging observations in optical emission lines were made with the Hubble Space Telescope's STIS spectrograph, operating in a "slitless" mode, complemented by WFPC2 images in several of the same lines. The NICMOS camera was used to image the knots in molecular hydrogen. These observations, when combined with other studies of molecular hydrogen and CO provide a complete characterization of the knots. They possess dense molecular cores of densities about 1,000,000 per cubic centimeter surrounded on the central star side by a zone of hot molecular hydrogen. The temperature of the molecular hydrogen emitting layer defies explanation either through detailed calculations for radiative equilibrium or for simplistic calculations for shock excitation. Further away from the core is the ionized zone, whose peculiar distribution of emission lines is explained by the expansion effects of material flowing through this region. The shadowed region behind the core is the source of most of the CO emission from the knot and is of the low temperature expected for a radiatively heated molecular region.

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arXiv2005

J-resolved He I emission predictions in the low-density limit

Determinations 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 is the simplest case and it forms the basis for the more complex situation where collisions are important. The formation of a recombination line is a two-step process, beginning with the capture of a continuum electron into a bound state and followed by radiative cascade to ground. The rate coefficient for capture from the continuum is obtained from photoionization cross sections and detailed balancing, while radiative transition probabilities determine the cascades. We have made every effort to use today's best atomic data. Radiative decay rates are from Drake's variational calculations, which include QED, fine structure, and singlet-triplet mixing. Certain high-$L$ fine-structure levels do not have a singlet-triplet distinction and the singlets and triplets are free to mix in dipole-allowed radiative decays. We use quantum defect or hydrogenic approximations to include levels higher than those treated in the variational calculations. [Abstract clipped]

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arXiv2005

Theoretical He I Emissivities in the Case B Approximation

We 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 large enough to have a significant effect not only on the interpretation of observed spectra of a wide variety of objects but also on determinations of the primordial helium abundance.

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arXiv2005

Molecular Hydrogen in Star-forming regions: implementation of its micro-physics in Cloudy

Much 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 and can be important as a spectral diagnostic of the gas. Modeling and interpretation of observations of such environments requires a quantitatively complete and accurate treatment of H2. Using this micro-physical model of H2, illustrative calculations of prototypical astrophysical environments are presented. This work forms the foundation for future investigations of these and other environments where H2 is an important constituent.

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