arXiv subjects

G. J. Ferland

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

At least 145 records · Page 8Linked to original sources

arXiv2004

Close-coupling calculations of low-energy inelastic and elastic processes in $^4$He collisions with H$_2$: A comparative study of two potential energy surfaces

The 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 atom-diatom collision processes. The BMP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the PES compared to that of the MR surface. We found significant differences in inelastic rovibrational cross sections computed on the two surfaces for processes dominated by large changes in target rotational angular momentum. In particular, the H$_2$($ν=1, j=0$) total quenching cross section computed using the BMP potential was found to be a factor of $~$1000 larger than that obtained with the MR surface.

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arXiv2004

The Origin of Fe II Emission in AGN

We 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 of the 2200-2800A Fe II UV bump unless there is considerable velocity structure corresponding to a microturbulent velocity parameter v_turb > 100 km/s for the LOC models used here. This could be either microturbulence in gas that is confined by some phenomenon such as MHD waves, or a velocity shear such as in the various models of winds flowing off the surfaces of accretion disks. The alternative way that we can find to simultaneously match both the observed shape and equivalent width of the Fe II UV bump is for the Fe II emission to be the result of collisional excitation in a warm, dense gas. Such gas would emit very few lines other than Fe II. However, since the collisionally excited gas would constitute yet another component in an already complicated picture of the BELR, we prefer the model involving turbulence. In either model, the strength of Fe II emission relative to the emission lines of other ions such as Mg II depends as much on other parameters (either v_turb or the surface area of the collisionally excited gas) as it does on the iron abundance. Therefore, the measurement of the iron abundance from the FeII emission in quasars becomes a more difficult problem.

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arXiv2004

Physical Conditions in Orion's Veil

Orion'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, the veil has two remarkable properties, high magnetic field (~100 microGauss) and a surprising lack of molecular hydrogen given its total hydrogen column density. Here we compute photoionization models of the veil to establish its gas density and its distance from the Trapezium. We use a greatly improved model of the hydrogen molecule that determines level populations in ~1e5 rotational/vibrational levels and provides improved estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our best fit photoionization models place the veil 1-3 pc in front of the star at a density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of non-thermal motions in at least one of the 21cm H I velocity components. Therefore, the veil is the first interstellar environment where magnetic dominance appears to exist. We find that the low ratio of molecular to atomic hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due to its proximity to the Trapezium stars and the absence of small grains in the region.

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arXiv2004

Grain size distributions and photo-electric heating in ionized media

Ever 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 we will use the spectral synthesis code Cloudy for this purpose. The grain model of this code has recently undergone a comprehensive upgrade. Part of this is the newly developed hybrid grain charge model, which will be described in detail. This model allows discrete charge states of very small grains to be modelled accurately while simultaneously avoiding the overhead of fully resolving the charge distribution of large grains, thus making the model both accurate and computationally efficient. A comprehensive comparison with the fully resolved charge state models of Weingartner & Draine (2001) shows that the agreement is very satisfactory for realistic size distributions. The effect of the grain size distribution on the line emission from photo-ionized regions is studied by taking standard models for an H II region and a planetary nebula and adding a dust component to the models with varying grain size distributions. A comparison of the models shows that varying the size distribution has a dramatic effect on the emitted spectrum, and affects the ionization balance as well. These results clearly demonstrate that the grain size distribution is an important parameter in photo-ionization models.

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arXiv2004

$^{12}$C/$^{13}$C ratio in planetary nebulae from the IUE archives

We 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 spectra of the same object were coadded to achieve improved signal-to-noise. The $^{13}$C hyperfine structure line at 1909.6 Åwas detected in NGC 2440. The $^{12}$C/$^{13}$C ratio was found to be $\sim4.4\pm$1.2. In all other objects, we provide an upper limit for the flux of the 1910 Åline. For 23 of these sources, a lower limit for the $^{12}$C/$^{13}$C ratio was established. The impact on our current understanding of stellar evolution is discussed. The resulting high signal-to-noise \ion{C}{3} spectrum helps constrain the atomic physics of the line formation process. Some objects have the measured 1907/1909 flux ratio outside the low-electron density theoretical limit for $^{12}$C. A mixture of $^{13}$C with $^{12}$C helps to close the gap somewhat. Nevertheless, some observed 1907/1909 flux ratios still appear too high to conform to the presently predicted limits. It is shown that this limit, as well as the 1910/1909 flux ratio, are predominantly influenced by using the standard partitioning among the collision strengths for the multiplet $^1S_0$--$^3P_J$ according to the statistical weights. A detailed calculation for the fine structure collision strengths between these individual levels would be valuable.

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arXiv2003

The Effects of Low-Temperature Dielectronic Recombination on the Relative Populations of the Fe M-Shell States

We examine the effects of low-temperature, or $Δ$$n = 0$, dielectronic recombination (DR) on the ionization balance of the Fe M-Shell (Fe IX through Fe XVI). Since $Δ$$n = 0$ rates are not available for these ions, we have derived estimates based on the existing rates for the first four ionization states of the CNO sequence and newly calculated rates for L-shell ions of 3rd row elements and Fe. For a range of ionization parameter and column density applicable to the intrinsic absorbers detected in {\it ASCA}, {\it Chandra}, and {\it XMM-Newton} observations of Seyfert galaxies, we generated two grids of photoionization models, with and without DR. The results show that the ionization parameter at which the population of an Fe M-shell ion peakscan increase in some cases by factor $>$ 2 when these rates are included. More importantly, there are dramatic changes in the range in ionization parameter over which individual M-shell ions contain significant fractions of the total Fe (e.g. $>$ 10%) in the plasma. These results may explain the mismatch between the range of Fe ionization states detected in the X-ray spectra of Seyferts, identified by the energies of the M-Shell Unresolved Transition Array, and those predicted by photoionization models of the X-ray absorbers that reproduce lines of second and third row elements. The results suggest that care should be taken in using 3rd and 4th row ions to constrain the physical conditions in photoionized X-ray plasmas until accurate DR rates are available. This underscores the importance of atomic physics in interpreting astronomical spectroscopy.

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arXiv2003

Grain Size Distributions and Photo-Electric Heating in Ionized Regions

In 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 the effect, and show that this effect is nothing short of dramatic. This makes the grain size distribution an important parameter in modeling of photo-ionized regions such as H II regions and planetary nebulae.

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arXiv2002

Temperature Variations from HST Spectroscopy of the Orion Nebula

We 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 measure the intrinsic line ratio [N II] 5756/6585 in order to estimate T_e and t_A^2 in the N^+ region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [N II] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5" square (matching the slit width). The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t_A^2 varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 K to 10232 K; t_A^2 varies from 0.00584 to 0.0175. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t_A^2, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II] T_e exceeds the average [O III] T_e confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by theta^1 Ori C; and indirectly, evidence of high electron density.

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arXiv2002

Chemical Abundances in Broad Emission Line Regions: The "Nitrogen-Loud" QSO 0353-383

The 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. Previously, it had been shown that the (normally) much weaker lines N III] 1750 and N IV] 1486 could be used in the same way. The redshift 1.96 QSO 0353-383 has long been known to have N III] and N IV] lines that are far stronger relative to Ly-alpha or C IV than in any other QSO. Because in this particular case these intercombination lines can be easily measured, this unusual object provides an ideal opportunity for testing whether the N V line is a valid abundance indicator. Using new observations of Q0353-383 made both with HST in the ultraviolet and from the ground in the visible passband, we find that intensity ratios involving the strengths of N V, N IV] and N III] relative to lines of He, C and O all indicate that nitrogen is overabundant relative to oxygen in Q0353-383 by a factor of ~15 compared to solar ratios. This agreement among the diagnostics supports the use of these lines for measuring BLR chemical abundances. If nitrogen behaves like a secondary element, such that N/O is proportional to O/H, then the extreme nitrogen enhancement in Q0353-383 implies a metallicity of ~15 times the solar value. Even if Q0353-383 represents an extreme outlier in the N/O proportional to O/H relation, the overall metallicity should still be at least five times solar. Unusually high metallicities in Q0353-383 might imply that we caught this object just as the gas-phase metallicity in the central part of its host galaxy has peaked, at a time when the interstellar gas supply is nearly exhausted and hence the fuel source for the central QSO is ready to shut off.

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arXiv2002

The Mass of Quasar Broad Emission Line Regions

We 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 less luminous Seyfert galaxies or include only a small fraction of the ionized/emitting volume of the BLR. The previous estimates also fail to include the large amounts of BLR gas that emit at low efficiency (in a given line), but that must be present based on reverberation and other studies. Very highly ionized gas, as well as partially ionized and neutral gas lying behind the ionization zones, are likely additional sources of mass within the BLR. The high masses found here imply that the chemical enrichment of the BLR cannot be controlled by mass ejection from one or a few stars. A significant stellar population in the host galaxies must be contributing. Simple scaling arguments based on normal galactic chemical enrichment and solar or higher BLR metallicities show that the minimum mass of the enriching stellar population is of order 10 times the BLR mass, or greater than 10^4-10^5 M_sun. More realistic models of the chemical and dynamical evolution in galactic nuclei suggest that much larger, bulge-size stellar populations are involved.

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arXiv2002

The physical conditions within dense cold clouds in cooling flows II

This 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 are research areas in themselves. We review the dominant physical processes in our previously published simulations, to clarify those issues that have caused confusion in the literature. We show that conditions in the core of an X-ray illuminated cloud are very different from those found in molecular clouds, largely because carbon remains substantially atomic and provides powerful cooling through its far infrared lines. We show how the results of the Opacity Project have had a major impact on our predictions, largely because photoionization cross sections of atoms and first ions are now calculated to be far larger than previously estimated. Finally we show that the predicted conditions are strongly affected by such complexities as microturbulence or the presence of small amounts of dust. Large masses of cold dense gas, in addition to the warmer molecular gas detected recently, could be present in cooling flows.

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arXiv2002

Temperature Variations from HST Imagery and Spectroscopy of NGC 7009

We present new HST/WFPC2 imagery and STIS long-slit spectroscopy of the planetary nebula NGC 7009. The primary goal was to obtain high spatial resolution of 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. The WFPC2 T_e map is rather uniform; almost all values are between 9000 - 11,000 K, with the higher T_e's closely coinciding with the inner He^++-zone. The results indicate very small values - <~ 0.01 - for t_A^2 throughout. Our STIS data allow an even more direct determination of T_e and t_A^2, albeit for a much smaller area than with WFPC2. We present results from binning the data along the slit into tiles that are 0.5'' square (matching the slit width). The average [O III] temperature using 45 tiles (excluding the central star and STIS fiducial bars) is 10,139 K; t_A^2 is 0.0035. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t_A^2, we cannot completely rule out temperature fluctuations along the line of sight as the cause of the large abundance discrepancy between heavy element abundances inferred from collisionally excited emission lines compared to those derived from recombination lines.

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arXiv2001

Metallicities and Abundance Ratios from Quasar Broad Emission Lines

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_o. Among the lines we consider, N III] 1750/O III] 1664, N V 1240/(C IV 1549 + O VI 1034) and N V/He II 1640 are the most robust diagnostics. We argue, in particular, that the average N V BEL is not dominated by scattered Ly-alpha photons from a broad absorption line wind. We then compare our calculated line ratios with observations from the literature. The results support earlier claims that the gas-phase metallicities near quasars are typically near or several times above the solar value. We conclude that quasar activity is preceded by, or coeval with, an episode of rapid and extensive star formation in the surrounding galactic (or proto-galactic) nuclei. Chemical evolution models of these environments suggest that, to reach Z > Z_o in well-mixed interstellar gas, the star formation must have begun > 10^8 yr before the observed quasar activity.

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arXiv2001

Grains in Photo-Ionized Environments

Ever 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 regions. A detailed code is necessary to model grains in a photo-ionized medium since the interactions of grains with their environment include a host of microphysical processes, and their importance can only be judged by performing a complete simulation. In this paper we will use the spectral synthesis code Cloudy for this purpose. A comprehensive upgrade of the grain model has been recently incorporated in Cloudy, and certain aspects of this upgrade will be discussed. Special emphasis will be on the new grain charge model. We will consider in detail the physics of grains in both ionized and neutral environments, and will present a calculation of photo-electric heating and collisional cooling rates for a range of physical conditions and grain materials and for a range of grain sizes (including a realistic size distribution). We conclude with a brief discussion of the problems currently hampering progress in this field. The new grain model will be used to model the silicate emission in the Ney-Allen nebula, and will help us better understand the nature of the grains in that part of the Orion complex.

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arXiv2000

The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ Relation

We 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 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 HII region has an [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that, regardless of its metallicity, it will have a negligibly small ICF. A similar, but metallicity dependent, result was found using the [O III] 5007/H$β$ ratio. These two results can be used as selection criteria to remove nebulae with potentially non-negligible ICFs. Using our metallicity independent criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of the rms scatter about the best fit $Y-Z$ line. A fit to the selected data results in a slight increase of the value of the primordial helium abundance.

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arXiv2000

Magnetic Confinement, MHD Waves, and Smooth Line Profiles in AGN

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. Line widths of contributing clouds or flow elements are much wider than their thermal widths, due to the presence of non-dissipative MHD 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 in velocity, is used to show that smoothness can be achieved for less than 80,000 clouds and may even be as low as a few hundred. We conclude that magnetic confinement has far reaching consequences for observing and modeling active galactic nuclei.

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arXiv2000

Current Development of the Photo-Ionization Code Cloudy

This 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.

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arXiv1999

The collision strength of the [Ne V] infrared fine-structure lines

The 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 to a discussion of the accuracy of these results. More in particular, based on an analysis of the spectra of NGC 3918 and NGC 6302, Clegg et al. (1987) and Oliva et al. (1996) have questioned R-matrix calculations for the infrared [Ne V] fine-structure transitions. Using improved flux measurements for the [Ne V] lines, we show that the conclusion that these collision strengths would be too high, is not correct. The discrepancies found by Clegg et al. (1987) can be explained by the inaccuracy of the [Ne V] 342.6 nm flux they adopted. The discrepancies found by Oliva et al. (1996) can be explained by the inaccuracy of the LRS flux for the [Ne V] 14.32 micron line. Based on the data presented in this paper there is no reason to assume that there are any problems with the R-matrix calculations for Ne^4+ of Lennon & Burke (1994). We show that the data are accurate at the 30 % level or better. This confirms the validity of the close coupling method.

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