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NASA NTRS1985

The theory of emission-line regions in active galactic nuclei

The inability of photoionization models to predict the observed amount of energy in the Fe II lines and the Balmer continuum has led to the hypothesis that an additional, nonphotoionized region is present to supplement the intensities of low ionization lines. The question of reddening is addressed in light of its influence on the interpretation of the lines and continuum. It was found that X-rays can raise the gas temperature enough for collisional excitation to affect the Balmer decrement, and that narrow line emission from AGN (ranging in ionization level from Seyfert galaxies to low-ionization nuclei) can be attributed to photoionization by a nonthermal continuum for appropriate ionization-parameter values.

Ferland, G. J.↗
NASA NTRS1987

Ionization Chamber Measures Extreme Ultraviolet

Ionization chamber operates in nearly total photon absorption as stable, self-calibrating detector of ionizing extreme ultraviolet radiation. Working gas of instrument is neon; photoionization properties well known and readily applicable to absolute measurements. Designed for measurements of solar ultraviolet flux aboard sounding rocket, instrument used on Earth to measure ultraviolet radiation in vacuum systems. Ionization chamber collects positive neon ions and electrons produced by irradiation of neon gas by ultraviolet photons. Approximately one ion produced by each photon; consequently, photoionization current nearly proportional to photon flux.

Carlson, Robert W.↗
NASA NTRS1986

An analysis at mesospheric coherent-scatter power enhancements during solar flare events

Solar flares produce increases in coherent-scatter power from the mesosphere due to the increase in free electrons produced by X-ray photoionization. Thirteen such power enhancements were observed at Urbana. When such an enhancement occurs at an altitude containing a turbulence layer with constant strength, the relative enhancement of electon density is estimated from the enhancement in power. Such estimates of enchanced electron density are compared with estimates of the X-ray photoionization at that altitude, deduced from geostationary satellite measurements. It is found that possible types ion-chemical reaction scheme may be distinguished, and the nonflare ion-pair production function may be estimated. The type of ion-chemical scheme and the nonflare ion-production function are shown to depend on the solar zenith angle.

Parker, J.↗
NASA NTRS1987

The ultraviolet and optical emission-line spectrum of III Zw 77

The high-ionization Seyfert I galaxy III Zw 77 has been simultaneously observed over the satellite ultraviolet to near-IR spectral regions. As in other AGN, the continuous energy distribution shows a roughly power-law decline across much of the optical region and a flatter-than-unity spectral index in the near-UV. The equivalent widths of the Balmer lines and Ly-alpha are very similar to those found in other AGN. The UV-to-optical emission-line spectrum for narrow lines is fairly similar to those deduced for Seyfert 2 galaxies. A surprising result is the detection of the semiforbidden O III 1661, 1666 A doublet with an intensity indicating high temperatures more suggestive of shock heating than photoionization equilibrium. An interpretation in terms of a multicomponent photoionized structure is proposed in which dense regions produce the UV lines and the lower density regions produce the optical lines.

Ferland, Gary J.↗
NASA NTRS1988

The origin of broad emission line clouds in active galactic nuclei

Numerous arguments suggest that the broad emission lines characteristic of many types of active galactic nuclei are formed in comparatively cool clouds embedded in a much hotter medium of roughly equal pressure. Here it is shown that a cycle exists in which cooler condensations form within the hot medium via a finite-amplitude thermal instability, drop to temperatures characteristic of photoionization equilibrium, are destroyed by hydrodynamic processes, and then return their material to the hot medium. Clouds of characteristic pressure about 10 to the 14th K/cu cm and thickness about 10 to the 23rd/sq cm are produced by this mechanism, in agreement with the pressure and thickness inferred from photoionization model analysis of observed emission-line ratios. Only a fraction of the hot medium's mass must encounter a finite-amplitude perturbation in order to produce the observed mass in clouds.

Krolik, Julian H.↗
NASA NTRS1988

The reionization of unshocked ejecta in SN 1006

The validity of carbon-deflagration models white dwarf models for type Ia supernovae is investigated by examining whether most of the iron in the center of SN 1006 exists in forms other than Fe II, such as iron grains or other ionic stages of iron, as the models require. The possible ways in which iron can be hidden in SN 1006 are reviewed, and an argument on observational grounds is made against any appreciable fraction of iron in grains or Fe I. Various mechanisms for ionizing unshocked iron beyond Fe II are discussed; the most probable mechanism for ionizing the iron appears to be photoionization by UV and X-ray emission from reverse-shocked ejecta. A detailed model for this reverse-shcok photoionization mechanism is described.

Hamilton, Andrew J. S.↗
NASA NTRS1988

Emission-line variability in Seyfert galaxies

Observations and theory of emission-line variability in Seyfert galaxies are reviewed in the context of standard photoionization models of the emission-line regions. Expected time scales for variability and systematic effects which can give misleading results are discussed. Published observations of emission-line variability are summarized. The implications of these observations are noted, particularly the physical consequences of broad-line regions that variability analysis indicates are an order of magnitude smaller than predicted by standard photoionization models.

Peterson, Bradley M.↗
NASA NTRS1988

Hydrogen line and continuum emission in young stellar objects. I - Excitation model

Two mechanisms that populate the n = 2 level of hydrogen after the Lyman continuum is depleted are identified. They are ionization of N I from its excited states, followed by charge-exchange between N II and H I, and Ly-beta line wing absorption. Both processes involve absorption of the sub-Lyman continuum between 11 and 13.6 eV. With population in level n = 2 thus maintained, the strong Brackett line fluxes observed are then produced as a result of Balmer photoionization. The magnitude of the sub-Lyman continuum dictates the fraction of Balmer continuum that will be absorbed. Numerical calculations for four young stellar objects with luminosity ranging from 10 to 10,000 solar are performed, and it is concluded that this two-step process of sub-Lyman continuum absorption followed by Balmer photoionization can account for the great majority of observed Brackett line fluxes. The location and mass of the emitting as as determined from the Brackett line fluxes are reported. The Br-alpha luminosity is calculated as a function of the mass loss rate.

Kwan, John↗
NASA NTRS1988

IUE observations of oxygen-rich supernova remnants

The IUE observations were used to determine the composition of the ejecta (especially C and Si abundances) and to test models for the ionization and excitation of the ejecta of two oxygen-rich supernova remnants (N132D in the Large Magellanic Cloud and 1E 0102-7219 in the Small Magellanic Cloud). Time-dependent photoionization by the EUV and X-ray radiation from 1E 0102-7219 can qualitatively explain its UV and optical line emission, but the density and ionization structures are complex and prevent a unique model from being specified. Many model parameters are poorly constrained, including the time dependence and shape of the ionizing spectrum. Moreover, the models presented are not self-consistent in that the volumes and densities of the optically emitting gas imply optical depths of order unity in the EUV, but absorption of the ionizing radiation was ignored. It is possible that these shortcomings reflect a more fundamental limitation of the model assumptions. It is assumed that the electron velocity distribution is Maxwellian and that the energy deposited by photoionization heats the electrons directly. The 500 eV electrons produced by the Auger process may excite or ionize other ions before they slow down enough to share their energy with other electrons. Many of the excitations would produce photons that could ionize lower ionization stages.

Blair, W. P.↗
NASA NTRS1990

Role of the methylene amidogen (H2CN) radical in the atmospheres of Titan and Jupiter

The methylene amidogen (H2CN) radical can be shown to be an important intermediate in models for the formation of HCN (via N + CH3) and the recombination of H to H2 (via H + HCN) on Titan as well as in models for the formation of HCN (via NH2 + C2H3) in the atmosphere of Jupiter. Experiments in our laboratory in a discharge flow system with mass spectrometric detection of both reactants and products have established that the major product channel (90 percent for the reaction N + CH3 is that leading to H2CN + H. The same result was obtained for N + CD3 yields D2CN + D. The rate constant for the reaction D + D2CN yields DCN + D2 was measured for the first time and k(298 K) greater than 7 x 10(exp -11) cu cm/s was found. The same result was obtained for the H atom reaction. This is the final step in the reaction sequence leading to HCN on both Titan and Jupiter and to formation of H2 from H on Titan. The first measurement of the ionization potentials for H2CN and D2CN was also made. From electron impact studies I.P. = (9.6 + or - 1.0) eV was obtained for both radicals. An upperlimit of I.P. less than 11.6 eV came from observations of the H2CN radical by photoionization mass spectrometry using an Ar resonance lamp (106.7 nm). Further photoionization experiments are planned using synchrotron radiation plus monochromator as a tunable vacuum UV light source.

Nesbitt, F. L.↗
NASA NTRS1989

Atomic data for opacity calculations. XI - The carbon isoelectronic sequence

Close-coupling calculations are carried out for radiative processes in neutral carbon and a number of carbon-like ions; energy levels, oscillator strengths, and photoionization cross sections have been computed for all bound states of the type 2 s(j)2p(k)nl with n not above 10 and 1 not above 3. The R-matrix method is employed to solve the coupled equations with a ten-state eigenfunction expansion for the parent ion C II and an eight-state expansion for the other boron-like target ions. A number of selected results for oscillator strengths are presented and compared with earlier data, as well as for photoionization cross sections with autoionizing resonance structures. Isoelectronic trends are discussed. The present results for the oscillator strengths of C I and N II are found to differ significantly from some earlier theoretical works for a number of transitions. However, the present C I f values are in excellent agreement with recent calculations and experimental results.

Luo, D.↗
NASA NTRS1990

Ultraviolet gas absorption and dust extinction toward M8

Interstellar absorption lines are analyzed using high-resolution IUE spectra of 11 stars in the young cluster NGC 6530 located in the M8 region. High-velocity clouds at -35 km/s and -60 km/s are seen toward all cluster stars. The components arise in gases that are part of large interstellar bubbles centered on the cluster and driven by stellar winds of the most luminous members. Absorption lines of species of different ionization states are separated in velocity. The velocity stratification is best explained as a 'champagne' flow of ionized gas away from the cluster. The C IV/Si IV ratios toward the hotter cluster members are consistent with simple photoionization models if the gas-phase C/Si ratio is increased by preferential accretion onto dust grains. High ion column densities in the central cluster decline with distance from W93, suggesting that radiation from a hot source near W93 has photoionized gas in the central cluster.

Boggs, Don↗
NASA NTRS1990

Lyman alpha radiation in external galaxies

The Ly alpha line of atomic hydrogen is often a luminous component of the radiation emitted by distant galaxies. Except for those galaxies which have a substantial central source of non-stellar ionizing radiation, most of the Ly alpha radiation emitted by galaxies is generated within regions of the interstellar medium which are photoionized by starlight. Conversely, much of the energy radiated by photoionized regions is carried by the Ly alpha line. Only hot, massive stars are capable of ionizing hydrogen in the interstellar medium which surrounds them, and because such stars are necessarily short-lived, Ly alpha emission traces regions of active star formation. Researchers argue that the strength of the Ly alpha emission observed from external galaxies may be used to estimate quantitatively the dust content of the emitting region, while the Ly alpha line profile is sensitive to the presence of shock waves. Interstellar dust particles and shock waves are intimately associated with the process of star formation in two senses. First, both dust particles and shock waves owe their existence to stellar activity; second, they may both serve as agents which facilitate the formation of stars, shocks by triggering gravitational instabilities in the interstellar gas that they compress, and dust by shielding star-forming molecular clouds from the ionizing and dissociative effects of external UV radiation. By using Ly alpha observations as a probe of the dust content in diffuse gas at high redshift, we might hope to learn about the earliest epochs of star formation.

Neufeld, David A.↗
NASA NTRS1990

Star formation in proto dwarf galaxies

The effects of the onset of star formation on the residual gas in primordial low-mass Local-Group dwarf spheroidal galaxies is studied by a series of hydrodynamical simulations. The models have concentrated on the effect of photoionization. The results indicate that photoionization in the presence of a moderate gas density gradient can eject most of the residual gas on a time scale of a few 10 to the 7th power years. High central gas density combined with inefficient star formation, however, may prevent mass ejection. The effect of supernova explosions is discussed briefly.

Noriega-Crespo, A.↗
NASA NTRS1990

Discourse for slide presentation: An overview of chemical detection systems

A brief overview of some of the analytical techniques currently used in monitoring and analyzing permanent gases and selected volatile organic compound in air are presented. Some of the analytical considerations in developing a specific method are discussed. Four broad groups of hardware are discussed: compound class specific personal monitors, gas chromatographic systems, infrared spectroscopic systems, and mass spectrometric residual gas analyzer systems. Three types of detectors are also discussed: catalytic sensor based systems, photoionization detectors, and wet or dry chemical reagent systems. Under gas chromatograph based systems five detector systems used in combination with a GC are covered: thermal conductivity detectors, photoionization detectors, Fourier transform infrared spectrophotometric systems, quadrapole mass spectrometric systems, and a relatively recent development, a surface acoustic wave vapor detector.

Peters, Randy Alan↗
NASA NTRS1992

Lunar and Asteroid Composition Using a Remote Secondary Ion Mass Spectrometer

Laboratory experiments simulating solar wind sputtering of lunar surface materials have shown that solar wind protons sputter secondary ions in sufficient numbers to be measured from low-altitude lunar orbit. Secondary ions of Na, Mg, Al, Si, K, Ca, Mn, Ti, and Fe have been observed sputtered from sample simulants of mare and highland soils. While solar wind ions are hundreds of times less efficient than those used in standard secondary ion mass spectrometry, secondary ion fluxes expected at the Moon under normal solar wind conditions range from approximately 10 to greater than 10(exp 4) ions cm(sup -2)s(sup -1), depending on species. These secondary ion fluxes depend both on concentration in the soil and on probability of ionization; yields of easily ionized elements such as K and Na are relatively much greater than those for the more electronegative elements and compounds. Once these ions leave the surface, they are subject to acceleration by local electric and magnetic fields. For typical solar wind conditions, secondary ions can be accelerated to an orbital observing location. The same is true for atmospheric atoms and molecules that are photoionized by solar EUV. The instrument to detect, identify, and map secondary ions sputtered from the lunar surface and photoions arising from the tenuous atmosphere is discussed.

Elphic, R. C.↗
NASA NTRS1993

The formation of low-ionization emission in the halo of NGC 891

Imaging and Spectroscopic study first revealed the presence of a diffuse ionized medium (DIM), having unusual excitation, pervading the lower halo of the edge-on spiral galaxy NGC 891. Emission from this DIM is strongest northeast of the nucleus, at radii between 2 and 8 kpc (hereafter region 1). The (N2)(lambda)6583/H(alpha) and (S2)(lambda) (lambda)6716,6731/H(alpha) ratios increase dramatically with z in region 1, from 0.6 and 0.5 respectively at z is approximately equal to 500 pc to 1.1 and 1.0 at z is approximately equal to 1 kpc, while nondetections of (O1)(lambda)6300 and (O3)(lambda)5007 emission yield upper limits of (O1)(lambda)6300/H(alpha) less than or equal to 0.05 and (O3)(lambda)5007/H(alpha) less than or equal to 0.15 for z less than 1 kpc. Previous photoionization models, using the radiation field from disk O and B stars, have been successful in reproducing the elevated (N2)(lambda)6583/H(alpha) and (S2)(lambda)(lambda)6716.6731/H(alpha) ratios observed. However, these radiation bounded models also produce significant (O3)(lambda)5007 emission, in conflict with the observed upper limit. Here, we report the results of new, matter bounded models for the photoionization of the DIM in region 1 of NGC 891.

Sokolowski, James↗
NASA NTRS1992

Infrared fine-structure line diagnostics of shrouded active galactic nuclei

The ultraluminous far-IR galaxies revealed by IRAS, quasar-like in luminosity but smothered in molecular gas, probably conceal either immense starbursts or luminous active nuclei. In both scenarios, these objects ought to produce copious infrared fine-structure emission with several lines comparable to H-beta in luminosity. We show how these lines, if detected, can be used to determine the electron densities and far-IR obscurations of shrouded photoionized regions and to constrain the shape and ionization parameter of the ionizing spectra. The presence of Ne v emission in particular will distinguish shrouded AGNs from shrouded starbursts. Since all active galaxies photoionize at least some surrounding material, these diagnostics can also be applied to active galaxies in general and will aid in studying how an active nucleus interacts with the interstellar medium of its host galaxy.

Voit, G. M.↗