Lyman-alpha Radiation Research Articles

Sensitivity of Predicted Tokamak Physics Parameters to Divertor Atomic/Molecular Data and Modeling

An interactive divertor and scrape-off layer plasma/two-dimensional neutrals/core plasma particle and power balance model has been used to investigate the importance of uncertainties or inadequacies in the data and modeling of atomic/molecular phenomena in the divertor to the calculation of the core and divertor plasma physics parameters in a tokamak. Treating recycling as being in the form of molecules rather than atoms as well as the inclusion of reabsorption of Lyman alpha radiation in the divertor are found to have large effects on the calculated plasma and neutral parameters throughout the divertor, scrape-off layer, and core. Whether the global parameters are changed if a significant fraction of the molecules are vibrationally excited has been tested; while molecular excitation does change parameters in the recycling region in front of the divertor plate, the effect on other divertor, scape-off layer, and core plasma parameters is negligible. Estimated uncertainties in the rates for charge-exchange and elastic scattering also have a very small effect on the global parameters. Inclusion of neutral-neutral scattering, while important for the calculation of local properties in the recycling region, has only a small effect on the global parameters.

High resolution measurements of neutral density and ionization rate in the Alcator C-Mod tokamak

Two new high resolution detectors have been installed on the Alcator C-Mod tokamak to measure the neutral density and ionization rate at the edge of the main chamber plasma. Using a silicon detector sensitive to UV light, and a very narrow filter with transmission peaking at 1216 Å, the Lyman alpha radiation emanating from neutral deuterium (and hydrogen) is measured. The detectors consist of 20 channel arrays which view the plasma tangentially 12.5 cm below the outer midplane, and 10 cm above the inner midplane. The imaging is performed using a 1 mm×3 mm slit, which gives a nominal radial resolution of 2 and 3 mm, respectively. The local emissivity is obtained via a standard Abel inversion technique. Employing well-known branching ratios, and using measured local electron density and temperature, the neutral density and ionization rate are inferred with similar radial resolution. Details of the setup and sensitivity of the results to plasma conditions are discussed.

Preliminary retrieval of solar wind latitude distribution from Solar Wind Anisotropies/SOHO observations

The Solar Wind Anisotropies (SWAN) instrument on board the SOHO spacecraft measures Lyman alpha radiation emanating mainly from neutral hydrogen gas in the solar neighborhood. This gas is part of the local interstellar cloud in which the Sun and the heliosphere are immersed. Measurements of Lyman alpha can be used to infer the local cloud characteristics like the velocity and the direction of the flow, gas temperature, and density. The strong interaction between the Sun and the neutral hydrogen gas also makes possible investigations of solar characteristics by Lyman alpha measurements. In this work we will concentrate on deriving the latitudinal distribution of solar‐induced ionization from SWAN Lyman alpha maps measured in 1996 at a time of the solar minimum. From the ionization we derive the distribution of the solar wind mass flux. SWAN Lyman alpha data show that the ionization and the mass flux are nearly flat for all solar latitudes except the narrow belt from −20° to 20° around the solar equator. In this region, ionization and the solar wind mass flux show a definite increase, which can be seen as an intensity depression in the Lyman alpha data from directions near the ecliptic. These results confirm earlier in situ measurements by Ulysses during the present minimum and Lyman alpha measurements by Prognoz satellites 20 years ago.

Simulation and methods for the data analysis of the Hydrogen-Deuterium-Absorption-Cell-experiment on Cassini

The Hydrogen-Deuterium-Absorption-Cell (HDAC) will be flown as part of the UltraViolet Imaging Spectrograph on the Cassini mission with the objective to determine the deuterium-to-hydrogen (D/H) ratio in the exosphere of Titan. The HDAC is a photometer designed to measure separately the H and D resonant scattered solar Lyman alpha radiation. A non-parametric reconstruction of the Lyman alpha line profiles by means of local linear filters has been developed for the data analysis. An apriori line profile has been statistically described. Applying a minimum variance criterion leads to systems of linear equations, whose solutions yield the filter coefficients. The quality of the line profiles reconstructed with the proposed algorithm in realistic conditions of observations shows that is workable for the real mission.

Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Atomic hydrogen Lyman alpha radiation (121.6 nm) has been measured in emission from the atmosphere of Ganymede with the Galileo ultraviolet spectrometer. An exospheric model with the following parameters has been fit to the observational data: atomic hydrogen density directly above the surface (radius 2634 km) equal to 1.5 × 104 atoms cm−3 scale height 2634 km, exospheric temperature 450 K. A model calculation of the photodissociation of water vapor from surface ice at 146 K is used to obtain the photodissociation rate necessary to supply the hydrogen atoms that are escaping from the exosphere of Ganymede. The calculated escape flux of atomic hydrogen is 7 × 108 atoms/cm² sec. Two alternate but speculative sources of the atomic hydrogen escaping from Ganymede are photodesorption of water ice by ultraviolet photons in the wavelength range 120.5–186.0 nm and sputtering of water ice by Jupiter's magnetospheric ion plasma.

Deuterium atomic cross section uncertainties and molecular recycling effects in the divertor region of tokamaks

How predictions of deuterium atomic reaction rates in tokamak divertors are altered by changes in cross section data which are comparable with the uncertainty in published data is investigated. An extended neutral diffusion theory calculation, which includes D atoms and ground and excited state D2 molecules, is performed on a fixed background plasma. For the atoms, the principal relevant mechanisms are elastic scattering, charge exchange, ionization, and recombination. It is evaluated that the charge exchange and electron processes are known to an accuracy better than +/−25%, and it is concluded that this level of accuracy is sufficient for the present state of divertor calculations. Of significant concern is the fact that classical elastic cross sections which are widely used in divertor calculations differ substantially from a more accurate set of cross sections based on quantum mechanics, and that the latter produce predictions of neutral density and ionization rate which differ by orders of magnitude from predictions based on the classical cross sections. Opacity of the plasma to Lyman alpha radiation influences transport for high density plasma situations. The explicit representation of incident particles recycling as molecules is shown to significantly affect the neutral atom density and reaction rates near the divertor plate. In particular, the dissociation of even a small fraction of excited molecules significantly reduces the plasma ion population and increases the neutral atom population just in front of the plate in nearly detached plasmas.

Determination of atomic hydrogen densities in gases by a deuterium based Lyman-alpha absorption method

We discuss the variant of a method for the determination of absolute hydrogen atom densities based on the absorption of the hydrogen Lyman-alpha line. Previously, we demonstrated that by using a simple vacuum ultraviolet spectrometer with low resolution an accuracy of better than 50% for the determined hydrogen atom densities can be obtained for transmissions ranging from 10% to 90%. For transmissions outside of this range excessive errors occur, thus, limiting the usefulness of the method to a certain range of hydrogen atom densities, depending on absorber length and temperature. This range of atomic hydrogen densities accessible to the measurement can be extended by the new method that consists of using well-defined mixtures of hydrogen and deuterium in the absorber. Using a source of either hydrogen or deuterium Lyman-alpha radiation, only one sort of atoms contributes to the absorption. Thus, by selecting an appropriate mixture of hydrogen and deuterium, the range of atom densities accessible to the measurement can be extended to higher densities. Using well-defined mixtures ranging from 1% hydrogen in deuterium to 2% deuterium in hydrogen as absorbers, we were able to determine hydrogen atom densities up to a factor 100 higher than those measured previously in pure hydrogen. Using mixtures down to the natural abundance of deuterium in hydrogen, the measurement of even higher atom densities seems possible. The effect of the difference of the energies of dissociation of hydrogen and deuterium has been investigated using a computer model of relevant chemical processes. In most situations, this effect is smaller than the experimental error. The main limitation of the method is its sensitivity to absorption by impurities, as many molecular gases absorb Lyman-alpha radiation.

The alpha Centauri Line of Sight: D/H Ratio, Physical Properties of Local Interstellar Gas, and Measurement of Heated Hydrogen (The 'Hydrogen Wall') Near the Heliopause

view Abstract Citations (234) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The α Centauri line of sight: D/H ratio, physical properties of local interstellar gas, and measurement of heated hydrogen (the "hydrogen wall") near the heliopause. Linsky, Jeffrey L. ; Wood, Brian E. Abstract The authors analyze high resolution spectra of the nearby (1.34 pc) stars α Cen A (G2 V) and α Cen B (K1 V), which were obtained with the GHRS on the HST. The observations consist of echelle spectra of the Mg II 2800 Å and Fe II 2599 Å resonance lines and the Lyman-α lines of hydrogen and deuterium. The interstellar gas has a velocity of v = -18.0±0.2 km s-1. The temperature and nonthermal velocity inferred from the Fe II, Mg II, and D I line profiles are T = 5400±500K and ξ = 1.20±0.25 km s-1, respectively. However, single component fits to the H I Lyman-α lines yield a Doppler parameter (bHI = 11.80 km s-1) that implies a significantly higher temperature of 8350K, and the velocity of the H I absorption is redshifted by about 2.2 km s-1 with respect to the Fe II, Mg II, and D I lines. The one component model of the interstellar gas suggests log NHI = 18.03±0.01 and D/H = (5.7±0.2)×10-6. The most sensible way to resolve the discrepancy between H I and the other lines is to add a second absorption component to the H I lines. This component is hotter, is redshifted relative to the primary component by 2 - 4 km s-1, and has a column density too low to be detected in the Fe II, Mg II, and D I lines. The authors propose that the gas responsible for this component is located near the heliopause, consisting of the heated H I gas from the interstellar medium that is compressed by the solar wind (the solar "hydrogen wall"). After considering the effects of a possible similar hydrogen wall around α Cen on the analysis, the authors derive for the solar hydrogen wall the parameters: log NHI = 14.74±0.24, bHI = 21.9±1.7 km s-1 (corresponding to T = 29,000±5000K) and vHI > -16 km s-1. Taking the solar hydrogen wall into account, the H I column density along the line of sight to α Cen is log NHI = 17.80±0.30, corresponding to nHI = 0.15 cm-3 (± a factor of 2), and D/H is in the range (0.5 - 1.9)×10-5. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177238 Bibcode: 1996ApJ...463..254L Keywords: Centaurus Constellation; Interstellar Matter; Interstellar Gas; Hydrogen; High Resolution; Chemical Properties; Hubble Space Telescope; Spectrographs; Temperature Measurement; D Lines; H Lines; Heliosphere; Lyman Alpha Radiation; Plasma Interactions; Solar Energy; Subgiant Stars; Solar Physics; Interstellar Matter: Deuterium; Interstellar Matter: UV Lines; Interstellar Matter: Solar Neighborhood; Heliosphere: Interstellar Clouds full text sources ADS | data products SIMBAD (10) MAST (1) ESA (1)

Formation of H(2s) atoms by excitation in 10 - 100 keV collisions

Cross sections for 2s excitation of H atoms in 10 - 100 keV collisions have been determined using a modulated crossed-beam technique. The measurements have been based on observations of the Lyman alpha radiation emitted during electric-field-induced decay of the metastable H(2s) collision products. The results extend the range of the 5 - 26 keV cross sections measured by Morgan and co-workers to intermediate energies where theoretical predictions based on close-coupling methods are known to be strongly dependent on the choice of the expansion basis. The present cross sections pass through a broad maximum at about 40 keV. Over the range 5 - 100 keV the available experimental data exhibit an undulatory structure similar to that predicted by some close-coupling calculations but good quantitative agreement is very limited. Close-coupling calculations which employ large basis sets and include a large number of projectile states at the expense of target states are shown to agree less satisfactorily with experiment than those which include only the dominant 1s capture projectile channel.

Effect of local interstellar medium hydrogen fractional ionization on the distant solar wind and interface region

The paper investigates the influence of the local interstellar medium (LISM) proton number density on the gasdynamics of LISM / solar wind interaction. It uses the self‐consistent axisymmetric model of Baranov and Malama (1993) for a constant value of the LISM neutral hydrogen number density (nH∞ = 0.14 cm−3). It is shown that the flow structure is asymmetric relative to the upwind and downwind directions. Numerical results are presented for four values of the proton number density in the LISM: np∞ = 0.005; 0.025; 0.1 and 0.28 cm−3. The results are given along the axis of symmetry only (in upwind and downwind directions). Velocity, number density, and temperature distributions of the plasma component as well as of the H atoms of the LISM are presented. It is shown that the influence of H atoms, penetrating from the LISM into the solar system on the solar wind may be strong enough to be observed by the “Voyager” spacecraft. The results show that the number density of the LISM's plasma component as well as of the solar wind has very large gradients in the interface region (between the bow and termination shocks). This effect may be very important for the interpretation of the kHz radiation detected by “Voyager”. Moreover, the asymmetric (in upwind and downwind directions) distributions of H atom number density, bulk velocity, and temperature may be observed on the basis of the scattered solar Lyman alpha radiation data.

Spacelab 2 measurement of the solar coronal helium abundance

The abundance of helium relative to hydrogen has been measured with the “Coronal Helium Abundance Spacelab Experiment” (CHASE) from the space shuttle Challenger in 1985. Previous solar measurements have proved difficult due to the temperature-sensitivity of the electron excitation rates for the observed lines. In this approach scattered Lyman Alpha (Lyα) radiation of belium and hydrogen formed in the corona were measured with a grazing-incidence spectrometer and compared with the intensity of the illuminating flux from the solar chromosphere. The abundance ratio by number of atoms was found to be 0.070 with an uncertainty of 0.011. Scattered light in the telescope is the main source of error.

Damped lyman-alpha systems versus cold + hot dark matter

view Abstract Citations (115) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Damped Lyman-Alpha Systems versus Cold + Hot Dark Matter Klypin, Anatoly ; Borgani, Stefano ; Holtzman, Jon ; Primack, Joel Abstract Although the cold + hot dark matter (CHDM) cosmology provides perhaps the best fit of any model to all the available data at the current epoch (z = 0), CHDM produces structure at relatively low redshifts and thus is very sensitive to the observed numbers of massive objects at high redshifts. Damped Lyα systems are abundant in quasar absorption spectra and provide possibly the most significant evidence for early structure formation, and thus a stringent constraint on CHDM. Using the numbers of halos in N-body simulations to normalize Press-Schechter estimates of the number densities of protogalaxies as a function of redshift, we find that CHDM with {OMEGA}_c_/{OMEGA}nu_/{OMEGA}_b_ = 0.6/0.3/0.1 is compatible with the damped Lyα data only at < 2.5, but that it is probably incompatible with the z > 3 damped Lyα data. The situation is uncertain because there is very little data for z > 3. The predictions of CHDM are quite sensitive to the hot (neutrino) fraction, and we find that {OMEGA}_c_/{OMEGA}nu_/{OMEGA}_b_ = 0.725/0.20/0.075 (and possibly even {OMEGA}_c_/{OMEGA}nu_/{OMEGA}_b_ = 0.675/0.25/0.075) is compatible with the z > 3 data. With one massive neutrino species, using {OMEGA}nu_ = 0.20 instead of 0.30 corresponds to lowering the neutrino mass from 7.0 to 4.7 eV, for H_0_ = 50 km s^-1^ Mpc^-1^ and T - 2.726 K. In CHDM, the higher redshift damped Lyα systems are predicted to have lower masses (~3 x 10^10^ M_sun_ at z = 3), a prediction which can be checked by measuring the velocity widths of the associated metal-line systems. Predictions for high-z objects crucially depend on the effects of limited resolution and the finite box size in N-body simulations or on the parameters of the Press-Schechter approximation, if it is used. By analyzing our numerical simulations with vastly different resolutions and box sizes as well as those of Ma & Bertschinger (1994), we show that for the CHDM models with {OMEGA}nu_ = 0.2-0.3 the Press-Schechter approximation should be used with Gaussian filter with δ_c_ = 1.5 if halos are defined with the mean overdensity larger than 200. If one tries to recover the total mass of a collapsed halo, a better value for the collapse parameter is δ_c_ = 1.40. We argue that nonlinear effects due to waves both longer and shorter than those considered in numerical simulations could probably result in δ_c_ as low as δ_c_ = 1.3. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175578 arXiv: arXiv:astro-ph/9405003 Bibcode: 1995ApJ...444....1K Keywords: Computational Astrophysics; Cosmology; Damping; Dark Matter; Lyman Alpha Radiation; Universe; Absorption Spectra; Approximation; Galactic Evolution; Galactic Halos; Many Body Problem; Neutrinos; Quasars; Simulation; Space Density; Astrophysics; COSMOLOGY: DARK MATTER; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; COSMOLOGY: THEORY; GALAXIES: FORMATION; GALAXIES: QUASARS: ABSORPTION LINES; Astrophysics; High Energy Physics - Phenomenology E-Print: submitted to ApJ 16 pages including 3 figures, uuencoded compressed postscript, SCIPP 94/09 full text sources arXiv | ADS |

Hubble Space Telescope imaging of a radio-quiet galaxy at redshift Z = 3.4

view Abstract Citations (17) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Imaging of a Radio-quiet Galaxy at Redshift Z = 3.4 Giavalisco, M. ; Macchetto, F. D. ; Madau, P. ; Sparks, W. B. Abstract We have observed with the Wide Field/Planetary Camera (WF/PC) on the Hubble Space Telescope (HST) a radio-quiet Lyα-emitting galaxy at redshift z = 3.428 (G2 below). The images probe the rest-frame UV light around 1250 A with an angular resolution of ~0.1", corresponding to 1.4 h_50_^-1^ kpc at redshift z = 3.4 (in this Letter we use q_0_ = 0 and H_0_ = 50 h_50_^-1^ km s^-1^ Mpc^-1^). The light profile of the central ~10 h_50_^-1^ kpc region is well fitted by an r^1/4^ law with r_e_~1.3 h_50_^-1^ kpc, suggesting a dynamically relaxed state. The outer regions are characterized by the presence of substructures, such as an elongated formation and low surface brightness nebulosities. The isophotal analysis shows no evidence of an AGN-like unresolved source in the center. The structural properties of G2 are consistent with a dynamically hot stellar system observed during an early phase of star formation, very likely the progenitor of an elliptical or the bulge of a spiral galaxy. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/187777 Bibcode: 1995ApJ...441L..13G Keywords: Active Galactic Nuclei; Big Bang Cosmology; Galactic Evolution; Lyman Alpha Radiation; Quasars; Spectral Energy Distribution; Star Formation; Starburst Galaxies; Ultraviolet Astronomy; Angular Resolution; Brightness Distribution; Cameras; Hubble Space Telescope; Red Shift; Spaceborne Astronomy; Stellar Luminosity; Astronomy; COSMOLOGY: OBSERVATIONS; GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: FORMATION; GALAXIES: FUNDAMENTAL PARAMETERS; GALAXIES: STARBURST full text sources ADS | data products NED (3) SIMBAD (2) MAST (1) ESA (1)

Disk galaxies near the line of sight to 3C 273 and conjectures on Lyman-alpha absorption

view Abstract Citations (30) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Disk Galaxies near the Line of Sight to 3C 273 and Conjectures on Lyman-Alpha Absorption Salpeter, E. E. ; Hoffman, G. Lyle Abstract Using data from the preceding paper (Hoffman, Lewis, & Salpeter 1995) and recent literature, we augment compilations by Salzer (1992) and by Morris et al. (1993) to select (by projected distance) 22 disk galaxies near the line of sight to the quasar 3C 273. These galaxies are correlated with the seven Lyα absorption lines (from HST observations) below 10,000 km s^-1^ on velocity scales typical of galaxy groups (a few hundred km s^-1^). For three of the seven lines an individual galaxy is close enough in position and velocity that it may provide the absorption. This includes the galaxy MGC 0-32-16, which has a projected separation of 204 kpc and a velocity difference of 93 km s^-1^ from the lowest velocity absorption line. For a fourth of the seven lines, there is a small group of galaxies fairly close by. For the remaining three of the seven absorption lines, however, no ordinary galaxies are nearby. Thus, while some of the absorption lines are likely to arise in extensions of ordinary galaxy disks, the majority must be caused by otherwise invisible objects. Whatever the physical nature of these objects, many of them are likely to reside in "clouds or filaments" in voidlike regions which also contain some ordinary galaxies or galaxy groups. As an alternative to "minihalos" for the invisible objects, we postulate an "almost invisible" class of galaxies ("Vanishing Cheshire Cats") which are born with central column densities already about 10 times smaller than for ordinary disk galaxies and which then suffer further mass loss from the inner disk. Recent observations that show a larger fraction of Lyman-limit systems with H I column density below 10^17.7^ cm^-2^ for z < 1 than for z > 2 provide further motivation for this model. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175334 Bibcode: 1995ApJ...441...51S Keywords: Absorption Spectra; Disk Galaxies; Lyman Alpha Radiation; Quasars; Velocity Distribution; Astronomical Models; Hubble Space Telescope; Line Of Sight; Astronomy; GALAXIES: CLUSTERS: INDIVIDUAL NAME: VIRGO; GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: QUASARS: ABSORPTION LINES; RADIO LINES: GALAXIES full text sources ADS | data products NED (30) SIMBAD (11)

Heating and ionization of stellar chromospheres by nonthermal proton beams: Implications for impulsive phase, redshifted Lyman-alpha radiation in stellar flares

We investigate the physical basis for the timescale of impulsive-phase, redshifted Lyman-alpha emission in stellar flares on the assumption that it is determined by energy losses in a nonthermal proton beam that is penetrating the chromosphere from above. The temporal evolution of ionization and heating in representative model chromospheres subjected to such beams is calculated. The treatment of 'stopping' of beam protons takes into account their interactions with (1) electrons bound in neutral hydrogen, (2) nuclei of neutral hydrogen, (3) free electrons, and (4) ambient thermal protons. We find that, for constant incident beam flux, the system attains an equilibrium with the beam energy input to the chromosphere balanced by radiative losses. In equilibrium, the beam penetration depth is constant, and erosion of the chromosphere ceases. If the redshifted, impulsive-phase stellar flare Lyman-alpha emission is produced by downstreaming hydrogen formed through charge exchange between beam protons and ambient hydrogen, then the emission should end when the beam no longer reaches neutral hydrogen. The durations of representative emission events calculated on this assumption range from 0.1 to 14 s. The stronger the beam, the shorter the timescale over which the redshifted Lyman-alpha emission can be observed.

Spectroscopy of the double quasars Q1343+266A, B: The relationship between galaxies and quasi-stellar object C IV absorption lines

view Abstract Citations (33) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopy of the Double Quasars Q1343+266A, B: The Relationship between Galaxies and C IV Absorption Lines Crotts, Arlin P. S. ; Bechtold, Jill ; Fang, Yihu ; Duncan, Robert C. Abstract We present spectra of QSOs Q1343+266A, B, covering Lyβ to C IV emission at 2 A resolution, and find further evidence that this system is not gravitationally lensed. We see, however, a large number of Lyα and some C IV absorption features common to both spectra. The frequency of C IV lines in common indicates that these absorbers have a radius close to the sight line separation, 39 h^-1^ kpc (q_0_ = 0.5), implying a spatial number density of these absorbers the same as current-day L > 0.3 L_*_ galaxies. Furthermore, the velocity mismatch for C IV systems indicates absorber masses in the range 10^11^ M_sun_ < M < 10^12^ M_sun_. These results confirm that typical galaxies cause QSO C iv-line absorption. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/187687 arXiv: arXiv:astro-ph/9409008 Bibcode: 1994ApJ...437L..79C Keywords: Absorption Spectra; Absorption Spectroscopy; Carbon; Lyman Spectra; Quasars; Ultraviolet Astronomy; Galaxies; Lyman Alpha Radiation; Lyman Beta Radiation; Space Density; Stellar Mass; Velocity Distribution; Astrophysics; GALAXIES: QUASARS: ABSORPTION LINES; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: Q1343; 266; Astrophysics E-Print: (8 pages, Accepted by Ap. J Letters) 9295 full text sources arXiv | ADS | data products NED (41) SIMBAD (2)

The He II Lyman-alpha opacity of the universe

view Abstract Citations (84) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The He II Lyman-Alpha Opacity of the Universe Madau, Piero ; Meiksin, Avery Abstract We discuss the recent Hubble Space Telescope FOC observations by Jakobsen et al. of a sharp drop in the spectrum of the high redshift (z = 3.29) quasar Q0302-003 at the rest-frame wavelength of 304 A, the Lyα line of He II. We assess two possible explanations for this flux reduction: line blanketing produced by He II in the Lyα forest, and the He II Gunn-Peterson effect from a diffuse intergalactic medium (IGM). The observed He II trough can be produced entirely by line blanketing only if the absorbing clouds are velocity broadened and/or their distribution extends to very low column densities. A significant contribution from absorption in a diffuse IGM is likely. We show that a steep ionizing UV background is required to explain the reported He II absorption and the simultaneous absence of H I. Such a soft metagalactic flux could arise from star-forming galaxies, decaying dark matter, or QSOs, after including the effect of the continuum opacity from intervening absorption systems on the integrated spectrum. We describe an alternative interpretation, that the He II reionization of the universe is not completed until z~3, well after the H II region network has fully percolated at z~5, and find that it too requires a steep ionizing spectrum for the individual sources. Photoionization from QSOs detected in optical surveys may satisfy the H I and He II limits if the Lyα forest contains a significant fraction of the baryons in the universe. If the ionizing background remains soft shortward of the H II edge, metal- line absorbers at high redshift must be ionized locally. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/187546 Bibcode: 1994ApJ...433L..53M Keywords: Absorption Spectra; Cosmology; Helium; Lyman Alpha Radiation; Opacity; Quasars; Ultraviolet Astronomy; Ultraviolet Spectra; Baryons; Faint Object Camera; Gunn Effect; Hubble Space Telescope; Hydrogen; Intergalactic Media; Photoionization; Astronomy; COSMOLOGY: DIFFUSE RADIATION; COSMOLOGY: OBSERVATIONS; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: QUASARS: ABSORPTION LINES full text sources ADS | data products SIMBAD (2) NED (1)

Predicted optical/ultraviolet line fluxes for the warm absorber in Abell 2256

view Abstract Citations (4) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Predicted Optical/Ultraviolet Line Fluxes for the Warm Absorber in Abell 2256 Pistinner, Shlomi ; Sarazin, Craig L. Abstract Recently Miyaji et al. (1993) detected excess soft X-ray absorption in the BBXRT spectrum of the cluster A2256. The absorption was due to the K-edge of oxygen; from the observed edge energy and the redshift of the cluster, Miyaji et al. argued that the primary absorber was oxygen in an ionization state between O^+2^ and O^+4^ at a temperature in the range 0.5-5 x 10^5^ K. Such gas should produce strong UV and optical line emission. We predict the strongest UV and optical line fluxes for this X- ray absorbing gas. The strongest UV lines include Lyα, C III] λ1909, C IV λ1549, O IV] λ1401, O V] λ1218, O VI λ1034, Si III λ1207, and Si IV λ1397 lines. These lines should be detectable with the International Ultraviolet Explorer, the Hubble Space Telescope, or the Hopkins Ultraviolet Telescope. In the optical the lines are weaker, but the [O II] λλ3726,3729, [O III] λλ4959,5007, [N II] λλ6567,6584, [Ne II] λλ3918,3969, [S II] λ6720, and [S III] λ9320 lines should be detected with moderate exposures if the warm absorber is near the lower limit of the allowed temperature range, T <~ 1.5 x 10^5^ K. In this limit the Hα and [S III] λ3720 lines will require longer exposure times, but should be detectable as well. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174668 Bibcode: 1994ApJ...433..577P Keywords: Cooling Flows (Astrophysics); Emission Spectra; Galactic Clusters; Intergalactic Media; Line Spectra; Lyman Alpha Radiation; Oxygen; Ultraviolet Spectra; Visible Spectrum; X Ray Absorption; Carbon; Computerized Simulation; Hubble Space Telescope; Iue; Neon; Nitrogen; Red Shift; Rosat Mission; Silicon; Sulfur; Astrophysics; GALAXIES: CLUSTERS: INDIVIDUAL ALPHANUMERIC: A2256; GALAXIES: COOLING FLOWS; GALAXIES: INTERGALACTIC MEDIUM; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (1) NED (1) MAST (1)

The formation of the solar He II 1640.4 A emission line

view Abstract Citations (39) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Formation of the Solar He II 1640.4 Angstrom Emission Line Wahlstrom, Cathrine ; Carlsson, Mats Abstract We explain the formation of the solar He II 1640.4 A Balmer-alpha emission line. Non-LTE (non-local thermodynamic equilibrium) modeling is performed including the effects of an incoming coronal radiation field, the overlapping emission profile of He II 304 A with the He I continuum and the pumping effect of hydrogen Lyman-alpha. We find that the line is formed by the photoionization from the ground state of He II, mainly by the incident coronal radiation, followed by cascade recombination into the n = 3 levels. The peak of the contribution functions is at low temperatures, 7000-10,000 K, with a very small contribution from collisional excitation at a temperature of 70,000 K. We obtain good agreement with the observed line widths, the observed fine structure and the relative strength of the fine structure components with the blue peak about 10% stronger than the red peak. It is shown that these results are insensitive to the assumptions of geometry in the line forming regions and to the filling factor. The relative intensities only depend on an optically thin 1640.4 A line and optically thick He II Lyman lines. The observed asymmetry also imposes an upper limit to the density of 30 times the density at 8000 K in a plane parallel model. It is shown that the total intensity is very sensitive to the assumed geometry and filling factors. This sensitivity is parameterized in terms of a multiplicative factor to the assumed coronal radiation field. In a plane-parallel model the total intensity comes out a factor of 4 too low with the adopted coronal radiation field but already an enhancement factor of 2 of the incident radiation field gives a total intensity in agreement with observations. Note that this enhancement factor is only a quantification of the disagreement with observations. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/174654 Bibcode: 1994ApJ...433..417W Keywords: Balmer Series; Emission Spectra; Solar Atmosphere; Solar Spectra; Star Formation; Stellar Models; Ultraviolet Astronomy; Ultraviolet Spectra; Chromosphere; Lyman Alpha Radiation; Mathematical Models; Solar Corona; Solar Radiation; Solar Physics; LINE: FORMATION; LINE: PROFILES; SUN: ATMOSPHERE; SUN: CHROMOSPHERE; SUN: UV RADIATION full text sources ADS |

The distribution of neutral hydrogen in the interstellar medium. 1: The data

view Abstract Citations (128) References (88) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Distribution of Neutral Hydrogen in the Interstellar Medium. I. The Data Fruscione, Antonella ; Hawkins, Isabel ; Jelinsky, Patrick ; Wiercigroch, Alexandria Abstract We compile, from the existing literature, the largest sample to date (842 data points) of hydrogen column density measurements, N(H I), of the gas in the interstellar medium. We include only results obtained from absorption measurements toward individual stars (594 in our sample) in an effort to construct a three-dimensional picture of the interstellar gas. We derive hydrogen column densities toward a fraction of the stars in the sample from published column density measurements of metal ions. A three-dimensional physical model derived from this data set will be presented in a companion paper. The observed stars span distances from a few parsecs to a few thousand parsecs, and more than half of the sample serves to describe the local interstellar medium within a few hundred parsecs of the Sun. Hydrogen column densities range from 1017 to 1022/sq cm. We describe here the various observational methods used to estimate the hydrogen column densities and present the table with the stellar and hydrogen column density data. The provided table is intended as a global reference work, not to introduce new results. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1994 DOI: 10.1086/192075 Bibcode: 1994ApJS...94..127F Keywords: Astronomical Catalogs; Extreme Ultraviolet Radiation; H I Regions; Interstellar Gas; Ultraviolet Astronomy; X Ray Astronomy; Extreme Ultraviolet Explorer Satellite; Ionized Gases; Iue; Lyman Alpha Radiation; Neutral Gases; Oao 3; Rosat Mission; Astronomy; ISM: ABUNDANCES; ASTRONOMICAL DATA BASES: MISCELLANEOUS; CATALOGS full text sources ADS | data products SIMBAD (595) MAST (2) CDS (2) Related Materials (1) Catalog: 2001yCat..20940127F