Strong Radiation Field Research Articles

X‐Ray Modeling of Very Young Early‐Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications

The Orion Trapezium is one of the youngest and closest star-forming regions within our Galaxy. With a dynamic age of ~3 × 105 yr, it harbors a number of very young hot stars, which likely are on the zero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectra in the wavelength range of 1.5-25 A of three of its X-ray-brightest members (Θ1 Ori A, C, and E) obtained with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We measured X-ray emission lines, calculated differential emission measure distributions (DEMs), and fitted broadband models to the spectra. The spectra from all three stars are very rich in emission lines, specifically from highly ionized Fe, which includes emission from Fe XVII to Fe XXV ions. A complete line list is included. This is a mere effect of high temperatures rather than an overabundance of Fe, which in fact turns out to be underabundant in all three Trapezium members. Similarly there is a significant underabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. The DEM derived from over 80 emission lines in the spectrum of Θ1 Ori C indicates three peaks located at 7.9, 25, and 66 MK. The emission measure varies over the 15.4 day period of the star. For the two phases observed, the low-temperature emission remains stable, while the high-temperature emission shows significant differences. The line widths seem to show a similar bifurcation, where we resolve some of the soft X-ray lines with velocities up to 850 km s-1 (all widths are stated as half-width at half-maximum), whereas the bulk of the lines remain unresolved with a confidence limit of 110 km s-1. The broadband spectra of the other two stars can be fitted with several collisionally ionized plasma model components within a temperature range of 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK for Θ1 Ori A. The high-temperature emissivity contributes over 70% to the total X-ray flux. None of the lines are resolved for Θ1 Ori A and E with a confidence limit of 160 km s-1. The influence of the strong UV radiation field on the forbidden line in the He-like triplets allows us to set an upper limit on distance of the line-emitting region from the photosphere. The bulk of the X-ray emission cannot be produced by shock instabilities in a radiation-driven and are likely the result of magnetic confinement in all three stars. Although confinement models cannot explain all the results, the resemblance of the unresolved lines and of the DEM with recent observations of active coronae in II Peg and AR Lac during flares is quite obvious. Thus we speculate that the X-ray production mechanism in these stars is similar, with the difference that the Orion stars may be in a state of almost continuous flaring driven by the wind. We clearly rule out major effects due to X-rays from a possible companion. The fact that all three stars appear to be magnetic and are near zero age on the main sequence also raises the issue of whether the Orion stars are simply different or whether young massive stars enter the main sequence carrying significant magnetic fields. The ratio log LX/Lbol using the wind component of the spectrum is -7 for the Trapezium stars, consistent with the expectation from O stars. This suggests that massive ZAMS stars generate their X-ray luminosities like normal O stars and magnetic confinement provides an additional source of X-rays.

High-resolution XMM-Newton X-ray spectra of τ SCORPII

The first high-resolution X-ray spectra of the B0.2V star τ Scorpii obtained during 22.5ks on Aug. 20, 2001 with the RGS Grating and the epic-mos ccd spectrometers on board XMM-Newton exhibit bright emission lines of the H- and He-like ions of C, N, O, Ne, Mg, and Si, as well as Fe xvii and Fe xviii lines. The emission line and continuum spectra have been simultaneously fitted. We obtain four temperatures: 1.6, 5.2, 8.2, and ≳20 MK, emission measures and abundances. The nitrogen lines are relatively strong: The N/O abundance ratio is ∼3x solar. No indication of a solar-type “FIP effect” was found for the other elements. The temperatures are confirmed by DEM modeling. According to the derived models L x (0.3–10 keV) = 3.2 × 1031 ergs −1 at a distance of 132 pc. The He-like forbidden and intercombination line ratios of N,O,Ne, and Mg are determined by the strong stellar UV radiation field and yield upper limits to the radial distances at which these lines originate. The soft (≲ 8 MK) component probably originates from shocks low in the wind that are produced by the common mechanism of radiation line-driven instabilities, consistent with the observed emission line profiles that are much narrower (≲ 500 km/s) than the broad lines (up to 1500 km/s) observed high up in the wind of ζ Puppis. The hot (∼ 20–40 MK) component is explained by a model involving dense clumps embedded in a wind that approaches high relative velocities (∼ 1400 – 1700 km/s) and the interaction produces strong shocks

Quantitative Spectroscopy of Supergiants

Blue supergiants of spectral types B and A are the visually brightest stars in spiral and irregular galaxies, with their most luminous members (at ) outshining entire dwarf galM 10 V axies. This characteristic allows us to use them as probes to study the local universe in great detail. In principle, already existing large telescopes and instrumentation facilitate quantitative spectroscopy of these objects as far as the Virgo and Fornax clusters of galaxies. Beyond their challenging stellar atmospheres and opportunities for testing sophisticated nonLTE physics, they offer numerous applications to modern astrophysics. Quantitative spectroscopy of supergiants will contribute to improving our understanding of massive-star evolution. Galactic abundance gradients and abundance patterns, as will be obtained from studies of large ensembles of supergiants in our own and other galaxies, will foster the understanding of galactochemical evolution. Finally, they are promising independent indicators for calibrating the extragalactic distance scale, by application of the wind momentum–luminosity and the flux-weighted gravity–luminosity relationships (R. P. Kudritzki et al. 1999, AA R. P. Kudritzki, F. Bresolin, & N. Przybilla 2003, ApJ, 582, L83). In view of this large potential, the objective of this thesis is to improve the status of quantitative spectroscopy of BA-type supergiants and to provide first applications on a sample of Galactic and extragalactic targets. It is shown that among the model atmospheres available at present, the best suited for analyses of supergiants are line-blanketed classical LTE models. An investigation of the impact of various parameters such as helium abundance and line blanketing on the atmospheric structure shows that for the most luminous objects, an accurate treatment of these parameters is essential for a quantitative analysis, whereas the less luminous supergiants react less sensitively. Spectrum synthesis is used to model the line spectra. It is the only technique capable of providing analyses of spectra of different qualities from low to high resolution and able to cope with heavy line blending at a broad range of signal-tonoise ratios. The stellar parameters are determined from purely spectroscopic indicators, from temperatureand gravity-sensitive ionization equilibria and the Balmer line wings. Elemental abundances are derived by modeling individual spectral features. Several tens of thousands spectral lines from 28 chemical species are included in the line formation, allowing almost the entire observed spectra to be reproduced. Non-LTE effects become important in blue supergiants, where a strong radiation field at low particle densities favors deviations from LTE. Comprehensive model atoms are therefore constructed for C i/ii, N i/ii, O i, and Mg i/ii in order to determine non-LTE level populations (N. Przybilla, K. Butler, & R. P. Kudritzki 2001, AA N. Przybilla & K. Butler 2001, AA N. Przybilla et al. 2000, AA N. Przybilla et al. 2001, A&A, 369, 1009). Highly accurate radiative and collisional atomic data recently determined for astrophysical and fusion research using the R-matrix method in the closecoupling approximation are incorporated. In addition, model atoms for H, He i, O ii, S ii/iii, Ti ii, and Fe ii are adopted from the literature, the atomic data being updated to more modern values in some cases. Thus, an improved treatment for the main elements of astrophysical interest is achieved. Extensive testing of the atomic data is performed for the nearby bright main-sequence standard Vega (A0 V), at welldetermined stellar parameters and atmospheric structure. A high-resolution and low-noise spectrum with large wavelength coverage from the visual to the near-IR is used for this purpose. Further tests are performed for the Galactic supergiants h Leo, HD 111613, HD 92207, and b Ori, with similar high-quality spectra in order to study the non-LTE effects across the parameter space. Accurate and consistent stellar parameters are derived for these. Non-LTE ionization equilibria of several elements—typically N i/ii, O i/ii, Mg i/ii, and S ii/iii—agree simultaneously, provided that a realistic treatment of line blocking is used. These parameters also constitute important input data for further studies of the stellar winds of these objects. Accounting for non-LTE reduces the random errors and removes systematic trends in the analyses. In particular, the improved treatment of electron collisions largely removes longstanding discrepancies in analyses of lines from different spin systems of a given ion. The computed non-LTE line profiles fit the observations well for the different species at a given elemental abundance. In the parameter range covered, all lines from He i, C i/ii, N i/ii, O i/ii, and S ii/iii are significantly

Radiation pressure and mass outflows from hot stars and quasars

We consider the radiation pressure effects in the most luminous astrophysical objects, stellar winds of the O-B stars and the broad-absorption-line (BAL) quasistellar object (quasar) (QSO) matter outflows. Although the objects have different physical natures, there are many similarities in the behaviours of their gas envelopes in the strong radiation fields. We discuss both smooth and structured stellar wind models, and the dynamic and kinetic instability of the winds. We consider also the dynamics of the absorbing gas in the BAL QSO, the nature of the line-locking effects and the sources of the outflowing gas.

A VLBA Search for a Stimulated Recombination Line from the Accretion Region in NGC 1275

The radio source 3C 84 in NGC 1275 has a two-sided structure on parsec scales. The northern feature, presumed to be associated with a jet moving away from Earth, shows strong evidence for free-free absorption. The ionized gas responsible for that absorption would be a source of detectable stimulated recombination line emission for a wide range of physical conditions. The VLBA has been used to search for the H65α hydrogen recombination line. The line is expected to be seen only against the northern feature, which contains a small fraction of the total radio flux density. This spatial discrimination significantly aids the search for a weak line. No line was seen, with upper limits of roughly 15% of the continuum over a velocity range of 1486 km s-1 with resolutions up to 6.6 km s-1. In the absence of a strong radiation field, this would imply that the free-free absorbing gas has a wide velocity width, is moving rapidly relative to the systemic velocity, or is concentrated in a thin high-density structure. All these possibilities are reasonably likely close to an active galactic nucleus (AGN). However, in the intense radiation environment of the AGN, even considering only the radiation we actually observe passing through the free-free absorbing gas, the nondetection is probably ensured by a combination of saturation and radiation damping.

High-resolution X-ray spectroscopy ofτ Scorpii (B0.2V) withXMM-Newton

We report the analysis of the first high-resolution X-ray spectrum of the B0.2V star τ Scorpii obtained with the Reflection Grating Spectrometers ( )a nd the-  spectrometers on board XMM-Newton. The spectrum exhibits bright emission lines of the H- and He-like ions of C, N, O, Ne, Mg, and Si, as well as Fe  &F e lines. Line fluxes have been determined. Simultaneous fits to the  and spectra were used to obtain four plasma temperatures, emission measures, and the overall elemental abundances. This multi-temperature fitting yielded temperatures of 1.6, 5.2, 8.2, and >20 MK. These temperatures are confirmed by DEM modelling. The nitrogen lines are relatively strong: the N/O abundance ratio is about 3× solar. No indication of a solar-type FIP effect was found for the other elements. According to the derived models the X-ray luminosity in the energy range 0.3-10 keV is Lx =3.2 ×10 31 erg s −1 at a distance of 132 pc. The sensitivity of the He-like forbidden and intercombination lines to a strong ultraviolet radiation field is used to derive upper limits to the radial distances at which lines of Mg  ,N e ,O  ,a nd N originate. The results suggest that the soft X-rays (<8 MK) originate from shocks low in the wind that are produced by the common mechanism of radiation line-driven instabilities. This is consistent with the observed emission line profiles that are much narrower (<500 km s −1 ) than the broad lines (up to 1500 km s −1 ) observed high up in the wind of ζ Puppis. The hot (∼20-40 MK) component may be explained by a model involving dense clumps embedded in a wind which sweeps past them at high relative velocity (∼1400-1700 km s −1 ). Such an interaction would produce the strong

A method of detecting capture -rays in a strong pulsed radiation field

We present a technology for diagnosing the D-T fusion process by detecting capture γ-rays. This technology provides an alternative route to diagnosing the D-T reaction process when a great deal of heavy Z materials surrounds the D-T region. A very important aspect of this paper is to focus on the methods of shielding low-energy γ-rays whose radiation intensity is 106 times higher than that of the capture γ-rays. Another aspect is about how to distinguish signal from noise. The result of a 50/1 signal-to-noise ratio indicates that the designed double-magnetic spectrograph is very successful for diagnosing the D-T fusion reaction process.

On the theory of the relativistic cross-sections for stimulated bremsstrahlung on an arbitrary electrostatic potential in the strong electromagnetic field

On the base of relativistic generalized eikonal approximation wave function the multiphoton cross-sections of a Dirac particle bremsstrahlung on an arbitrary electrostatic potential and strong laser radiation field are presented. In the limit of the Born approximation the ultimate analytical formulas for arbitrary polarization of electromagnetic wave have been obtained and numerically analyzed.

Physical Properties of Giant Molecular Clouds in Nearby Starburst Galaxies

Physical Properties of Giant Molecular Clouds in Nearby Starburst Galaxies

Single-photon double ionization of an atom in a strong radiation field

Double ionization is studied by direct numerical integration of the time-dependent Schrodinger equation for a model two-electron system in the field of an electromagnetic wave in the case when the photon energy exceeds the energy required for the removal of both electrons of the atom. The probabilities of single-electron and double ionization are obtained as functions of the radiation intensity. The energy spectra and double pulse distributions of photoelectrons are analyzed. It is shown that single-photon ionization plays a significant role only in the limiting cases of weak and strong fields. The obtained results are used in an analysis of the contributions from different channels to the double-ionization process (in particular, for clarifying the role of the electron-electron interaction). The results of numerical calculations are compared with the analytic model of the phenomenon.

The nuclear bulge of the Galaxy

We analyse IRAS and COBE DIRBE data at wavelengths between 2.2 and 240 of the central 500 pc of the Galaxy and derive the large-scale distribution of stars and interstellar matter in the Nuclear Bulge. Models of the Galactic Disk and Bulge are developed in order to correctly decompose the total surface brightness maps of the inner Galaxy and to apply proper extinction corrections. The Nuclear Bulge appears as a distinct, massive disk-like complex of stars and molecular clouds which is, on a large scale, symmetric with respect to the Galactic Centre. It is distinguished from the Galactic Bulge by its flat disk-like morphology, very high density of stars and molecular gas, and ongoing star formation. The Nuclear Bulge consists of an R-2 Nuclear Stellar Cluster at the centre, a large Nuclear Stellar Disk with radius pc and scale height pc, and the Nuclear Molecular Disk of same size. The total stellar mass and luminosity of the Nuclear Bulge are and , respectively. About 70% of the luminosity is due to optical and UV radiation from young massive Main-Sequence stars which are most abundant in the Nuclear Stellar Cluster. For the first time, we derive a photometric mass distribution for the central 500 pc of the Galaxy which is fully consistent with the kinematic mass distribution. We find that the often cited R-2 distribution holds only for the central ~30 pc and that at larger radii the mass distribution is dominated by the Nuclear Stellar Disk which has a flatter density profile. The total interstellar hydrogen mass in the Nuclear Bulge is , distributed in a warm inner disk with pc and a massive, cold outer torus which contains more than 80% of this mass. Interstellar matter in the Nuclear Bulge is very clumpy with ~90% of the mass contained in dense and massive molecular clouds with a volume filling factor of only a few per cent. This extreme clumpiness, probably caused by the tidal stability limit in the gravitational potential of the Nuclear Bulge, enables the strong interstellar radiation field to penetrate the entire Nuclear Bulge and explains the relatively low average extinction towards the Galactic Centre. In addition, we find of cold and dense material outside the Nuclear Bulge at positive longitudes and at negative longitudes. This material is not heated by the stars in the Nuclear Bulge and gives rise to the observed asymmetry in the distribution of interstellar matter in the Central Molecular Zone.

Far Ultraviolet Spectroscopic ExplorerObservations of the Blue Compact Dwarf Galaxy Markarian 59

New Far Ultraviolet Spectroscopic Explorer (FUSE) far-UV spectroscopy of the nearby metal-deficient (Z☉/8) cometary blue compact dwarf (BCD) galaxy Markarian (Mrk) 59 is discussed. The data are used to investigate element abundances in its interstellar medium. The H I absorption lines are characterized by narrow cores that are interstellar in origin and by broad wings that are stellar in origin. The mean interstellar H I column density is ~7 × 1020 cm-2 in Mrk 59. No H2 lines are seen and N(H2) is 1015 cm-2 at the 10 σ level. The lack of diffuse H2 is due to the combined effect of a strong UV radiation field that destroys the H2 molecules and a low metallicity that leads to a scarcity of dust grains necessary for H2 formation. P Cygni profiles of the S VI λλ933.4, 944.5 and O VI λλ1031.9, 1037.6 lines are seen, indicating the presence of very hot O stars and a stellar wind terminal velocity of ~1000 km s-1. By fitting the line profiles with multiple components having each a velocity dispersion b = 7 km s-1 and spanning a radial velocity range of 100 km s-1, some of which can be saturated, we derive heavy element abundances in the neutral gas. We find log N(O I)/N(H I) = -5.0 ± 0.3 or [O I/H I] = -1.5 for the neutral gas, about a factor of 10 below the oxygen abundance of the supergiant H II region, implying self-enrichment of the latter.

(Sub)millimetre emission from NGC 1569: An abundance of very small grains

We present new data of the dwarf galaxy NGC 1569 at 450 $\mu$m, 850 $\mu$m and 1200$\mu$m taken with SCUBA at the JCMT and the bolometer array at the IRAM 30m telescope. After including data from IRAS at 12, 25, 60 and 100 $\mu$m, we have successfully fitted the dust grain population model of D\'esert et al. (1990) to the observed midinfrared-to-millimeter spectrum. The fit requires a combination of both large and very small grains exposed to a strong radiation field as well as an enhancement of the number of very small grains relative to the number of large grains. We interpret this as the consequence of large grain destruction due to shocks in the turbulent interstellar medium of NGC 1569. The contribution of polyaromatic hydrocarbons (PAH's) is found to be negligible. Comparison of the dust emission maps with an HI map of similar resolution shows that both dust and molecular gas distributions peak close to the radio continuum maximum and at a minimum in the HI distribution. From a comparison of these three maps and assuming that the gas-to-dust mass ratio is the same everywhere, we estimate the ratio of molecular hydrogen column density to integrated CO intensity to be about 25 -- 30 times the local Galactic value. The gas-to-dust ratio is 1500 -- 2900, about an order of magnitude higher than in the Solar Neighbourhood.

FOAM COMPOSITIONS FOR IMPROVING RADIATION CONDITIONS DURING DECOMMISSIONING OF NUCLEAR OBJECTS

It is suggested that foam compositions containing a film-producing material be used to increase worker safety and decrease aerosol emissions during disassembly of contaminated equipment and decommissioning of nuclear objects. Confining films make it possible to simplify the technology for depositing protective coatings in the presence of strong radiation fields and in inaccessible locations. The mechanisms of the formation of confining films on stainless steel from foam based on polyvinyl alcohol and OP-10 preparation are determined and the isolating properties for contamination of surfaces by fission products are estimated.

Detection of the unusual 3.5 μm feature in the Herbig Be star MWC 297

We present spectroscopic observations of MWC 297 with medium spectral resolution in the 2.1-4.1 μm region, that show the unusual emission band at 3.53 μm as well as gaseous emission lines of the Brackett, Pfund and Humphrey series of hydrogen. A unique aspect of the measured 3.53 μm emission band is the fact that, within our detection limit, it is not accompanied by the 3.3 and 3.4 μm emission band. We suggest that the 3.53 μm feature could be emitted by highly dehydrogenized carbonaceous dust particles that were processed under the influence of the strong radiation field of the central star.

ITAL]Chandra[/ITAL] Detection of Doppler-shifted X-Ray Line Profiles from the Wind of ζ Puppis (O4 [CLC]f[/CLC

We report on a 67 ks High-Energy Transmission Grating observation of the optically brightest early O star z Puppis (O4 f). Many resolved X-ray lines are seen in the spectra over a wavelength range of 5–25 A u . Chandra has sufficient spectral resolution to study the velocity structure of isolated X-ray line profiles and to distinguish the individual forbidden, intercombination, and resonance (fir) emission lines in several He-like ions, even where the individual components are strongly Doppler-broadened. In contrast to X-ray line profiles in other hot stars, z Pup shows blueshifted and skewed line profiles, providing the clearest and most direct evidence that the Xray sources are embedded in the stellar wind. The broader the line, the greater the blueward centroid shift tends to be. The N vii line at 24.78 A u is a special case, showing a flat-topped profile. This indicates that it is formed in regions beyond most of the wind attenuation. The sensitivity of the He-like ion fir lines to a strong UV radiation field is used to derive the radial distances at which lines of S xv ,S ixiii ,M gxi ,N eix, and O vii originate. The formation radii correspond well with a continuum optical depth of unity at the wavelength of each line complex, indicating that the X-ray line emission is distributed throughout the stellar wind. However, the S xv emission lines form deeper in the wind than expected from standard wind-shock models. Subject headings: line: profiles — stars: early-type — stars: individual ( z Puppis) — stars: mass loss — stars: winds, outflows — X-rays: stars

Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains

We present a method for calculating the infrared emission from a population of dust grains heated by starlight, including very small grains for which stochastic heating by starlight photons results in high transients. Because state-to-state transition rates are generally unavailable for complex molecules, we consider model PAH, graphitic, and silicate grains with realistic vibrational mode spectra and realistic radiative properties. The vibrational density of states is used in a statistical-mechanical description of the emission process. Unlike previous treatments, our approach fully incorporates multiphoton heating effects, important for large grains or strong radiation fields. We discuss how the temperature of the grain is related to its vibrational energy. By comparing with an exact statistical calculation of the emission process, we determine the conditions under which the thermal and the continuous cooling approximations can be used to calculate the emission spectrum. We present results for the infrared emission spectra of PAH grains of various sizes heated by starlight. We show how the relative strengths of the 6.2, 7.7, and 11.3um features depend on grain size, starlight spectrum and intensity, and grain charging conditions. We show results for grains in the cold neutral medium, warm ionized medium, and representative conditions in photodissociation regions. Our model results are compared to observed ratios of emission features for reflection nebulae and photodissociation regions, the Milky Way, normal spiral galaxies, and starburst galaxies.

Non-LTE line formation for MgI/II: Abundances and stellar parameters

An extensive model atom for non-LTE line-formation calculations for neutral and singly-ionized magnesium is presented, taking into account recent improvements in the atomic data. As a test and first application of the model, stellar parameters and magnesium abundances are determined on the basis of line-blanketed LTE model atmospheres for three stars: Vega (A0 V), η Leo (A0 Ib) and HD 92207 (A0 Iae) using high and high resolution spectra at visual and near-IR wavelengths. The ionization equilibrium of Mgi/ii proves to be a sensible temperature indicator for early A-type stars at all luminosities. Evidence is given that in late A and early F-type supergiants ( K) the determination of accurate stellar parameters is hampered by the presence of a pressure inversion region in the model atmospheres at line-formation depths. The Mgi/ii lines in the observations are reproduced simultaneously by the calculated line profiles with high accuracy. For Vega spectral synthesis in the UV region of the Mgi/ii resonance lines also proves excellent consistency with the results from the visual. The dependence of the non-LTE effects on the atmospheric parameters is discussed with special emphasis on the supergiants where a strong radiation field at low particle densities favours deviations from LTE, especially in the minor ionic species of neutral magnesium. Non-LTE corrections turn out to be small in Mgi -typically 0.3 dex -even in supergiants, but are essential for an accurate effective temperature determination. From the Mgii spectrum, only the features at 4481 and 7877-96 Å react sensitively to non-LTE effects. Furthermore, the influence of microturbulence on the statistical-equilibrium calculations is investigated. The line strengths are found to be systematically affected.

ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

In front of the LMC molecular hydrogen is found in absorption near 0 km s -1 , being local disk gas, near +60 km s -1 in an intermediate velocity cloud, and near +120 km s -1 , being a high velocity halo cloud. The nature of the gas is discussed based on four Orfeus far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only $2.5^\circ$, rather fluffy. The fractional abundance of H 2 varies from $\log f=\log [N({\rm H}_2)/(2\cdot N({\rm H}_2)+N(\rm{\ion{H}{i}}))]=-5.4$ to -3.3. Metal depletions (up to -1.7 dex for Fe) are typical for galactic disk gas. In the intermediate and high velocity gas an apparent underabundance of neutral oxygen points to an ionization level of the gas of about 90% . H 2 is detected in intermediate and high velocity gas towards HD 269546. In the intermediate velocity gas we find an H 2 column density of $\log(N)\simeq15.6$. The H 2 excitation indicates that the line of sight samples a cloud at a temperature below 150 K. Column densities are too small to detect the higher UV pumped excitation levels. The high velocity H 2 ($\log(N)\simeq15.6$) is highly excited and probably exposed to a strong radiation field. Its excitation temperature exceeds 1000 K. Due to the radial velocity difference between the halo gas and the Milky Way disk, the unattenuated disk radiation is available for H 2 excitation in the halo. We do not find evidence for an intergalactic origin of this gas; a galactic as well as a Magellanic Cloud origin is possible.

Properties of the Photodissociated Gas in NGC 6334

To investigate the properties of photodissociated gas in strong radiation fields, the massive star formation region NGC 6334 has been mapped with the Far-Infrared Imaging Fabry-Perot Interferometer (FIFI) instrument in the far-infrared fine-structure transitions [C II] 158 μm, [O I] 146 μm, and [O I] 63 μm. Bright [C II] 158 μm emission is found throughout the cloud, whereas the [O I] 146 μm emission is associated only with the star-forming ridge. Comparison of the relative intensities of the lines with single-component photodissociation region models suggests densities of nH ~ 104 cm-3. The models imply unphysically large radiation fields for three sources, particularly for NGC 6334A, which is probably caused by self-absorption in the [O I] 63 μm line. An alternative method for estimating physical conditions, based on the correlation between [C II] 158 μm and CO line intensities, is explored. This method implies hydrogen column densities of NH 1022-23 cm-2, which agree well with those from molecular excitation models. The relative distribution of the [C II] 158 μm and [O I] 146 μm emission supports clumpy photodissociation region models that suggest the [C II] 158 μm emission arises from interclump gas and thus should be more extended than the [O I] 146 μm emission that arises from the denser clumps. The spatial coincidence of [C II] 158 μm emission peaks with minima in the molecular gas emission indicates that at least some of the molecular holes contain photodissociated gas. The relative pressures of the ionized, photodissociated, and molecular gas are compared: the photodissociated and molecular gas are in approximate pressure equilibrium, but the ionized gas is overpressurized by at least an order of magnitude at the continuum sources.