By radiation transfer models we show that the optical properties of grains are poorly constrained by observations of reflection nebulae. The interstellar medium is known to be hierarchically clumped from a variety of observations (molecules, H I, far-infrared). We have performed radiative transfer through four-tiered hierarchically clumped dust in a sphere surrounding a central star. Our models have realistic power spectra of the projected density distributions (index ∼ −3). The input parameters are the albedo (a) and phase parameter (g) of the dust, the radial optical depth of the sphere averaged over all directions (τ0), and the detailed random distribution of the dust clumps within the sphere. The outputs are the stellar extinction, optical depth, and flux of scattered light as seen from various viewing angles. Observations of a reflection nebula provide the extinction and scattered flux as viewed from one particular direction. Hierarchical geometry has a large effect on the flux of scattered light emerging from a nebula for a particular extinction of the exciting star. There is a very large spread in both scattered fluxes and stellar extinctions for any distribution of dust. Consequently, an observed (τext, τsca) can be fitted by a wide range of albedos. There are lower limits on a set by the scattered flux. As an example, in the best observed reflection nebula, NGC7023, a(1300 Å) must be larger than ∼ 0.5 if the scattered flux from Witt et al (1993) and a reasonable value for the optical depth within the nebula are adopted. However, the same observations can be fitted with a = 0.8 and 0.6 ≤ g ≤ 0.85, the entire range we considered. With hierarchical geometry it is not completely safe to determine even relative optical constants from multiwavelength observations of the same reflection nebula. The problem is that the geometry effectively changes with wavelength as the opacity of the clumps varies. Limits on the implications of observing the same object in various wavelengths are discussed briefly. Henry (2002) uses a recipe to determine the scattered flux from a star with a given extinction. It is claimed to be independent of the geometry. It provides

We investigate the diffuse absolute calibration of the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (SST) at 8.0 mu m using a sample of 43 H II regions with a wide range of morphologies near l = 312 degrees. For each region we carefully measure sky-subtracted, point-source-subtracted, areally integrated IRAC 8.0-mu m fluxes and compare these with Midcourse Space eXperiment (MSX) 8.3-mu m images at two different spatial resolutions, and with radio continuum maps. We determine an accurate median ratio of IRAC 8.0-mu m/MSX 8.3-mu m fluxes, of 1.55 +/- 0.15. From robust spectral energy distributions of these regions we conclude that the present 8.0-mu m diffuse calibration of the SST is 36 per cent too high compared with the MSX validated calibration, perhaps due to scattered light inside the camera. This is an independent confirmation of the result derived for the diffuse calibration of IRAC by the Spitzer Science Centre (SSC).From regression analyses we find that 843-MHz radio fluxes of H II regions and mid-infrared (MIR) fluxes are linearly related for MSX at 8.3 mu m and Spitzer at 8.0 mu m, confirming the earlier MSX result by Cohen & Green. The median ratio of MIR/843-MHz diffuse continuum fluxes is 600x smaller in non-thermal than thermal regions, making it a sharp discriminant. The ratios are largely independent of morphology up to a size of similar to 24 arcmin. We provide homogeneous radio and MIR morphologies for all sources. MIR morphology is not uniquely related to radio structure. Compact regions may have MIR filaments and/or diffuse haloes, perhaps infrared counterparts to weakly ionized radio haloes found around compact H II regions. We offer two IRAC colour-colour plots as quantitative diagnostics of diffuse H II regions.

We have compared Monte Carlo photoionization models of H II regions with a uniform density distribution with models with the same central stars and chemical compositions but with a three-dimensional hierarchical density distribution consisting of clumps within clumps, on a four-tier scheme. The purpose is to compare the abundances of He, N, O, Ne and S obtained by standard analyses (emission-line strengths and measured mean temperatures from [O III] and [N II]) with the abundances in our models. We consider stellar temperatures in the range 37.5-45 kK and ionizing photon luminosities from 10(48) to 10(51) s(-1).Clumped models have different ionic abundances than uniform. For hot stars, (He-0/H+) is 2-3 per cent, much larger than with uniform models. This amount of He-0 is independent of metallicity and so impacts the determination of the primordial abundance of He. The total abundances of O, Ne and S obtained by the usual methods of analysis, using T([O III]) for high stages of ionization and T([N II]) for low, are about as accurate for clumped models as for uniform, and within similar to 20 per cent of the true values. If T([O III]) is used for analysing all ions, the derived (O/H) is similar to 40-60 per cent too large for cool stars but is good for hot stars. Uniform models have similar errors, so the clumping does not change the accuracy of abundance analysis.The physical causes of the ionic abundance errors are present in real nebulae. In clumped models, helium ionizing radiation from zones of high ionization (low He-0 and low ultraviolet opacity) can penetrate nearby regions near the edge of the ionized zone. This effect allows He-0 to absorb more stellar photons than in uniform or radially symmetrical geometries. In turn, these absorptions compete with O+, etc., for those energetic stellar photons.

We present extinction curves that include data down to far- ultraviolet wavelengths ( FUV; 1050 - 1200 angstrom) for nine Galactic sight lines. The FUVextinction was measured using data from the Far Ultraviolet Spectroscopic Explorer. The sight lines were chosen for their unusual extinction properties in the infrared through the ultraviolet; that they probe a wide range of dust environments is evidenced by the large spread in their measured ratios of total to selective extinction, R-V = 2: 43 3: 81. We find that extrapolation of the Fitzpatrick & Massa relationship from the ultraviolet appears to be a good predictor of the FUVextinction behavior. We find that predictions of the FUVextinction based on the Cardelli, Clayton, & Mathis ( CCM) dependence on RV give mixed results. For the seven extinction curves well represented by CCM in the infrared through ultraviolet ( x < 8 mu m (-1)), the FUVextinction is well predicted in three sight lines, overpredicted in two sight lines, and underpredicted in two sight lines. A maximum entropy method analysis using a simple three- component grain model shows that seven of the nine sight lines in the study require a larger fraction of grain materials to be in dust when FUVextinction is included in the models. Most of the added grain material is in the form of small ( radii <= 200 angstrom) grains.

We apply our three-dimensional photoionization code to model Wisconsin H alpha Mapper observations of the H II region surrounding the O9.5 V star zeta Oph. Our models investigate the porosity of the interstellar medium around zeta Oph and the effects of three-dimensional densities on the H alpha surface brightness and variation in the [ N II] lambda 6583/ H alpha line ratio. The zeta Oph H II region has a well-characterized ionizing source, so it is an excellent starting point for three-dimensional models of diffuse ionized gas. We investigate various hierarchically clumped density structures, varying the overall smoothness within the clumping algorithm. By simulating the observations, we estimate the porosity of the medium in the vicinity of zeta Oph and find that within the context of our hierarchically clumped models, around 50%-80% of the volume is occupied by clumps surrounded by a low-density smooth medium. We also conclude that in order for O stars to ionize the diffuse warm ionized medium, the O star environment must be more porous than that surrounding zeta Oph, with clumps occupying less than one-half of the interstellar volume. Our clumpy models have irregular boundaries, similar to observed H II regions. However, in observed H II regions, it is difficult to identify the precise location of the boundary because of the foreground and/or background emission from the widespread warm ionized medium. This complicates the interpretation of the predicted rapid rise of some emission-line ratios near the edge of uniform density H II regions and, combined with the three-dimensional clumpy nature of the interstellar medium, may explain the apparent lack of distinctive emission-line ratios near H I-H II interfaces.

We present a new planetary nebula, first identified in images from the Australia Telescope Compact Array, although not recognized at that time. Recent observations with the Spitzer Space Telescope during the GLIMPSE Legacy program have rediscovered the object. The high-resolution radio and infrared images enable the identification of the central star or its wind, the recognition of the radio emission as thermal, and the probable presence of polycyclic aromatic hydrocarbons in and around the source. These lead to the conclusion that G313.3+00.3 is a planetary nebula. This object is of particular interest because it was discovered solely through radio and mid-infrared imaging, without any optical ( or near-infrared) confirmation, and acts as a proof of concept for the discovery of many more highly extinguished planetary nebulae. G313.3+00.3 is well resolved by both the instruments with which it was identified and suffers extreme reddening due to its location in the Scutum-Crux spiral arm.

We present an extinction analysis of nine reddened/comparison star pairs in the Large and Small Magellanic Clouds (LMC and SMC) based on Far-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Magellanic Clouds using spectral data for wavelengths as short as 950 Å. We supplement these two with data from four regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinct characters of extinction in the Clouds and Milky Way, our results are generally analogous to those found for Galactic curves—namely, that the FUSE portions of each extinction curve are described reasonably well by Fitzpatrick & Massa curves fitted only to longer wavelength data and lack any dramatic new extinction features, and any deviations from the Cardelli, Clayton, & Mathis (CCM) formalism continue into FUV wavelengths. A maximum entropy method analysis of all of these curves suggests that LMCAvg and SMC wing sight lines, whose extinction parameters more closely resemble those for Galactic paths, require more silicon and/or carbon in dust than current abundance measurements would indicate are available. The requirements for LMC2 and SMC bar sight lines do not fully tax the available reservoirs, in part because large grains contribute less to the extinction in these directions. An intermediate product of this extinction analysis is the measurement of new H2 abundances in the Magellanic Clouds. Collectively considering Cloud sight lines that possess significant H2 column densities, E(B - V)/N(H ) ratios are reduced by significant factors relative to the Galactic mean, whereas the corresponding E(B - V)/N(H2) values more closely resemble their Galactic counterpart. These trends reflect the fact that among these sight lines f(H2) values are lower than those common in the Milky Way for paths with similar degrees of reddening.

We have developed a three-dimensional Monte Carlo photoionization code tailored for the study of Galactic HII regions and the percolation of ionizing photons in diffuse ionized gas. We describe the code, our calculation of photoionization, heating and cooling, and the approximations we have employed for the low-density HII regions we wish to study. Our code gives results in agreement with the Lexington HII region benchmarks. We show an example of a two-dimensional shadowed region and point out the very significant effect that diffuse radiation produced by recombinations of He has on the temperature within the shadow.

We demonstrate that the observed increase of some nebular line ratios with height above the midplane in the diffuse ionized gas (DIG) in the Milky Way and other galaxies is a natural consequence of the progressive hardening of the radiation field far from the midplane ionizing sources. To obtain increasing temperatures and line ratios away from the midplane, our photoionization simulations of a multicomponent interstellar medium do not require as much additional heating (over and above that from photoionization) as previous studies that employed one-dimensional, spherically averaged models. Radiation leaking into the DIG from density bounded H II regions is generally harder in the H-ionizing continuum and has its He-ionizing photons suppressed compared to the ionizing source of the H it region. In line with other recent investigations, we find that such leaky H II region models can provide elevated temperatures and line ratios, and a lower He+ fraction in the DIG. For a composite model representing the relative spectral types of O stars in the solar neighbourhood, we find that the natural hardening of the radiation field reaching large heights in our simulations can explain most of the observed line ratios. However, additional heating is required to reproduce the largest line ratios in the DIG.

The luminous, massive star formation region RCW 49, located in the southern Galactic plane, was imaged with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope as part of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) program. The IRAC bands contain polycyclic aromatic hydrocarbon (PAH) features at 3.3, 6.2, 7.7, and 8.6 mum, as well as the Bralpha line. These features are the major contributors to the diffuse emission from RCW 49 in the IRAC bands. The Spitzer IRAC images show that the dust in RCW 49 is distributed in a network of fine filaments, pillars, knots, sharply defined boundaries, bubbles, and bow shocks. The regions immediately surrounding the ionizing star cluster and W-R stars are evacuated of dust by stellar winds and radiation. The IRAC images of RCW 49 suggest that the dust in RCW 49 has been sculpted by the winds and radiation from the embedded luminous stars in the inner 5' (inner similar to6 pc) of the nebula. At projected angular radii phi>5' from the central ionizing cluster, the azimuthally averaged infrared intensity falls off as similar tophi(-3). Both high-resolution radio and mid-IR images suggest that the nebula is density bounded along its western boundary. The filamentary structure of the dust in RCW 49 suggests that the nebula has a small dust filling factor and, as a consequence, the entire nebula may be slightly density bounded to H-ionizing photons.

The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire ( GLIMPSE), a Space Infrared Telescope Facility ( SIRTF) Legacy Science Program, will be a fully sampled, confusion-limited infrared survey of 2/3 of the inner Galactic disk with a pixel resolution of similar to1."2 using the Infrared Array Camera at 3.6, 4.5, 5.8, and 8.0 mum. The survey will cover Galactic latitudes \b\ less than or equal to 1degrees and longitudes \l\ = 10degrees- 65degrees (both sides of the Galactic center). The survey area contains the outer ends of the Galactic bar, the Galactic molecular ring, and the inner spiral arms. The GLIMPSE team will process these data to produce a point-source catalog, a point-source data archive, and a set of mosaicked images. We summarize our observing strategy, give details of our data products, and summarize some of the principal science questions that will be addressed using GLIMPSE data. Up-to-date documentation, survey progress, and information on complementary data sets are available on the GLIMPSE Web site.

By radiation transfer models we show that the optical properties of grains are poorly constrained by observations of reflection nebulae. The interstellar medium is known to be hierarchically clumped, from a variety of observations (molecules, H I, far-infrared). We have performed radiative transfer through four-tiered, hierarchically clumped dust in a sphere surrounding a central star. Our models have realistic power spectra of the projected density distributions (index ~-3). The input parameters are the albedo (a) and phase parameter (g) of the dust, the radial optical depth of the sphere averaged over all directions (τ0), and the detailed random distribution of the dust clumps within the sphere. The outputs are the stellar extinction, optical depth, and flux of scattered light as seen from various viewing angles. Observations of a reflection nebula provide the extinction and scattered flux as viewed from one particular direction. Hierarchical geometry has a large effect on the flux of scattered light emerging from a nebula for a particular extinction of the exciting star. There is a very large spread in both scattered fluxes and stellar extinctions for any distribution of dust. Consequently, an observed (τext, τsca) can be fitted by a wide range of albedos. There are lower limits on a set by the scattered flux. As an example, in the best-observed reflection nebula, NGC 7023, a(1300 Å) must be higher than ~0.5 if the scattered flux from Witt et al. and a reasonable value for the optical depth within the nebula are adopted. However, the same observations can be fitted with a = 0.8 and 0.6 ≤ g ≤ 0.85, the entire range we considered. With hierarchical geometry it is not completely safe to determine even relative optical constants from multiwavelength observations of the same reflection nebula. The problem is that the geometry effectively changes with wavelength as the opacity of the clumps varies. Limits on the implications of observing the same object in various wavelengths are discussed briefly. Henry uses a recipe to determine the scattered flux from a star with a given extinction. It is claimed to be independent of the geometry. It provides considerably more scattering for given dust optical properties than our models, probably leading to an underestimate of the grain albedos from the UV diffuse Galactic light.

Sight lines through high- and intermediate-velocity clouds allow measurements of ionic gas-phase abundances A at very low values of H I column density N(H I). Present observations cover over 4 orders of magnitude in N(H I). Remarkably, for several ions we find that the A versus N(H I) relation is the same at high and low column densities and that the abundances have a relatively low dispersion (factors of 2-3) at any particular N(H I). Halo gas tends to have slightly higher values of A than disk gas at the same N(H I), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for Na I; using Na I as a predictor of N(H I) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the interstellar medium are (1) because of clumping, over sufficiently long path lengths N(H I) is a reasonable measure of the local density of most of the H atoms along the sight line; (2) the destruction of grains does not mainly take place in catastrophic events such as strong shocks but is a continuous function of the mean density; (3) the cycling of the ions becoming attached to grains and being detached must be rapid, and the two rates must be roughly equal under a wide variety of conditions; and (4) in gas that has a low average density the attachment should occur within denser concentrations.

There are now observations of several emission lines from the "warm ionized medium" (WIM or, equivalently, the "diffuse ionized gas") of the local interstellar medium, from the Perseus arm in the Milky Way, and also in several other galaxies. Interesting features of these observations include the great strength of [N (II)] lambda 6563 (similar toH alpha in some cases) and the fact that CS nl lambda 6717/[N (II)] lambda 6583 is almost the same (similar to0.6-0.7) in all locations and objects. Other line ratios (e.g., [O (II)] lambda 5007/H beta) vary considerably.This paper presents simple photoionization models that reproduce the observed spectra, providing extra heating beyond that supplied by photoionization is assumed. The same extra heating was used for models of all stellar temperatures being combined together, although it could easily depend on T*.With observed gas-phase abundances (not solar), the line ratios in the Local arm at b = 0 degrees are fitted with no extra heating and S/H = 13 ppm, as opposed to solar (similar to 20 ppm). Local gas observed at b = -35 degrees requires extra heating of about Gamma (-25)=0.75, where Gamma (-25) is the extra heating in units of 10(-25) ergs H-1 s(-1). In the Perseus arm there are similar results: little extra heating is required at \z\ = 500 pc, and Gamma (-25) = 3.0 is needed at \z\ = 1.2 kpc. To fit the observations, the gas-phase composition in the Perseus arm must be reduced as required by the Galactic abundance gradient observed for H (II) regions. The requirements for NGC 891 (the best observed other galaxy at \z\ = 1 kpc and 2 kpc are similar to the Perseus arm: little or no extra heating near the plane (1 kpc in this case) and Gamma (-25) similar to 3 at \z\ = 2 kpc. In NGC 891 there is also an increase of lambda 5007/H alpha with \z\ that can only come about if most of the ionizing radiation is supplied by very hot stars (type O4: T* similar to 50,000 K). Either their radiation must propagate from the plane to high \z\ through very little intervening matter, or else the stars are located at high \z\. The total power requirement of the extra heating is less than or similar to 15% of the power to photoionize the WIM without extra heating.Extra heating enhances [O (II)] lambda 3727/H beta. Figure 1 shows that there is a spread in the predicted values, but the ratio can serve as a useful diagnostic of extra heating. The [S (II)] lambda lambda 9065, 9531 lines (see Fig. 2) are not useful in diagnosing extra heating.

Interstellar grains may be composite collections of particles of distinct materials, including voids agglomerated together. We determine the various optical cross sections of such composite grains, given the optical properties of each constituent, using an approximate model of the composite grain. We assume it consists of many concentric spherical layers of the various materials, each with a specified volume fraction. In such a case the usual Mie theory can be generalized and the extinction, scattering, and other cross sections determined exactly. We find that the ordering of the materials in the layering makes some difference to the derived cross sections, but averaging over the various permutations of the order of the materials provides rapid convergence as the number of shells (each of which is filled by all of the materials proportionately to their volume fractions) is increased. Three shells, each with one layer of a particular constituent material, give a very satisfactory estimate of the average cross section produced by larger numbers of shells. We give the formulae for the Rayleigh limit (small size parameter) for multilayered spheres and use it to propose an "effective medium theory" (EMT), in which an average optical constant is taken to represent the ensemble of materials. Multilayered models are used to compare the accuracies of several EMTs already in the literature. EMTs are worse for predicting scattering cross sections than extinction, and considerably worse for predicting g, the mean cosine of the angle of scattering. However, the angular distribution of the scattered radiation depends sensitively on the assumed grain geometry and should be taken with caution for any grain theory. Our computation is vastly simpler than discrete multipole calculations and may be easily applied for practical modeling of the extinction and scattering properties of interstellar grains.

We present results from the analysis of Hubble Space Telescope Faint Object Spectrograph (FOS), WFPC1, IUE, and Cerro Tololo Inter-American Observatory 4 m observations of the morphology, physical conditions, and chemical abundances in the anomalous H II region N88A in the Small Magellanic Cloud. Not only is N88A unusual among SMC H II regions because it contains much dust, it also is found to have a high electron density and complex ionization structure. The derived reddening curve for the nebula is flatter in the UV than the general SMC extinction curve, suggesting the absence of small grains. A detailed abundance analysis, using both empirical emission-line diagnostics and photoionization model comparisons, indicates that carbon and silicon are enriched in the nebula, while He, O, N, Ne, and Ar are similar in abundance to other SMC H II regions. There is no evidence for the large fluctuations in temperature or density that are seen to occur in some comparably dense planetary nebulae, so we believe that our rather traditional analyses are well founded. We conclude that SMC N88A is a very young H II region forming out of a small dusty molecular cloud that is being disrupted by star formation in a larger OB association in the area. The high carbon and silicon abundances in N88A are attributed to photoevaporation of dust grains existing in the molecular cloud material—rather than being ejected from recent stellar mass loss from stars in N88A itself.