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The diffuse high-latitude H-alpha background is widely believed to be predominantly the result of in-situ recombination of ionized hydrogen in the warm interstellar medium of the Galaxy. Instead, we show that both a substantial fraction of the diffuse high-latitude H-alpha intensity in regions dominated by Galactic cirrus dust and much of the variance in the high-latitude H-alpha background are the result of scattering by interstellar dust of H-alpha photons originating elsewhere in the Galaxy. We provide an empirical relation, which relates the expected scattered H-alpha intensity to the IRAS 100um diffuse background intensity, applicable to about 81% of the entire sky. The assumption commonly made in reductions of CMB observations, namely that the observed all-sky map of diffuse H-alpha light is a suitable template for Galactic free-free foreground emission, is found to be in need of reexamination., Comment: 26 pages, 5 figures, Accepted for publication in ApJ

The proto-planetary Red Rectangle nebula is powered by HD 44179, a spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the primary source of both luminosity and current mass loss. Here, we present the results of a seven-year, eight-orbit spectroscopic monitoring program of HD 44179, designed to uncover new information about the source of the Lyman/far-ultraviolet continuum in the system as well as the driving mechanism for the bipolar outflow producing the current nebula. Our observations of the H-alpha line profile around the orbital phase of superior conjunction reveal the secondary component to be the origin of the fast (max. v~560$ km s$^{-1}$) bipolar outflow in the Red Rectangle. The variation of total H-alpha flux from the central H II region with orbital phase also identifies the secondary or its surroundings as the source of the far-ultraviolet ionizing radiation in the system. The estimated mass of the secondary (~0.94 M$\sun$) and the speed of the outflow suggest that this component is a main sequence star and not a white dwarf, as previously suggested. We identify the source of the Lyman/far-ultraviolet continuum in the system as the hot, inner region (T$_{max} \ge 17,000$ K) of an accretion disk surrounding the secondary, fed by Roche lobe overflow from the post-AGB primary at a rate of about $2 - 5\times10^{-5}$ M$\sun$ yr$^{-1}$. The total luminosity of the accretion disk around the secondary is estimated to be at least 300 L$\sun$, about 5% of the luminosity of the entire system. (abridged), Comment: Accepted for publication in ApJ

We present narrow-band images of the Red Rectangle (RR) nebula which reveal the distinct morphologies of this intriguing nebula in different optical emission bands. The morphology of the RR nebula in blue luminescence (BL) and extended red emission (ERE) are almost mutually exclusive. We also present the optical detection of the circum-binary disk of the RR in the light of the BL. The total intensities from the two optical band emissions (BL and ERE) when summed over the nebula are of comparable magnitude. Their spatial distributions with respect to the embedded illumination sources lead us to suggest that they may be attributed to different ionization stages of the same family of carriers., Accepted to ApJ

The carrier of the dust-associated photoluminescence process causing the extended red emission (ERE) in many dusty interstellar environments remains unidentified . Several competing models are more or less able to match the observed broad, unstructured ERE band. We now constrain the character of the ERE carrier further by determining the wavelengths of the radiation that initia tes the ERE. Using the imaging capabilities of the Hubble Space Telescope, we have resolved the width of narrow ERE filaments appearing on the surfaces of externally illuminated molecular clouds in the bright refle ction nebula NGC 7023 and compared them with the depth of penetration of radiation of known wavelengths into the same cloud surfaces. We identify photons with wavelengths shortward of 118 nm as the source of ERE initiation, not to be confused with ERE excitation, however. There are strong indications from the well-studied ERE in the Red Rectangle nebula and in the high|b| Galactic cirrus that the photon flux with wavelengths shortw ard of 118 nm is too small to actually excite the observed ERE, even with 100% quantum efficiency. We conclude, therefore, that ERE excitation results from a two-step process. The first step, involving far-UV photons with E > 10.5 eV, leads to the creation of the ERE carrier, most likely through photo-ionization or photo-di ssociation of an existing precursor. The second step, involving more abundant near-UV/optical photons, consists of the optical pumping of the previously created carrier, followed by subsequent de-excitation via photoluminescence. The latter process can occur many times for a single particle, depending upon the lifetime of the ERE carrier in its active state. While none of the previously proposed ERE models can match these new constraints, we note that under interstellar conditions most polycyclic aromatic hydrocarbon (PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH di-cations is consistent with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations deserve further study as potential carriers of the ERE. Subject headings:dust, extinction ‐ ISM: individual (NGC 7023) ‐ ISM: lines and bands ‐ radiation mechanisms: nonthermal ‐ reflection nebulae

Blue Luminescence (BL) was first discovered in a proto-planetary nebula, the Red Rectangle (RR) surrounding the post-AGB star HD 44179. BL has been attributed to fluorescence by small, 3-4 ringed neutral polycyclic aromatic hydrocarbon (PAH) molecules, and was thought to be unique to the RR environment where such small molecules are actively being produced and shielded from the harsh interstellar radiation by a dense circumstellar disk. In this paper we present the BL spectrum detected in several ordinary reflection nebulae illuminated by stars having temperatures between 10,000 -- 23,000 K. All these nebulae are known to also exhibit the infrared emission features called aromatic emission features (AEFs) attributed to large PAHs. We present the spatial distribution of the BL in these nebulae. In the case of Ced~112, the BL is spatially correlated with mid-IR emission structures attributed to AEFs. These observations provide evidence for grain processing and possibly for in-situ formation of small grains and large molecules from larger aggregates. Most importantly, the detection of BL in these ordinary reflection nebulae suggests that the BL carrier is an ubiquitous component of the ISM and is not restricted to the particular environment of the RR.

Following our initial discovery of blue luminescence in the spectrum of the Red Rectangle (RR) and its identification as fluorescence by small three- to four-ringed polycyclic aromatic hydrocarbon (PAH) molecules, we report on the spatial correlation between the blue luminescence and the 3.3 micron emission, commonly attributed to small, neutral PAH molecules, and on the newly-derived UV/optical attenuation curve for the central source of the RR, HD 44179. Both results provide strong additional evidence for the presence of small PAH molecules with masses of less than 250 a.m.u. in the RR, which supports the attribution of the blue luminescence to fluorescence by the same molecules. We contrast the excellent spatial correlation of the two former emissions with the distinctly different spatial distribution of the extended red emission (ERE) and of the dust-scattered light within the RR. The UV/optical attenuation curve of the central star is unlike any interstellar extinction curve and is interpreted as resulting from circumstellar opacity alone. Major contributions to this opacity are absorptions in broad bands in the mid-UV, contributing to the electronic excitation of the luminescing PAH molecules, and a sharp ionization discontinuity near 7.5 eV in the far-UV, which places a sharp upper limit on the masses of the PAH molecules that are responsible for this absorption. The strength of the far-UV absorption leads to an abundance of the PAH molecules of 10^{-5} relative to hydrogen in the RR. Such small PAHs are perhaps unique to the environment in the RR, where they are shielded from harsh radiation by the dense circmstellar disk.

We present results of new Monte Carlo calculations made with the DIRTY code of radiative transfer of stellar and scattered radiation for a dusty giant late-type galaxy like the Milky Way, which illustrate the effect of the attenuation of stellar light by internal dust on the integrated photometry of the individual bulge and disk components. Here we focus on the behavior of the attenuation function, the color excess, and the fraction of light scattered or directly transmitted towards the outside observer as a function of the total amount of dust and the inclination of the galaxy, and the structure of the dusty interstellar medium (ISM) of the disk. We confirm that dust attenuation produces qualitatively and quantitatively different effects on the integrated photometry of bulge and disk, whatever the wavelength. In addition, we find that the structure of the dusty ISM affects more sensitively the observed magnitudes than the observed colors of both bulge and disk. Finally, we show that the contribution of the scattered radiation to the total monochromatic light received by the outside observer is significant, particularly at UV wavelengths, even for a two-phase, clumpy, dusty ISM. Thus understanding dust scattering properties is fundamental for the interpretation of extragalactic observations in the rest-frame UV., 62 pages, 28 eps-figures, 1 table, accepted for publication in ApJ Main Journal

Here we report our discovery of a band of blue luminescence (BL) in the Red Rectangle (RR) nebula. This enigmatic proto-planetary nebula is also one of the brightest known sources of extended red emission as well as of unidentified infra-red (UIR) band emissions. The spectrum of this newly discovered BL is most likely fluorescence from small neutral polycyclic aromatic hydrocarbon (PAH) molecules. PAH molecules are thought to be widely present in many interstellar and circumstellar environments in our galaxy as well as in other galaxies, and are considered likely carriers of the UIR-band emission. However, no specific PAH molecule has yet been identified in a source outside the solar system, as the set of mid-infra-red emission features attributed to these molecules between the wavelengths of 3.3 micron and 16.4 micron is largely insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs is more specific as to size, structure, and charge state of a PAH molecule. If the carriers of this near-UV/blue fluorescence are PAHs, they are most likely neutral PAH molecules consisting of 3-4 aromatic rings such as anthracene (C14H10) and pyrene (C16H10). These small PAHs would then be the largest molecules specifically identified in the interstellar medium., 4 pages, 4 figures, Accepted for publication in ApJL (LaTeX, uses emulateapj.sty)

We have obtained new STIS/HST spectra to search for structure in the ultraviolet interstellar extinction curve, with particular emphasis on a search for absorption features produced by polycyclic aromatic hydrocarbons (PAHs). The presence of these molecules in the interstellar medium has been postulated to explain the infrared emission features seen in the 3-13 $\mu$m spectra of numerous sources. UV spectra are uniquely capable of identifying specific PAH molecules. We obtained high S/N UV spectra of stars which are significantly more reddened than those observed in previous studies. These data put limits on the role of small (30-50 carbon atoms) PAHs in UV extinction and call for further observations to probe the role of larger PAHs. PAHs are of importance because of their ubiquity and high abundance inferred from the infrared data and also because they may link the molecular and dust phases of the interstellar medium. A presence or absence of ultraviolet absorption bands due to PAHs could be a definitive test of this hypothesis. We should be able to detect a 20 \AA wide feature down to a 3$\sigma$ limit of $\sim$0.02 A$_V$. No such absorption features are seen other than the well-known 2175 \AA bump.

We present our analysis of UV attenuation by internal dust of a large sample (N=906 galaxies) of Lyman Break Galaxies (LBGs). Using spectral energy distributions (SEDs) from the P\'EGASE galaxy spectral evolution model we apply dust attenuation corrections to the G-R colors using the Witt & Gordon (2000) models for radiative transfer in dusty galactic environments to arrive at the UV attenuation factors. We show that the dust in the LBGs exhibit SMC-like characteristics rather than MW-like, and that the dust geometry in these systems is most likely to be represented by a clumpy shell configuration. We show that the attenuation factor exhibits a pronounced dependence on the luminosity of the LBG, $a_{1600}\propto (L/L_\sun)^\alpha$, where $0.5\leq\alpha\leq1.5$. The exponent $\alpha$ depends on the initial parameters of the stellar population chosen to model the galaxies and the dust properties. We find that the luminosity weighted average attenuation factor is likely to be in the range from $5.7-18.5$, which is consistent with the upper limits to the star formation rate at $2, Comment: 17 pages, 12 figures, uses AASTEX, accepted for publication in the Astrophysical Journal

We obtained low-resolution, long-slit 5300--9100 A spectroscopy of NGC4826 (a nearby galaxy with an absorbing dust lane (DL) asymmetrically placed across its bulge, associated with several HII regions) with a slit encompassing its bulge, positioned across its nucleus. The wavelength-dependent effects of absorption and scattering by dust in the lane are evident when comparing the observed stellar SEDs of pairs of positions symmetrically located with respect to the nucleus, one on the DL side and one on the opposite side of the bulge, by assuming that the intrinsic ISRF is axi-symmetric. We analyzed these SED ratios through the multiple-scattering radiative transfer model of Witt and Gordon and we discovered strong residual Extended Red Emission (ERE) from a region of the DL within a distance of 13 arcsec from the nucleus, adjacent to a broad, bright HII region. ERE is an established phenomenon in the literature interpreted as originating from photoluminescence by nanometer-sized clusters, illuminated by UV/optical photons. The complex radial variation of the ERE band-integrated intensity and of the ERE-to-scattered light band-integrated intensity ratio with the optical depth of the model derived for the DL and with the strength and hardness of the illuminating ISRF is reproduced consistently through the theoretical interpretation of the photophysics of the ERE carrier by Smith and Witt. When examined within the context of ERE observations in a variety of Galactic dusty environments (e.g. the diffuse ISM, reflection nebulae, planetary nebulae and the Orion Nebula), we conclude that the ERE photon conversion efficiency in NGC4826 is as high as found elsewhere, but that the size of the actively luminescing nanoparticles there is about twice as large as those thought to exist in the Galactic diffuse ISM., Comment: 57 pages, 11 Postscript figures, accepted for publication in ApJ

Interstellar dust contains a component which reveals its presence by emitting a broad, unstructured band of light in the 540 to 950 nm wavelength range, referred to as Extended Red Emission (ERE). The presence of interstellar dust and ultraviolet photons are two necessary conditions for ERE to occur. This is the basis for suggestions which attribute ERE to an interstellar dust component capable of photoluminescence. In this study, we have collected all published ERE observations with absolute-calibrated spectra for interstellar environments, where the density of ultraviolet photons can be estimated reliably. In each case, we determined the band-integrated ERE intensity, the wavelength of peak emission in the ERE band, and the efficiency with which absorbed ultraviolet photons are contributing to the ERE. The data show that radiation is not only driving the ERE, as expected for a photoluminescence process, but is modifying the ERE carrier as manifested by a systematic increase in the ERE band's peak wavelength and a general decrease in the photon conversion efficiency with increasing densities of the prevailing exciting radiation. The overall spectral characteristics of the ERE and the observed high quantum efficiency of the ERE process are currently best matched by the recently proposed silicon nanoparticle (SNP) model. Using the experimentally established fact that ionization of semiconductor nanoparticles quenches their photoluminescence, we proceeded to test the SNP model by developing a quantitative model for the excitation and ionization equilibrium of SNPs under interstellar conditions for a wide range of radiation field densities., 42 p., incl. 8 fig. Accepted for publication by ApJ

We present the DIRTY radiative transfer model in this paper and a companion paper. This model computes the polarized radiative transfer of photons from arbitrary distributions of stars through arbitrary distributions of dust using Monte Carlo techniques. The dust re-emission is done self-consistently with the dust absorption and scattering and includes all three important emission paths: equilibrium thermal emission, non-equilibrium thermal emission, and the aromatic features emission. The algorithm used for the radiative transfer allows for the efficient computation of the appearance of a model system as seen from any viewing direction. We present a simple method for computing an upper limit on the output quantity uncertainties for Monte Carlo radiative transfer models which use the weighted photon approach., Comment: 8 pages, 3 figures, accepted to the ApJ

To examine a recently proposed hypothesis that silicon nanoparticles are the source of extended red emission (ERE) in the interstellar medium, we performed a detailed modeling of the mean Galactic extinction in the presence of silicon nanoparticles. For this goal we used the appropriate optical constants of nanosized Si, essentially different from those of bulk Si due to quantum confinement. It was found that a dust mixture of silicon nanoparticles, bare graphite grains, silicate core-organic refractory mantle grains and three-layer silicate-water ice-organic refractory grains works well in explaining the extinction and, in addition, results in the acceptable fractions of UV/visible photons absorbed by silicon nanoparticles: 0.071-0.081. Since these fractions barely agree with the fraction of UV/visible photons needed to excite the observed ERE, we conclude that the intrinsic photon conversion efficiency of the photoluminescence by silicon nanoparticles must be near 100%, if they are the source of the ERE., Comment: Latex2e, uses emulateapj.sty (included), multicol.sty, epsf.sty, 6 pages, 3 figures (8 Postscript files), accepted for publication in ApJ Letters, complete Postscript file is also available at http://physics.technion.ac.il/~zubko/eb.html#SNP1

In 1992 the Far-Ultraviolet Space Telescope (FAUST) provided measurements of the ultraviolet (140-180nm) diffuse sky background at high, medium, and low Galactic latitudes. A significant fraction of the detected radiation was found to be of Galactic origin, resulting from scattering by dust in the diffuse interstellar medium. To simulate the radiative transfer in the Galaxy, we employed a Monte Carlo model which utilized a realistic, non-isotropic radiation field based on the measured fluxes (at 156nm) and positions of 58,000 TD-1 stars, and a cloud structure for the interstellar medium. The comparison of the model predictions with the observations led to a separation of the Galactic scattered radiation from an approximately constant background, attributed to airglow and extragalactic radiation, and to a well constrained determination of the dust scattering properties. The derived dust albedo a = 0.45 ± 0.05 is substantially lower than albedos derived for dust in dense reflection nebulae and star-forming regions, while the phase function asymmetry g = 0.68 ± 0.10 is indicative of a strongly forward directed phase function. We show the highly non-isotropic phase function to be responsible, in conjunction with the non-isotropic UV radiation field, for the wide range of observed correlations between the diffusely scattered Galactic radiation and the column densities of neutral atomic hydrogen. The low dust albedo is attributed to a size distribution of grains in the diffuse medium with average sizes smaller than those in dense reflection nebulae.

Measures of the depletion of interstellar elements from the gas phase are usually derived by assuming that the general composition of the interstellar medium is identical to that of the Sun. A compilation of stellar composition data, including B stars as well as field F and G stars, however, calls this assumption into question. In this Letter we consider the impact on derived depletions if the reference abundances are derived from stars in the solar neighborhood rather than from the Sun, and we discuss the implications for current models of the interstellar dust. Using recent, accurate gas-phase column densities for ζ Ophiuchi, we show that the systematically lower depletions resulting from our revised cosmic abundances are in conflict with most dust models because insufficient quantities of raw materials are available to explain the observed extinction. A Kramers-Kronig analysis for the ζ Oph line of sight shows that the revised depletions are consistent with the required opacity of interstellar dust only if the density of the grain material is near 1 g cm-3, suggesting that the grain structure must be open (i.e., the grains must by fluffy, porous, or fractal in structure).

During NRL's Far Ultraviolet Cameras experiemnt on STS-39, four images of the giant reflection nebula encompassing the Upper Scorpius subgroup of the Sco OB2 association were obtained in two ultraviolet bandpasses with lambda(sub eff) = 1362 A and 1769 A. From these images and IUE and TD-1 stellar spectra, the ratio of nebular to stellar flux was calculated. The ratio ranged from 0.577 to 0.921 at 1362 A and 0.681 to 0.916 at 1769 A with the spread in the ratio arising mainly from uncertainties in the sky background. In order to analyze these images, a model utilizing Monte Carlo techniques to describe radiative transfer in a spherical nebula with asymmetrically distributed stars was developed by elaborating on previous work by Witt. This model was used to determine the range of albedos reproducing the observed nebular-to-stellar flux ratios while allowing the scattering phase function asymmetry to vary between 0.0 and 0.8. The resulting albedos were 0.47-0.70 at 1362 A and 0.55-0.72 at 1769 A.

New measurements of the reduction of the V-band surface brightness across the prominent dust feature in the galaxy NGC 4826 are compared with corresponding increases in the V-K' color within the context of radiative transfer models invoking both absorption and scattering. The K'-band surface brightness is found to be higher than expected from standard dust models. We interpret the difference as resulting from a high effective dust albedo at K', with a likely value in excess of 0.8, provided the near-IR extinction curve in NGC 4826 is identical to the Galactic one. The high effective albedo may result from scattering by dust with a maximum grain size at least twice as large as assumed by standard models, a conclusion already indirectly hinted at by recent studies of dust star-forming regions and reflection nebulae. At least part of the high effective albedo at K' may result from near-IR nonequilibrium continuum emission attributable to very small grains.

We report on new laboratory results which relate directly to the observation of strongly enhanced extended red emission (ERE) by interstellar dust in H2 photodissociation zones. The ERE has been attributed to photoluminescence by hydrogenated amorphous carbon (HAC). We are demonstrating that exposure to thermally dissociated atomic hydrogen will restore the photoluminescence efficiency of previously annealed HAC. Also, pure amorphous carbon (AC), not previously photoluminescent, can be induced to photoluminesce by exposure to atomic hydrogen. This conversion of AC into HAC is greatly enhanced by the presence of UV irradiation. The presence of dense, warm atomic hydrogen and a strong UV radiation field are characteristic environmental properties of H2 dissociation zones. Our results lend strong support to the HAC photoluminescence explanation for ERE.

We report the first observational determination of the nebular-to-stellar flux ratio out to wavelengths as short as 1000 A for the reflection nebula NGC 7023, using a combination of spectroscopic data from Voyager 2 and from the Hopkins Ultraviolet Telescope (HUT) on the Astro 1 mission. After accounting for contributions by H2 fluorescence to the nebular flux, the wavelength variation of the ratio of the residual dust scattered flux to that observable from the embedded star is consistent with a reduction of the dust albedo by 25 percent over the wavelength interval 1300-1000 A. This is most easily interpreted, if the particles responsible for the FUV rise in the extinction curve are low-albedo, not necessarily zero-albedo, grains. The direct proportionality between the nebular surface brightness, measured at 22 arcsec offset by HUT, and the nebular flux suggests that the phase function of scattering does not vary in the 2000-1000 A range.

We present and discuss models for the transfer of ultraviolet, visual, and near-infrared radiation within a variety of spherical geometries which are intended to approximate different types of dusty environments within galaxies. Among those objects that our models approximate are normal and starburst galaxies, active galactic nuclei and QSOs, colliding systems with a widely dispersed interstellar medium, and elliptical galaxies. Our mathematical technique is a complete spherically symmetric, three-dimensional Monte Carlo simulation which covers a wide range of dust optical depths for 10 photometric bands

We have completed a long-slit, spectrophotometric survey of the optical continuum emission of 20 planetary nebulae (PNs) covering a wide range of C/O ratios and have detected the broad, red emission band known as extended red emission (ERE) in seven of these objects. We have therefore added PNs to the growing list of astronomical objects which are known to emit ERE. In PNs where ERE is detected, the band is found to have an average integrated intensity of 1.9 × 10 -3 ergs/(cm 2 × s × sr) and to contribute approximately 15% of the flux in the continuum between 550 and 820 nm

The Red Rectangle nebula is the strongest known source of the so-called extended red emission (ERE), seen in this object in association with a series of as yet unidentified sharp emission features near 5800 A. New long-slit CCD spectroscopy throughout this biconical nebula discloses that the sharp features and ERE arise in spatially distinct environments, suggesting their respective origins in different emitters

Les observations spectroscopiques CCD du continuum optique de la nebuleuse planetaire NGC 7027 sont presentees. La distribution spatiale de la bande d'emission rouge etendue (ERE) attribuee a la photoluminescence des carbones amorphes hydrogenes est analysee. L'intensite moyenne integree de la bande ERE est determine.

Les observations spectroscopiques de 34 nebuleuses reflechissantes sont presentees. L'intensite de l'emission rouge etendue, detectee dans 19 de ces nebuleuses, ou sa limite superieure, pour celle ou elle n'a pas ete detectee, sont determinees. Les mecanimes de cette emission sont analyses.

New BVI color-difference measurements are reported for the reflection nebulae NGC 7023, NGC 2068, and CED 201. The data are obtained through BVI imaging with a CCD detector and are analyzed together with previously published measurements for the reflection nebula NGC 2023. All four nebulae are substantially redder in the (V, I) range than expected on the basis of dust scattering alone, a result of the presence of extended emission in the I band. The relative strength of the I excess reaches a maximum in nebular regions where the color in the (B, V) range is bluest. These facts are interpreted in terms of a model which explains the extended I emission as a luminescence process excited by UV radiation from the illuminating star(s). Model fits identify the wavelength region between 1800 and 2500 A of the illuminating star's spectrum as the source of the energy of excitation. It appears possible that the 2200 A band in the dust-extinction curve is a gate for the excitation energy, and it is likely that the I excess emission and the extended near-IR emission recently discovered in reflection nebulae are similar in nature and origin.

High-resolution photographs of the Orion Nebula have been obtained in UV bands centered near 1400, 1820, 2240 and 2620 by a 31-cm telescope mounted on an Astrobee sounding rocket. Reduction of the data by image processing, photometric calibration and error analysis steps was used to derive a matrix of values of specific intensity in absolute units. Data indicate that the Orion Nebula is primarily a reflection nebula in the UV, so that morphology is determined by the distribution of dust with respect to the illuminating stars. Ultraviolet color gradients for each color relative to the 1820-A image indicate reddening towards the outer parts of the nebula, as well as fairly symmetric dust scattering in the Theta 1 and Theta 2 Ori region. The color gradients also imply a monotonic change in scattering phase function asymmetry between 2620 and 1400 A. Total nebular flux in each bandpass is observed to be about 2.5 times the stellar flux from the Trapezium, indicating a high scattering efficiency.

New measurements of the ultraviolet surface brightness of the night sky in 71 fields in the galactic longitude range 65degree< or =l/sub i//sub i/< or =145degree are presented. The data were obtained with the Orbiting Astronomical Observatory (OAO-2) at nine wavelengths between 1500 A and 4200 A and have been corrected for the contributions due to zodiacal light and integrated starlight. The residual brightnesses were analyzed with radiative transfer models for the diffuse galactic light which incorporate a z-dependent source function. The results qualitatively confirm earlier findings for this wavelength region, yielding a wavelength dependent albedo of the interstellar grains of approximately ..cap alpha..=0.7 +- 0.1 longward of lambda3000, ..cap alpha..=0.35 +- 0.05 around the pronounced minimum near lambda2200, and ..cap alpha..=0.6 +- 0.05 at lambda1550. The true absorption nature of the bump in the interstellar extinction curve near lambda2200, as well as the increase of the albedo shortward of lambda2000 are thus reconfirmed. The phase function asymmetry factor is found to lie between g=0.6 and g=0.9 for the entire wavelength range, indicating the interstellar grains are strongly forward scattering. (AIP)

Surface-brightness measurements are presented that cover the region of Orion in nine intermediate-width bandpasses ranging from 4250 to 1550 A. The existence of an extended ultraviolet reflection nebulosity in this area is confirmed, and the characteristics of its spectrum and spatial distribution are derived. The observations are consistent with a model in which the dense molecular cloud complex in Orion is illuminated by the foreground Orion aggregate of early-type stars. The interpretation is complicated by the fact that foreground dust may contribute to the observed scattered light. The scattering particles in the cloud appear to exhibit a wavelength-dependent albedo similar to that found for interstellar grains in general, with a strong indication that the phase function changes to a less forward-scattering form in the ultraviolet.

The paper presents spectroscopic data which identify the source of the photometrically detected extended red emission in the reflection nebulae NGC 2023 and NGC 7023 as emission in the form of a broad luminescence band, reaching its peak in the 6600-6800 A wavelength region. This feature is identified as luminescence from hydrogenated amorphous carbon, excited by short-wavelength photons from the central illuminating stars. In NGC 2023 the luminescence is found to be notably enhanced in a filamentary shell structure surrounding HD 39703, which is spatially coincident with a region exhibiting fluorescent molecular hydrogen emission as well as 3.3-micron emission. From BVRI surface photometry of NGC 2023 and NGC 7023 it is concluded that in regions of enhanced luminescence, presumably regions containing hot atomic hydrogen, the luminescence band shifts toward shorter wavelengths, while in more remote molecular regions more luminescent energy emerges in the long-wavelength tail of the band. This observed behavior is consistent with laboratory results concerning luminescence of hydrogenated amorphous carbon under varying conditions. 27 references.

Using the photographic unsharp masking technique and image cancellation, the high-frequency surface brightness fluctuations in the reflection nebula NGC 2023 have been imaged over 406 arcsec x 510 arcsec field. Maps with 1 arcsec resolution, centered on the illuminating star HD 37903, are presented for the photometric B and R bands. A system of narrow filamentary structures observable only in the R band are identified as extended red emission (ERE) features. These features are not associated with any apparent density enhancements in the nebula. Arguments are presented which link the ERE filaments to regions within the nebula where photodissociation of molecular hydrogen occurs.

The study deals with the extensive nebulosity in the Trapezium region of Orion. An entrance slit was placed at the focal plane of rocketborne telescope, creating a long-slit spectrograph with a wavelength resolution of 22 A FWHM and a spatial resolution of 14 in. along the slit. Data reduction is outlined, the structure of the Orion Nebula is discussed, and radiative transfer considerations are presented.

IUE measurements have been obtained for the UV extinction characteristics of 20 reddened stars previously reported to have anomalously high or low A(4430)/E(B-V) ratios, in order to conduct a first statistical study of the correlations between these ratios and interstellar extinction features in the UV. A marginally significant positive correlation is noted between A(4430)/E(B-V) and the normalized strength of the 2175 A extinction jump, but no correlation is found with the total extinction at the wavelength and a marginally negative correlation is found with the extinction rise at 1250 A. The hypothesis that 4430 A diffuse absorption is produced by very small interstellar grains is not supported by the present observations.

A surface brightness study of the reflection nebula NGC 2023 covering the 3500-10,000 A wavelength region performed with uvby photoelectric photometry and BVRI imaging with a CCD detector is reported along with VRI photometry of a cluster of embedded red stars. The nebular radiation in the 3500-5500 A region is dust-scattered starlight originating in the star HD 37903. The embedded red stars are probably pre-main sequence stars. The nebular surface brightness in R and I exceeds that expected on the basis of a reasonable radiative transfer model by factors of two and more than three, respectively. The excess radiation is extended across the nebula in a manner similar to the scattered light. The extended red emission may be interpreted as the high-frequency extension of extended emission discovered by Sellgren, Werner, and Dinerstein (1983) in the 2-5 micron region in NGC 2023 and two other reflection nebulae.

New surface brightness measurements in the reflection nebulae associated with the Pleiades stars 17, 20, and 23 Tau have been carried out with the IUE satellite. Each nebula was observed at two angular offsets south of the respective illuminating star. At the larger offsets (40, 40, and 60 arcsec respectively) each nebula exhibits a flux distribution which rises much more steeply with decreasing wavelengths than the corresponding stellar spectrum. At the smaller offsets (20 arcsec) the nebular spectra are more similar to those of the stars and fail to show the steep rise at wavelengths less than 2200 A. Independent observations of interstellar absorption lines as well as theoretical arguments by several previous authors lead to a model wherein the reflection nebulosity near the Pleiades stars is caused by a thin sheet of interstellar matter in front of the stars, at an approximate distance of a few x 10 to the -2nd pc. In the context of such a model the nebular spectra obtained near 17, 20, and 23 Tau can be understood only if the phase-function asymmetry of scattering declines with decreasing wavelengths throughout the spectral range. This result indicates that the role of scattering in the UV by particles in the 10 to the -6th cm size range appears more important than is generally recognized by current models.

The separation of the observed reddening of HD 200775 into intrinsic and interstellar components is rediscussed in the light of new surface-brightness data for NGC 7023, the reflection nebula surrounding HD 200775. Appropriate correction for the nebular contribution to reported ultraviolet flux measurements of HD 200775 leads to new values due to dust reddening: E(B-V) sub D = 0.44, and due to intrinsic reddening: E(B-V) sub I = 0.13-0.26. The newly derived extinction curve for HD 200775 is characterized by an abnormally weak interstellar 2200 A absorption feature, and the deduced nebular brightness in the far-ultraviolet is consistent either with dust in NGC 7023 having a relatively high albedo (a approximately equal to 0.6) and a nearly isotropic phase function (g approximately equal to 0.2), or with low albedo dust (a approximately equal to 0.3) with a forward-throwing phase function (g approximately equal to 0.6).

Spectrophotometric observations of two reflection nebulae and their illuminating stars obtained with the International Ultraviolet Explorer Satellite have provided the first evidence for the presence of a scattering contribution to the 2175 A interstellar extinction band. Lower than normal far-UV extinction for the stars embedded in the nebulae indicates that the nebulae have a dust particle size distribution that is dominated by larger particles. The strength of the 2175 A band is larger than normal in both cases. The scattering is found to dominate the long-wavelength wing of the band, without shifting the central wavelength of the band by more than 20 A toward longer wavelengths. These observations are taken to indicate that the solid particles responsible for the 2175 A band can be considerably larger than the Rayleigh limit in some interstellar locations. The absence of a notable shift in the central wavelength of the band in such large particles presents a new severe constraint for models of interstellar grains.

High-dispersion IUE spectra have been obtained of HD 38087, a star associated with reflection nebulosity where 2175 A scattering has previously been observed. The presence of 2175 A in emission implies unusually large grains, an attempt was made to see how these unusual grains may have affected the depletions of gas-phase elements onto dust in the line of sight. Even though the observed scattering region constitutes only a fraction of the total column density of dust, it is expected that the present line-of-sight analysis provides useful information on the gas and dust in the scattering nebulosity. Somewhat larger overall depletions than normal are found, and it is found that the depletions of certain elements (manganese and zinc) are enhanced relative to the normal pattern of element-to-element depletions, suggesting that grain growth has occurred with some elements sticking to grains preferentially. The molecular fraction in the line of sight is low, in accord with similar lines of sight having low far-ultraviolet extinction and large depletions.

The subject of color differences between reflection nebulae and their illuminating stars is reexamined in the light of developments of observational techniques, permitting accurate surface-brightness photometry over an expanded spectral region from the UV to the IR. Color-color diagrams for reflection nebulae can yield useful information about the wavelength dependence of the scattering properties of nebular dust without excessive sensitivity to the specific nebular geometry or the presence of multiple scattering, resulting in considerable savings in computational efforts. As an illustration, the color-difference method was applied to existing data for the Merope nebula, covering the spectral region 1550-5500 A. Strong evidence for a monotonically changing phase function of scattering at wavelengths less than or equal to 3500 A is found. The result is interpreted in the context of a plausible geometry for the Merope environment as providing support for a bimodal size distribution of nebular dust grains.

Resonant fluorescence of molecular hydrogen has been observed in the reflection nebula IC 63 with the IUE satellite. Both the noncoherent resonant scattering and the dissociative continuum are shown to agree well with molecular theory. Extended red emission is also found in this nebula and may be physically related to the dissociation of H2.

The wavelength dependence of ultraviolet extinction in the Pleiades dust clouds has been determined from IUE observations of HD 23512, the brightest heavily reddened member of the Pleiades cluster. There is evidence for an anomalously weak absorption bump at 2200 A, followed by an extinction rise in the far ultraviolet with an essentially normal slope. A relatively weak absorption band at 2200 A and a weak diffuse absorption band at 4430 A seem to be common characteristics of dust present in dense clouds. Evidence is presented which suggests that the extinction characteristics found for HD 23512 are typical for a class of extinction curves observed in several cases in the Galaxy and in the LMC.

IUE observations of the reflection nebula NGC 7023 and the illuminating star, HD 200775, in the spectral range from 1300 to 3100 A are combined with ground-based measurements of the nebular brightness distribution at 3500, 4100, 4700, and 5500 A to determine the scattering properties of the nebular dust grains. Total nebular fluxes are derived from existing data obtained by satellite and ground-based observations. The relevance of far-IR fluxes from the nebula is considered, and models are derived that are applicable to NGC 7023 both with respect to the ratio of nebular to stellar fluxes and with respect to the nebular surface brightness distribution. It is shown that the average grain albedo in the UV is about 0.54 and that the albedo increases to a level of about 0.6 at 1400 A after reaching a minimum of approximately 0.4 near 2200 A. The results suggest that isotropically scattering particles of high albedo make a significant contribution to interstellar scattering in the far-UV.