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Far-ultraviolet scattering by dust in Orion
We have modelled diffuse far-ultraviolet (FUV) spectrum observed by theFar Ultraviolet Spectroscopic Explorer (FUSE) near M42 as the scatteringof the starlight from the Trapezium stars by dust in front of thenebula. The dust grains are known to be anomalous in Orion withRV= 5.5 and these are the first measurements of the FUVoptical properties of the grains outside of `normal' Milky Way dust. Wefind an albedo varying from 0.3 +/- 0.1 at 912 Å to 0.5 +/- 0.2 at1020 Åwhich is consistent with theoretical predictions.
| Rocket and Far Ultraviolet Spectroscopic Explorer Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflectionnebula IC 405 obtained by a rocket-borne long-slit spectrograph and theFar Ultraviolet Spectroscopic Explorer (FUSE). Both data sets show arise in the ratio of the nebular surface brightness to stellar flux(S/F*) of approximately 2 orders of magnitude toward the blueend of the far-UV bandpass. Scattering models using simple dustgeometries fail to reproduce the observed S/F* for realisticgrain properties. The high spectral resolution of the FUSE data revealsa rich fluorescent molecular hydrogen spectrum ~1000" north of the starthat is clearly distinguished from the steady blue continuum. TheS/F* remains roughly constant at all nebular pointings,showing that fluorescent molecular hydrogen is not the dominant cause ofthe blue rise. We discuss three possible mechanisms for the ``bluedust'': differential extinction of the dominant star (HD 34078), unusualdust-grain properties, and emission from nebular dust. We conclude thatuncertainties in the nebular geometry and the degree of dust clumpingare most likely responsible for the blue rise. As an interestingconsequence of this result, we consider how IC 405 would appear in aspatially unresolved observation. If IC 405 were observed with a spatialresolution of less than 0.4 pc, for example, an observer would infer afar-UV flux that was 2.5 times the true value, giving the appearance ofa stellar continuum that was less extinguished than radiation from thesurrounding nebula, an effect that is reminiscent of the observedultraviolet properties of starburst galaxies.
| Scattering by Interstellar Dust Grains. I. Optical and Ultraviolet
Scattering and absorption properties at optical and ultravioletwavelengths are calculated for an interstellar dust model consisting ofcarbonaceous grains and amorphous silicate grains. Polarization as afunction of scattering angle is calculated for selected wavelengths fromthe infrared to the vacuum ultraviolet. The widely usedHenyey-Greenstein phase function provides a good approximation for thescattering phase function at wavelengths between ~0.4 and 1 μm butfails to fit the calculated phase functions at shorter and longerwavelengths. A new analytic phase function is presented. It is exact atlong wavelengths and provides a good fit to the numerically calculatedphase function for λ>0.27 μm. Observational determinationsof the scattering albedo and show considerabledisagreement, especially in the ultraviolet. Possible reasons for thisare discussed.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Interstellar Dust Grains
This review surveys the observed properties of interstellar dust grains:the wavelength-dependent extinction of starlight, including absorptionfeatures, from UV to infrared; optical luminescence; infrared emission;microwave emission; optical, UV, and X-ray scattering by dust; andpolarization of starlight and of infrared emission. The relationshipbetween presolar grains in meteorites and the interstellar grainpopulation is discussed. Candidate grain materials and abundanceconstraints are considered. A dust model consisting of amorphoussilicate grains, graphite grains, and polycyclic aromatic hydrocarbonsis compared with observed emission and scattering. Some issuesconcerning evolution of interstellar dust are discussed.
| Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023
The reflection nebula NGC 2023 was observed by a rocket-borne long-slitimaging spectrograph in the 900-1400 Å bandpass on 2000 February11. A spectrum of the star, as well as that of the nebular scatteredlight, was recorded. Through the use of a Monte Carlo modeling process,the scattering properties of the dust were derived. The albedo is low,0.2-0.4, and decreasing toward shorter wavelengths, while the phasefunction asymmetry parameter is consistent with highlyforward-scattering grains, g~0.85. The decrease in albedo, while theoptical depth increases to shorter wavelengths, implies that the far-UVrise in the extinction curve is due to an increase in absorptionefficiency.
| The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component that reveals its presence byemitting a broad unstructured band of light in the 540-950 nm wavelengthrange, referred to as extended red emission (ERE). The presence ofinterstellar dust and ultraviolet photons are two necessary conditionsfor ERE to occur. This is the basis for suggestions that attribute EREto an interstellar dust component capable of photoluminescence. In thisstudy, we have collected all published ERE observations withabsolute-calibrated spectra for interstellar environments, where thedensity of ultraviolet photons can be estimated reliably. In each case,we determined the band-integrated ERE intensity, the wavelength of peakemission in the ERE band, and the efficiency with which absorbedultraviolet photons are contributing to the ERE. The data show thatradiation is not only driving the ERE, as expected for aphotoluminescence process, but is modifying the ERE carrier, asmanifested by a systematic increase in the ERE band's peak wavelengthand a general decrease in the photon conversion efficiency withincreasing densities of the prevailing exciting radiation. The overallspectral characteristics of the ERE and the observed high quantumefficiency of the ERE process are currently best matched by the recentlyproposed silicon nanoparticle (SNP) model. Using the experimentallyestablished fact that ionization of semiconductor nanoparticles quenchestheir photoluminescence, we proceeded to test the SNP model bydeveloping a quantitative model for the excitation and ionizationequilibrium of SNPs under interstellar conditions for a wide range ofradiation field densities. With a single adjustable parameter, the crosssection for photoionization, the model reproduces the observations ofERE intensity and ERE efficiency remarkably well. The assumption thatabout 50% of the ERE carriers are neutral under radiation conditionsencountered in the diffuse interstellar medium leads to a prediction ofthe single-photon ionization cross section of SNPs with average diameter3.5 nm of <=3.4×10-15 cm2. The shift ofthe ERE band's peak wavelength toward larger values with increasingradiation density requires a change of the size distribution of theactively luminescing ERE carriers through a gradual removal of thesmaller particles by size-dependent photofragmentation. We propose thatheat-assisted Coulomb decay of metastable, multiply charged SNPs is sucha process, which selectively removes the smaller components of anexisting SNP size distribution.
| The UV spectrum of nebulae
This paper presents an analysis of the UV spectrum of some nebulae withclearly identified illuminating stars, all observed by the IUEsatellite.The data show remarkable properties of the UV spectrum of thenebulae. Each spectrum is the product of the star spectrum and a linearfunction of 1/lambda. There is no peculiar behaviour in the spectrums at2200 A: no bump created in the spectrum of a nebula and no excess ofscattering. When moving away from the star, the surface brightness of anebula decreases as the inverse of the square of the angular distance tothe star.These results can logically be interpreted in terms ofscattering of starlight. They imply constant properties of theinterstellar grains in the UV and in the directions of space sampled bythe nebulae, and probably a strong forward scattering phase function.There is no evidence for any particular type of grain which wouldspecifically extinguish starlight at 2200 A. Concerning the UV spectrumof a star, this may imply a revisal of the traditional interpretation ofthe 2200 A bump.
| Extended Red Emission from Carbon Clusters in Interstellar Clouds
We have simulated extended red emission (ERE) spectra using a model inwhich this emission arises as photoluminescence from small carbonparticles of mixed sp^2/sp^3 hybridized bonding characteristics. Theemission efficiency from such particles can be highly efficient whentheir size is such that geminate recombination of photoexcited electronhole pairs is enhanced. The amplitude and emission profile of the EREemission from the diffuse interstellar medium and nebular sources suchas NGC 2327 and NGC 7027 can be reproduced with a range of averageparticle size and size distribution. These carbon particles arecomponents of the same mixtures that yield an accurate fit to theinterstellar 2175 Å extinction peak and suggest that ERE emission,the 2175 Å absorption, and infrared absorption at 3.4 μm mayarise from the same carbonaceous components of interstellar matter.
| Looking for Distributed Star Formation in L1630: A Near-Infrared (J, H, K) Survey
We have carried out a simultaneous, multiband (J, H, K) survey over anarea of 1320 arcmin2 in the L1630 region, concentrating on the regionaway from the dense molecular cores with modest visual extinctions(<=10 mag). Previous studies by Lada and coworkers showed that starformation in L1630 occurs mainly in four localized clusters, which inturn are associated with the four most massive molecular cores. The goalof this study is to look for a distributed population ofpre--main-sequence stars in the outlying areas outside the knownstar-forming cores. More than 60% of the pre--main-sequence stars in theactive star-forming regions of NGC 2024 and NGC 2023 show anear-infrared excess in the color-color diagram. In the outlying areasof L1630, excluding the known star-forming regions, we found that among510 infrared sources with the near-infrared colors (J-H and H-K)determined and photometric uncertainty at K better than 0.10 mag, thefraction of the sources with a near-infrared excess is 3%--8%; thesurface density of the sources with a near-infrared excess is less thanone-seventh of that found in the distributed population in L1641, and1/20 of that in the young cluster NGC 2023. This extremely low fractionand low surface density of sources with a near-infrared excess stronglyindicates that recent star formation activity has been very low in theoutlying region of L1630. The sources without a near-infrared excesscould be either background/foreground field stars or associated with thecloud, but formed a long time ago (more than 2 Myr). Our results areconsistent with McKee's model of photoionization-regulated starformation.
| Radiative Transfer Analysis of Far-Ultraviolet Background Observations Obtained with the Far Ultraviolet Space Telescope
In 1992, the Far Ultraviolet Space Telescope provided measurements ofthe ultraviolet (140--180 nm) diffuse sky background at high, medium,and low Galactic latitudes. A significant fraction of the detectedradiation was found to be of Galactic origin, resulting from scatteringby dust in the diffuse interstellar medium. To simulate the radiativetransfer in the Galaxy, we employed a Monte Carlo model that utilized arealistic, nonisotropic radiation field based on the measured fluxes (at156 nm) and positions of 58,000 TD-1 stars, and a cloud structure forthe interstellar medium. The comparison of the model predictions withthe observations led to a separation of the Galactic scattered radiationfrom an approximately constant background, attributed to airglow andextragalactic radiation, and to a well-constrained determination of thedust scattering properties. The derived dust albedo a = 0.45 +/- 0.05 issubstantially lower than albedos derived for dust in dense reflectionnebulae and star-forming regions, while the phase function asymmetry g =0.68 +/- 0.10 is indicative of a strongly forward-directed phasefunction. We show the highly nonisotropic phase function to beresponsible, in conjunction with the nonisotropic UV radiation field,for the wide range of observed correlations between the diffuselyscattered Galactic radiation and the column densities of neutral atomichydrogen. The low dust albedo is attributed to a size distribution ofgrains in the diffuse medium with average sizes smaller than those indense reflection nebulae.
| Exploring southern nebulae.
Not Available
| Dust scattering in radio galaxies.
We describe a model of dust scattering and polarization of anisotropicnuclear radiation in the UV-optical range, which we have developedhaving in mind the AGN unification scheme. The model is applied tospectrophotometric and spectropolarimetric data of radio galaxies withthe aim of understanding the dust properties, the scattering geometryand the stellar ages, which are left as free parameters. The goal is toconfirm and develop the unified scheme for radio loud AGN and to getclues on the evolution of dust and stars in active galaxies.
| A Survey of Near-Infrared Emission in Visual Reflection Nebulae
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..102..369S&db_key=AST
| Scattering Properties of the Dust in the Reflection Nebula IC 435
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...446L..97C&db_key=AST
| The origin of variations in the 2175 A extinction bump
In this paper, small graphite grains are assumed to produce the 2175 Ainterstellar extinction bump. The principal observational characteristicof the bump in the invariance of its central wavelength (2174 +/- 9 A)among various lines of sight through diverse environments. On the otherhand, its Full Width at Half Maximum (FWHM) varies from about0.80/micrometer to 1.2/micrometer among the same lines of sight. In thispaper, the variations in bump width among different sight lines isattributed to coatings on the graphite grains. The necessary propertiesof the coating material are investigated. Simple neutral polycycliaromatic hydrocarbons (PAHs) have measured laboratory absorptionsremarkably similar to those required to explain the broadening of thebump. Only 1.5% of cosmic carbon in PAHs is required to produce verybroad bumps. Alternatively, about 2%-4% of the combined abundances ofthe refractory elements Fe and Mg, more highly depleted onto grains indense regions than in the diffuse interstellar medium, could explain themantles if their absorption rises rapidly with energy, as it does forMgO. Coatings of amorphous carbon, hydrogenated or not, diamond-likebonded carbon, or water ice do not have published optical constants thatvary rapidly enough in wavenumber to produce the variations.
| Scattering and polarization of light by rough and porous interstellar grains
Interaction of light with rough or porous interstellar grains isstudied. Scattering diagrams and polarization curves are presented forseveral roughness and porosity rates. Obsidian and glassy carbonparticles are considered for size parameters either much smaller than orof the same order as the wavelength of the incident light. In the lastcase scattering diagrams and polarization curves are drasticallyaffected by roughness and porosity. Implications on the structure ofsome reflection nebulae are discussed. Also, the results suggest theexistence of interstellar grains larger than usually accepted.
| The albedo and scattering phase function of interstellar dust and the diffuse background at far-ultraviolet wavelengths
The diffuse FUV background in a variety of targets has been observedwith a nebular spectrometer designed specifically to eliminate potentialsources of contamination. The intensity of the background near theGalactic plane is found to be much lower than previously published. Thedata are analyzed using a radiative transfer model with a limited set offree parameters, including those which describe the scatteringproperties of the interstellar dust. Strong evidence is presented thatscattering of starlight by Galactic dust associated with neutralhydrogen produces the major part of the FUV diffuse background. Fewassumptions are necessary to conclude that the albedo of the grains islow in the FUV. The grains, at least at high latitude, scatter fairlyisotropically in the FUV. The low FUV albedo and the usual phase factorg are not consistent with predictions of standard models for the sizedistribution and composition of interstellar grains.
| The excitation of 12 micron emission from very small particles
The results of IRAS observations of visual reflection nebulaeilluminated by stars with effective temperatures between 3000 and 33,000K are presented. The ratio of the energy radiated in the IRAS 12-micronsband to the total energy radiated at far-infrared wavelengths (60 and100 microns) is about 0.2, with little or no dependence on thetemperature of the illuminating star for stellar temperatures betweenabout 5000 and 33,000 K. This is interpreted as requiring that visualphotons as well as ultraviolet photons are capable of exciting12-microns emission from very small particles.
| Spectroscopy of extended red emission in reflection nebulae
This paper reports the results of a spectroscopic survey of reflectionnebulae, aimed at studying the characteristics of the broad emissionfeature responsible for the extended red emission (ERE) observed inthese objects. The ERE band was detected and its strength, centralwavelength, and width were measured in 12 nebulae, while upper limitswere recorded in 12 further objects. The maximum ERE band intensity wasfound to vary from object to object within an extremely narrow range,while the associated scattered light intensities differed by nearly twoorders of magnitude. The ERE band is interpreted as arising from thephotoluminescence of hydrogenated amorphous carbon (HAC) grains whichbecome rehydrogenated and gain luminescence efficiency in narrow H2photodissociation zones. These zones are probably thin shells around theexciting stars. Observed changes in the central wavelengths and widthsof the ERE bands detected in different parts of a given nebula, and fromnebula to nebula, support the HAC model and are explained as arisingfrom variations in the degree of hydrogenation to the solid grains.
| UV Scattering Properties of Dust Grains in the Reflection Nebula IC 435
Not Available
| I.A.U. Symp. 135 - Interstellar Dust - Mountain-View - 1988JUL26
Not Available
| Interstellar dust and the IUE
Understanding of the physical nature of interstellar dust, as modifiedby observations of extinction obtained with IUE, is considered.Properties of extinction laws along various lines of sight, withemphasis on the bump, are reviewed. Correlations of the UV andoptical/near infrared extinction are discussed. Other diagnostics ofinterstellar dust, such as the near infrared emission bands,polarization, far infrared thermal emission, and depletions arementioned, and theories are summarized.
| CCD surface photometry of bright reflection nebulae
Surface brightness measurements in the B, V, R, and I photometric bandsare presented for 14 reflection nebulae. The analysis of nebula-starcolor differences leads to the conclusion that excess emission in the Iband beyond that expected from scattering is a common phenomenon amongreflection nebulae illuminated by B stars. An ultraviolet-poweredfluorescence mechanism is suggested. Both the absolute and the relativeV surface brightnesses of the nebulae in the sample are analyzed. Thedata can be explained, if the nebulae arise in moderately denseinterstellar clouds with illuminating stars embedded at an optical depthlevel of order unity and with dust of high albedo and with a stronglyforward-directed phase function. It is concluded that bright reflectionnebulae must arise under almost optimal scattering conditions, whichapparently are found when newly formed low-mass star clusters are stillembedded in the material from which they originated.
| Light scattering in the IUE telescope
The scattered light intensity measured in May 1986 in the IUE telescopenear eta UMa over radial distances between 15" and 120" is presented.The scattering profile of the IUE follows a power law with exponent -2.1for r greater than 40" and is steeper between 10" and 40". The fractionof starlight scattered decreases smoothly by a factor 3 from 1300 to2900A. About 10% of the target light is scattered to outside the IUElarge aperture and also 10% is scattered over the 40 pixels inside thelarge aperture.
| Reflection nebulae observed with the IUE (review)
The IUE measurements of scattering of starlight by interstellar dustnear Zeta Ori are reviewed. The wavelength dependence of the dust albedoand phase function are derived. A phase-function value between 0.1 and0.8 is found, with a value near 0.6 at visual wavelengths. This meansthat all dust appears to be forward scattering. The albedo appears to besmaller towards shorter wavelengths than in the visual.
| Detection of scattering in the 2175 A interstellar band
Spectrophotometric observations of two reflection nebulae and theirilluminating stars obtained with the International Ultraviolet ExplorerSatellite have provided the first evidence for the presence of ascattering contribution to the 2175 A interstellar extinction band.Lower than normal far-UV extinction for the stars embedded in thenebulae indicates that the nebulae have a dust particle sizedistribution that is dominated by larger particles. The strength of the2175 A band is larger than normal in both cases. The scattering is foundto dominate the long-wavelength wing of the band, without shifting thecentral wavelength of the band by more than 20 A toward longerwavelengths. These observations are taken to indicate that the solidparticles responsible for the 2175 A band can be considerably largerthan the Rayleigh limit in some interstellar locations. The absence of anotable shift in the central wavelength of the band in such largeparticles presents a new severe constraint for models of interstellargrains.
| The large system of molecular clouds in Orion and Monoceros
Emission is noted over about one-eighth of an 850-sq deg region centeredon Orion and Monoceros that has been surveyed in the J = 1 to 0 line ofCO; most of the emission arises from giant molecular clouds associatedwith Orion A and B, and Mon R2. A much smaller area was surveyed forC-13O emission. A comparison of cloud masses obtained by threeindependent methods indicates that CO luminosity is as accurate ameasure of cloud mass as other indicators. The possible relationshipsamong clouds in the survey are discussed, including the conjecture thatthe overall Orion complex of clouds is a much larger system thanpreviously considered, incorporating most of the clouds in the presentsurvey.
| Neutral hydrogen in isolated galaxies. IV - Results for the Arecibo sample
A standard sample for the comparison of the H I content of galaxies invarious intergalactic environments is presently defined by means ofobservations of 324 isolated galaxies lying in the declination rangeaccessible to the Arecibo 305-m telescope. Both mapping and single pointspectra are used to compute the integral properties of these galaxies.Neutral hydrogen was detected in 288 of the 324 galaxies surveyed, andit is noted that the optical diameter of a spiral disk is bettercorrelated with the hydrogen mass than the morphological type. When usedto define a measure of H I content, the isolated galaxy sample canpredict 'normalcy' with an accuracy that carries a standard error ofabout 0.20 in the log of the H I mass, if a dependence on disk size, aswell as type, is taken into account.
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