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Observational Properties of Rotationally Excited Molecular Hydrogen in Translucent Lines of Sight
The Far Ultraviolet Spectroscopic Explorer (FUSE) has allowed precisedeterminations of the column densities of molecular hydrogen(H2) in Galactic lines of sight with a wide range ofpathlengths and extinction properties. However, survey studies of linesof sight with greater extinction have been mostly restricted to thelow-J states (lower total angular momentum) in which most molecularhydrogen is observed. This paper presents a survey of column densitiesfor the molecular hydrogen in states of greater rotational excitation (J>= 2) in Galactic lines of sight with log N(H2) >~ 20.This study is comprehensive through the highest excited state detectablein each line of sight. J = 5 is observed in every line of sight, and wedetect J = 7 in four lines of sight, J = 8 in one line of sight, andvibrationally excited H2 in two lines of sight. We comparedthe apparent b-values and velocity offsets of the higher-J statesrelative to the dominant low-J states and we found no evidence of anytrends that might provide insight into the formation of higher-JH2, although these results are the most affected by thelimits of the FUSE resolution. We also derive excitation temperaturesbased on the column densities of the different states. We confirm thatat least two distinct temperatures are necessary to adequately describethese lines of sight, and that more temperatures are probably necessary.Total H2 column density is known to be correlated with othermolecules; we explore if correlations vary as a function of J forseveral molecules, most importantly CH and CH+. Finally, webriefly discuss interpretations of selected lines of sight by comparingthem to models computed using the Meudon PDR code.
| A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
A study of gas-phase element abundances reported in the literature for17 different elements sampled over 243 sight lines in the local part ofour Galaxy reveals that the depletions into solid form (dust grains) areextremely well characterized by trends that employ only three kinds ofparameters. One is an index that describes the overall level ofdepletion applicable to the gas in any particular sight line, and theother two represent linear coefficients that describe how to derive eachelement's depletion from this sight-line parameter. The information fromthis study reveals the relative proportions of different elements thatare incorporated into dust at different stages of grain growth. Anextremely simple scheme is proposed for deriving the dust contents andmetallicities of absorption-line systems that are seen in the spectra ofdistant quasars or the optical afterglows of gamma-ray bursts. Contraryto presently accepted thinking, the elements sulfur and krypton appearto show measurable changes in their depletions as the general levels ofdepletions of other elements increase, although more data are needed toascertain whether or not these findings are truly compelling. Nitrogenappears to show no such increase. The incorporation of oxygen into solidform in the densest gas regions far exceeds the amounts that can takethe form of silicates or metallic oxides; this conclusion is based ondifferential measurements of depletion and thus is unaffected byuncertainties in the solar abundance reference scale.Based in large part on published observations from (1) the NASA/ESAHubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555, (2) theFar Ultraviolet Spectroscopic Explorer (FUSE) mission operated by JohnsHopkins University, supported by NASA contract NAS5-32985, and (3) TheCopernicus satellite, supported by NASA grant NAGW-77 to PrincetonUniversity.
| Molecular Hydrogen in the Far Ultraviolet Spectroscopic Explorer Translucent Lines of Sight: The Full Sample
We report total abundances and related parameters for the full sample ofthe Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogenin 38 translucent lines of sight. New results are presented for the"second half" of the survey involving 15 lines of sight to supplementdata for the first 23 lines of sight already published. We assess thecorrelations between molecular hydrogen and various extinctionparameters in the full sample, which covers a broader range ofconditions than the initial sample. In particular, we are now able toconfirm that many, but not all, lines of sight with shallow far-UVextinction curves and large values of the total-to-selective extinctionratio, RV = AV /E(B -V)—characteristic of larger than average dust grains—areassociated with particularly low hydrogen molecular fractions (f_H_2).In the lines of sight with large RV , there is in fact a widerange in molecular fractions, despite the expectation that the largergrains should lead to less H2 formation. However, we seespecific evidence that the molecular fractions in this sub-sample areinversely related to the estimated strength of the UV radiation fieldand thus the latter factor is more important in this regime. We haveprovided an update to previous values of the gas-to-dust ratio,N(Htot)/E(B - V), based on direct measurements ofN(H2) and N(H I). Although our value is nearly identical tothat found with Copernicus data, it extends the relationship by a factorof 2 in reddening. Finally, as the new lines of sight generally showlow-to-moderate molecular fractions, we still find little evidence forsingle monolithic "translucent clouds" with f_H_2 \sim 1.
| The Search for H- in Astrophysical Environments
The negative ion H- is widely understood to be important inmany astrophysical environments, including the atmospheres of late-typestars like the Sun. However, the ion has never been detectedspectroscopically outside the laboratory. A search for thefar-ultraviolet autodetaching transitions of H- ininterstellar and circumstellar matter seems to be the best hope fordirectly detecting this ion. We undertook a highly sensitive searchusing data from the FUSE instrument. We concentrated on two types ofsight lines: planetary nebulae, where model calculations suggest asufficient abundance of H- to be determined, and translucentclouds, where H- might form on dust grains as an intermediatestep in molecular hydrogen formation. Upper limits on H-abundances were set.
| The role of the charge state of PAHs in ultraviolet extinction
Aims: We explore the relation between the charge state of polycyclicaromatic hydrocarbons (PAHs) and the extinction curve morphology. Methods: We fit extinction curves with a dust model includingcore-mantle spherical particles of mixed chemical composition (silicatecore, sp2 and sp3 carbonaceous layers), and anadditional molecular component. We use exact methods to calculate theextinction due to classical particles and accurate computed absorptionspectra of PAHs in different charge states, for the contribution due tothe molecular component, along five different lines of sight. Results: A combination of classical dust particles and mixtures ofreal PAHs satisfactorily matches the observed interstellar extinctioncurves. Variations of the spectral properties of PAHs in differentcharge states produce changes consistent with the varying relativestrengths of the bump and non-linear far-UV rise.
| New Insights on Interstellar Gas-Phase Iron
In this paper we report on the gas-phase abundance of singly ionizediron (Fe II) for 51 lines of sight, using data from FUSE. Fe II columndensities are derived by measuring the equivalent widths of several UVabsorption lines and subsequently fitting those to a curve of growth.Our derivation of Fe II column densities and abundances creates thelargest sample of iron abundances in moderately to highly reddened linesof sight explored with FUSE, lines of sight that are on average morereddened than lines of sight in previous Copernicus studies. We presentthree major results. First, we observe the well-established correlationbetween iron depletion and and also find trendsbetween iron depletion and other line-of-sight parameters [e.g.,f(H2), EB-V, and AV], and we examinethe significance of these trends. Of note, a few of our lines of sightprobe larger densities than previously explored and we do not seesignificantly enhanced depletion effects. Second, we present twodetections of an extremely weak Fe II line at 1901.773 Å in thearchival STIS spectra of two lines of sight (HD 24534 and HD 93222). Wecompare these detections to the column densities derived through FUSEspectra and comment on the line's f-value and utility for future studiesof Fe II. Finally, we present strong anecdotal evidence that the Fe IIf-values derived empirically through FUSE data are more accurate thanprevious values that have been theoretically calculated, with theprobable exception of f1112.
| The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration Based on Synthetic He I Lines
We derive projected rotational velocities (v sin i) for a sample of 156Galactic OB star members of 35 clusters, H II regions, and associations.The He I lines at 4026, 4388, and 4471 Å were analyzed in order todefine a calibration of the synthetic He I full widths at half-maximum(FWHMs) versus stellar v sin i. A grid of synthetic spectra of He I lineprofiles was calculated in non-LTE using an extensive helium model atomand updated atomic data. The v sin i values for all stars were derivedusing the He I FWHM calibrations, but also, for those target stars withrelatively sharp lines, v sin i values were obtained from best-fitsynthetic spectra of up to 40 lines of C II, N II, O II, Al III, Mg II,Si III, and S III. This calibration is a useful and efficient tool forestimating the projected rotational velocities of O9-B5 main-sequencestars. The distribution of v sin i for an unbiased sample of early Bstars in the unbound association Cep OB2 is consistent with thedistribution reported elsewhere for other unbound associations.
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
We report on the abundance of interstellar neutral nitrogen (N I) for 30sight lines, using data from the Far Ultraviolet Spectroscopic Explorer(FUSE) and the Hubble Space Telescope (HST). N I column densities arederived by measuring the equivalent widths of several ultravioletabsorption lines and subsequently fitting those to a curve of growth. Wefind a mean interstellar N/H of 51+/-4 ppm. This is below the mean foundby Meyer et al. of 62+4-3 ppm (adjusted for adifference in f-values). Our mean N/H is similar, however, to the(f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for alarger sample of sight lines with larger hydrogen column densitiescomparable to those in this study. We discuss the question of whether ornot nitrogen shows increased gas-phase depletion in lines of sight withcolumn densities logN(Htot)>~21, as claimed by Knauth etal. The nitrogen abundance in the line of sight toward HD 152236 isparticularly interesting. We derive very small N/H and N/O ratios forthis line of sight that may support a previous suggestion that membersof the Sco OB1 association formed from an N-deficient region.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Non-LTE abundances of magnesium, aluminum and sulfur in OB stars near the solar circle
Non-LTE abundances of magnesium, aluminum and sulfur are derived for asample of 23 low-v sin i stars belonging to six northern OBassociations of the Galactic disk within 1 kpc of the Sun. Theabundances are obtained from the fitting of synthetic line profiles tohigh resolution spectra. A comparison of our results with HII regionabundances indicates good agreement for sulfur while the cepheidabundances are higher. The derived abundances of Mg show good overlapwith the cepheid results. The aluminum abundances for OB stars aresignificantly below the cepheid values. But, the OB star results show adependence with effective temperature and need further investigation.The high Al abundances in the cepheids could be the result of mixing. Adiscussion of the oxygen abundance in objects near the solar circlesuggests that the current mean galactic oxygen abundance in this regionis 8.6-8.7 and in agreement with the recently revised oxygen abundancein the solar photosphere. Meaningful comparisons of the absolute S, Aland Mg abundances in OB stars with the Sun must await a reinvestigationof these elements with 3D hydrodynamical model atmospheres for the Sun.No abundance gradients are found within the limited range ingalactocentric distances in the present study. Such variations would beexpected only if there were large metallicity gradients in the disk.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| Gas-Phase Iron Abundances and Depletions in Translucent Interstellar Lines of Sight from Far Ultraviolet Spectroscopic Explorer Observations of Fe II Lines
Far Ultraviolet Spectroscopic Explorer (FUSE) wavelength coverageincludes several weak- to moderate-strength lines of Fe II, allowing thedetermination, through curve-of-growth analysis, of accurate Fe IIabundances and hence iron depletions. We have analyzed Fe II absorptionlines toward 18 of the reddened stars included in the FUSE survey ofmolecular hydrogen abundances in translucent clouds. Our analysis isbased on equivalent width measurements and curves of growth, aided bythe fact that some of the observed lines are weak enough to be on thelinear part of the curve of growth. In interpreting our abundance anddepletion results, we have combined our data with those of an earliersurvey of interstellar iron abundances and depletions in diffuse clouds,based on Copernicus data. The principal result of our survey is thatiron depletions, known from earlier work to increase with averageline-of-sight density for diffuse clouds, do not continue to increasewith either density or extinction in translucent clouds; i.e., there isno significant trend of increasing depletion with increasing extinctionor molecular fraction. This may be due to the fact that our data setdoes not probe lines of sight with greater average volume densities thanthose that were covered by the previous Copernicus-based survey of irondepletions. We conclude by reevaluating the definition of translucentclouds, based on the lack of enhanced iron depletions in our sample.
| Chemical Abundances of OB Stars in Five OB Associations
We present LTE abundances of magnesium, aluminum, sulfur, and iron andnon-LTE abundances of carbon, nitrogen, oxygen, and silicon for a sampleof 15 slowly rotating B stars belonging to five OB associations: CygOB3, Cyg OB7, Lac OB1, Vul OB1, and Cep OB3. These OB associations lieon the Galactic plane and are situated within 3 kpc of the Sun. Of theeight elements sampled, non-LTE abundances for C, N, O, and Si, as wellas LTE abundances for Al and Fe, generally show subsolar abundances,with typical underabundances of ~0.2-0.4 dex. The LTE abundances for Mgand S tend to fall closer to solar values in the five associations.Whether the somewhat larger abundances derived for Mg and S, relative tothe other six elements studied, are significantly different will requirefurther work, while the modest, but persistent, underabundances(relative to solar) found for the other elements confirm a number ofprevious studies of young disk OB stars lying relatively near to theSun. The five associations studied here do not span a significant rangeof Galactocentric distances; however, their derived abundances agreewith what would be expected based upon previous studies that have mappedabundance versus Galactocentric distance and measured abundancegradients in the Milky Way disk.
| Interstellar extinction.
Not Available
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| A survey of vacuum-ultraviolet extinction curves based on International Ultraviolet Explorer spectra
This paper presents the interstellar extinction curves derived from theIUE spectra with the aid of the already published 'artificialstandards'. The variety of possible shapes of the curves, demonstratedearlier on spectra from the TD/1 satellite, is fully confirmed.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Environment Dependence of Interstellar Extinction Curves
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...274..439J&db_key=AST
| Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
The literature on all OB associations was reviewed, and their IRAS pointsource content was studied, between galactic longitude 55 and 150 deg.Only one third of the 24 associations listed by Ruprecht et al. (1981)have been the subject of individual studies designed to identify thebrightest stars. Distances to all of these were recomputed using themethod of cluster fitting of the B main sequence stars, which makes itpoossible to reexamine the absolute magnitude calibration of the Ostars, as well as for the red supergiant candidate stars. Also examinedwas the composite HR diagram for these associations. Associations withthe best defined main sequences, which also tend to contain very youngclusters, referred to here as OB clusters, have extremely few evolved Band A or red supergiants. Associations with poorly defined mainsequences and few OB clusters have many more evolved stars. They alsoshow an effect in the upper HR diagram referred to as a ledge byFitzpatrick and Garmany (1990) in similar data for the Large MagellanicCloud. It is suggested that the differences in the associations are notjust observational selection effects but represent real differences inage and formation history.
| Infrared dust and millimeter-wave carbon monoxide emission in the Orion region
The far-infrared dust emission seen by the IRAS satellite in the Orionregion is analyzed as a function of the local radiation field intensity,and the dust temperature and opacity are compared with (C-12)O and(C-13)O emission. The infrared radiation is interpreted within theframework of a single-component large grain model and a multicomponentgrain model consisting of subpopulations of grains with size-dependenttemperatures. A strong dependence of the 100-micron optical depthderived is found using the large grain model on the averageline-of-sight dust temperature and radiation field. In the hotenvironment surrounding high-luminosity sources and H II regions, alldust along the line-of-sight radiates at 100 microns, and thedust-to-gas ratio, based on the 100-micron opacity and I(/C-13/O),appears to be in agreement with the standard value, about 1 percent bymass. A relationship is found between the inferred dust-to-gas ratio andthe radiation field intensity responsible for heating the dust which canbe used to estimate the gas column density from the dust opacity derivedfrom the 60- and 100-micron IRAS fluxes.
| Empirical temperature calibrations for early-type stars
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
| Hydrogenated amorphous carbon-coated silicate particles as a source of interstellar extinction
A new model for interstellar grains has recently been proposed in whichlarge and small silicate grains are mantled with thin layers ofhydrogenated amorphous carbon (HAC or a-C:H) and amorphous carbon (a-C).A complete description of the grain model is presented for the firsttime. The optical properties of small inhomogenous particles arediscussed and new expressions for carbon depletion, bump strength andalbedo are derived. It is shown, in particular, that the wide variety ofinterstellar extinction curves can be explained by variations in carbondepletion, and model fits to normal and extreme cases of extinction arepresented. The model enables a direct measurement of carbon depletion tobe made from the extinction data and implies that carbon depletion in aninterstellar cloud is determined by the recent history of that cloud.Thus, on this model, grains respond to and react with their environment,rather than being immutable, inert entities in the interstellar gas.
| An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction
In this paper the properties of interstellar extinction in thefar-ultraviolet region are examined utilizing IUE extinction curves fora primary data sample of 45 reddened Milky Way OB stars. These resultsare combined with those derived for the 2175 A bump in a previous study.It is found that IUE extinction curves can be represented by linearcombinations of a Lorentzian-like 2175 A bump profile, a well-determinedFUV curvature term, and an underlying linear component. The parametersof the linear component are strongly correlated and therefore only fivefree parameters are required to fit all of the curves in our sample.Three parameters describe the 2175 A bump, one parameter describes thelinear background, and one parameter describes the strength of the FUVcurvature term. The shape of the FUV curvature is found to be identical,to within the observational errors, for all the curves in the sample -which spans virtually the entire range of extinction curve morphologiesobserved in the Milky Way.
| Atlas of the wavelength dependence of ultraviolet extinction in the Galaxy
The paper presents a collection of 115 extinction curves derived fromlow-dispersion IUE spectra. The spectra have been reduced with the useof techniques designed to reduce the effects of random noise and theinfluence of residual spectral features due to classification mismatch.The magnitudes of other instrumental and interpretational uncertaintiesare estimated. The extinction curves are presented with normalization toE(B-V) = 1 and the FUV portion (below 170 nm) is also shown for E(13-17)= 1. The atlas includes examples of extinction originating in thediffuse medium and several major nebulae and dense clouds.
| An analysis on the shapes of ultraviolet extinction curves. I - The 2175 A bump
IUE data are used to investigate the properties of the 2175 Ainterstellar extinction bump toward 45 reddened Milky Way stars. Usinganalytic fitting functions, parameters are derived which describe thecentral position, FWHM, and strength of the bump. The bump position isvery stable, with an extreme deviation which is still significantlylarger than the measurement uncertainty, indicating true variability.The bump FWHM has a large range of intrinsic values, 0.77/micron to1.25/micron. The normalized bump strength varies by more than a factorof three. There are no convincing correlations among the three bumpparameters, but the width of the bump is strongly correlated with thedust grain environments. Dense quiescent regions yield broad bumps,while diffuse interstellar medium and regions of recent star formationyield narrower bumps. The absence of any correlation between bump widthand central position argues strongly against graphite grains in the sizerange prescribed by the Mathis, Rumpl, and Nordsieck (1977) model ascarriers of the bump.
| The extinction law in the open cluster NGC 457 and the intrinsic energy distribution of Phi Cassiopeiae (F0 Ia)
Five early B-type stars near the main-sequence turnoff in NGC 457 havebeen observed at low dispersion with the short-wavelength prime and thelong-wavelength redundant cameras of the IUE satellite. The equivalentwidths of spectral features that are particularly strong and sensitiveto temperature and luminosity were computed in the cluster stars and in20 lightly reddened stars of types O9-B3 and luminosity classes III-V.The comparison of the equivalent widths provides a reliable method forfinding matching pairs. Having identified the best comparison star foreach program star, binned fluxes were used to determine the meanextinction curve. In order to cover the visible region, monochromaticfluxes of Phi Cas were derived from observations with the intensifiedReticon scanner mounted on the No. 2 0.9 m telescope of KPNO, and theywere dereddened with the mean extinction curve of Savage and Mathis.Thus, the intrinsic energy distribution of Phi Cas were determined from1500 to 5800 A for use in a detailed model-atmosphere analysis.
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