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FUSE Measurements of Far-Ultraviolet Extinction. III. The Dependence on R(V) and Discrete Feature Limits from 75 Galactic Sightlines
We present a sample of 75 extinction curves derived from Far UltravioletSpectroscopic Explorer (FUSE) far-ultraviolet spectra supplemented byexisting International Ultraviolet Explorer (IUE) spectra. Theextinction curves were created using the standard pair method based on anew set of dereddened FUSE+IUE comparison stars. Molecular hydrogenabsorption features were removed using individualized H2models for each sightline. The general shape of the FUSE extinction (8.4μm-1 < λ-1 < 11μm-1) was found to be broadly consistent withextrapolations from the IUE extinction (3.3 μm-1< λ-1 < 8.6 μm-1)curve. Significant differences were seen in the strength of the far-UVrise and the width of the 2175 Å bump. All the FUSE+IUE extinctioncurves had positive far-UV slopes giving no indication that the far-UVrise was turning over at the shortest wavelengths. The dependence ofA(λ)/A(V) versus R(V)-1 in the far-UV using thesightlines in our sample was found to be stronger than tentativelyindicated by previous work. We present an updated R(V)-dependentrelationship for the full UV wavelength range (3.3μm-1 <= λ-1 <= 11μm-1). Finally, we searched for discrete absorptionfeatures in the far-ultraviolet. We found a 3σ upper limit of~0.12A(V) on features with a resolution of 250 (~4 Å width) and3σ upper limits of ~0.15A(V) for λ-1 <9.6 μm-1 and ~0.68A(V) forλ-1>9.6 μm-1 on featureswith a resolution of 104 (~0.1 Å width).
| 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.
| CN column densities and excitation temperatures
We analyse abundances and rotational temperatures of the interstellar CNmolecule. We have calculated the column densities and excitationtemperatures of the molecule along 73 lines of sight basing on ouroriginal measurements of the B 2?+ -X2?+ (0,0) vibrational band recorded in highsignal-to-noise ratio spectra and also for 88 directions based onmeasurements already available in literature. We compare the columndensities obtained from different bands of CN molecule available toground-based instruments. The obtained excitation temperatures in theanalysed directions show always an excess over the cosmic microwavebackground radiation (CMBR) temperature.
| 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.
| Interstellar gas, dust and diffuse bands in the SMC
Aims.In order to gain new insight into the unidentified identity of thediffuse interstellar band (DIB) carriers, this paper describes researchinto possible links between the shape of the interstellar extinctioncurve (including the 2175 Å bump and far-UV rise), the presence orabsence of DIBs, and physical and chemical conditions of the diffuseinterstellar medium (gas and dust) in the Small Magellanic Cloud (SMC). Methods: We searched for DIB absorption features in VLT/UVES spectra ofearly-type stars in the SMC whose reddened lines-of-sight probe thediffuse interstellar medium of the SMC. Apparent column density profilesof interstellar atomic species (Na i, K i, Ca ii and Ti ii) areconstructed to provide information on the distribution and conditions ofthe interstellar gas. Results: The characteristics of eight DIBsdetected toward the SMC wing target AzV 456 arestudied and upper limits are derived for the DIB equivalent widthstoward the SMC stars AzV 398, AzV214, AzV 18, AzV 65 andSk 191. The amount of reddening is derived for theseSMC sightlines, and, using RV and the H i column density,converted into a gas-to-dust ratio. From the atomic column densityratios we infer an indication of the strength of the interstellarradiation field, the titanium depletion level and a relative measure ofturbulence/quiescence. The presence or absence of DIBs appears to berelated to the shape of the extinction curve, in particular with respectto the presence or absence of the 2175 Å feature. Our measurementsindicate that the DIB characteristics depend on the local physicalconditions and chemical composition of the interstellar medium of theSMC, which apparently determine the rate of formation (and/or)destruction of the DIB carriers. The UV radiation field (viaphotoionisation and photo-destruction) and the metallicity (i.e. carbonabundance) are important factors in determining diffuse band strengthswhich can differ greatly both between and within galaxies.Based on observations collected with VLT/UVES at the European SouthernObservatory, Paranal, Chile (ESO programs 67.C-0281 and 71.C-0679).Tables [see full text]-[see full text] and Fig. [see full text]are only available in electronic form at http://www.aanda.org
| 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
| Diffuse Atomic and Molecular Clouds
Diffuse interstellar clouds have long been thought to be relativelydevoid of molecules, because of their low densities and high radiationfields. However, in the past ten years or so, a plethora of polyatomicmolecules have been observed in diffuse clouds, via their rotational,vibrational, and electronic transitions. In this review, we propose anew systematic classification method for the different types ofinterstellar clouds: diffuse atomic, diffuse molecular, translucent, anddense. We review the observations of molecules (both diatomic andpolyatomic) in diffuse clouds and discuss how molecules can be utilizedas indicators of the physical and chemical conditions within theseclouds. We review the progress made in the modeling of the chemistry inthese clouds, and the (significant) challenges that remain in thisendeavor. We also review the evidence for the existence of very largemolecules in diffuse clouds, and discuss a few specific clouds ofparticular interest.
| VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue in one sight line but is lower by factors of 10-15 in two others.The abundance of CH in the Magellanic Clouds thus appears to depend onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| FUSE Measurements of Far-Ultraviolet Extinction. II. Magellanic Cloud Sight Lines
We present an extinction analysis of nine reddened/comparison star pairsin the Large and Small Magellanic Clouds (LMC and SMC) based onFar-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date,just two LMC sight lines have probed dust grain composition and sizedistributions in the Magellanic Clouds using spectral data forwavelengths as short as 950 Å. We supplement these two with datafrom four regions distinguished by their IR through UV extinction curvesand grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinctcharacters of extinction in the Clouds and Milky Way, our results aregenerally analogous to those found for Galactic curves-namely, that theFUSE portions of each extinction curve are described reasonably well byFitzpatrick & Massa curves fitted only to longer wavelength data andlack any dramatic new extinction features, and any deviations from theCardelli, Clayton, & Mathis (CCM) formalism continue into FUVwavelengths. A maximum entropy method analysis of all of these curvessuggests that LMCAvg and SMC wing sight lines, whose extinctionparameters more closely resemble those for Galactic paths, require moresilicon and/or carbon in dust than current abundance measurements wouldindicate are available. The requirements for LMC2 and SMC bar sightlines do not fully tax the available reservoirs, in part because largegrains contribute less to the extinction in these directions. Anintermediate product of this extinction analysis is the measurement ofnew H2 abundances in the Magellanic Clouds. Collectivelyconsidering Cloud sight lines that possess significant H2column densities, E(B-V)/N(H I) ratios are reduced by significantfactors relative to the Galactic mean, whereas the correspondingE(B-V)/N(H2) values more closely resemble their Galacticcounterpart. These trends reflect the fact that among these sight linesf(H2) values are lower than those common in the Milky Way forpaths with similar degrees of reddening.Based on observations with the NASA-CNES-CSA Far-UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS-32985.
| Diffuse Interstellar Bands Toward HD 62542
Diffuse interstellar bands (DIBs) have been detected for the first timealong the peculiar translucent line of sight toward HD 62542, whichpasses through a diffuse cloud core. Although only a small fraction (18out of more than 300) of generally weak DIB features have been shown tocorrelate with C2 and C3 (the ``C2DIBs''), it is predominantly these DIBs that are observed toward HD62542. The typically strong DIBs λλ5780 and 5797 aredetected but are significantly weaker than toward other lines of sightwith similar reddening. Other commonly observed DIBs (such asλλ4430, 6270, and 6284) remain noticeably absent. Theseobservations further support the suggestion that the line of sighttoward HD 62542 crosses only the core of a diffuse cloud and show thatthe correlation between the C2 DIBs and small carbon chainsis maintained in environments with very large fractions of molecularhydrogen, fH2>0.8. A comparison of CH, CN,C2, and C3 column densities and C2 DIBstrengths toward HD 62542, HD 204827, and HD 172028 suggests that theline of sight toward HD 204827 passes through a diffuse cloud coresimilar to that seen toward HD 62542, as well as what might be referredto as a diffuse cloud envelope. This indicates that the bare core towardHD 62542 may not have significantly different relative chemicalabundances from other diffuse cloud cores and that the C2DIBs may serve as a diagnostic of such cores.
| FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far-ultravioletwavelengths (FUV; 1050-1200 Å) for nine Galactic sight lines. TheFUV extinction was measured using data from the Far UltravioletSpectroscopic Explorer. The sight lines were chosen for their unusualextinction properties in the infrared through the ultraviolet; that theyprobe a wide range of dust environments is evidenced by the large spreadin their measured ratios of total to selective extinction,RV=2.43-3.81. We find that extrapolation of the Fitzpatrick& Massa relationship from the ultraviolet appears to be a goodpredictor of the FUV extinction behavior. We find that predictions ofthe FUV extinction based on the Cardelli, Clayton, & Mathis (CCM)dependence on RV give mixed results. For the seven extinctioncurves well represented by CCM in the infrared through ultraviolet(x<8 μm-1), the FUV extinction is well predicted inthree sight lines, overpredicted in two sight lines, and underpredictedin two sight lines. A maximum entropy method analysis using a simplethree-component grain model shows that seven of the nine sight lines inthe study require a larger fraction of grain materials to be in dustwhen FUV extinction is included in the models. Most of the added grainmaterial is in the form of small (radii <~ 200 Å) grains.Based on observations with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Ultraviolet Extinction Properties in the Milky Way
We have assembled a homogeneous database of 417 ultraviolet (UV)extinction curves for reddened sight lines having InternationalUltraviolet Explorer spectra. We have combined these with optical andTwo Micron All Sky Survey photometry, allowing estimates of the ratio oftotal-to-selective extinction, RV, for the entire sample.Fitzpatrick-Massa (FM) parameters have also been found for the entiresample. This is the largest study of parameterized UV extinction curvesyet published, and it covers a wide range of environments, from densemolecular clouds to the diffuse interstellar medium, with extinctionsAV ranging from 0.50 to 4.80. It is the first to extend farbeyond the solar neighborhood and into the Galaxy at large, with 30sight lines having distances greater than 5 kpc. Previously, the longestsight lines with FM parameters and RV extended ~1 kpc. Wefind that (1) the Cardelli, Clayton, & Mathis (CCM) extinction lawapplies for 93% of the sight lines, implying that dust processing in theGalaxy is efficient and systematic, (2) the central wavelength of the2175 Å bump is constant, and (3) the 2175 Å bump width isdependent on environment. Only four sight lines show systematicdeviations from CCM: HD 29647, 62542, 204827, and 210121. These sightlines all sample dense, molecule-rich clouds. The new extinction curvesand values of RV allow us to revise the CCM law.
| Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
It is well known that the sight line toward HD 204827 in the clusterTrumpler 37 shows a UV extinction curve that does not follow the averageGalactic extinction relation. However, when a dust component, foregroundto the cluster, is removed, the residual extinction curve is identicalto that found in the SMC within the uncertainties. The curve is verysteep and has little or no 2175 Å bump. The position of HD 204827in the sky is projected onto the edge of the Cepheus IRAS bubble. Inaddition, HD 204827 has an IRAS bow shock, indicating that it may beembedded in dust swept up by the supernova that created the IRAS bubble.Shocks due to the supernova may have led to substantial processing ofthis dust. The HD 204827 cloud is dense and rich in carbon molecules.The 3.4 μm feature indicating a C-H grain mantle is present in thedust toward HD 204827. The environment of the HD 204827 cloud dust maybe similar to the dust associated with HD 62542, which lies on the edgeof a stellar wind bubble and is also dense and rich in molecules. Thissight line may be a Rosetta Stone if its environment can be related tothose in the SMC having similar dust.
| Observations of Rotationally Resolved C3 in Translucent Sight Lines
The rotationally resolved spectrum of theA1Πu<--X1Σ+g000-000 transition of C3, centered at 4051.6 Å, hasbeen observed along 10 translucent lines of sight. To interpret thesespectra, a new method for the determination of column densities andanalysis of excitation profiles involving the simulation and fitting ofobserved spectra has been developed. The populations of lower rotationallevels (J<=14) in C3 are best fitted by thermaldistributions that are consistent with the kinetic temperaturesdetermined from the excitation profile of C2. Just as in thecase of C2, higher rotational levels (J>14) ofC3 show increased nonthermal population distributions inclouds that have been determined to have total gas densities below ~500cm-3.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Dust Grain Size Distributions from MRN to MEM
Employing the maximum entropy method (MEM) algorithm, we fitinterstellar extinction measurements that span the wavelength range0.125-3 μm. We present a uniform set of MEM model fits, all using thesame grain materials, optical constants, and abundance constraints. Inaddition, we are taking advantage of improved UV and IR data and betterestimates of the gas-to-dust ratio. The model fits cover the entirerange of extinction properties that have been seen in the Galaxy and theMagellanic Clouds. The grain models employed for this presentation arethe simplistic homogeneous sphere models (i.e., those of Mathis, Rumpl,& Nordsieck in 1977) with two (graphite, silicate) or three(graphite, silicate, amorphous carbon) components. Although such usageis only a first step, the results do provide interesting insight intothe use of grain size as a diagnostic of dust environment. We find thatthe SMC bar extinction curve cannot be fitted using carbon grains alone.This is a challenge to the recent observational result indicating littlesilicon depletion in the SMC.
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
| 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.''
| Polarimetry toward the IRAS Vela Shell. I. The Catalog
We have obtained linear polarization measurements of stars along thewestern side of the IRAS Vela Shell toward HD 62542. From 16 CCD fieldsdistributed along the ionization front (I-front) we have built a catalogof 856 objects with polarization signal-to-noise ratio larger than 10.We detect very significant levels of polarization and hence anappreciable magnetic field throughout the region. Composite polarizationmaps around the I-front are shown. In some regions the polarizationvectors are parallel to the I-front, but a perpendicular trend is alsoevident along parts of the front. In addition, the polarization patternseems to be affected by gas streaming inside the cloud.
| Unusually Weak Diffuse Interstellar Bands toward HD 62542
As part of an extensive survey of diffuse interstellar bands (DIBs), wehave obtained optical spectra of the moderately reddened B5 V star HD62542, which is known to have an unusual UV extinction curve of the typeusually identified with dark clouds. The typically strongest of thecommonly cataloged DIBs covered by the spectra-those at 5780, 5797,6270, 6284, and 6614 Å-are essentially absent in this line ofsight, in marked contrast with other lines of sight of similarreddening. We compare the HD 62542 line of sight with others exhibitinga range of extinction properties and molecular abundances and interpretthe weakness of the DIBs as an extreme case of deficient DIB formationin a dense cloud whose more diffuse outer layers have been strippedaway. We comment on the challenges these observations pose foridentifying the carriers of the diffuse bands.
| 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.
| The incompatibilities between the standard theory of interstellar extinction and observations
In this paper I review a series of observations which do not agree withthe standard interpretation of the extinction curve. The consequence isthat light we receive from a reddened star must be contaminated bystarlight scattered at very small angular distances from the star. Thetrue extinction curve is a straight line from the near infrared to thefar-UV. If so, all interstellar grains models must be questioned.Another conclusion concerns the average properties of interstellargrains which seem much more uniform than previously thought.
| The relation between far-UV and visible extinctions
For directions of sufficient reddening (/E(B-V)>~0.25), there is asimple relation between the slope of the extinction curve in the far-UVand /E(B-V). Regardless of direction, the far-UV extinction curve isproportional to 1/λn e-2E(B-V)/λ(/λ in μm, /n=4), in accordance with the idea that reddenedstars spectra are contaminated by scattered light (Zagury, 2001b). Thisrelation is not compatible with the standard theory of extinction whichstates that far-UV and visible extinctions are due to different classesof particle. In that model the two (far-UV and visible) extinctions varythus independently according to the proportion of each type of particle.In preceding papers I have shown that the standard theory cannot explainUV observations of nebulae, and is contradicted by the UV spectra ofstars with very low reddening: for how long shall the standard theory beconsidered as the interpretation of the extinction curve?
| The spectrum of HD46223
The spectrum of HD46223 was established from the optical to the far UVand normalized by the spectrum of a non-reddened star of same spectraltype. The resulting spectrum is separated into two components. One isthe direct starlight. The second is an additional component of lightscattered at small angles. In the optical the spectrum is dominated bydirect starlight which decreases exponentially due to the linearextinction ~e-2E(B-V)/λ. Scattered light begins to benoticeable in the near-UV. The near-UV rise of the scattered light isinterrupted in the 2200Å bump region. The wavelength dependence ofthe scattered light is established in the far-UV, where scattered lightdominates the extinction curve. A 1/λ4 dependence isfound, proving the presence of grains small compared to UV wavelengths.The mathematic expressions of the different components mentioned abovegive a good fit to the extinction curve in the direction of the star. Ona mathematic standpoint the fit can be completed by a Lorentzien for the2200Å bump region. The physical interpretation of the bump may bemore difficult to achieve since the paper shows the possibility thatonly scattered light is extinguished in the bump region. Consequencesfor the grain size distribution which is necessary to explain thedifferent aspects of scattering in interstellar clouds, for the value ofRV, and problems raised by this interpretation of thespectrum of HD46223, are considered at the end of this paper.
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