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The Abundance of Carbon Dioxide Ice in the Quiescent Intracloud Medium
We present new observations with the Infrared Spectrograph on board theSpitzer Space Telescope of the solid-CO2 absorption featurenear 15 μm in the spectra of eight field stars behind the Tauruscomplex of dark clouds. Solid CO2 is detected in six lines ofsight. New results are combined with previous data to investigate thecorrelation of CO2 column density with those of other majorice constituents (H2O and CO) and with extinction.CO2 is shown to display a ``threshold extinction'' effect,i.e., a minimum extinction (A0=4.3+/-1.0 mag) required fordetection, behavior similar to that previously reported forH2O and CO. We find a particularly tight correlation throughthe origin between N(CO2) and N(H2O), confirmingthat these species form in tandem and coexist in the same (polar) icelayer on the grains. The observed composition of the mantles is broadlyconsistent with the predictions of photochemical models with diffusivesurface chemistry proposed by Ruffle & Herbst. Comparison of ourresults for Taurus with published data for Serpens indicates significantdifferences in ice composition consistent with enhanced CO2production in the latter cloud. Our results also place constraints onthe distribution of elemental oxygen between ices and other potentialreservoirs. Assuming a constant N(H) to extinction ratio, we show that~65% of the solar O abundance is accounted for by summing thecontributions of ices (~26%), refractory dust (~30%) and gas-phase CO(~9%). If the Sun is an appropriate standard for the interstellarmedium, the ``missing'' oxygen may reside in atomic O I gas and/or(undetected) O2 within the ices.
| On the interstellar extinction law toward young stars
We have determined the atomic hydrogen column density N HI toward all ofthe young stars from the Taurus-Auriga-Perseus star-forming complex forwhich the corresponding spectra are available in the Hubble SpaceTelescope archive (nine stars) by analyzing the Lyα line profile.We show that the stars studied, except DR Tau, lie not far from the edgeof the gaseous cloud of the star-forming region closest to us or, moreprecisely, inside the outer H I shell of the cloud. This shell with acolumn density of N HI ≃ 6 × 1020 cm-2 surrounds themolecular gas of the cloud composed of a diffuse component (theso-called diffuse screen) in which dense, compact TMC-1 cores areembedded. The properties of the dust grains toward the stars that lie atthe front edge of the cloud most likely differ only slightly from thoseof the interstellar dust outside star-forming regions. This casts doubton the validity of the hypothesis that the extinction curve toward youngstars has an anomalously low amplitude of the 2175 Åbump—such an extinction curve is observed for the field stars HD29647 and HD 283809 toward which the line of sight passes through theTMC-1 core.
| The dust-eliminated shape of quasar spectra in the near-infrared: a hidden part of the big blue bump
The near-infrared shape of the big blue bump component in quasar spectrahas till now been essentially unknown. It usually cannot be observeddirectly, due to the strong hot dust emission that dominates quasarspectra longward of ~1 μm. However, this is quite an important partof the spectrum theoretically. At least bare-disc models provide quite arobust prediction for the overall continuum shape in the near-infrared.Self-gravity should become important in the outer,near-infrared-emitting regions of the putative disc, possibly leaving asignature of disc truncation in the near-infrared. We propose here thatthis important part of the spectrum can be revealed for the first timeby observing polarized flux from normal quasars. At least in somepolarized quasars, the emission lines are all unpolarized and so thepolarized flux should originate interior to the broad-line region, andtherefore also interior to the dust-emitting region. This can then beused to eliminate the dust emission. We present the results ofnear-infrared polarimetry for three such quasars (Ton202, 4C 37.43 andB2 1208+32). The data for Ton202 have the highest signal-to-noise ratio,and the near-infrared polarized flux in this case is measured to havequite a blue shape, Fν~ν+0.42+/-0.29,intriguingly consistent with the simple multitemperature blackbody,bare-disc prediction of ν+1/3. All these data, althoughstill with quite low signal-to-noise ratio for the other two objects,demonstrate the unique potential of the technique with future betterdata. We also present similar data for other quasars and radio galaxies,and briefly discuss the nature of the polarization.
| The Mass Accretion Rates of Intermediate-Mass T Tauri Stars
We present Hubble Space Telescope ultraviolet spectra and supportingground-based data for a sample of nine intermediate-mass T Tauri stars(IMTTSs; 1.5-4 Msolar). The targets belong to threestar-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZOri, P2441, and V1044 Ori in the Ori OB1c association surrounding theOrion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring aroundλ Ori. The supporting ground-based observations include nearlysimultaneous UBV(R I)C photometry, 6 Å resolutionspectra covering the range 3900-7000 Å, optical echelleobservations in the range 5800-8600 Å, and K-band near-infraredspectra. We use these data to determine improved spectral types andreddening corrections and to obtain physical parameters of the targets.We find that an extinction law with a weak 2175 Å feature but highvalues of AUV/AV is required to explain thesimultaneous optical-UV data; the reddening laws for two B-type starslocated behind the Taurus clouds, HD 29647 and HD 283809, meet theseproperties. We argue that reddening laws with these characteristics maywell be representative of cold, dense molecular clouds. Spectral energydistributions and emission-line profiles of the IMTTSs are consistentwith expectations from magnetospheric accretion models. We compare oursimultaneous optical-UV data with predictions from accretion shockmodels to get accretion luminosities and mass accretion rates (M) forthe targets. We find that the average mass accretion rate for IMTTSs is~3×10-8 Msolar yr-1, a factor of~5 higher than that for their low-mass counterparts. The new data extendthe correlation between M and stellar mass to the intermediate-massrange. Since the IMTTSs are evolutionary descendants of the Herbig Ae/Bestars, our results put limits to the mass accretion rates of theirdisks. We present luminosities of the UV lines of highly ionized metalsand show that they are well above the saturation limit for magneticallyactive cool stars but correlate strongly with accretion luminosity,indicating that they are powered by accretion, in agreement withprevious claims but using a sample in which reddening and accretionluminosities have been determined self-consistently. Finally, we findthat the relation between accretion luminosity and Brγ luminosityfound for low-mass T Tauri stars extends to the intermediate-massregime.
| The Ultraviolet Extinction Curve of Intraclump Dust in Taurus (TMC-1): Constraints on the 2175 Å Bump Absorber
Ultraviolet extinction curves for three early-type stars that sampledust in the Taurus dark cloud in the vicinity of TMC-1 are analyzed toexamine the contribution of internal (intraclump) dust to the totalline-of-sight extinction. One of our selected targets samples only dustin the ``diffuse-screen'' component of the cloud, where the dust grainslack ice mantles and have optical properties similar to average``diffuse-ISM'' dust. Comparison with data for more heavily reddenedstars that also sample dust within TMC-1 (where grains are mantled)allows us to determine the intraclump extinction. Our principal resultis that the 2175 Å ``extinction bump'' is extremely weak or absentinside the clump. The feature present in the observed extinction curvesappears to arise almost entirely in the diffuse screen. This dramaticreduction in bump amplitude in the high-density environment isconsistent with the prediction of the graphite model that the feature issuppressed by hydrogenation. Amplitude reduction is accompanied by amore subtle change in the profile of the bump, specifically, an increasein width. Comparison of Taurus and other clouds strongly suggests thatthe increase in width results from accumulation of surface coatings onthe carrier grains rather than from particle clustering.
| 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.
| The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction. I. Probing Small Molecular Polycyclic Aromatic Hydrocarbons
We have obtained new Hubble Space Telescope/Space Telescope ImagingSpectrograph spectra to search for structure in the ultravioletinterstellar extinction curve, with particular emphasis on a search forabsorption features produced by polycyclic aromatic hydrocarbons (PAHs).The presence of these molecules in the interstellar medium has beenpostulated to explain the infrared emission features seen in the 3-13μm spectra of numerous sources. Ultraviolet (UV) spectra are uniquelycapable of identifying specific PAH molecules. We obtained highsignal-to-noise ratio UV spectra of stars that are significantly morereddened than those observed in previous studies. These data put limitson the role of small (30-50 carbon atoms) PAHs in UV extinction and callfor further observations to probe the role of larger PAHs. PAHs are ofimportance because of their ubiquity and high abundance inferred fromthe infrared data, and also because they may link the molecular and dustphases of the interstellar medium. A presence or absence of UVabsorption bands due to PAHs could be a definitive test of thishypothesis. We should be able to detect a 20 Å wide feature downto a 3 σ limit of ~0.02 AV. No such absorption featuresare seen other than the well-known 2175 Å bump.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface
Observations of interstellar linear polarization in the spectral range0.35-2.2 μm are presented for several stars reddened by dust in theTaurus region. Combined with a previously published study by Whittet etal., these results represent the most comprehensive data set availableon the spectral dependence of interstellar polarization in this nearbydark cloud (a total of 27 sight lines). Extinction data for these andother reddened stars in Taurus are assembled for the same spectralrange, combining published photometry and spectral classifications withphotometry from the Two Micron All Sky Survey. The polarization andextinction curves are characterized in terms of the parametersλmax (the wavelength of maximum polarization) andRV (the ratio of total to selective extinction),respectively. The data are used to investigate in detail the question ofwhether the optical properties of the dust change systematically as afunction of environment, considering stars observed throughprogressively more opaque (and thus progressively denser) regions of thecloud. At low visual extinctions (03, real changes in grainproperties occur, characterized by observed RVvalues in the range 3.5-4.0. A simple model for the development ofRV with AV suggests thatRV may approach values of 4.5 or more in thedensest regions of the cloud. The transition between ``normal''extinction and ``dense cloud'' extinction occurs at AV~3.2, avalue coincident with the threshold extinction above whichH2O-ice is detected on grains within the cloud. Changes inRV are thus either a direct consequence ofmantle growth or occur under closely similar physical conditions. Dustin Taurus appears to be in a different evolutionary state compared withother nearby dark clouds, such as ρ Oph, in which coagulation is thedominant physical process.
| 1-4 Micron Spectrophotometry of Dust in the Taurus Dark Cloud: Water Ice Distribution in Heiles Cloud 2
We have conducted near infrared spectroscopy of 61 background starstoward Heiles Cloud 2 in the Taurus molecular cloud complex. We used alow-dispersion spectrometer, PASP2, which can simultaneously obtain thespectrum with a wavelength coverage between 1.3 and 4.2 μm. For 56 of61 objects, the visual extinction (AV) and theoptical depth of water ice at λ=3.1 μm (τICE)have been estimated: for 50 of 56 objects, these were systematicallyestimated from our data only. In order to investigate the water icedistribution in Heiles Cloud 2, we have constructed a ``water ice map''in which τICE is plotted at the position of each object.The water ice map is then compared with the C18O (J=1-0) mapobtained by millimeter observations performed by Sunada & Kitamura.We find that the distribution of water ice is closely correlated withthat of C18O. Strong water ice absorption is seen only towardthe dense C18O clumps, while less water ice absorption isdetected toward the outer region of the cloud. There is anAV threshold for the positive ice detection(AV0), as suggested by previous observations, but with asignificant scatter; AV0=2-5 mag. The scatter might be causedby the different contribution of the inner water-containing portion ofthe cloud along the line of sight. The value of τICEincreases with increasing of AV and the slope ofΔτICE/ΔAV is 0.067,consistent with the previously observed values for the Taurus molecularcloud.
| Determining the extinction through dark clouds
We discuss the problem of a consistent determination of the visualextinction to objects in and behind a dark cloud. The use ofnear-infrared colours is discussed, in particular the H-K colours. Weconcentrate on the uncertainties, with emphasis on the extinction lawand on the intrinsic colours of the young stars embedded in a darkcloud. The cases of the two intensively studied nearby star-formingclouds, Taurus and Ophiuchus, are addressed.
| Ices and extinction through the Taurus and Ophiuchus clouds
A detailed intercomparison is made between published observations of H_2O and CO ices towards the Taurus and Ophiuchus dark clouds. The columndensities of the ices are intercompared, and each compared to the visualextinction through the clouds, A_v. It is neither clear that the twoclouds have different or well defined threshold extinctions for thesurvival of ice mantles, nor that, for each of the clouds, thethresholds for the survival of CO and H_2 O mantles are different. Theinclusion of new objects in Taurus (Teixeira et al. 1998) introduces alarge scatter in the relations between those quantities relative toresults obtained by previous authors. Lines-of-sight towards deeplyembedded Young Stellar Objects in Taurus appear to show an enhancementin the amount of water-ice relative to lines-of-sight towards fieldstars behind that cloud. While for A_v < 14 mag there is a tightcorrelation between the water-ice column density, N_s(H_2 O), and A_v,the inclusion of those new objects reveals a discontinuity in therelation between N_s(H_2 O) and A_v. The interpretation of thisdiscontinuity is discussed.
| Stromvil Photometry: Peculiar Stars and Anomalous Reddening
Possibilities of the Stromvil photometric system in identifying peculiarstars of various types are reviewed. The system can identify in thepresence of interstellar reddening the following types of peculiarstars: F--G--K--M subdwarfs, G--K--M metal-deficient giants, coolcarbon, barium and zirconium stars, chemically peculiar B and A stars,emission-line stars (Be, Ae/Be, WR, T Tauri, etc.), white dwarfs, afraction of horizontal-branch stars and many types of unresolvedbinaries. Also, the system can identify stars affected by anomalousinterstellar reddening and classify them correctly.
| Interstellar Polarization in the Taurus Dark Clouds: Wavelength-dependent Position Angles and Cloud Structure near TMC-1
Systematic variations with wavelength in the position angle ofinterstellar linear polarization of starlight may be indicative ofmultiple cloud structure along the line of sight. We use polarimetricobservations of two stars (HD 29647 and HD 283809) in the generaldirection of TMC-1 in the Taurus dark cloud to investigate grainproperties and cloud structure in this region. We show the data to beconsistent with a simple two-component model in which generalinterstellar polarization in the Taurus cloud is produced by a widelydistributed cloud component with relatively uniform magnetic fieldorientation; light from stars close to TMC-1 suffers additionalpolarization arising in one (or more) subcloud(s) with larger averagegrain size and magnetic field directions different from the generaltrend. Toward HD 29647 in particular, we show that the unusually lowdegree of visual polarization relative to extinction is due todepolarization associated with the presence of distinct cloud componentsin the line of sight with markedly different magnetic fieldorientations. Stokes parameter calculations allow us to separate thepolarization characteristics of the individual components. Results arefitted with the Serkowski empirical formula to determine the degree andwavelength of maximum polarization. Whereas lambda max values in thewidely distributed material are similar to the average (0.55 mu m) forthe diffuse interstellar medium, the subcloud in the line of sight to HD283809, the most heavily reddened star in our study, has lambda max ~0.73 mu m, indicating the presence of grains ~30% larger than thisaverage. Our model also predicts detectable levels of circularpolarization toward both HD 29647 and HD 283809.
| Grain Alignment in the Taurus Dark Cloud
Variations in the polarization efficiency (p/A) of interstellar grainsas a function of environment place vital constraints on models for themechanism of alignment. In this Letter, polarimetric observations ofbackground field stars are used to investigate alignment in the TaurusDark Cloud for extinctions in the magnitude range 0 < AK < 2.5 (0< AV < 25). Results show a strong systematic trend in polarizationefficiency with extinction, well represented by a power law p/A ~A-0.56. A number of possible interpretations of this result arediscussed. Assuming magnetic alignment of the grains, the observed trendmay be influenced by such factors as small-scale magnetic fieldstructure, variation of magnetic field strength and coupling of gas anddust temperatures as functions of density, and systematic changes in theefficiency of suprathermal spin as a function of grain surfaceproperties and H/H2 fraction within the dark cloud.
| High-Resolution Studies of Solid CO in the Taurus Dark Cloud: Characterizing the Ices in Quiescent Clouds
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...455..234C
| Grain Mantles in the Taurus Dark Cloud
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.263..749S&db_key=AST
| Systematic variations in the wavelength dependence of interstellar linear polarization
New observations of the wavelength dependence of interstellar linearpolarization, p(lambda), which were performed to investigate theinfluence of the environment on the effective size distribution of thealigned polarizing particles, are presented. Optical and IR measurementswere obtained simultaneously in eight photometric passbands between Uand K, giving a coherent data set for a total of 105 reddened stars. Thecontention that variations of K and lambda-max are correlated isconfirmed, and the empirical linear relationship between K andlambda-max found by Wilking et al. (1980, 1982, 1983) is revised to K +0.01 +/-0.05 + (1.66 +/-0.09)lambda-max. There is some cosmic scatter ofthe data about this line. The same linear dependence of K on l-max seenoverall provides a consistent representation of the systematicpolarization in changes within individual regions with rather differingenvironments. It is argued that the grain size distribution in denseregions is modified by coagulation which removes the smaller particleswithout major modification of the larger ones.
| Diffuse interstellar bands in the Taurus dark clouds
This paper presents strengths and profiles of seven diffuse interstellarbands measured in a group of field stars situated behind the Taurus darkcloud complex. Comparison with the current 'band family' theories showsthat, within a single complex, the relative proportions of differentfamily carriers can vary in rather close unions: although correlationswith extinction are poor in all cases, some pairs of bands arenonetheless strongly interrelated. Band strengths relative to reddeningare investigated and found to be reasonably well fitted by a model inwhich all of the DIB production takes place in a surface layer of thecloud. These results are compared with existing data for the Rho Ophcloud. Band production efficiency varies in the same way in bothregions, falling rapidly with increasing E(B-V) to a limiting value afactor of 3-10 below the norm for the diffuse ISM. Comparison of grainproperties and the UV radiation field in the two cases shows that therelative weakness of the diffuse bands in dark clouds is driven bychanging grain properties rather than by species or other ionizedmolecules.
| Infrared spectroscopy of dust in the Taurus dark clouds - Ice and silicates
Low-resolution spectra of the 3 micron water-ice features of 22 starsand the 10 micron silicate dust features of 16 stars are presented forstars in the direction of the extensive dark cloud complex in Taurus,including both dust-embedded objects and background field stars seenthrough the cloud. A very close linear correlation is found between thepeak optical depth in the 3 micron feature and visual extinction, A(v),for field stars. The detection of ice in all cases where A(v) exceeds athreshold value of 3.3 + or - 0.1 mag serves as a basis for models ofvolatile mantle growth on grains in the dark cloud environment. Thesilicate feature is poorly correlated with A(v), and its relativeweakness per unit A(v) in the spectra of field stars with strong icebands may reflect the dilution of silicates as a fraction of total grainmass as mantles accrete in the dark cloud.
| Photoelectric photometry of the stars HD 29647 and HDE 283809 in the Taurus dark cloud
The present photometric study of the B-type stars HD 29647 and HDE283809 indicates their brightness and energy distribution to be constantto within 1-2 percent. A determination is also made of HDE 283809'sinterstellar extinction law on the basis of both Vilnius and UBVRJHKLsystem photometric data, showing that grain sizes exceed the average byabout 10 percent. The interstellar extinction law for the two stars isnoted to be the same in the IR despite being very different in thenear-UV. The new HDE 283809 spectra confirm the earlier classification,and indicate an absence of emission in the H lines; observed energydistribution peculiarities in this spectrum apparently originate ininterstellar or circumstellar dust rather than in the star itself.
| Magnetic field structure in the Taurus dark cloud
Optical and infrared polarimetry of sources in the direction of theTaurus cloud are obtained in order to study the magnetic field in thiscloud and its possible role in the cloud's evolution. Most of the starsare background giant stars whose light shines through the cloud and ispolarized by the cloud material. The transverse component of themagnetic field, as delineated by the polarization vectors, is generallyperpendicular to the galactic plane, and the stratified structure of thecloud could be due to the effect of the magnetic field during the earlystages of collapse. Three of the 13 embedded stars are stronglypolarized with position angles nearly perpendicular to those of nearbyfield stars. The polarization of these stars is most likely intrinsic,and the direction of polarization indicates that the materialsurrounding these stars may be magnetic i.e., that the magnetic field isfrozen in this material.
| HDE 283809 - A type B star toward the Taurus dark cloud
Spectroscopic and photometric observations of the heavily reddened starHDE 283809 located in the direction of the Taurus dark cloud show thatit belongs to spectral type B3; but at the same time it has a strong CaII K line and a peculiar spectral energy distribution, with diminishedviolet and ultraviolet intensity but enhanced red and infrared emission.If the star does belong to the Taurus star-formation complex, itspeculiarity might result from instability or the presence of acircumstellar envelope. If it is a galactic background star, it could bea binary, with the secondary component a late-type giant.
| U, B, V, R, I stellar photometry in the field of the Taurus dark clouds
A program of photoelectric stellar photometry has been carried out inthe field of the Taurus dark clouds with a view to establishing thedistance of the clouds and the absorption law in that region of the sky.It is found that the distance of the dark clouds accompanying the Tau T1and Tau T3 associations is 132 plus or minus 10 pc. Within theabsorption region 0-4 m, the Taurus dark clouds are characterized by anearly normal absorption law (to within 5-10%). In the field of the darkclouds a group of F0-G0 stars close to the main sequence is found whosespace density is three to eight times that in the solar neighborhood.
| Interstellar extinction in the dark Taurus clouds. I
The results of photoelectric photometry of 74 stars in the Vilniusseven-color system in the area of Taurus dark clouds with coordinates(1950) 4h20m-4h48m +24.5 deg to +27 deg are presented. Photometricspectral types, absolute magnitudes, color excesses, interstellarextinctions and distances of the stars are determined. The dark cloudKhavtassi 286, 278 and the surrounding absorbing nebulae are found toextend from 140 to 175 pc from the sun. The average interstellarextinction on both sides of the dark cloud is of the order of 1.5m. Noevidence of the existence of several absorbing clouds situated atvarious distances is found.
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