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Blueshifted diffuse interstellar bands in the spectrum of HD 34078
In this paper, we report the very first observation of diffuseinterstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur),are blueshifted with respect to the normal position that they have inother objects, where the rest-wavelength velocity frame is determinedusing very sharp interstellar atomic lines or molecular features. Onlyreasonably narrow DIBs seemingly show this effect, which is absent inbroader ones. The result is confirmed independently using threedifferent spectrographs attached to two different telescopes.
| 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 Loopy Ultraviolet Line Profiles of RU Lupi: Accretion, Outflows, and Fluorescence
We present far-ultraviolet (FUV) spectra of the classical T Tauri starRU Lup covering the 912-1710 Å spectral range, as observed by theHubble Space Telescope STIS and the Far Ultraviolet SpectroscopicExplorer satellite. We use these spectra, which are rich in emission andabsorption lines, to probe both the accreting and outflowing gas.Absorption in the Lyα profile constrains the extinction toAV~0.07 mag, which we confirm with other diagnostics. Weestimate a mass accretion rate of (5+/-2)×10-8Msolar yr-1 using the optical-NUV accretioncontinuum. The accreting gas is also detected in bright, broad lines ofC IV, Si IV, and N V, which all show complex structures across the lineprofile. Many other emission lines, including those of H2 andFe II, are pumped by Lyα. RU Lup's spectrum varies significantlyin the FUV; our STIS observations occurred when RU Lup was brighter thanseveral other observations in the FUV, possibly because of a high massaccretion rate.
| 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.
| ARIES imaging polarimeter.
An Imaging Polarimeter has been fabricated for use with liquid- N2cooled CCD camera and is designed to suit 104-cm Sampurnanand telescopewith an f/13 focus at Aryabhatta Research Institute of ObservationalSciences (ARIES), Naini Tal. The instrument measures the linearpolarisation in broad B, V and R band and has a field of view ~ 20' x20'. We are presenting here some observations regarding the polarisationof some polarised as well as unpolarised stars with a view to show theperformance of our polarimeter.
| 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.
| 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.
| Astral magnetic fields-as observed in starforming nurseries, in stars, and in the Solar system
``Cherchez le champ magnétique'' is trendy in astronomy. Themagnetic field takes the hourglass shape in gravitationally contractingyoung stellar objects, helically wraps up the jets of protostars, guidesparticles in stellar coronae, explodes in stellar flares, causes unrestin pulsar quakes and creates planetary aurorae. In stars and pulsars,dipolar dynamo magnetic fields play a dominant rôle in the gasdynamics. In planetary disks, the field is toroidal or archimedeanspiral. Remanent magnetism is found in meteorites and asteroids. Theastral magnetic fields can reach 1018 G in magnetars,dwarfing anything we can do on Earth in stable conditions(107 G) or in pulsed conditions (1010 G).Magnetism plays a physical rôle in starforming clouds and itsconcomittent structures, from stellar nurseries to protostars, down toplanets and asteroids. Starting with star-forming clouds (~10 pc=32light-years=31×1016 m), this reviews coversprotostellar systems (~1 cpc), circumstellar space (~1 mpc), masers,interplanetary space (~1 μpc), pulsars, stars, planets (~1 npc),asteroids (~30 km; ~1 ppc), and meteorites (~0.3 m; ~10 apc).
| 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.
| The Spatial Distribution of Fluorescent H2 Emission near T Tauri
New subarcsecond far-UV observations of T Tau with Hubble SpaceTelescope STIS show spatially resolved structures in the2''×2'' area around the star. The structuresare apparent in multiline emission of fluorescent H2 pumpedby Lyα. One emission structure follows the cavity walls observedaround T Tau N in scattered light in the optical. A temperature of>=1000 K is required to have a high enough population in theH2 to produce the observed fluorescent lines; in the coolenvironment of the T Tau system, shock heating is required to achievethis temperature at distances of a few tens of AU. FluorescentH2 along the cavity wall represents the best evidence to datefor the action of low-density, wide opening angle outflows drivingcavities into the molecular medium at scales <=100 AU. A southernregion of emission consists of two arcs, with shape and orientationsimilar to the arcs of H2 2.12 μm and forbidden-lineemission crossing the outflow associated with the embedded system T TauS. This region is located near the centroid of forbidden-line emissionat the blueshifted lobe of the north-south outflow.
| 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.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Observations of C3 in Translucent Sight Lines
The A1Πu<--X1Σ+g transition of the simplestpolyatomic carbon chain molecule, C3, at 4051.6 Å hasbeen searched for toward reddened stars where abundant C2 hadbeen reported and toward other stars with high color excess. Absorptionfrom C3 has been detected toward 15 stars with color excessE(B-V) from 0.33 to 1.12. The observed C3 column densities,ranging from 1012 to 1013 cm-2, arewell correlated with the corresponding C2 column densities,with N(C2)/N(C3)~40, indicating their closechemical relation. The carbon-rich sight line toward HD 204827 (forwhich no previous C2 observation had been reported) has byfar the highest C3 and C2 column densities. Thechemistry of formation of C3 from C2 is discussed.A search for the next strongest 020-000 vibronic band was unsuccessfulas a result of the low Franck-Condon factor and interference with astellar line. Searches for C4 and C5 werenegative.
| Radiative accelerations on Ne in the atmospheres of late B stars
Radiative accelerations on Ne are calculated for the atmospheres ofmain-sequence stars with 11 000 <=Teff<= 15 000 K. Thisrange corresponds to that of the mercury-manganese (HgMn) stars. Thecalculations take into account neon fine structure as well as shadowingof neon lines using the entire Kurucz line list, bound-bound, bound-freeand free-free opacity of H, He and C as well as some non-LTE effects.Non-LTE effects are found to modify the radiative acceleration by afactor of the order of 102 in the outer atmosphere and arecrucial for d m < 10-3 g cm-2. The dependenceof the radiative accelerations on the Ne abundance, effectivetemperature and gravity is studied. Radiative accelerations are found tobe well below the gravitational acceleration over the entire range ofTeff and it is predicted that in stable atmospheres devoid ofdisturbing motions, Ne should sink and be observed as under-abundant.This agrees with recent observations of low Ne abundances in HgMn stars.
| 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.
| 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?
| Astrophysical and astrochemical insights into the origin of life.
Not Available
| An elemental abundance analysis of the mercury manganese star HD 29647
The sharp-lined mercury manganese (HgMn) star HD 29647, which is locatedbehind the outer envelope of the Taurus Molecular Cloud 1, is ofinterest for both its stellar properties and its utility as a probe ofinterstellar gas and dust along its line of sight. In this paper wereview the properties of the star, summarize its line identificationsand present an abundance analysis based on spectrograms obtained at theMcDonald Observatory and the Observatoire de Haute-Provence. This starhas elemental abundances similar to those of other HgMn stars exceptthat its He/H ratio is closer to solar, possibly indicating a young age,and that it, like HR 7775, is overabundant in selected elementsincluding the rare earths. The stellar radial velocity, unfortunately,closely matches that of the foreground cloud.
| 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.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| 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.
| The distribution of OH in Taurus Molecular Cloud-1
The column density of the OH radical in Taurus Molecular Cloud-1(TMC-1), reaches its maximum close to the cyanopolyyne peak. Acomparison with previously published maps of other molecules shows thatOH has a similar distribution as HC_3N and CCS, but differs largely fromCS, SO, NH_3 and N_2H+. The OH/C18O column densityratio is, however, almost constant along the dense ridge of TMC-1,suggesting that the fractional OH abundance does not change much, thederived value being about 10-7. This high number confirmsthat in dense gas OH is mainly formed by the electron recombination ofH_3O+. The approximate constancy of the fractional OHabundance is in accordance with a flat density distribution as has beenderived recently (\cite{pratap}). Previous SO/CS maps together with somerecent modelling results seem to be in conflict with the idea that thecyanopolyyne peak in the southeastern part of the cloud would bechemically less evolved than the ammonia maximum in the northwest.Therefore we discuss the possibility that the OH maximum represents theso called `radical peak', which occurs when freezing on to grainsurfaces starts to be the dominant factor controlling the chemicalcomposition and reactions (\cite{pdbrown}). It turns out that thegreater part of the data accumulated so far, including the present OHobservations, fit the `old' picture where a slightly higher density anda more advanced chemical state prevail in the northwestern part of theTMC-1 ridge. The `satellite' lines towards two locations in the cloudshow enhanced 1612 MHz emission. We suggest that this is due tonon-thermal excitation by far-infrared radiation from dust, heated bythe embedded young stars in the neighbourhood of the TMC-1 ridge.
| Particle Shapes and the 2175 Å Feature
The Galactic interstellar extinction curve exhibits a pronounced featureat 2175 Å. The most striking characteristic of this feature is therelatively fixed nature of its central wavelength, while the width ofthe feature may vary. Small graphite particles are often proposed as thecarrier of this UV bump. However, graphite grains of a single size andshape cannot reproduce the observed width and peak characteristicssimultaneously. Here, we investigate to what extent thesecharacteristics may be reproduced by invoking a shape distribution ofprolate and oblate graphite particles. The graphite constants calculatedby Draine and Lee do not yield the narrowest 2175 Å features thathave been observed. We have recalculated the dielectric function ofastronomical graphite such that a spherical grain of radius 50 Åproduces a feature with a peak at x0=4.6 μm-1and a width γ=0.8 μm-1. Using grains in the Rayleighapproximation, we have calculated extinction curves for combinations ofprolate, spherical, and oblate grain shapes and successfully reproducethe stationary nature of the peak. In addition, such shape variations dogive a standard deviation in the width of the feature of ~1/2 theobserved standard deviation. However, the absolute values of thecalculated widths fall short by ~10% compared with observed widths. Theobserved lack of correlation between peak position and width is notreproduced by the shape variations considered here.
| 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.
| A Large-Scale Objective-Prism and X-Ray Survey in Taurus-Auriga
We present the results of a moderately deep, wide-field optical/X-raysurvey extending over ~60 deg in the Taurus-Auriga molecular cloudcomplex. Our observations are sensitive enough to allow us to detectlower mass and older pre-main-sequence stars that could have been missedin previous surveys. We identify four new T Tauri stars, two in the areaof L1500 and L1503, one in L1538, and one in L1544. X-rays were detectedfrom three near-IR sources, one of them a very reddened Class II T Tauristar. Four X-ray sources have no known optical/IR known counterpart;they could be very reddened pre-main-sequence stars. Our findings areconsistent with the idea that little (if any) star formation has takenplace in L1537 and L1538. Although we found one slightly older T Tauristar (~3-4 Myr old), our survey yields no evidence for any significantnumber of T Tauri stars, regardless of mass and age, outside the maingroups of young stars in Taurus. We conclude that the M stars comprisingthe low-mass end of the ROSAT All-Sky Survey (RASS) source populationmust be older than ~10 Myr and most probably unrelated to the ongoingprocess of star formation in Taurus. Instead, the bulk of the RASSsources seen toward regions like Taurus seem to trace the history ofstar formation over a longer period in the solar vicinity. Researchreported herein based on observations made with the 1 m Schmidttelescope at Observatorio Nacional de Llano del Hato, Venezuela,operated by the Centro de Investigaciones de Astronomía (CIDA).
| A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV)extinction using data from the IUE final archive. Our new analysis takesadvantage of the improved signal-to-noise ratio of the IUE NEWSIPSreduction, the exclusion of stars with very low reddening, the carefulselection of well-matched comparison stars, and an analysis of theeffects of Galactic foreground dust. Differences between the averageextinction curves of the 30 Dor region and the rest of the LMC arereduced compared with previous studies. We find that there is a group ofstars with very weak 2175 Å bumps that lie in or near the regionoccupied by the supergiant shell, LMC 2, on the southeast side of 30Dor. The average extinction curves inside and outside LMC 2 show a verysignificant difference in 2175 Å bump strength, but their far-UVextinctions are similar. While it is unclear whether or not theextinction outside the LMC 2 region can be fitted with the relation ofCardelli, Clayton, & Mathis (CCM), sight lines near LMC 2 cannot befitted with CCM because of their weak 2175 Å bumps. While theextinction properties seen in the LMC lie within the range of propertiesseen in the Galaxy, the correlations of UV extinction properties withenvironment seen in the Galaxy do not appear to hold in the LMC.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bika |
| Rektaszcenzió: | 04h41m08.05s |
| Deklináció: | +25°59'34.0" |
| Vizuális fényesség: | 8.477 |
| Távolság: | 177.305 parszek |
| RA sajátmozgás: | 13.9 |
| Dec sajátmozgás: | -10.7 |
| B-T magnitude: | 9.477 |
| V-T magnitude: | 8.56 |
Katalógusok és elnevezések:
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