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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.
| 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.
| 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
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| 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.
| 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.
| 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.
| Polarimetric investigation of background stars in the region of T and RY Tau
Electropolarimetric observations of 97 background stars in the TTauri/RY Tauri region, obtained with the 40-cm Cassegrain telescope ofthe Biurakan Astrophysical Observatory in December 1982, are reportedand interpreted in terms of local-magnetic-field effects oncometary-nebula bending. The data are presented in a table, and thedependence of polarization position angle on declination is establishedin a graph and attributed to a continuous variation in the direction ofthe magnetic field. From the magnitude of the effect, however, it isconcluded that the observed bending of the cometary nebulae in theregion results from the combined influence of the local magnetic momentand the magnetic moment of the star itself (as proposed by Vardanian,1983) rather than from the local moment alone.
| On interstellar reddening law in the Taurus dark clouds
The interstellar reddening law in the Taurus dark clouds is investigatedfrom visual, infrared, and ground based ultraviolet photometricobservations of heavily reddened stars HD 29647, HDE 283701, and HDE283812. The star HD 29647 located in the center of a very dense darkcloud Khavtassi 278 shows a reddening law which is anomalous in thewavelengths shorter than 400 nm. When combined with observations by Snowand Seab (1980) from the IUE it can be represented by a single straightline from 1 micron to 250 nm. The two other stars situated in less denseparts of the Taurus dark clouds show normal reddening law.
| 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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Наблюдательные данные и астрометрия
| Созвездие: | Телец |
| Прямое восхождение: | 04h34m54.97s |
| Склонение: | +27°12'11.3" |
| Видимая звёздная величина: | 9.602 |
| Собственное движение RA: | -0.3 |
| Собственное движение Dec: | -8.6 |
| B-T magnitude: | 10.64 |
| V-T magnitude: | 9.688 |
Каталоги и обозначения:
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