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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.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| 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.
| A new optical extinction law and distance estimate for the Taurus-Auriga molecular cloud
This paper presents optical spectrophotometry of field stars projectedon the Taurus-Auriga molecular cloud. We derive extinction laws forheavily reddened stars in this sample and show that the reddening lawthrough the dark cloud is nearly identical to the standard law forlambda-lambda(3600-6100) and AV approximately less than 3mag. Our spectroscopic parallaxes suggest a distance of 140 +/- 10 pcfor the northern portion of the cloud and show no compelling evidencefor a substantial variation in distance across the leading edge of thecloud.
| 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.
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