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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.
| Testing Protoplanetary Disk Alignment in Young Binaries
We present K-band (2.2 μm) imaging polarimetry that resolves 19 TTauri binary and multiple systems in the Taurus-Auriga andScorpius-Ophiuchus star-forming regions. We observed systems withprojected separations 1.5"-7.2" (~200-1000 AU) in order to determine therelative orientation of the circumstellar disks in each binary system.Scattered light from these disks is polarized, allowing us to deduce theposition angle of the disk on the sky from the position angle ofpolarization even though our observations do not resolve the disksthemselves. We detected measurable polarization (typically 0.5%-2%, withtypical uncertainty 0.1%) from both stars in 14 of the systems observed.In eight of the nine binary systems, the two stars' polarizationposition angles are within 30° of each other, inconsistent withrandom orientations. In contrast, the five triple and quadruple systemsappear to have random disk orientations when comparing the polarizationposition angles of the widest pair in the system; the close pairs areunresolved in all but one system. Our observations suggest that disks inwide (200-1000 AU) binaries are aligned with each other within<~20° but not perfectly coplanar. However, we cannot conclusivelyrule out random relative disk orientations if the observed polarizationsare significantly contaminated by interstellar polarization. Even in thepresence of interstellar polarization our observations securely excludecoplanar disks. These results provide constraints on possible binaryformation mechanisms if the observed orientations are primordial. On theother hand, models of disk-binary interactions indicate that the disksmay have had time to decrease their relative inclinations sinceformation. If the common orientation of the disks in these binaries is atracer of the binary orbital plane, then our results also havesignificance for the stability of planetary orbits, suggesting thatplanetary systems in wide binaries should be stable over 109yr timescales.
| 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.
| 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.
| 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.
| UBV photometry of stars whose positions are accurately known. IV
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...68..211O&db_key=AST
| 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.
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