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The Efficiency of Grain Alignment in Dense Interstellar Clouds: a Reassessment of Constraints from Near-Infrared Polarization
We present the results of a detailed study of interstellar polarizationefficiency (as measured by the ratiopλ/τλ) toward molecular clouds,with the aim of discriminating between grain alignment mechanisms indense regions of the interstellar medium. The data set includes bothcontinuum measurements in the K (2.2 μm) passband and values based onice and silicate spectral features. Background field stars are used toprobe polarization efficiency in quiescent regions of dark clouds,yielding a dependence on visual extinction well-represented by a powerlaw(pλ/τλ~[AV]-0.52),in agreement with previous work. No significant change in this behavioris observed in the transition region between the diffuse outer layersand dense inner regions of clouds, where icy mantles are formed, and weconclude that mantle formation has little or no effect on the efficiencyof grain alignment. The field-star data are used as a template forcomparison with results for embedded young stellar objects (YSOs). Thelatter generally exhibit greater polarization efficiency compared withfield stars at comparable extinctions, some displaying enhancements inpλ/τλ by factors of up to ~6with respect to the power-law fit. Of the proposed alignment mechanisms,that based on radiative torques appears best able to explain the data.The attenuated external radiation field appears adequate to account forthe observed polarization in quiescent regions for extinctions up toAV~10 mag. Radiation from the embedded stars themselves mayenhance alignment in the lines of sight to YSOs. Enhancements inpλ/τλ observed in the icefeatures toward several YSOs are of greatest significance, as theydemonstrate efficient alignment in cold molecular clouds associated withstar formation.
| 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.
| 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.
| 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.
| 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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Observation and Astrometry data
| Constellation: | Taureau |
| Right ascension: | 04h43m05.03s |
| Declination: | +26°14'35.1" |
| Apparent magnitude: | 8.677 |
| Distance: | 257.069 parsecs |
| Proper motion RA: | -12.8 |
| Proper motion Dec: | -17.9 |
| B-T magnitude: | 10.715 |
| V-T magnitude: | 8.846 |
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INHABITED SKY
