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Observational Constraints on Interstellar Grain Alignment
We present new multicolor photopolarimetry of stars behind the SouthernCoalsack. Analyzed together with multiband polarization data from theliterature, probing the Chamaeleon I, Musca, ? Opiuchus, R CrA, andTaurus clouds, we show that the wavelength of maximum polarization(?max) is linearly correlated with the radiationenvironment of the grains. Using far-infrared emission data, we showthat the large scatter seen in previous studies of?max as a function of AV is primarily dueto line-of-sight effects causing some AV measurements to notbe a good tracer of the extinction (radiation field strength) seen bythe grains being probed. The derived slopes in ?maxversus AV, for the individual clouds, are consistent with acommon value, while the zero intercepts scale with the average values ofthe ratios of total to selective extinction (RV) for theindividual clouds. Within each cloud we do not find direct correlationsbetween ?max and RV. The positive slope isconsistent with recent developments in theory and indicating alignmentdriven by the radiation field. The present data cannot conclusivelydifferentiate between direct radiative torques and alignment driven byH2 formation. However, the small values of?max(AV=0), seen in several clouds, suggesta role for the latter, at least at the cloud surfaces. The scatter inthe ?max versus AV relation is found to beassociated with the characteristics of the embedded YSOs in the clouds.We propose that this is partially due to locally increased plasmadamping of the grain rotation caused by X-rays from the YSOs.
| The distance and structure of the Coalsack. I - Photometric data
The results of UBV and H-beta photometry for 284 stars of spectral typesB to F located in the area of the dark cloud Coalsack are presented.Most of the present stars are brighter than 10.0 mag and are classifiedin the MK system of Houk and Cowley (1975). For each star, the bestvalues for the extinction and distance modulus are provided, withtypical estimated errors of + or - 0.08 mag for the extinction and + or- 0.32 mag for the distance modulus. Possible misclassifications andspectral peculiarities are noted for several of the stars.
| Stromgren and H-beta photometry of stars earlier than G0 in the Southern Coalsack direction
Four-color and H-beta photometry have been obtained for 236 starsearlier than G0 in order to study the color excess distribution in theSouthern Coalsack direction. The beta histogram of the stars reveals arelative absence of stars in the range of A4 to A9. The (b-y)distribution for B-type stars suggests the existence of three differenttypes of stars located at different distances.
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