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Differential polarisation studies of the deffuse interstellar band and Be stars Dinshaw, Nadine


A microcomputer-controlled polarisation analyser has been designed and built at the Dominion Astrophysical Observatory specifically for the 1.83-m telescope. It is mounted before the Cassegrain spectrograph entrance, and is capable of providing spectropolarime-try at the full (--, 0.15 A) resolution of the spectrograph-detector system. The device uses a polarising beam splitter for selecting a plane of polarisation, and a quarter-wave plate to remove most 90%) of the effects of instrumental polarisation. The throughput of the analyser is --, 40%. The analyser may be rotated at a rate of up to 12.5° s'; position angles of the analyser may be set to within +0.5°. We present the results of three studies carried out with the polarisation analyser: if the diffuse interstellar bands arise in the same grains which produce the optical continuum polarisation, whereby the bands represent spectral regions of enhanced extinction, then a corresponding enhancement in the polarisation is expected. Differential polarization measurements in four of the strongest diffuse features in the spectra of HD 183143, 55 Cyg and Per revealed no polarisation structure through any of the bands at levels significantly lower than those predicted. From our results, we have established upper limits to the variation of polarisation in the 5780, 5797, 6177 and 6284 A bands of 0.01, 0.01, 0.02 and0.03%, respectively. We conclude that the diffuse features are not associated with the grains responsible for the continuum polarisation. Intermediate-sized grains (0.02 < a

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