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Differential polarisation studies of the deffuse interstellar band and Be stars

University of British Columbia

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 <0.1pm) which are poorly aligned or spherical are still consistent with our results as well as other observational properties of the bands. The absence of polarisation tends to favour a molecular origin for the diffuse bands. Attempts to measure differential polarisation associated with the line profile variations of two OB stars are also described. Analysis of the travelling subfeatures in the H/3 line of C Oph and the H/3 and He I \4921 lines of E Per failed to reveal polarisation structure with upper limits of 0.08% and 0.1%, respectively. Differential polarisation measurements in the 11,3 emission line of three intrinsically polarised Be stars are described. The stars Per and -y Cas exhibit significant polarization changes across the HO line. These changes are characterised by a decrease of polarization toward the center of the line and an increase at the line center itself. Our observations also show strong evidence for variations in the plane of polarisation across the line. We argue that unpolarised line emission cannot completely account for the observed polarization changes. Indeed, some of the polarisation structure especially in the line wings appears to be caused by absorption processes in a rotating circumstellar envelope. No significant change of polarisation was detected in the line of 28 Cyg.

Thesis/Dissertation

application/pdf

2008-09-09

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http://hdl.handle.net/2429/1729

Astronomy

University of British Columbia

UBCV

DSpace