Polarization Profile Near the 4430 Band

Extract

It has been reported by several authors that the extinction profile near the wavelengths of unidentified diffuse features shows fine structure (Brück and Nandy, 1970; Walker, 1971; York, 1971; Bromage, 1972); the question arises whether these features also exhibit fine structure in wavelength dependence of polarisation. It was first suggested by Greenberg et al. (1970, 1971) that interstellar bands produced by grain impurities may be polarized differently from the neighbouring continuum. Similar conclusions were also derived by Wickramasinghe and Nandy (1971) and Kelly (1971). Our first attempt to derive a profile of polarization against wavelength across the 4430 Å band was reported earlier (Nandy and Seddon, 1970). In this method we have used a Wollaston prism at the Cassegrain focus of the 36″ Telescope of the Royal Observatory, Edinburgh to produce orthogonally polarised spectra on the photographic plate. The results for the star 55 Cyg (Figure 1) indicated a definite profile. It was felt than an extension of the programme to acquire further profiles and to establish instrumental polarisation to a greater accuracy was necessary.