On the polarimetric variability of bright O-type stars

¹ University Observatory, Acre Road, Glasgow, G20 0TL, Scotland, UK
² Limber Observatory, PO Box 63599, Pipe Creek, Texas, 78063–3599, USA e-mail: mcdavid@limber.org
³ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098SJ Amsterdam, The Netherlands e-mail: huib@astro.uva.nl

Corresponding author: D. Clarke, d.clarke@astro.gla.ac.uk

Received: 16 January 2001
Accepted: 13 December 2001

Abstract

Polarimetric data associated with multi-parameter observational campaigns of selected bright O-type stars and their variable winds, are analysed in relation to the outcomes of the UV and optical spectroscopic studies. For the stars Per and Cep, individual measurement uncertainties are with nightly mean uncertainties of . Although variability is apparent on a night-to-night basis, with differences in , no correlations are found between these and the periodic behaviours associated with the stellar Si IV and Hα lines. Similar polarimetric variability is seen in the data for the standard star Cas used as a reference in this observing campaign. It is suggested that all of these low level fluctuations are not intrinsic to the stars but are engendered by structured instrumental polarization in the diffraction pattern and depolarization effects in combination with inconsistent target acquisition and with variable seeing conditions in the Earth's atmosphere. Reassessment of older data for Cep from Hayes ([CITE]) also supports this thesis.