A&A 489, 911-921 (2008)
DOI: 10.1051/0004-6361:200809521

Polarimetric measures of selected variable stars

N. M. Elias II^{1, 2}, R. H. Koch³, and R. J. Pfeiffer<sup>4

1</sup>  Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
    e-mail: n.elias@lsw.uni-heidelberg.de
²  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³  210 Roberts Road, Ardmore, PA 19003, USA
    e-mail: rhkoch@earthlink.net
⁴  Department of Physics, The College of New Jersey, Ewing, NJ 08628-0718, USA
    e-mail: pfeiffer@tcnj.edu

Received 6 February 2008 / Accepted 1 July 2008

Abstract
Aims. The purpose of this paper is to summarize and interpret unpublished optical polarimetry for numerous program stars that were observed over the past decades at the Flower and Cook Observatory (FCO), University of Pennsylvania. We also make the individual calibrated measures available for long-term comparisons with new data.
Methods. We employ three techniques to search for intrinsic variability within each dataset. First, when the observations for a given star and filter are numerous enough and when a period has been determined previously via photometry or spectroscopy, the polarimetric measures are plotted versus phase. If a statistically significant pattern appears, we attribute it to intrinsic variability. Second, we compare means of the FCO data to means from other workers. If they are statistically different, we conclude that the object exhibits long-term intrinsic variability. Third, we calculate the standard deviation for each program star and filter and compare it to the standard deviation estimated from comparable polarimetric standards. If the standard deviation of the program star is at least three times the value estimated from the polarimetric standards, the former is considered intrinsically variable. All of these statements are strengthened when variability appears in multiple filters.
Results. We confirm the existence of an electron-scattering cloud at L₁ in the Per system, and find that LY Aur and HR 8281 possess scattering envelopes. Intrinsic polarization was detected for Nova Cas 1993 as early as day +3. We detected polarization variability near the primary eclipse of 32 Cyg. There is marginal evidence for polarization variability of the  Cepheid type star  Peg. The other objects of this class exhibited no variability. All but one of the  Cepheid objects (ES Vul) fall on a tight linear relationship between linear polarization and E(B-V), in spite of the fact that the stars lay along different lines of sight. This dependence falls slightly below the classical upper limit of Serkowski, Mathewson, and Ford.

Key words: polarization -- binaries: close -- Cepheids -- circumstellar matter -- accretion, accretion disks -- dust, extinction

© ESO 2008

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