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Astron. Astrophys. 362, 1020-1040 (2000)

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6. Polarization

Polarimetric data were obtained at the Crimean Observatory in 5 colors (UBVRI) and at McDonald Observatory in a B filter over six nights simultaneously with the spectroscopic campaign. In this paper only results derived from observations in the B filter are reported, after correction for the interstellar component determined by Poeckert et al. (1979). As in the spectroscopy, a rapid variation superimposed on a mid-term one was detected in the degree of polarization (Fig. 5d). The position angle was constant, and no significant periodicities could be found.

A positive correlation of decreasing polarization with decreasing absorption equivalent width of He1  6678 is suggested by Fig. 5a and c. Fig. 16a shows all B polarization data points together with the individual He1  6678 equivalent width data points at the most nearly matching times. The average time difference was 0.01 d and the greatest time difference was 0.04 d. Fig. 16b shows graphically the degree of correlation between the two quantities, which were normalized for comparison. The Spearman rank-order correlation coefficient is 0.533. The corresponding value of the complementary error function indicates a probability of 0.003% that the observed correlation would be found if the data were selected at random.

[FIGURE] Fig. 16. a: All intrinsic B polarization data points together with the He I 6678 equivalent widths observed at most nearly the same times. b Correlation between the B polarization and the He1  6678 equivalent width. Both quantities were normalized for comparison

The simultaneous decrease in He1  6678 equivalent width and B polarization might trace the dissipation of an expanding ring of gas ejected from the star, which would produce a drop in polarization due to the reduction in number density of scattering electrons as the gas expanded. At the same time, the accompanying decrease in opacity of the expanding gas would dilute the polarization by allowing more transmission of unpolarized light directly from the star. Geometrical dilution would also contribute to the polarization decrease as the scattering material moved farther away from the star.

The Spearman rank-order test shows no statistically significant correlation between the B polarization and the equivalent width, V intensity peak, or R intensity peak of the H[FORMULA] line profile.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000