3. New and old polarization measurements
3.1. GD 229
In a search of the long-term variations West (1989), compared his linear polarization measurements with the first data published by Kemp et al. (1974). These first observations were not sufficienly accurate, and it was not possible to judge about variability by comparing the R band position angle obtained by Kemp et al. (1974) with the value obtained by West (87o 12o and 94.2o 1o, respectively). More accurate observations of GD 229 were published by Landstreet & Angel, 1974; Efimov, 1981; Angel et al., 1981) but these escaped from West's consideration. In Table 1, we present all broad-band polarization data available from the literature, including our latest measurements. One can see from this table that there is an obvious difference in position angle measured in the period 1973-1976 (Landstreet & Angel, 1974; Efimov, 1981; Angel et al., 1981) and that measured by West. The B band position angle has rotated from 80o to 98o, while the degree of polarization remained at approximately the same value (about 2.4% - 2.9%).
Table 1. Polarization of GD 229
A comparison of our new data with that obtained by West (1989) shows further rotation of the position angle in all passbands by 15o - 20o, except for U band, where the position angle deviates from those observed in other wavelength bands at both epochs. The degree of polarization has increased in all passbands by factor of 1.5. There is also a remarkable change in the circular polarization, especially in the U band, to a much larger value of 3.2%, as compared to -0.1%. The sign of polarization has changed as well. The difference between our and West's data is depicted also in Fig. 1 as a wavelength dependence of circular polarization, linear polarization and position angle of linear polarization.
3.2. G 240-72
All available broadband linear polarization data are presented in Table 2. As is seen, B band position angle obtained by Angel et al., (1974) and that obtained by West are virtually identical within the errors of determination (47.5o 1.5o vs. 49o 4o). However, the new value, which we measured in 1997, is 37o 2o. The systematic decrease of position angle in comparison with the West's data is clearly seen in all passbands, although only in B and V bands the observational accuracy is good enough to make a definite judgement. The degree of polarization remained within the errors at the same value in U, B, R and I bands, but there is some indication of probable increase in V band (1.3% 0.1% vs. 1.9% 0.1%).
Table 2. Linear polarization of G 240-72
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999