Astron. Astrophys. 332, 149-154 (1998)

5. Circularly polarized radio emission

Some of the observations of our sample also contain measurements of the left hand (LHC) and right hand (RHC) circular polarizations. From these data we have selected only those with error bars lower than 5 . Moreover, according to Mutel et al. (1985), only the low level emission is polarized, therefore we have rejected, from the polarization data set, all data having a total flux density larger than 100 mJy.

The spectral analysis results are shown in Fig. 4 (see Appendix for details). Two relevant periods are present: one at 6.5 days in both spectra and one at 57 4 days in the PDM analysis and at 56 4 in the FFT one. The peak at 6.5 days is clearly related to the stellar rotation and can be easily explained in the following way. The longitudinal component of the magnetic field in the top of a non meridian loop changes sign at any central meridian transit of the loop. The electrons spiralling along these field lines are therefore alternately producing LHC and RHC polarization during each half rotation. The period of 12.88 days, previously determined by Neidhöfer et al (1993) on a subset of the present data was therefore a subharmonic of the 6.44 day period.

Fig. 4. Spectra by FFT and PDM for the circular polarization. Bottom: circular polarization data folded with a period of 57.14 days

While the 6.5 days period is due to a purely geometric effect, that at 57.14 days could be related to the magnetic field activity. It is in fact, within their error bars, twice the 25.5 day period previously determined. In the lowest panel of Fig. 4 the observed polarization percentages, plotted versus phase, turn out to be well represented by a sinusoidal curve of .

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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