Volume 558, October 2013
|Number of page(s)||16|
|Section||Stellar structure and evolution|
|Published online||26 September 2013|
Variations of Stokes IQUV profiles in the 5190–5196 Å wavelength region. The four prominent features at 5191–5193 Å are blends of Fe-peak and REE lines. The other two lines around 5195 Å belong to Fe. This plot is characteristic of the complex polarization profiles observed in the spectra of HD 24712. The format of this figure is similar to Fig. 2.
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Previous HARPSpol observations demonstrated an exceptional polarimetric sensitivity of this instrument. For example, Piskunov et al. (2011) could put an upper limit of ~10-5 on the circular and linear polarization signal in the lines of the bright inactive star α Cen A. These results demonstrate that the instrument does not suffer from a noticeable spurious polarization. Intrinsically unpolarized spectral features, such as telluric lines, also show no polarization artifacts down to ~10-3 in our observations of HD 24712. However, these measurements do not directly constrain the polarimetric precision, i.e., the uncertainty with which strong polarization signals can be measured with this instrument. Our observational data for HD 24712 and the employed polarimetric demodulation method allow us to derive not only the Stokes parameters but also the null polarization spectra. The latter can be used as a diagnostic tool to detect and characterize spurious polarization and crosstalks. In this section we present a detailed analysis of the mean Stokes VQU null profiles.
With the help of the multiprofile LSD code we calculated LSD null profiles for Fe-peak elements, REEs and for the full line mask. The results of these calculations, which are illustrated in Fig. A.1, demonstrate the presence of very weak signals in all of the null LSD profiles. The amplitude of these signals in the LSD Q, U, and V null profiles is 12, 25, and 60 times smaller than the corresponding LSD Stokes profiles. The signals present in the null Q and U profiles are similar in shape and correlate in amplitude with the Stokes V signal. This might be an indication of a crosstalk from circular to linear polarization.
On the other hand, the null LSD V profiles show a more complicated picture that most likely is a combination of several factors. Instead of a clear rotational phase dependence as in the case of the null LSD Q and U profiles, we see only a semi-regular variability. Thus, we cannot attribute the null Stokes V signal to a crosstalk from Q and U to V. An alternative possibility is that the Stokes V null spectrum is dominated by a spurious signature appearing due to a variation of the barycentric velocity in the course of a fairly long sequence of polarimetric sub-exposures. In that case, the Stokes V null signature would be proportional to the derivative of Stokes V profile itself times the velocity change between the first and last sub-exposure.
We used a weighted linear least-squares fitting procedure to estimate the crosstalk levels. In the case of circular to linear polarization, we found a crosstalk level of 0.37 ± 0.01% for Stokes Q and 0.5 ± 0.01% for Stokes U. Figure A.1 shows that the null linear polarization signatures are indeed very well approximated by the scaled Stokes V profile. A similar fit attempt to reproduce the null Stokes V profile by a superposition of the Stokes Q and U profiles is less successful and requires crosstalk levels of 4–8%.
Summarizing this crosstalk analysis, we can state that the detected spurious polarimetric signals cannot significantly affect our measurements because their level is 15 to 60 times smaller than the corresponding LSD Stokes profiles. Nevertheless, in the subsequent measurements using the LSD Stokes profiles (Sect. 6.1) we continue using null profiles to assess possible systematic errors due to non-ideal polarimetric measurements and artifacts in data processing.
Analysis of the LSD null profiles of HD 24712. Thin black lines show mean profiles derived from the null Stokes Q (left panel), U (middle panel), and V (right panel) spectra. The thick red lines show an attempt to reproduce these spurious polarization signatures with scaled LSD Stokes V profiles (QUpanels) and a linear combination of the LSD Stokes Q and U profiles (Vpanel).
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© ESO, 2013
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