SIMBAD references

We have discovered that the magnetic variable 49 Cam displays a remarkable linear polarization, with periodic variations larger than those of other Ap stars. The time variation of the Stokes parameters has been established with good accuracy for the standard B band (Figs. 1 and 2) and additional measurements in the U and V bands already give some insight into the spectral variation of the polarization (Fig. 3). 49 Cam is expected to be closer than about 50 pc and its polarization, like that of most neighbouring stars, suffers little contamination from the interstellar polarization. The hypothetical presence of continuum polarization due to a circumstellar shell seems unlikely, given both the spectral dependence of the polarization and the negative result of our polarimetry bearing on several A-shell stars. Therefore, we think that the polarization of 49 Cam has to be understood, as for the other Ap stars, as the result of magnetic intensification in spectral lines. The interpretation of the polarization variation is much easier if one knows the magnetic phase derived from the classical longitudinal magnetic field measurements. Owing to new measurements obtained at the University of Western Ontario, we have improved the knowledge of the ephemeris of 49 Cam and we can propose a more accurate rotation period (4.2866 m_p0.0005 days). The specific shape of the linear polarization diagram makes it difficult to interpret our observations in the customary framework of the oblique rotating dipole. Modifications of this model are necessary: as a first step, we have introduced local weighting functions which can represent either a change of the actual field structure, or, simply, a change of the saturation of the spectral lines. In particular, one can reproduce fairly well the variations of the polarization of 49 Cam if the magnetic signal is not symmetrical with respect to the magnetic axis and has some peculiarities in the polar regions (Figs. 4 and 5). In fact, the problem is not to find a magnetic solution but rather that there are too many possible solutions. While we cannot claim that we have already found a firm model for 49 Cam, there are serious guidelines for progress of the interpretation. In any case, it is clear that this star is one of the most peculiar objects among the 40 Ap stars considered in our investigation.