Astron. Astrophys. 336, 604-612 (1998)

5. Conclusion

A preliminary synoptic study has been made of a sample of young solar type stars with the aim of detecting activity via polarimetry. Previous studies, both theoretical and observational, suggest that the levels of any polarization are likely to be small. As the stars are effectively solar type in terms of their luminosity and display high apparent brightness, they are relatively close and unlikely to be contanimated by interstellar polarization.

For the work reported here, both the linear and circular components have been investigated with typical measurement uncertainties in . This high level of accuracy causes problems with the assessment and removal of instrumental polarization due to the inadequacy of the catalogued unpolarized and polarized standard stars. In addition, even if the reference stars provided well calibrated values, an in-ordinate amount of time from the observational run would be required to perform the measurements to the required levels of accuracy. Consequently the tabulated polarimetric values for the programme stars may be subject to systematic errors of the order of the uncertainties of their measurements but this in no way negates the investigations and detections of any variable intrinsic polarization. There is no evidence of any instability in the instrumental polarization which could erroneously be interpreted as stellar variability.

The sample of investigated stars is small and the time window for the measurements is very limited. Of the stars suspected of displaying polarimetric variability, there are no similarities of behaviour. The most remarkable recorded event was for SAO154972 (TU Pyx) which displayed a sudden change in polarization from a level to in a time interval of about 15 minutes. There is also a strong indication of circular polarization associated with this star, suggesting the presence of magnetic activity. It is important that this star be included as a prime target in future polarimetric programmes.

Measurements of HD 115383 (59 Vir) on four nights display polarization variations which are coherent in the B and V bands, confirming that this originates in the star. The fact that the variation in V has larger amplitude than for B emphasises the notion that the behaviour truly occurs in the star and is not caused by any experimental instability, the instrumental polarization being much smaller in V than in B.

There is also evidence for a detection of circular polarization in the V band. The data for HD 155555 also display night-to-night variations in the V band but not in B; these may be related to previously monitored photometric variability which is also most strongly apparent in the V band.

The data for HD 1835 are very limited but there is marginal evidence for detection of a weak polarization in the B band.

All of these stars require further observations to characterise their polarimetric behaviour and to confirm that the observed effects are related to active regions traversing the projected disk as a result of stellar rotation or to other causes.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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