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

1. Introduction

The optical radiation from young stars at both ends of the spectral classification sequence frequently exhibits intrinsic linear polarization (Menard & Bastien, 1992, Jain & Bhatt, 1995 - and references therein). It is now well established that at least 85% (or even as many as 100%) of such stars exhibit temporal polarimetric variability on a wide range of time scales (Menard & Bastien, 1992, Yudin & Evans, 1998). These include Herbig Ae/Be stars (HAEBE spectral class from B to early F) and T Tauri type stars, ranging between later F stars to early M). There is, however, little or no information on the polarimetric properties for intermediate spectral type stars ranging between late F and early G8, the so-called solar-type stars. In catalogues of young stars (Herbig & Bell, 1988 and Thé et al., 1994) this latter category represents about 15% of the total objects and most of them have not been studied polarimetrically.

Recently it has been shown that the polarimetric properties of young stars might be considered from the point of view of their circumstellar evolution (Tamura & Sato, 1989 and Yudin, 1994). As suggested by Yudin (1994), at least one subgroup of HAEBE stars, namely those with algol-like brightness minima (Grinin, 1988), represents a late stage of circumstellar evolution with fragmented circumstellar disks forming protoplanetary condensations. Such asymmetries would affect the scattering with an associated polarization being generated in the observed light. At present this subgroup contains about 10 objects which have spectral classes mainly A. No such behaviour has been found in T Tauri type stars.

As a result of magnetic activity, young solar-type stars exhibit chromospheric disturbances which can be monitored by K-index photometry. The renowned observational program at Mt Wilson (see Vaughan et al., 1981) is a prime source of information for solar-type stars on rotation periods, typically a few days, and on magnetic cycles with periods of years. Radiation emanating from active regions is likely to be partially polarized for a variety of reasons, one such polarigenic mechanism being the integration of Zeeman split lines with differential saturation (see Leroy and LeBorgne, 1989). The expected wavelength dependence of the degree of any polarization, , generated by this mechanism has been explored by Leroy (1989). Predictions of the magnitude of p to be observed globally from stars are generally not sanguine for the promotion of extensive polarimetric studies and there has been controvosy about some of the reported observations - see discussions in Clarke and Fullerton (1996). Fox (1995) has modelled the behaviour of star spots using Rayleigh scattering as the polarigenic mechanism and suggests that their presence might be detected with polarimetric accuracies , a challenging but not impossible task. On the positive and observational side, Tinbergen and Zwaan (1981) have used statistical arguments to suggest that polarimetric phenomena, at low levels, are fairly common in middle spectral type stars and with reports of polarimetric variability of individual objects (see e.g. Piirola, 1977 [ Her - F9V] and Kemp et al., 1986 [ Boö - K1.5 III]), there is evidence for promoting further polarimetric studies of solar-type stars, albeit with the requirement of extremely high precision.

Recently, an attempt to record rotationally induced polarimetric variability has been undertaken by Gullbring & Gahm (1995) but with null results, an outcome which may have been influenced by the short run of observations and/or measurement uncertainties which were too large. Clarke and Fullerton (1996) report on novel measurements of the global radiation from the Sun near to the solar maximum and have recorded changes in p just larger than 0.0001 at the time of the appearance of large spots traversing the disk, promoting the possibilities of making solar-stellar connections by polarimetry.

A further dimension to polarimetry of stars in the middle part of the spectral sequence is offered by the measurements of circular polarization resulting from the presence of magnetic field structures. Kemp et al. (1987), for example, have recorded variations of the circular polarization of the RV CVn star  And and Elias and Dorren (1990) have recorded both linear and circular polarization variations in the young solar-type star HD 129333, thought to have spots covering 6% of its surface, these being responsible for low amplitude light variations. It is important to note that, according to Bastien et al. (1989), circular polarization has been detected in most of the investigated young T Tauri stars, whereas for young HAEBE stars, circular polarization was detected for only a few objects (Yudin & Evans, 1998 - and references therein).

With the above ideas in mind, a polarimetric investigation of some southern hemisphere objects has been undertaken with aim of detecting variability in solar-type stars and the results are reported below.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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