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Astron. Astrophys. 327, 145-154 (1997)

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1. Introduction

In this paper we report the continuation of our photo-polarimetric investigations of young stars with non-periodic Algol-type minima, of which UX Ori is a prototype (Bibo and Thé 1991). An important special feature of their polarimetric activity is the clear anti-correlation between brightness and polarization variations (Grinin et al. 1991). This behaviour of the linear polarization is predicted in the framework of the variable CS extinction model and is a result of the screening of the young star by opaque dust fragments (clouds) in its protoplanetary disk (Grinin 1986). When such a star is observed we measure the sum of the direct and the scattered stellar radiation. The former is variable due to the presence of the small clouds which, at times, intercept the stellar light. The latter is constant if it is mostly due to circumstellar dust distributed in a much larger volume, of which the clouds occupy only a small fraction.

The observations at deep minima are especially important. In such episodes the CS dust clouds, intersecting the line of sight, act as a natural coronograph. It allows the observations of the weak scattered radiation of the young protoplanetary disk, somewhat contaminated by direct stellar radiation. High linear polarization (5-8%) which is systematically observed at deep minima indicates that the protoplanetary disks of the UX Ori type stars (below we will use the abbreviation "UXORs" suggested by Herbst et al. 1994) are observed edge-on or somewhat inclined to the line-of-sight. This is probably the main reason of their large amplitude Algol-type variability (Grinin 1992). The investigations of such objects are of great interest for the study of the physics of young stars.

Because of the unpredictable character of the Algol-type minima long-term photo-polarimetric monitoring of the UXORs is needed to cover the whole interval of the brightness changes. Below we present the results of the coordinated photo-polarimetric multi-band observations of the HAeBe star RR Tau made at the Crimean Astrophysical Observatory (CAO) in cooperation with the National Optical Observatory of Japan (Dodaira). Unlike previous objects of our program (UX Ori, BF Ori, WW Vul, CQ Tau etc.) which all are isolated HAeBe stars 1, RR Tau is a classical HAeBe star associated with a weak nebulosity (Herbig 1960).

According to Herbig (1960) the spectral type of RR Tau is B8-9. Strom et al. (1972) estimated it as A3-A5. The difference is caused by the use of different spectral lines for the spectral classification.

The variability range of RR Tau is about [FORMULA] from [FORMULA] to [FORMULA] (Herbig & Bell 1989). The photometric behaviour of this star has been investigated by Rössiger & Wenzel (1974) who observed a complex behaviour of RR Tau in the colour-magnitude diagram [FORMULA] vs. V (the so-called "turnaround" of the colour-indices). Later a similar "turnaround" in the colour-magnitude diagrams [FORMULA] vs. V and [FORMULA] vs. V at deep minima was also observed by Zajtseva (1986). She noted that the star at deep minima can be even bluer in [FORMULA] than at the maximum brightness state.

The published polarimetric observations of RR Tau are fragmentary and were not accompanied by synchronous photometry (Breger, 1974; Garrison & Anderson, 1978; Vrba et al. 1979; Hillenbrand et al. 1992). An exception are the works by Kardopolov & Rspaev (1989) and Kardopolov et al. (1994). They observed a decrease of brightness of RR Tau from V = 10.m8 to V = 13.m2 accompanied by an increase of the linear polarization from [FORMULA] up to [FORMULA] and by changes of the position angle within about [FORMULA].

In Sect. 2 we present the results of the multi-year photo-polarimetric monitoring of RR Tau which cover the whole interval of its brightness changes 2. The analysis of the observational data and their numerical modeling are given in Sects. 3 and 4. The main results are summarized in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998