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

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5. Conclusion

The main results obtained in this work can be summarized as follows:

1. The anti-correlation between the linear polarization and the brightness variations of the classical HAeBe star RR Tau, observed earlier by Kardopolov and Rspaev (1989) and Kardopolov et al. (1994), is confirmed in the larger interval of brightness changes, which cover practically all the observed variability range (about 100 times in the U pass-band!). Such a behaviour agrees well with the predicted results in the model of variable CS screening of a young star, in which the opaque dust fragments of its protoplanetary disk are causing this effect.

2. In the framework of this model the minimum of the brightness variations are controlled by scattered radiation due to CS dust, which dominates at deep minima. In the case of RR Tau this radiation is weaker compared with other UXORs; it is only about 3% of the stellar radiation in the V pass-band.

3. The position angle of the intrinsic linear polarization of RR Tau coincides within about [FORMULA] with the direction of the local magnetic field estimated from the polarization map of this region. Such a coincidence means that the equatorial plane of the circumstellar disk-like envelope of RR Tau is perpendicular to the magnetic lines. A similar feature was found earlier for several other UXORs: (Grinin et al. 1988, 1991, 1995). From the point of view of the modern scenario of star formation (see Mestel 1985) such an orientation indicates an important rôle of the magnetic field at the initial phase of collapse of the protostellar cloud.

4. Two flare-like events of RR Tau were observed near J.D. 9224-9225 and 9245. The colour-magnitude variations during both "flares" follows with similar slopes the upper parts of the colour-magnitude diagrams of RR Tau [FORMULA] vs. V, [FORMULA] vs. V and [FORMULA] vs. V caused by CS reddening. This means that both "flares" were caused by the occurrence of short-living holes, almost free of dust, in the line-of-sight. However, such an interpretation does not explain the blue colour [FORMULA] of the "flares" which is probably connected with Balmer jump emission. Both these phenomena: formation of the short-living holes free of dust and the additional hydrogen emission are probably different manifestations of the same phenomenon connected with violent non-stationary processes in the neighbourhood of the star.

5. The numerical simulation of the colour-magnitude diagrams and the intrinsic linear polarization of RR Tau based on the Mie theory support our previous conclusion (Grinin et al. 1991) that the CS dust envelopes around UXORs are strongly flattened. This result is important in the light of current discussions of models of CS envelopes around HAeBe stars (see Hartmann et al. 1993 and references therein). It means that the geometry of the dust envelopes of these stars (many of them are HAeBe stars) does not differ principally from those of T Tauri stars. Their form is disk-like.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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