9 Conclusions

We have presented the results of statistical study of correlations between polarization and photometric behaviour for a representative group of young stars of various types. It is shown that for 85% of the sample stars there is a general relation between the degree of optical polarzation and the infrared colour index $(V-L)_{\rm obs}$ and the colour excess E(V-L) due to the contribution of a circumstellar dust shell. We show that on average HAEBE stars exhibit larger near IR excesses than TT stars due to the difference in the amount of hot dust in their CS shells. Clear differences are found also in the polarization distribution for the groups of TT and HAEBE stars that occur for the same reason. On the other hand, most of the young early-type stars which might be classified as Vega-type or $\beta$-Pic type or post HAEBE stars (i.e. comparable young stars near the end of the pre-MS evolutionary phase or young MS stars) as well as young MS solar-type stars show statistically smaller near IR excesses and polarization than HAEBE and TT stars. From the analysis of polarimetric and photometric behaviour of Vega-type stars we show that this group is at present inhomogeneous. Some objects might be considered as genuinely MS stars and they have small polarization and no IR excesses whereas others have a little higher level of polarization and some near IR excesses. It is felt that the latter objects are in the transition stage from HAEBE stars to young MS stars. The behaviour of HAEBE and TT stars with Algol-like minima of brightness indicates that most of them are positioned on the diagram in the region in which 85% of all young stars are located.

We show that none of Be stars, Mira Ceti stars, early-type supergiants and MS stars lie within the relation derived for young objects but were distributed in separate groups on the diagram which reflects either absence of hot dust in their envelopes (if it exists) or differences in CS shell configuration.

All available data on the rotational velocities for young stars allow us to conclude that no correlation exists between $p_{\rm obs}$ and $v\sin{i}$ for TT stars and for HAEBE stars. Slight changes in $v\sin{i}$ distributions for TT $\Longleftrightarrow$young solar-type stars are well explained in terms of the canonical theory of stellar evolution. For $v\sin{i}$ distributions in HAEBE and classical Be stars it is reasonably safe to suggest that these distributions diverge considerably. Classical Be stars are much more rapid rotators even in comparison with young Herbig Be and B Vega-type stars.

Analysis of polarimetric characteristics of early-type stars from our sample indicates that a few different groups of objects may be distinguished taking into account the value of polarization and polarimetric variability as well as the values of near IR excesses. We suppose that this differentiation is caused by the changes in structure and geometry of dust circumstellar shells around the stars and these changes are caused by evolution of the dust circumstellar environment.

We also found that most TT stars which are accompanied by "active" CS disks show significantly larger polarization and near IR excesses than other TT stars and possibly average polarization and IR excesses decrease with the disk's evolution. We suggest that the dissipation of CS matter with decreasing optical depth may be well explained in terms of a combination of infall and outflow.

As follows from the analysis there is a definite sequence of changes in polarization values from the group of young stars with flat or rising IR spectra through the group with decreasing IR spectra to the groups with small IR excesses and further to the group of stars which contains the objects which are close to or on the MS.

Finally we conclude that there is a clear evidence of changes in polarimetric behaviour of stars during the evolution from young stars to MS.

Acknowledgements

I'd like to express my sincere thanks to Dr. M.A. Pogodin (Pulkovo Observatory) for useful discussions and to Prof. A. Evans (Keele University) and Dr. G. Penny (Glasgow University) for the help in the improving of the text. The research described in this paper was made possible in part by the 99-02-16336 grant of RFBR. This research has made use of the VizieR Service and SIMBAD database, operated at Centre de Données Astronomiques de Strasbourg, France.