2 Previous studies of correlations between polarimetric and photometric data for young stars

The first attempt to compare the observed polarimetric characteristics of young TT stars with the colour indices V-R, V-I, V-J, V-H, V-K, V-L, V-M and V-N(Bastien [1982]) led to detection of a correlation between p and the IR colour indices. In a following paper (Bastien [1985]) a statistical analysis of similar data on the polarization of 85 TT stars was made and correlations between p and the IR colour indices and the colour excess [V-L] was found. Note however that the correlation coefficients obtained by Bastien ([1985]) were small and not significant in some cases. Nevertheless the main conclusion of Bastien ([1985]) was as follows:
The polarization is well correlated with IR colour indices and colour excesses, implying that most of the IR excesses are due to the same dust grains which are responsible for the polarization.

A little later, the comprehensive analysis of correlation between p and photometric characteristics of young stars of different types (TT and HAEBE stars) was fulfilled by Yudin ([1988]). In the framework of his study a correlation between p and IR colour excesses E(V-J), E(V-H), E(V-K), E(V-L), E(V-M) was investigated (where these colour excesses were calculated using the following formula:

$$E(V-L)=(V-L)_{\rm obs} - (V-L)_{0} - (A_{V} - A_{L})$$ (1)

and (V-L)₀ is the normal colour index corresponding to the spectral type of the star, A_V and A_L are the amount of interstellar absorption in the respective photometric bands). It is important that E(V-L) determines the relative contribution of the circumstellar shell to the IR emission in comparison with the stellar photosphere in the given IR band.

As a result of the work of Yudin ([1988]) two main conclusions were made:

1. Common linear dependencies of $\log p$ on the IR colour excesses are exhibited for 80% of young stars in both types (TT and HAEBE), and the correlation coefficients of these dependencies were high (up to 0.93);
2. Dust with a temperature of $\approx 900~\mathrm{K}$ (i.e. dust which is responsible for IR excesses in L and M bands) makes the maximum contribution to the optical polarization of young stars.

Moreover, it was also noted that the $\log p/E(V-L)$ diagram can provide a criterion for selecting stars that are undergoing pre-MS evolution. This conclusion was made on the basis of a comparison between the position of young stars as well as Be stars and red giants on the above mentioned diagram.

Another important point is that in the first paper Bastien ([1982]) advanced the suggestion that stars with more highly polarized radiation are younger, but later (Bastien [1985]) he rejected his own suggestion. Yudin ([1988]) however showed that more young stars have more highly polarized emission, on average, and suggested that probably polarization of young stars originating in the circumstellar dust shells decreases as the stars evolved toward the MS.

At approximately the same time Tamura & Sato ([1989]) discussed the changes in IR polarization of TT stars in terms of evolutionary changes in the optical thickness of their circumstellar disks. They showed that "the IR polarization is caused by CS dust grains around TT stars and there is a positive correlation between the IR polarization and colour excess while no correlation is found between polarization and reddening". They also suggested a possible evolutionary sequence in the IR polarization of young stellar objects.

Later Yudin ([1990]) showed that there is a certain sequence in the CS shell evolution of different types of young stars in the IRAS colour-colour diagram and tried to discuss the changes in optical polarization of young stars in this context. This approach was also recently applied to the analysis of the evolutionary status for the large group of early-type young stars and candidate members by Yudin & Evans ([1998]).

The bank of polarimetric data (available at present) for young stars allows us to make a new statistically significant investigation of evolutionary changes in polarization, taking into account new results in the theory of CS shell evolution.