1 Introduction

It is well established that most of the pre-main sequence (pre-MS) stars in the range of spectral classes from early B to earlier F (so-called young Herbig Ae/Be stars; hereafter HAEBE) and from later F to M (so-called young T Tauri type stars; hereafter TT) show the variability in their polarization as well as the variability of their photometric and spectroscopic characteristics. As has been established by many observations, the intrinsic polarization of radiation is a common behaviour for most young stars (see for example Menard & Bastien [1992]; Yudin & Evans [1998] and references therein). It is not surprising because there is now a consensus that young stars exhibit dust and gas circumstellar (CS) shells whose shape may range from spherically symmetric envelopes, through geometrically and optically thick disks, to geometrically and optically thin (narrow) disks. Polarimetry which is more sensitive to the geometry and physical conditions of the material in the vicinity of the stars may help to solve many challenging questions in a theory of circumstellar evolution. However to gain a better understanding of the processes which may take place in CS shells a statistical analysis of correlations between the polarimetric and photometric characteristics should be made. Note that to make a detailed explanation for polarimetric variability and correlations between p and other characteristics of individual stars some specific processes in CS shells are often discussed (see for example Beskrovnaya et al. [1998] - presence of azimuthal inhomogeneities forming in different CS regions of a gaseous envelope; or Grinin et al. [1996]; Yudin et al. [1999] - fragmentation of large comet-like bodies in the vicinity of a star with ejection of small dust particles). However, as a rule, a nonvariable (on a short-time scale) component of polarization due to the presence of nonspherical dust envelopes exists in most young objects. Thus, to create a self consistent "star+shell" model for an individual object and for conclusions on the global changes of the dust distribution in CS shells of young stars the investigation of correlations between different kinds of observational characteristics (for representative groups of the objects which are on the different or similar stages of evolution) is most important. Such investigations have much potential for yielding information on classification criteria. As a result this may lead to a significant change in our knowledge of star and "star+shell" evolution.

The general aim of the proposed work is to compare all available polarimetric data for young stars with their photometric behaviour in the near and far IR, thus establishing real correlations between polarization and stages of their circumstellar shells evolution. Justification for this work is that since 1985-1988 numerous data on the polarimetry of different type young stars have been obtained. On the other hand, the lists of TT and HAEBE stars were re-established and many new objects were re-classified as young stars (Thé et al. [1994]). In addition, new hypotheses on CS shells' evolution have been considered in the past (mainly in connection with changes in the SED) (see for example Malfait et al. [1998]). In Sect. 2 we discuss briefly the results of previous studies of correlations between polarization and other observational characteristics of young stars. The accumulated lists of HAEBE and TT stars with available polarimetric and photometric data are presented in the Appendices 1-5. In Sect. 3, new correlation dependencies between $\log p$and IR colour excesses are investigated for different groups of young stellar objects (HAEBE and TT stars, young solar-type and Vega-type stars, HAEBE stars with Algol-like minima of brightness). The histograms of polarization and near IR excesses distributions for the above mentioned groups are compared. In Sect. 4 the polarization for different type stars is discussed in context with their projected rotational velocities. A comparison of this behaviour between the young stars and more evolved classical Be stars is made. For different types of stars which exhibit polarization of radiation and near IR excesses but are on a different stage of evolution (classical Be stars, red giants and supergiants, supergiants of early spectral types, peculiar B[e] stars, MS stars in solar neighborhood) their location at the $\log p/E(V-L)$ diagram are investigated in Sect. 5 and compared with the behaviour in young stars. In Sect. 6 main mechanisms of polarization in young stars are discussed, also simple calculations of expected polarization in the framework of a dust model are made. Some astrophysical aspects of the $\log p/E(V-L)$ diagram constructed here like that of evolutionary changes in CS shells of low- and intermediate mass young stars are discussed in Sect. 7. In Sect. 8 polarization of young stars is investigated in the context of their spectral energy distributions in the near and far IR.