CIRCUMSTELLAR ENVIRONMENT, INFRARED EXCESS AND VARIABLE EXTINCTION IN YOUNG STARS .2. DISCUSSION

In the present paper, we continue the analysis of the model of inhomogeneous dust envelope in young stars. Based on the results of Paper I, we suggest that most of the comet-like clouds are revolving in elongated orbits. This naturally produces the IR spectral energy distribution with the slope lambda F-lambda similar to lambda(-1) typical of many T Tauri stars and Herbig Ae/Be stars. Our suggestion agrees with the conclusion of Voshchinnikov and Grinin (1991) done from the analysis of observations of Ae/Be star WW Vul. We explain the evolutionary trend of the spectral slope in the IR as due to the fact that the clouds, closer to the star, should be firstly destroyed by tidal perturbations. We also give some preliminary analysis of the problem of cloud stability. Our model explains the nature of the extreme young objects with IR luminosities much larger than the photospheric luminosities, assuming that these objects are obscured by a large number of clouds, so that at each time there are similar to 5-10 screening clouds on the line of sight to the star. We find some indication that the number of screening clouds is a decreasing function of stellar age. The consequence of our interpretation is that the clouds, rather than the accreting disks, are formed in a very early stage of the stellar evolution, presumably, at the end of accretion phase. We support the interpretation of Voshchinnikov and Grinin (1991) that the existence of quasi homogeneous dust shells around young stars is explained by the destruction of clouds, which are unstable near the star. We suggest that the comet-like clouds can be proto-comets. If so, the observations of Grinin and coauthors can be the first direct evidence of the process of comet formation near young stars, which takes place in an early stage of the stellar evolution on the periphery of accreting molecular cloud. We predict that young stars should be variable in the far IR (so called, stochastic variability) with an amplitude of few percent and timescale of few days at 12 mu m. In principle, such a variability could be detected in the IRAS data. The best candidates for far LR variability studies are Herbig Ae/Be stars with spectral slopes in the IR lambda F-lambda similar to lambda(-1) - lambda(-2). Our model predicts the existence of anomalous extinction law in CTTS, caused by clumpy structure of their dusty winds. We find some observational indications supporting this prediction. We compare our model with the observed spectral energy distribution of three Herbig Ae/Be stars and show that the model fits well their spectra from about 3000 Angstrom and up to 1 mm, as well as their 10 mu m features.