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The protoplanetary disks of very young intermediate mass stars

Several observational and theoretical programmes were carried out in the framework of the extension phase of our project. They concentrated on the study of spectroscopic, polarimetric and photometric behaviour of the Herbig Ae/Be (HAEBE) and UX Ori-type stars (UXORs) and on the possible connection of their activity with the properties of their circumstellar (CS) matter.

An important result of the previous photopolarimetric observations of UX Ori-type stars started in the Crimean Astrophysical Observatory in 1986 was the evidence that their large amplitude variability is caused by variable circumstellar extinction and is a result of the edgeon orientation of their protoplanetary disks relatively to the observer (Grinin et al. 1991; Grinin 1992). Both these hypothesis were supported by new observations.

Several UXORs were observed by Natta et al. (1977) in the millimeter wavelengths (at 1.3 mm) on the IRAM 30m telescope. The importance of this spectral region is due to the fact that the CS dust disks are optically thin at these wavelengths, so that the mass of CS dust, Md, can be simply derived from the observed flux and does not depend on the orientation of the disk.

Mearurements were combined with data from the literature to examine a sample of 30 young stars with spectral types FO-B9, i.e. Herbig Ae stars, about half of which display UXor characteristics. No systematic differences were found in Md between the two groups of stars. Moreover, no statistically significant correlation between Md and stellar age was found, and the amplitude of the photometric variability appeared to be independent of age. It was proposed that UXORs phenomena do not characterize a more evolved environment. They are probably common to the majority of stars in our sample, but are observed only when the line-of-sight is close to the equatorial plane of an aspherical circumstellar nebula.

In the framework of the observational program of high resolution spectroscopic, study of UXORs with the Nordic Optical Telescope it was observed at the first time the rapid (during two days!) variability of the forbidden line [OI] 6300/6364 Å in the spectrum of the classical Herbig Ae/Be star RR Tau: during the deep minimum of RR Tau on Dec. 24 of 1996 the intensity of this line increased by about three times with comparison to its intensity during the star bright state. Such a variability gives a new independent evidence that the photometric activity of UXORs is the result of screening of the stellar radiation from the observer by opaque circumstellar dust clouds.

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