Spectral variability of UX Ori in deep minima: Modelling of eclipses

Context. Young stars in the UX Ori family (UXORs) demonstrate irregular brightness variability with varying degrees of depth and duration due to the specific orientation of their nearly edge-on discs with respect to the line of sight. It is supposed that a dusty screen located near the sublimation zone hides the star from the observer by rising above the plane of the disc. During such eclipses the spectrum of the star changes significantly, something that permits us to study both the nearest surroundings of the young star and the nature of the screening objects. The properties and features in the spectrum of UX Ori (a Herbig Ae star of spectral type A3III) have been considered in detail in our previous papers. Aims. Thanks to long-term observations with the Nordic Optical Telescope (NOT) we found evidence of a substantial change in the Hα line profile of UX Ori: from double-peaked when the star is in its bright state to single-peaked when in deep minimum. Using the spectra obtained during different brightness states we calculated models of the star’s eclipses that can explain the behaviour of the Hα line. Methods. To solve the problem we calculated models of the emission regions in the nearest vicinity of the star, taking into consideration the zones of magnetospheric accretion and magneto-centrifugal disc wind, the radiation scattered on the circumstellar dust, and the extended disc wind beyond the dust sublimation radius. Line profiles were calculated for the star in its normal (bright) state, as well as during dust eclipses by introducing obscuring gas and dust screens at different positions. Results. Our modelling reproduces well the observed Hα line profiles, and indicates that the transformation of the Hα line during eclipses is due to the action of two factors: (a) by obscuring the emission of the magnetosphere of the star and that of the inner region of the disc wind with the dusty screen, and (b) by increasing the contribution of the emission from the periphery zone beyond the screen into the total emission. The possibility of applying this model to other UXORs is discussed.