Modelling UX Ori Star Eclipses based on Spectral Observations with the Nordic Optical Telescope. I. RR Tau

TL;DR

This study models spectral variability during eclipses of the UX Ori star RR Tau, suggesting that dust clouds and disc winds influenced by magnetic fields cause deep fadings and spectral changes.

Contribution

It introduces two obscuration scenarios and compares them with spectral observations, providing insights into the causes of eclipses in UX Ori stars.

Findings

Vertical dust screen explains flux and line asymmetry variability.

Azimuthal dust movement explains some spectral features.

Main causes of eclipses likely involve structured disc winds or magnetic dust lifting.

Abstract

Based on observations obtained with the Nordic Optical Telescope we investigate the spectral variability of the Herbig Ae star RR Tau. This star belongs to the UX Ori family, characterized by very deep fadings caused by the screening of the star with opaque fragments (clouds) of the protoplanetary discs. At the moments of such minima one observes strong spectral variability due to the fact that the dust cloud occults, for an observer, not only the star but also a part of the region where the emission spectrum originates. We calculated a series of obscuration models to interpret the observed variability of the H-alpha line parameters. We consider two main obscuration scenarios: (1) the dust screen rises vertically above the circumstellar disc, and (2) the screen intersects the line-of-sight moving azimuthally with the disc. In both cases the model of the emission region consists of a compact magnetosphere and a magneto-centrifugal disc wind. Comparison with observations shows that the first scenario explains well the variability of the radiation flux, the equivalent width, as well as the asymmetry of the H-alpha line during eclipses, while the second scenario explains them only partly. This permits us to suggest that in the case of RR Tau, the main causes of the eclipses are either a structured disc wind, or the charged dust lifted along the field lines of the poloidal component of the magnetic field of the circumstellar disc.