Non-LTE Monte Carlo Radiative Transfer. III. The thermal properties of Tilted and Warped Be Star Discs

TL;DR

This study models tilted and warped Be star discs using Monte Carlo radiative transfer, revealing how tilt affects temperature asymmetry and observable features like H-alpha line shape and polarization, with implications for disc geometry diagnostics.

Contribution

It introduces detailed 3D models of tilted and warped Be star discs, analyzing their thermal properties and observational signatures across multiple spectral types and tilt angles.

Findings

Disc tilting causes temperature asymmetry in the disc.

Observable features like H-alpha line shape and polarization depend on tilt orientation.

Tilt effects on temperature are minimal, but significantly influence certain observables.

Abstract

We use the three-dimensional Monte Carlo radiative transfer code HDUST to model Be stars where the disc is tilted from the equatorial plane of the star. We compute 128 models across 4 spectral types, B0, B2, B5 and B8, tilting the disc by $0^o$, $10^o$, $20^o$, and $40^o$, while varying disc density according to spectral type. We also compute every model for an average and high stellar rotation rate. We first discuss non-tilted disc temperatures and show its non-linear dependence on stellar and disc parameters. We find that tilting the disc minimally affects the density-weighted average disc temperature, but tilting does create a temperature asymmetry in disc cross sections, which is more pronounced for a faster rotation rate. We also investigate the effect tilting has on $V$-band magnitude, polarization, and the H$\alpha$ line. Tilting the disc does affect these observables, but the changes are entirely dependent on the position of the observer relative to the direction of tilt. We find the observables that distinguish tilting from a change in density or geometry are the H$\alpha$ line shape, where it can transition between single-peaked and double-peaked, and the polarization position angle, whose value is dependent on the projected major elongation axis of the disc on the sky. We also present one early and one late-type model with warped discs. We find their temperature structure varies a small amount from the uniformly tilted models, and the different observables correspond to different tilt angles, consistent with their expected volume of origin within the disc.