Computing the Photometric and Polarimetric Variability of Be Stars

TL;DR

This study models the photometric and polarimetric variability of classical Be star disks, revealing how density enhancements and disk tilts influence observable polarization and brightness variations.

Contribution

It introduces detailed models of Be star disks with density enhancements and tilts, predicting their effects on observable variability using advanced radiative transfer simulations.

Findings

Density-enhanced disks cause up to 0.2% polarization variability.

Tilted disks at ~30° produce polarization angle changes of ~60°.

Brightness and color variations align with observed data.

Abstract

We investigate variations in the linear polarization as well as in the V-band and B-band colour-magnitudes for classical Be star disks. We present two models: disks with enhanced disk density and disks that are tilted or warped from the stellar equatorial plane. In both cases, we predict variation in observable properties of the system as the disk rotates. We use a non-LTE radiative transfer code BEDISK (Sigut & Jones) in combination with a Monte Carlo routine that includes multiple scattering (Halonen et al.) to model classical Be star systems. We find that a disk with an enhanced density region that is one order of magnitude denser than the disk's base density shows as much as $\sim$0.2% variability in the polarization while the polarization position angle varies by $\sim$8\deg. The $\Delta$V magnitude for the same system shows variations of up to $\sim$0.4 magnitude while the $\Delta$(B-V) colour varies by at most $\sim$0.01 magnitude. We find that disks tilted from the equatorial plane at small angles of $\sim$30\deg more strongly reflect the values of polarization and colour-magnitudes reported in the literature than disks tilted at larger angles. For this model, the linear polarization varies by $\sim$0.3%, the polarization position angle varies by $\sim$60\deg, the $\Delta$V magnitude varies up to 0.35 magnitude, and the $\Delta$(B-V) colour varies up to 0.1 magnitude. We find that the enhanced disk density models show ranges of polarization and colour-magnitudes that are commensurate with what is reported in the literature for all sizes of the density enhanced regions. From this, we cannot determine any preference for small or large density enhanced regions.