A New Diagnostic of the Radial Density Structure of Be Disks

TL;DR

This paper introduces a novel diagnostic method using polarization measurements across the Balmer jump and V-band to analyze the radial density structure of Be star disks, supported by simulations and observations.

Contribution

It proposes a new polarization-based diagnostic tool for probing the radial mass distribution in Be star disks, validated through Monte Carlo simulations.

Findings

Polarization across the Balmer jump traces inner disk density.

V-band polarization reflects total disk scattering mass.

Loop structures in polarization diagrams indicate mass decretion activity.

Abstract

We analyze the intrinsic polarization of two classical Be stars in the process of losing their circumstellar disks via a Be to normal B star transition originally reported by Wisniewski et al. During each of five polarimetric outbursts which interrupt these disk-loss events, we find that the ratio of the polarization across the Balmer jump (BJ+/BJ-) versus the V-band polarization traces a distinct loop structure as a function of time. Since the polarization change across the Balmer jump is a tracer of the innermost disk density whereas the V-band polarization is a tracer of the total scattering mass of the disk, we suggest such correlated loop structures in Balmer jump-V band polarization diagrams (BJV diagrams) provide a unique diagnostic of the radial distribution of mass within Be disks. We use the 3-D Monte Carlo radiation transfer code HDUST to reproduce the observed clockwise loops simply by turning "on/off" the mass decretion from the disk. We speculate that counter-clockwise loop structures we observe in BJV diagrams might be caused by the mass decretion rate changing between subsequent "on/off" sequences. Applying this new diagnostic to a larger sample of Be disk systems will provide insight into the time-dependent nature of each system's stellar decretion rate.