Predicted observational effects of rapid rotation for Be stars

TL;DR

This study systematically assesses how rapid rotation and gravity darkening modify Be-star observables by employing 3D NLTE radiative-transfer models across spectral subtypes B0–B8, disk densities, and viewing inclinations. Using the hdust code, the authors quantify changes in V-band magnitude, B-V color, H-alpha equivalent width, and UV/V-band polarization, revealing strong, subtype- and density-dependent effects. Key findings include rotation-induced photometric displacement tracks, complex H-alpha EW responses that hinge on inclination and disk density, and polarization behavior that is strongly impacted by gravity darkening, with the polarization color serving as a rotation diagnostic. The results underscore the need to account for rotation when interpreting Be-star observations and highlight UV spectropolarimetry as a valuable approach for constraining rotation rates in rapidly rotating Be stars.

Abstract

We conduct a systematic study on the effects of rapid rotation on predicted Be star observables. We use the three-dimensional Monte Carlo radiative transfer code, \textsc{hdust}, to model a comprehensive range of Be star subtypes at varying rotation rates. Using these models, we predict $V$ magnitude and photometric color, H$α$ line profiles, and polarization at UV wavelengths as well as in the $V$-band for Be stars from B0 to B8. For each spectral subtype, we investigate the effects of disk density on the produced observables. We find that reddening and brightening effects of gravity darkening may cause rapidly-rotating stars to appear more evolved than they truly are. Rotational effects on the H$α$ line profile shape may reduce line intensity for Be stars viewed at low inclinations and increase line intensity for those viewed at high inclinations. Additionally, rapid rotation can significantly impact the measured equivalent width of the line produced by a star with a moderate to high density disk, especially at high inclinations. When the star-disk system is viewed near edge-on, gravity darkening can result in stronger H$α$ emission than would otherwise be expected for a disk of a given density. We also find that the competing effects of rapid rotation and H\,\textsc{i} opacity cause the slope of the polarized continuum (the polarization color) to be very sensitive to changes in the stellar rotation rate. This quantity offers a strong diagnostic for the rotation rate of Be stars.

Figures (32)

• Figure 1: Absolute $V$-band magnitude trends with increasing $W$ for B2 models with no disk (top left), low density disk (top right), moderate density disk (bottom left) and high density disk (bottom right). The inclination angles are indicated in the legend in the top left panel.
• Figure 2: Absolute $V$-band magnitude for the B8 diskless (left) and high density disk (right) models. The inclination angles are indicated in the legend in the left panel.
• Figure 3: Top: color-magnitude diagrams showing rotational displacement fans for the diskless models (left) in addition to the moderate density models (right). The legend in the left panel indicates the color for each tested inclination angle. Bottom: high density disks for B5 (left) and B8 (right) stars. Different inclination angles are denoted by different markers, shown in the legend in the left panel, and the rotation rate is indicated through the colorbar.
• Figure 4: Left: H$\alpha$ equivalent width trends with increasing $W$ for B2 models with high density disks (top left), moderate density disks (center left) and low density disks (bottom left), at various inclinations as indicated in the legend. Right: Normalized H$\alpha$ line profiles for B2 models at different rotation rates viewed at a constant inclination of 45$^{\circ}$. Profiles are shown in order of decreasing density, from high density at the top to no disk at the bottom. A vertical shift has been applied between densities to represent them on the same axes. The rotation rates are indicated in the legend.
• Figure 5: Unnormalized H$\alpha$ lines for the B2 models at an inclination of 5$^{\circ}$ (left), 45$^{\circ}$ (center), and 85$^{\circ}$ (right). In the top panels, we present models for high density (top), moderate density (center) and low density (bottom) disks, while the bottom panels show the diskless models. The rotation rates are indicated in the legend.