Be Star Disk Models in Consistent Vertical Hydrostatic Equilibrium

A popular model for the circumstellar disks of Be stars is that of a geometrically thin disk with a density in the equatorial plane that drops as a power law of distance from the star. It is usually assumed that the vertical structure of such a disk (in the direction parallel to the stellar rotation axis) is governed by the hydrostatic equilibrium set by the vertical component of the star's gravitational acceleration. Previous radiative equilibrium models for such disks have usually been computed assuming a fixed density structure. This introduces an inconsistency as the gas density is not allowed to respond to temperature changes and the resultant disk model is not in vertical, hydrostatic equilibrium. In this work, we modify the {\sc bedisk} code of \citet{sig07} so that it enforces a hydrostatic equilibrium consistent with the temperature solution. We compare the disk densities, temperatures, H$\alpha$ line profiles, and near-IR excesses predicted by such models with those computed from models with a fixed density structure. We find that the fixed models can differ substantially from the consistent hydrostatic models when the disk density is high enough that the circumstellar disk develops a cool ($T\lesssim10,000$K) equatorial region close to the parent star. Based on these new hydrostatic disks, we also predict an approximate relation between the (global) density-averaged disk temperature and the $T_{\rm eff}$ of the central star, covering the full range of central Be star spectral types.Comment: 25 pages; 11 figure

The Spatially Resolved H\alpha-Emitting Wind Structure of P Cygni

High spatial resolution observations of the H\alpha-emitting wind structure associated with the Luminous Blue Variable star P Cygni were obtained with the Navy Prototype Optical Interferometer (NPOI). These observations represent the most comprehensive interferometric data set on P Cyg to date. We demonstrate how the apparent size of the H\alpha-emitting region of the wind structure of P Cyg compares between the 2005, 2007 and 2008 observing seasons and how this relates to the H\alpha line spectroscopy. Using the data sets from 2005, 2007 and 2008 observing seasons, we fit a circularly symmetric Gaussian model to the interferometric signature from the H\alpha-emitting wind structure of P Cyg. Based on our results we conclude that the radial extent of the H\alpha-emitting wind structure around P Cyg is stable at the 10% level. We also show how the radial distribution of the H\alpha flux from the wind structure deviates from a Gaussian shape, whereas a two-component Gaussian model is sufficient to fully describe the H\alpha-emitting region around P Cyg.Comment: 27 pages, 6 figues, accepted for publication in A

The Infrared Continuum Sizes of Be Star Disks

We present an analysis of the near-infrared continuum emission from the circumstellar gas disks of Be stars using a radiative transfer code for a parametrized version of the viscous decretion disk model. This isothermal gas model creates predicted images that we use to estimate the HWHM emission radius along the major axis of the projected disk and the spatially integrated flux excess at wavelengths of 1.7, 2.1, 4.8, 9, and 18 ?m. We discuss in detail the effect of the disk base density, inclination angle, stellar effective temperature, and other physical parameters on the derived disk sizes and color excesses. We calculate color excess estimates relative to the stellar V -band flux for a sample of 130 Be stars using photometry from 2MASS and the AKARI infrared camera all-sky survey. The color excess relations from our models make a good match of the observed color excesses of Be stars. We also present our results on the projected size of the disk as a function of wavelength for the classical Be star ? Tauri, and we show that the model predictions are consistent with interferometric observations in the H, K', and 12 \mu m bands

The Variability of Halpha Equivalent Widths in Be Stars

Focusing on B-emission stars, we investigated a set of H$\alpha$ equivalent widths calculated from observed spectra acquired over a period of about 4 years from 2003 to 2007. During this time, changes in equivalent width for our program stars were monitored. We have found a simple statistical method to quantify these changes in our observations. This statistical test, commonly called the F ratio, involves calculating the ratio of the external and internal error. We show that the application of this technique can be used to place bounds on the degree of variability of Be stars. This observational tool provides a quantitative way to find Be stars at particular stages of variability requiring relatively little observational data.Comment: 8 figures accepted by The Astronomical Journa

INVESTIGATING BE STAR DISKS USING LONG-BASELINE INTERFEROMETRY

RESUMEN Las estrellas Be forman una subclase de estrellas-B, dondeéstas poseen estructuras gaseosas tipo-disco. Históricamente, la presencia de un disco fue detectado a través de líneas de emisión presentes en el espectro, o vía el exceso del IR detectado en la distribución de energía espectral originada de estas fuentes. Sin embargo, la interferometría de línea de gran base es elúnico método de observación disponible hoy en día, que puede ser utilizado para resolver espacialmente las regiones circunestelares de estas estrellas, usando la emisión de las líneas o del contínuo de los discos. Se revisa una muestra de los resultados interferométricos dominantes que desempeñaron un rol principal apoyando la visión actual de estos sistemas, discos planos que rotan conectados con las estrellas que a su vez rotan rápidamente, son usualmente citados como un estándar observacional. Se presentan ejemplos de trabajos y resultados observacionales de varios interferómetros incluyendo trabajos recientes. Se discuten brevemente posibles direcciones futuras. ABSTRACT Be stars form a subclass of B-type stars, where the stars possess gaseous disk-like structures. Historically, the presence of a disk was detected through line emission present in the spectrum, or through IR excess detected in the spectral energy distribution originating from these sources. However, long-baseline interferometry is the only observational method available today that can be used to spatially resolve the circumstellar regions of Be stars using either line or continuum emission from the disks. A sample of key interferometric results that played a major role in supporting the current view of these systems, where commonly flat, rotationally supported disks connected to rapidly rotating stars are quoted as an observational standard are reviewed. Examples of observational work and results from various interferometric instruments including recent work are presented. Possible future directions are also briefly discussed