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

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

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

An interferometric study of the post-AGB binary 89 Herculis I Spatially resolving the continuum circumstellar environment at optical and near-IR wavelengths with the VLTI, NPOI, IOTA, PTI, and the CHARA Array

Binary post-AGB stars are interesting laboratories to study both the evolution of binaries as well as the structure of circumstellar disks. A multiwavelength high angular resolution study of the prototypical object 89 Herculis is performed with the aim of identifying and locating the different emission components seen in the SED. A large interferometric data set, collected over the past decade and covering optical and near-IR wavelengths, is analyzed with simple geometric models. Combining the interferometric constraints with the photometry and the optical spectra, we reassess the energy budget of the post-AGB star and its circumstellar environment. We report the first (direct) detection of a large (35-40%) optical circumstellar flux contribution and spatially resolve its emission region. Given this large amount of reprocessed and/or redistributed optical light, the fitted size of the emission region is rather compact and fits with(in) the inner rim of the circumbinary dust disk. This rim dominates our K band data through thermal emission and is rather compact, emitting significantly already at a radius of twice the orbital separation. We interpret the circumstellar optical flux as due to a scattering process, with the scatterers located in the extremely puffed-up inner rim of the disk and possibly also in a bipolar outflow seen pole-on. A non-LTE gaseous origin in an inner disk cannot be excluded but is considered highly unlikely. This direct detection of a significant amount of circumbinary light at optical wavelengths poses several significant questions regarding our understanding of both post-AGB binaries and the physics in their circumbinary disks. Although the identification of the source of emission/scattering remains inconclusive without further study on this and similar objects, the implications are manifold.Comment: Accepted for publication in A&A, 16 pages, 15 figure

The DDO IVC Distance Project: Survey Description and the Distance to G139.6+47.6

We present a detailed analysis of the distance determination for one intermediate Velocity Cloud (IVC G139.6+47.6) from the ongoing DDO IVC Distance Project. Stars along the line of sight to G139.6+47.6 are examined for the presence of sodium absorption attributable to the cloud, and the distance bracket is established by astrometric and spectroscopic parallax measurements of demonstrated foreground and background stars. We detail our strategy regarding target selection, observational setup, and analysis of the data, including a discussion of wavelength calibration and sky subtraction uncertainties. We find a distance estimate of 129 (+/- 10) pc for the lower limit and 257 (+211-33) pc for the upper limit. Given the high number of stars showing absorption due to this IVC, we also discuss the small-scale covering factor of the cloud and the likely significance of non-detections for subsequent observations of this and other similar IVC's. Distance measurements of the remaining targets in the DDO IVC project will be detailed in a companion paper.Comment: 10 pages, 6 figures, LaTe