81 research outputs found
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
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
(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 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 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
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