Christopher Tycner

Properties of the Hα-emitting Circumstellar Regions of Be Stars

Long-baseline interferometric observations obtained with the Navy Prototype Optical Interferometer of the Hα-emitting envelopes of the Be stars η Tau and β CMi are presented. For compatibility with the previously published interferometric results in the literature of other Be stars, circularly symmetric and elliptical Gaussian models were fitted to the calibrated Hα observations. The models are adequate for characterizing the angular distribution of the Hα-emitting circumstellar material associated with these Be stars. To study the correlations between the various model parameters and the stellar properties, the model parameters for η Tau and β CMi were combined with data for other Be stars from the literature. After accounting for the different distances to the sources and stellar continuum flux levels, it was possible to study the relationship between the net Hα emission and the physical extent of the Hα-emitting circumstellar region. A clear dependence of the net Hα emission on the linear size of the emitting region is demonstrated, and these results are consistent with an optically thick line emission that is directly proportional to the effective area of the emitting disk. Within the small sample of stars considered in this analysis, no clear dependence on the spectral type or stellar rotation is found, although the results do suggest that hotter stars might have more extended Hα-emitting regions.

A SYSTEMATIC STUDY OF Hα PROFILES OF Be STARS

We present a set of theoretical Hα emission-line profiles of Be stars, created by systematically varying model input parameters over a wide range of accepted values. Models were generated with a non-LTE radiative transfer code that incorporates a non-isothermal disk structure and a solar-type chemical composition. The theoretical Hα emission-line profiles were compared to a large set of Be star spectra with the aim of reproducing their global characteristics. We find that the observed profile shapes cannot be used to uniquely determine the inclination angle of Be star+disk systems. Drastically different profile shapes arise at a given inclination angle as a direct result of the state of the gas, and self-consistent disk physical conditions are therefore crucial for interpreting the observations.

Constraining Disk Parameters of Be Stars using Narrowband Hα Interferometry with the Navy Prototype Optical Interferometer

Interferometric observations of two well-known Be stars, gamma Cas and phi Per, were collected and analyzed to determine the spatial characteristics of their circumstellar regions. The observations were obtained using the Navy Prototype Optical Interferometer equipped with custom-made narrowband filters. The filters isolate the H-alpha emission line from the nearby continuum radiation, which results in an increased contrast between the interferometric signature due to the H-alpha-emitting circumstellar region and the central star. Because the narrowband filters do not significantly attenuate the continuum radiation at wavelengths 50 nm or more away from the line, the interferometric signal in the H-alpha channel is calibrated with respect to the continuum channels. The observations used in this study represent the highest spatial resolution measurements of the H-alpha-emitting regions of Be stars obtained to date. These observations allow us to demonstrate for the first time that the intensity distribution in the circumstellar region of a Be star cannot be represented by uniform disk or ring-like structures, whereas a Gaussian intensity distribution appears to be fully consistent with our observations.Abstract : Interferometric observations of two well-known Be stars, gamma Cas and phi Per, were collected and analyzed to determine the spatial characteristics of their circumstellar regions. The observations were obtained using the Navy Prototype Optical Interferometer equipped with custom-made narrowband filters. The filters isolate the H(alpha) emission line from the nearby continuum radiation, which results in an increased contrast between the interferometric signature due to the H(alpha)-emitting circumstellar region and the central star. Because the narrowband filters do not significantly attenuate the continuum radiation at wavelengths 50 nm or more away from the line, the interferometric signal in the H(alpha) channel is calibrated with respect to the continuum channels. The observations used in this study represent the highest spatial resolution measurements of the H -emitting regions of Be stars obtained to date. These observations allow us to demonstrate for the first time that the intensity distribution in the circumstellar region of a Be star cannot be represented by uniform disk or ringlike structures, whereas a Gaussian intensity distribution appears to be fully consistent with our observations.

Constraining the Physical Parameters of the Circumstellar Disk of χ Ophiuchi

We present a numerical model describing a circularly symmetric gaseous disk around the Be star χ Ophiuchi. The model is constrained by long-baseline interferometric observations that are sensitive to the Hα Balmer line emission from the disk. For the first time, our interferometric observations spatially resolve the inner region of the circumstellar disk around χ Oph, and we use these results to place a constraint on the physical extent of the Hα-emitting region. We demonstrate how this in turn results in very specific constraints on the parameters that describe the variation of the gas density as a function of radial distance from the central star.

A PARAMETER STUDY OF CLASSICAL Be STAR DISK MODELS CONSTRAINED BY OPTICAL INTERFEROMETRY

We have computed theoretical models of circumstellar disks for the classical Be stars κ Dra, β Psc, and υ Cyg. Models were constructed using a non-LTE radiative transfer code developed by Sigut & Jones (2007), which incorporates a number of improvements over previous treatments of the disk thermal structure, including a realistic chemical composition. Our models are constrained by direct comparison with long-baseline optical interferometric observations of the Hα-emitting regions and by contemporaneous Hα line profiles. Detailed comparisons of our predictions with Hα interferometry and spectroscopy place very tight constraints on the density distributions for these circumstellar disks.

THE CIRCUMSTELLAR ENVELOPE OF ¿ TAURI THROUGH OPTICAL INTERFEROMETRY

Abstract : We present optical interferometric observations of the Be star Zeta Tauri obtained using the Navy Prototype Optical Interferometer (NPOI). The multichannel capability of the NPOI allows a high-quality internal calibration of the squared visibilities corresponding to the H(alpha) emission from the circumstellar environment. The observations suggest a strong departure from circular symmetry and thus are described by an elliptical Gaussian model. We use a nonlinear least-squares fit to the data to obtain the likeliest parameters and the corresponding uncertainties are determined using a Monte Carlo simulation. We obtain 3.14 +/- 0.21 mas for the angular size of the major axis -62.3 deg +/- 4.4 deg for the position angle and 0.310 +/- 0.072 for the axial ratio. By comparing our results with those already in the literature we conclude that the model parameters describing the general characteristics of the circumstellar envelope of Zeta Tau appear to be stable on timescales of years. We also compare our results with the known parameters describing the binary nature of Zeta Tau and we conclude that the envelope surrounds only the primary component and is well within its Roche lobe.

THE ALGOL TRIPLE SYSTEM SPATIALLY RESOLVED AT OPTICAL WAVELENGTHS

Interacting binaries typically have separations in the milliarcsecond regime, and hence it has been challenging to resolve them at any wavelength. However, recent advances in optical interferometry have improved our ability to discern the components in these systems and have now enabled the direct determination of physical parameters. We used the Navy Prototype Optical Interferometer to produce for the first time images resolving all three components in the well-known Algol triple system. Specifically, we have separated the tertiary component from the binary and simultaneously resolved the eclipsing binary pair, which represents the nearest and brightest eclipsing binary in the sky. We present revised orbital elements for the triple system, and we have rectified the 180° ambiguity in the position angle of Algol C. Our directly determined magnitude differences and masses for this triple star system are consistent with earlier light curve modeling results.

Navy Prototype Optical Interferometer Imaging of Line Emission Regions of beta Lyrae Using Differential Phase Referencing

We present the results of an experiment to image the interacting binary star β Lyrae with data from the Navy Prototype Optical Interferometer using a differential phase technique to correct for the effects of the instrument and atmosphere on the interferometer phases. We take advantage of the fact that the visual primary of β Lyrae and the visibility calibrator we used are both nearly unresolved and nearly centrally symmetric, and consequently have interferometric phases near zero. We used this property to correct for the effects of the instrument and atmosphere on the phases of β Lyrae and to obtain differential phases in the channel containing the Hα emission line. Combining the Hα-channel phases with information about the line strength, we recovered complex visibilities and imaged the Hα emission using standard radio interferometry methods. Our images show the position of the Hα-emitting regions relative to the continuum photocenter as a function of orbital phase, indicating a major axis line of nodes along Ω = 249° ± 4°. The orbit is smaller than previously predicted, a discrepancy that can be alleviated if we assume that the system is at a larger distance, or if the stellar continuum contribution to the Hα channel was underestimated. We do not detect a jet in the Hα images, which may be due to the limited resolution of the observations along the direction perpendicular to the orbital plane. We find that the differential phase results are consistent with those obtained from a more standard analysis using squared visibilities (V 2s) and closure phases, which also indicate an Hα disk radius of 0.6 ± 0.1 mas, and ΔV = 1.30 ± 0.1 and ΔR = 1.20 ± 0.1 mag for the magnitude difference between the stars.

Spectral‐Line Profiles in Daytime Skylight

The 1.2 m telescope and spectrograph at our ElginÐeld Observatory was used to coude� investigate the suitability of daytime skylight as a reference spectrum for astrophysical work. We show that with sufficient care, skylight does give the Nux (or disk-integrated) spectrum of the Sun, allowing direct comparison with other stars. The line proÐles in skylight become shallower with increasing angular separa- tion from the Sun up to B100i, and then for larger angles they deepen slightly again. The amplitude of this variation is typically 3%E4% of the depth of the line. If the proÐles are normalized to their central depths, their shapes remain unaltered. This implies that astrophysical analyses based on the shapes of spectral lines or ratios of depths of spectral lines in skylight should not be seriously a†ected. However, equivalent widths are not conserved ; they can be reduced by up to the same 3%E4% unless care is taken to observe the sky close to the Sun or unless the angular variation is measured, as described here, and corrections applied. The skylight variations can be explained as a combination of aerosol and Rayleigh-Brillouin scattering. There is no discernible dependence with altitude of the Sun, ruling out a signiÐcant ground-albedo e†ect.

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

Context. Binary post asymptotic giant branch (post-AGB) stars are interesting laboratories to study both the evolution of binar ies as well as the structure of circumstellar disks. Aims. 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 spectral energy distribution. Methods. A large interferometric data set, collected over the past de cade and covering optical and near-infrared (near-IR) wavelengths, is analyzed in combination with the spectral energy distribution (SED) and flux-calibrated optical spectra. I n this first paper only simple geometric models are applied to fit the interfero metric data. Combining the interferometric constraints with the photometry and the optical spectra, we re-assess the energy budget of the post-AGB star and its circumstellar environment. Results. We report the first (direct) detection of a large (35-40%) opt ical 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 emissi on region is rather compact and fits with(in) the inner rim of the circumbinary du st disk. This rim dominates our K band data through thermal emission and is rather compact, emitting significantly already at a ra dius of twice the orbital separation. We interpret the circu mstellar optical flux as due to a scattering process, with the scatterers locat ed in the extremely puffed-up inner rim of the disk and possibly also in a bipolar outflow seen pole-on. A non local thermodynamic equi librium (non-LTE) gaseous origin in an inner disk cannot be excluded but is considered highly unlikely. Conclusions. This direct detection of a significant amount of circumbinar y 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.