G. C. Gilbreath

Vega is a Rapidly Rotating Star

Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard. Although its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visible spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers and it has unusually shaped absorption line profiles, leading some to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations that show that Vega has the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93 per cent of its breakup speed. In addition to explaining the unusual brightness and line shape peculiarities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vegas age and opening the possibility that its debris disk could be substantially older than previously thought.

First Observations with a Co-phased Six-Station Optical Long-Baseline Array: Application to the Triple Star ? Virginis

We report on the first successful simultaneous combination of six independent optical telescopes in an interferometric array. This is double the number of independent telescopes, and 5 times the number of independent baselines, heretofore combined simultaneously. This was accomplished with the Navy Prototype Optical Interferometer at Lowell Observatory, near Flagstaff, Arizona. We describe the main technologies demonstrated, including hybrid six-way beam combination, nonredundant multiple optical path modulation for fringe separation, and the fringe detection electronics. To test the array’s suitability for high

Resolving the effects of rotation in altair with long-baseline interferometry

We report the successful fitting of a Roche model, with a surface temperature gradient following the von Zeipel gravity darkening law, to observations of Altair made with the Navy Prototype Optical Interferometer. We confirm the claim by Ohishi et al. that Altair displays an asymmetric intensity distribution due to rotation, the first such detection in an isolated star. Instrumental effects due to the high visible flux of this first magnitude star appear to be the limiting factor in the accuracy of this fit, which nevertheless indicates that Altair is rotating at 0.90 ? 0.02 of its breakup (angular) velocity. Our results are consistent with the apparent oblateness found by van Belle et al. and show that the true oblateness is significantly larger owing to an inclination of the rotational axis of ~64? to the line of sight. Of particular interest, we conclude that instead of being substantially evolved as indicated by its classification, A7 IV-V, Altair is only barely off the zero-age main sequence and represents a good example of the difficulties rotation can introduce in the interpretation of this part of the HR diagram.

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.

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.

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.

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.

Free-space optical communications research and demonstrations at the U.S. Naval Research Laboratory.

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

Free space optical communications research at the U.S. Naval Research Laboratory

Free space optical (FSO) communication has enjoyed a renewal of interest in the past decade driven by increasing data rate requirements and decreasing amounts of radio frequency spectrum. These needs exist in both the commercial and military sectors. However military communications requirements differ in other ways. At the U.S. Naval Research Laboratory (NRL) we have been conducting research on FSO communications for over ten years with an emphasis on tactical applications. NRLs FSO research has covered propagation studies in the maritime domain, new component development, and systems demonstrations. In addition NRL has developed both conventional, direct, laser communications systems and retro-reflecting systems. In this paper we review some of this work and discuss possible future applications of FSO communications.

The Hyades Binary θ2 Tauri: Confronting Evolutionary Models with Optical Interferometry

We determine the masses and magnitude difference of the components of the Hyades spectroscopic binary θ2 Tauri. We find that both components appear to be less massive and/or brighter than predicted from some recent evolutionary models. The rapid rotation and unknown rotational inclination of both components introduce uncertainty in their luminosities and colors, but not enough to reconcile both of them with the evolutionary models. We measured the visual orbit with the Mark III optical interferometer and the Navy Prototype Optical Interferometer and combined it with the Hipparcos proper-motion-based parallax to find a total system mass Σ of 4.03 ± 0.20 ⊙. We also combined our visual orbit with three recent spectroscopic orbits to find three spectroscopically based estimates of Σ and compared these to the Σ from the visual orbit and parallax. We chose the spectroscopic orbit that agreed best and used its mass ratio to estimate individual masses A,B of 2.15 ± 0.12 and 1.87 ± 0.11 ⊙. From the interferometry, we determine Δm = 1.13 ± 0.05 mag across the 450-850 nm band. The parallax then implies absolute V magnitudes MA,B of 0.48 ± 0.05 and 1.61 ± 0.06 mag. If the components are rotating near breakup velocity and seen nearly pole-on, the true luminosities may be as faint as 1.03 and 2.13 mag; even in that case, however, the secondary is too blue by ~0.07 mag in B - V.