Nicholas M. Elias

Constraints on the Geometry of Circumstellar Envelopes: Optical Interferometric and Spectropolarimetric Observations of Seven Be Stars

We have obtained contemporaneous optical interferometry and spectropolarimetry of seven Be stars. The interferometry was done using both continuum and narrow Ha line -lters to observe the circumstel- lar envelope emission. The envelopes of all seven stars were resolved interferometrically in Ha. Of these seven stars, the envelopes of four were not circularly symmetric and showed clear evidence of elongation. The position angles of the major axis of the elongation were in good agreement with the disk orientation inferred from the intrinsic polarization data, which samples material within a few stellar radii, and also agreed with previous results from the radio, which samples material out as far as 100 stellar radii but at lower resolution. This -nding indicates that the envelope alignment persists over 2 orders of magnitude in radius. Minimum inclination angle estimates from the interferometry are presented for six of the seven stars. Under the assumption that the envelopes are fairly thin circularly symmetric disks, there is a straightfor- ward explanation of the interferometric results. The stars with the greatest elongations, f Tau, / Per, and t Per, are seen nearly equator-on; c Cas is seen at a more intermediate latitude, consistent with previous results; and 48 Per and g Tau, which are only slightly asymmetric, are nearly pole-on. b CMi had insuf- -cient coverage to determine whether its envelope is asymmetric. These results are consistent with inde- pendent indications of the inclinations based on the polarimetry. Spectral diagnostics have been used in the past to classify Be stars as pole-on or as shell stars (usually interpreted to indicate an equator-on orientation). Our inclination angle for 48 Per is consistent with its pole-on classi-cation by Slettebak, and the three equator-on stars have all been previously classi-ed as shell stars. However, both c Cas and g Tau have also been previously classi-ed as shell stars, but we -nd that they have intermediate and pole-on orientations, respectively. We interpret this -nding as evidence that, while equator-on stars may preferentially be shell stars, not all shell stars are actually equator-on. Our results show that the Ha emission region extends up to about 12 stellar radii, possibly depending on spectral type. The size of this region correlates with the equivalent width of the Ha emission. Our results also provide strong constraints for allowed models of Be star envelopes. In particular, for f Tau, we derive an upper limit to the disk opening angle of 20i, which limits the vertical extent of the envelope. We also -nd that the polarization position angle is perpendicular to the interferometric major axis in all cases. This rules out envelope models that are both optically and geometrically thick, since these models produce polarization parallel to the plane of the disk. We conclude that results from the combined interferometry and polarimetry strongly favor the disk paradigm for Be stars over mildly ellip- soidal models. Subject headings: circumstellar matter E stars: emission-line, Be E techniques: interferometric E techniques: polarimetric

ANGULAR DIAMETERS OF STARS FROM THE MARK III OPTICAL INTERFEROMETER

Observations of 85 stars were obtained at wavelengths between 451 and 800 nm with the Mark III Stellar Interferometer on Mount Wilson, near Pasadena, California. Angular diameters were determined by fitting a uniform-disk model to the visibility amplitude versus projected baseline length. Half the angular diameters determined at 800 nm have formal errors smaller than 1%. Limb-darkened angular diameters, effective temperatures, and surface brightnesses were determined for these stars, and relationships between these parameters are presented. Scatter in these relationships is larger than would be expected from the measurement uncertainties. We argue that this scatter is not due to an underestimate of the angular diameter errors; whether it is due to photometric errors or is intrinsic to the relationship is unresolved. The agreement with other observations of the same stars at the same wavelengths is good; the width of the difference distribution is comparable to that estimated from the error bars, but the wings of the distribution are larger than Gaussian. Comparison with infrared measurements is more problematic; in disagreement with models, cooler stars appear systematically smaller in the near-infrared than expected, warmer stars larger.

Navy Prototype Optical Interferometer Observations of the Double Stars Mizar A and Matar

We present a detailed analysis of two spectroscopic binaries based on new observations obtained with the Navy Prototype Optical Interferometer (NPOI). While the data calibration needs refinement, first results show the impressive potential of NPOI, both in terms of speed and precision, for imaging and modeling the orbits of spectroscopic binaries. We determine the orbital parameters of Mizar A (ζ1 Ursae Majoris) and Matar (η Pegasi), and derive masses and luminosities using published radial velocities and Hipparcos trigonometric parallaxes. The results on Mizar A are compared to earlier work done with the Mark III interferometer, while data from this instrument were combined with NPOI data in the Matar analysis.

Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer

We have measured the angular diameters of 50 F, G, K, and M giant and supergiant stars using the Navy Prototype Optical Interferometer at wavelengths between 649 and 850 nm and using three baselines with lengths up to 37.5 m. Uniform-disk diameters, obtained from fits to the visibility amplitude, were transformed to limb-darkened diameters through the use of limb-darkening coefficients for plane-parallel stellar atmosphere models. These limb-darkened diameters are compared with those measured with the Mark III optical interferometer and with those computed by the infrared flux method. Sources of random and systematic error in the observations are discussed.

Direct Confirmation of Stellar Limb Darkening with the Navy Prototype Optical Interferometer

Using three elements of the Navy Prototype Optical Interferometer and observing in 20 spectral channels covering 520-850 nm, we have implemented a phase bootstrapping technique in which short baselines with high visibilities are used to keep the longer baselines with low visibilities in phase. Using this method, we have been able to extend the spatial frequency coverage beyond the first zero of the stellar visibility function for two K giants α Arietis, and α Cassiopeiae. The data are inconsistent with a uniform-disk model and confirm the presence of limb-darkened radial profiles. Adopting a particular limb-darkening law enables us to determine the diameter with small formal errors (one part in 1000). In addition, we have measured closure phases for both stars. The closure phases show a jump of 180° at the first zero in the visibility amplitude, which was expected.

Photon orbital angular momentum in astronomy

Context. Photon orbital angular momentum (POAM) has been created in the laboratory, yet it is still relatively unknown. How does POAM manifest itself in astronomy? Are there any applications for measuring astrophysical POAM? Aims. In this paper, I 1) explain POAM in an astronomical context; 2) define the POAM observables for astronomy; 3) create generic systems-based calculi that describe how POAM propagates from celestial sphere to detector; 4) use the calculi with several astronomical instruments as examples of their utility; 5) demonstrate an application for astrophysical POAM measurements; and 6) relate POAM to existing astronomical instruments and concepts. Methods. Electric fields are expanded into azimuthal Fourier components, and the intensities are expanded into correlations or rancors. The source electric fields are spatially incoherent. In the systems-based calculi, the inputs are located on the celestial sphere, the system is represented by propagation through free space and instrument, and the outputs are located in a specific plane. The diffraction and point-spread function expansions are very generic and can be used with any type of instrument. I employ these examples to demonstrate the calculi (in order of increasing difficulty): free space, single telescopes, interferometers, coronagraphs, and rancorimeters. Results. The azimuthal Fourier components of the electric field correspond to POAM vortex states. Rancors contain less information than correlations, yet they are easier to measure and can be used in many applications. Propagation through an aberrated telescope applies external torque, which may be expressed in terms of Zernike polynomials. I prove that a sectored phase mask in a focal-plane coronagraph applies torque to the low-order states, producing a null. Also, I prove that a Michelson interferometer is inherently capable of filtering POAM; e.g., tracking 180 ◦ from the central fringe eliminates even states, producing a null. A limited rancorimeter can be created by placing a focal-plane wedge mask in a coronagraph. The resulting rancors can be used to perform super-Rayleigh observations of unresolved unresolved objects, such as binary stars. There are three types of source POAM: intrinsic, structure, and pointing. Instrumental POAM, which must be calibrated, includes optical aberrations and atmospheric turbulence. Conclusions. This paper represents the starting point for future research: 1) making a priori predictions about the intrinsic POAM of astronomical sources; 2) designing ground- and space-based POAM-measuring instruments; 3) understanding existing instruments in terms of POAM; 4) minimizing the effects of random noise on POAM; and 5) calibrating all types of instrumental POAM.

Multichannel Optical Aperture Synthesis Imaging of zeta1 URSAE Majoris with the Navy Prototype Optical Interferometer

We have used the Navy Prototype Optical Interferometer (NPOI) to obtain the first multichannel optical aperture synthesis images of a star. We observed the spectroscopic binary ~ 1 Ursae Majoris at 6 to 10 milliarcseconds separation during seven nights, using three interferometric baselines and 19 spectral channels (A.A. 520-850 nm) of the NPOI. After editing, a typical90 sec scan yielded fringe visibilities at 50 spatial frequencies and closure phases at 15 wavelengths. Three to five scans were obtained each night. The separations and position angles are in good agreement with the visual orbit obtained with the Mark III interferometer (Hummel et al. 1995, AJ, 110, 376) but show small systematic difference that can be used to improve the orbit. The closure phase data provide a sensitive measure of the magnitude difference between the components. These results demonstrate the power of broad-band interferometric observations for fast imaging and the utility of vacuum delay lines for simultaneous observations over a wide band. These observations are the first to produce simultaneous visibilities and closure phases with a separate-aperture optical interferometer, and the second to produce closure phase images, following the results from COAST reported by Baldwin et al. (1996, A&A, 306, L13). The angular resolution here is the highest ever achieved at visual wavelengths, exceeding by an order of magnitude the best thus far achieved by any single-aperture optical telescope. We generated complex visibilities and closure phases (the data types commonly used in radio interferometry) from the optical data and used standard radio interferometry techniques to produce these images. However, the fundamental observables of optical interferometry, the squared visibility amplitude and the closure phase, require the development of new analysis techniques.

The Asymmetric Envelope of gamma Cassiopeiae Observed with the MK III Optical Interferometer

We present interferometric observations of the Be star γ Cas in the light of the Hα emission line. Northsouth baselines ranging from 4.2 to 23.6 m have been used over a large range of hour angles. With the resulting coverage of the u-v plane, we have been able for the first time to measure the asymmetry of the Hα emitting region directly. The brightness distribution on the sky can be fit by an elliptical Gaussian model with an axial ratio of 0.74 and an angular diameter (FWHM of the major axis) of 3.2 milliarcsec (mas). The radial brightness distribution is not well-constrained by our data; elliptical uniform disk or ring models also give acceptable fits

New Perspectives on AX Monocerotis

AX Moncerotis is a 232d, noneclipsing, interacting binary star that consists of a K giant, a Be-like giant, and large amounts of circumstellar material. The K star is almost certainly a synchronous rotator and is probably in contact with its critical lobe. The Be star was believed to be a rapid rotator based on extremely wide absorption lines, but new spectra show that these lines arise from the circumstellar environment. Hydrogen emission, also circumstellar, is many times stronger than the continuum. Near-ultraviolet light curves exhibit a 0.5 mag dip near phase 0.75, but there is no such variability at longer wavelengths. Gas flow trajectories from the cusp of the K star toward the Be star provide a simple explanation for the photometric and spectroscopic behavior. We may have found a decreasing orbital period, but more data are necessary to confirm this result. We present several models for AX Mon based on (1) new and archival visible photometry, (2) archival ultraviolet spectroscopy, (3) new and archival visible spectroscopy, (4) new visible polarimetry, and (5) new radio photometry. Future observations, including optical interferometry, are proposed.

ζ Orionis A is a Double Star

A close, 4th magnitude companion to ζ Orionis A has been resolved with the Navy Prototype Optical Interferometer at Lowell Observatory. This confirms an indication of multiplicity from observations with the stellar intensity interferometer at Narrabri 26 years ago. The new component in the multiple system ADS 4263, ζ Orionis Ab, is 2 mag fainter than Aa, which is a supergiant of type O9.5. During 1998 February and March, the pair had a mean separation of 42 mas. Orbital motion was subsequently detected, but the corresponding arc allows only a preliminary ephemeris.