S. T. Ridgway

First results from the chara array. II. A description of the instrument

The CHARA Array is a six 1 m telescope optical/IR interferometric array located on Mount Wilson, California, designed and built by the Center for High Angular Resolution Astronomy of Georgia State University. In this paper we describe the main elements of the Array hardware and software control systems, as well as the data reduction methods currently being used. Our plans for upgrades in the near future are also described.

Imaging the Surface of Altair

Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we constructed a near-infrared image of the rapidly rotating hot star Altair with a resolution of <1 milliarcsecond. The image clearly reveals the strong effect of gravity darkening on the highly distorted stellar photosphere. Standard models for a uniformly rotating star cannot explain our findings, which appear to result from differential rotation, alternative gravity-darkening laws, or both.

First results from the chara array. I. An interferometric and spectroscopic study of the fast rotator α leonis (Regulus)

We report on K-band interferometric observations of the bright, rapidly rotating star Regulus (type B7 V) made with the CHARA Array on Mount Wilson, California. Through a combination of interferometric and spectroscopic measurements, we have determined for Regulus the equatorial and polar diameters and temperatures, the rotational velocity and period, the inclination and position angle of the spin axis, and the gravity darkening coefficient. These first results from the CHARA Array provide the first interferometric measurement of gravity darkening in a rapidly rotating star and represent the first detection of gravity darkening in a star that is not a member of an eclipsing binary system.

First Results from the CHARA Array. IV. The Interferometric Radii of Low-Mass Stars

We have measured the angular diameters of six M dwarfs with the CHARA Array, a long-baseline optical interferometer located at Mount Wilson Observatory. Spectral types range from M1.0 V to M3.0 Vand linear radii from 0.38 to 0.69 R� . These results are consistent with the seven other M dwarf radii measurements from optical interferometry and with those for 14 stars in eclipsing binary systems. We compare all directly measured M dwarf radii to model predictions and find that current models underestimate the true stellar radii by up to 15%‐20%. The differences are small among the metal-poor stars but become significantly larger with increasing metallicity. This suggests that theoretical models for low-mass stars may be missing some opacity source that alters the computed stellar radii.

Interferometric observations of the supergiant stars

We report the observations in the K band of the red supergiant star α Orionis and of the bright giant star α Herculis with the FLUOR beamcombiner at the IOTA interferometer. The high quality of the data allows us to estimate limb-darkening and derive precise diameters in the K band which combined with bolometric fluxes yield effective temperatures. In the case of Betelgeuse, data collected at high spatial frequency although sparse are compatible with circular symmetry and there is no clear evidence for departure from circular symmetry. We have combined the K band data with interferometric measurements in the L band and at 11.15 µm. The full set of data can be explained if a 2055 K layer with optical depths τK = 0.060 ± 0.003, τL = 0.026 ± 0.002 and τ11.15 µm = 2.33 ± 0.23 is added 0.33 Rabove the photosphere providing a first consistent view of the star in this range of wavelengths. This layer provides a consistent explanation for at least three otherwise puzzling observations: the wavelength variation of apparent diameter, the dramatic difference in limb darkening between the two supergiant stars, and the previously noted reduced effective temperature of supergiants with respect to giants of the same spectral type. Each of these may be simply understood as an artifact due to not accounting for the presence of the upper layer in the data analysis. This consistent picture can be considered strong support for the presence of a sphere of warm water vapor, proposed by Tsuji (2000) when interpreting the spectra of strong molecular lines.

\alpha

We present submilliarcsecond resolution imaging and modeling of two nearby rapid rotators α Cephei and α Ophiuchi, obtained with the CHARA array—the largest optical/IR interferometer in the world. Incorporating a gravity-darkening model, we are able to determine the inclination, the polar and equatorial radius and temperature, as well as the fractional rotation speed of the two stars with unprecedented precision. The polar and equatorial regions of the two stars have ∼2000 K temperature gradient, causing their apparent temperatures and luminosities to be dependent on their viewing angles. Our modeling allow us to determine the true effective temperatures and luminosities of α Cep and α Oph, permitting us to investigate their true locations on the H-R diagram. These properties in turn give us estimates of the masses and ages of the two stars within a few percent of error using stellar evolution models. Also, based on our gravity-darkening modeling, we propose a new method to estimate the masses of single stars in a more direct way through V sin i measurements and precise geometrical constraint. Lastly, we investigate the degeneracy between the inclination and the gravity-darkening coefficient, which especially affects the modeling of α Oph. Although incorporating V sin i has lifted the degeneracy to some extent, higher-resolution observations are still needed to further constrain the parameters independently.

Orionis and

We present the firstK 0 -band,long-baseline interferometric observations of the northern Be starsCas,� Per,� Tau, andDra. The measurements were made with multiple telescope pairs of the CHARA Array interferometer and in every case the observations indicate that the circumstellar disks of the targets are resolved. We fit the interferometric visibilities with predictions from a simple disk model that assumes an isothermal gas in Keplerian rotation. We derive fitsof thefourmodelparameters(diskbasedensity,radialdensityexponent,disknormalinclination,andpositionangle) for each of the targets. The resulting densities are in broad agreement with prior studies of the IR excess flux, and the resultingorientationsgenerallyagreewiththosefrominterferometricHandcontinuumpolarimetricobservations.We find that the angular size of the K 0 diskemissionis smaller thanthatdeterminedfor the Hemission, and weargue that thedifferenceisthe resultof a larger Hopacityandtherelativelylarger neutral hydrogenfractionwithincreasingdisk radius. All the targets are known binaries with faint companions, and we find that companions appear to influence the interferometric visibilities in the cases ofPer andDra. We also present contemporaneous observations of the H� , H� ,andBremissionlines.Syntheticmodelprofilesoftheselinesthatarebasedonthesamediskinclinationandradial densityexponentasderivedfromtheCHARA Arrayobservationsmatchtheobservedemissionlinestrengthif thedisk base density is reduced by � 1.7 dex.

\alpha

Near-infrared interferometers have recently imaged a number of rapidly rotating A-type stars, finding levels of gravity darkening inconsistent with theoretical expectations. Here, we present new imaging of both a cooler star ??Cas (F2IV) and a hotter one ? Leo (B7V) using the CHARA array and the MIRC instrument at the H band. Adopting a solid-body rotation model with a simple gravity darkening prescription, we modeled the stellar geometric properties and surface temperature distributions, confirming that both stars are rapidly rotating and show gravity darkening anomalies. We estimate the masses and ages of these rapid rotators on L-R pol and H-R diagrams constructed for non-rotating stars by tracking their non-rotating equivalents. The unexpected fast rotation of the evolved sub-giant ??Cas offers a unique test of the stellar core-envelope coupling, revealing quite efficient coupling over the past ~0.5?Gyr. Lastly, we summarize all our interferometric determinations of the gravity darkening coefficient for rapid rotators, finding that none match the expectations from the widely used von Zeipel gravity darkening laws. Since the conditions of the von Zeipel law are known to be violated for rapidly rotating stars, we recommend using the empirically derived ??= 0.19 for such stars with radiation-dominated envelopes. Furthermore, we note that no paradigm exists for self-consistently modeling heavily gravity-darkened stars that show hot radiative poles with cool convective equators.

Herculis with FLUOR at IOTA

We present the first resolved images of the eclipsing binary β Lyrae, obtained with the CHARA Array interferometer and the MIRC combiner in the H band. The images clearly show the mass donor and the thick disk surrounding the mass gainer at all six epochs of observation. The donor is brighter and generally appears elongated in the images, the first direct detection of photospheric tidal distortion due to Roche lobe filling. We also confirm expectations that the disk component is more elongated than the donor and is relatively fainter at this wavelength. Image analysis and model fitting for each epoch were used for calculating the first astrometric orbital solution for β Lyrae, yielding precise values for the orbital inclination and position angle. The derived semimajor axis also allows us to estimate the distance of β Lyrae; however, systematic differences between the models and the images limit the accuracy of our distance estimate to about 15%. To address these issues, we will need a more physical, self-consistent model to account for all epochs as well as the multiwavelength information from the eclipsing light curves.

Imaging and Modeling Rapidly Rotating Stars: α Cephei and α Ophiuchi

Optical and infrared interferometers definitively established that the photometric standard Vega (={alpha} Lyrae) is a rapidly rotating star viewed nearly pole-on. Recent independent spectroscopic analyses could not reconcile the inferred inclination angle with the observed line profiles, preferring a larger inclination. In order to resolve this controversy, we observed Vega using the six-beam Michigan Infrared Combiner on the Center for High Angular Resolution Astronomy Array. With our greater angular resolution and dense (u, v)-coverage, we find that Vega is rotating less rapidly and with a smaller gravity darkening coefficient than previous interferometric results. Our models are compatible with low photospheric macroturbulence and are also consistent with the possible rotational period of {approx}0.71 days recently reported based on magnetic field observations. Our updated evolutionary analysis explicitly incorporates rapid rotation, finding Vega to have a mass of 2.15{sup +0.10}{sub -0.15} M{sub Sun} and an age 700{sup -75}{sub +150} Myr, substantially older than previous estimates with errors dominated by lingering metallicity uncertainties (Z = 0.006{sup +0.003}{sub -0.002}).