Ellyn K. Baines

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.

CHARA Array K'-Band Measurements of the Angular Dimensions of Be Star Disks

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.

CHARA ARRAY MEASUREMENTS OF THE ANGULAR DIAMETERS OF EXOPLANET HOST STARS

Wehavemeasuredtheangulardiametersforasampleof 24exoplanethoststarsusingGeorgiaStateUniversity’s CHARA Array interferometer. We use these improved angular diameters together with Hipparcos parallax measurements to derive linear radii and to estimate the stars’ evolutionary states. Subjectheadingginfrared:stars — planetarysystems — stars:fundamentalparameters — techniques:interferometric Online material: extended figure, machine-readable table

THE CHARA ARRAY ANGULAR DIAMETER OF HR 8799 FAVORS PLANETARY MASSES FOR ITS IMAGED COMPANIONS

HR 8799 is an hF0 mA5 γ Doradus-, λ Bootis-, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799s limb-darkened angular diameter to be 0.342 ± 0.008 mas (an error of only 2%). By combining our measurement with the stars parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 ± 0.06 R_☉), effective temperature (7193 ± 87 K, consistent with F0), luminosity (5.05 ± 0.29 L_☉), and the extent of the habitable zone (HZ; 1.62-3.32 AU). These improved stellar properties permit much more precise comparisons with stellar evolutionary models, from which a mass and age can be determined, once the metallicity of the star is known. Considering the observational properties of other λ Bootis stars and the indirect evidence for youth of HR 8799, we argue that the internal abundance, and what we refer to as the effective abundance, is most likely near solar. Finally, using the Yonsei-Yale evolutionary models with uniformly scaled solar-like abundances, we estimate HR 8799s mass and age considering two possibilities: 1.516^(+0.038)_(–0.024) M_☉ and 33^(+7)_(–13.2) Myr if the star is contracting toward the zero-age main sequence or 1.513^(+0.023)_(–0.024_ M_☉ and 90^(+381)_(–50) Myr if it is expanding from it. This improved estimate of HR 8799s age with realistic uncertainties provides the best constraints to date on the masses of its orbiting companions, and strongly suggests they are indeed planets. They nevertheless all appear to orbit well outside the HZ of this young star.

ANGULAR DIAMETERS AND EFFECTIVE TEMPERATURES OF 25 K GIANT STARS FROM THE CHARA ARRAY

Using Georgia State Universitys Center for High Angular Resolution Astronomy Array interferometer, we measured angular diameters for 25 giant stars, six of which host exoplanets. The combination of these measurements and Hipparcos parallaxes produces physical linear radii for the sample. Except for two outliers, our values match angular diameters and physical radii estimated using photometric methods to within the associated errors with the advantage that our uncertainties are significantly lower. We also calculated the effective temperatures for the stars using the newly measured diameters. Our values do not match those derived from spectroscopic observations as well, perhaps due to the inherent properties of the methods used or because of a missing source of extinction in the stellar models that would affect the spectroscopic temperatures.

ELEVEN EXOPLANET HOST STAR ANGULAR DIAMETERS FROM THE CHARA ARRAY

We directly measured the angular diameters for 11 exoplanet host stars using Georgia State Universitys CHARA Array interferometer and calculated their linear radii and effective temperatures. The sample tends toward evolving or evolved stars and includes one dwarf, four subgiants, and six giants. We then estimated masses and ages for the stars using our effective temperatures combined with metallicity measurements from the literature.

Angular Diameters of the G Subdwarf μ Cassiopeiae A and the K Dwarfs σ Draconis and HR 511 from Interferometric Measurements with the CHARA Array

Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5?V subdwarf ??Cas?A, the first such determination for a halo population star. We compare this result to new diameters for the higher metallicity K0?V stars, ??Dra and HR?511, and find that the metal-poor star, ??Cas?A, has an effective temperature ( -->Teff = 5297 ? 32 K), radius ( -->R = 0.791 ? 0.008 R?), and absolute luminosity ( -->L = 0.442 ? 0.014 L?) comparable to those of the other two stars with later spectral types. We show that stellar models show a discrepancy in the predicted temperature and radius for ??Cas?A, and we discuss these results and how they provide a key to understanding the fundamental relationships for stars with low metallicity.

Direct Measurement of the Radius and Density of the Transiting Exoplanet HD 189733b with the CHARA Array

We have measured the angular diameter of the transiting extrasolar planet host star HD 189733 using the CHARA optical/IR interferometric array. Combining our new angular diameter of 0.377 +/- 0.024 mas with the Hipparcos parallax leads to a linear radius for the host star of 0.779 +/- 0.052 R☉and a radius for the planet of 1.19 +/- 0.08 RJup. Adopting the mass of the planet as derived by its discoverers, we derive a mean density of the planet of 0.91 +/- 0.18 g cm^-3. This is the first determination of the diameter of an extrasolar planet through purely direct means.

NAVY PRECISION OPTICAL INTERFEROMETER MEASUREMENTS OF 10 STELLAR OSCILLATORS

Using the Navy Precision Optical Interferometer, we measured the angular diameters of 10 stars that have previously measured solar-like oscillations. Our sample covered a range of evolutionary stages but focused on evolved subgiant and giant stars. We combined our angular diameters with Hipparcos parallaxes to determine the stars’ physical radii, and used photometry from the literature to calculate their bolometric fluxes, luminosities, and effective temperatures. We then used our results to test the scaling relations used by asteroseismology groups to calculate radii and found good agreement between the radii measured here and the radii predicted by stellar oscillation studies. The precision of the relations is not as well constrained for giant stars as it is for less evolved stars. Subject headings: visible: stars, stars: fundamental parameters, techniques: interferometric)

The Search for Stellar Companions to Exoplanet Host Stars Using the CHARA Array

Most exoplanets have been discovered via radial velocity studies, which are inherently insensitive to orbital inclination. Interferometric observations will show evidence of a stellar companion if it is sufficiently bright, regardless of the inclination. Using the CHARA Array, we observed 22 exoplanet host stars to search for stellar companions in low-inclination orbits that may be masquerading as planetary systems. While no definitive stellar companions were discovered, it was possible to rule out certain secondary spectral types for each exoplanet system observed by studying the errors in the diameter fit to calibrated visibilities and by searching for separated fringe packets.