Confirming Fundamental Parameters of the Exoplanet Host Star epsilon Eridani Using the Navy Optical Interferometer

We measured the angular diameter of the exoplanet host star epsilon Eridani using the Navy Optical Interferometer. We determined its physical radius, effective temperature, and mass by combining our measurement with the star's parallax, photometry from the literature, and the Yonsei-Yale isochrones (Yi et al. 2001), respectively. We used the resulting stellar mass of 0.82 +/- 0.05 M_Sun plus the mass function from Benedict et al. (2006) to calculate the planet's mass, which is 1.53 +/- 0.22 M_Jupiter. Using our new effective temperature, we also estimated the extent of the habitable zone for the system.Comment: Accepted for publication in The Astrophysical Journa

Inspection and Characterization of Exoplanet Using the CHARA Array

Until the last decade or so, our entire knowledge of planets around Sun-like stars consisted of those in our own Solar System. This is no longer the case. Over 200 planets have been discovered through radial velocity surveys and photometric studies, both of which depend on observing the planet\u27s effects on its host star. Much of our knowledge of the planets orbiting these stars is uncertain, based on assumptions about the stars\u27 masses and the planets\u27 orbital inclinations. This dissertation is comprised of two main sections. The first involves measuring the angular diameters for a sample of exoplanet host stars using Georgia State University\u27s CHARA Array in order to learn more about the nature of these stars. These direct angular measurements are not dependent on the exoplanet systems\u27 inclinations or the masses of the stars. Improved angular diameters lead to linear diameters when combined with HIPPARCOS parallax measurements, which in turn tell us of the stars\u27 ages and masses. Of the 82 exoplanet systems observable with the CHARA Array, 31 host stars were observed and stellar angular diameters were measured for 26 systems. In the special case of an exoplanet system with a transiting planet, this direct measurement of the star\u27s angular diameter leads to a direct measurement of the planet\u27s diameter, when the planet-to-star-radii ratio is known from photometric studies. This was done for HD 189733. The star\u27s angular diameter is 0.377 +/- 0.024 mas, which produces a stellar linear radius of 0.779 +/- 0.052 R_Sun and a planetary diameter of 1.19 +/- 0.08 R_Jupiter. The second part of this project involved the inspection of the exoplanet systems for stellar companions masquerading as planets. From radial velocity studies alone, it is impossible to distinguish between a planet in a high-inclination orbit and a low-mass stellar companion in a low-inclination orbit. Using the CHARA Array, it was possible to rule out certain secondary spectral types for each exoplanet system observed by studying the errors in the diameter fit and searching for separated fringe packets. While no definitive stellar companions were found, two expolanet systems, upsilon Andromedae and rho Coronae Borealis, exhibited behavior that were not consistent with the host star being a simple limb-darkened disk

Ruling Out Possible Secondary Stars to Exoplanet Host Stars Using the CHARA Array

Of the over 450 exoplanets known to date, more than 420 of them have been discovered using radial velocity studies, a method that tells nothing about the inclination of the planet's orbit. Because it is more likely that the companion is a planetary-mass object in a moderate- to high-inclination orbit than a low-mass stellar object in a nearly face-on orbit, the secondary bodies are presumed to be planets. Interferometric observations allow us to inspect the angular diameter fit residuals to calibrated visibilities in order to rule out the possibility of a low-mass stellar companion in a very low-inclination orbit. We used the Center for High Angular Resolution Astronomy (CHARA) Array interferometer to observe 20 exoplanet host stars and considered five potential secondary spectral types: G5 V, K0 V, K5 V, M0 V, and M5 V. If a secondary star is present and is sufficiently bright, the effects of the added light will appear in interferometric observations where the planet will not. All secondary types could be eliminated from consideration for 7 host stars and no secondary stars of any spectral type could be ruled out for 7 more. The remaining 6 host stars showed a range of possible secondary types.Comment: Accepted for publication in the Astronomical Journa

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 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.Comment: 26 pages, 5 tables, 8 figure

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 O/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 Rsol 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.Comment: 14 pages, 5 figures, to be published in Astrophysical Journal Letter

The CHARA Array Angular Diameter of HR 8799 Favors Planetary Masses for Its Imaged Companions

HR 8799 is an hF0 mA5 gamma Doradus, lambda Bootis, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799's limb-darkened angular diameter to be 0.342 +/- 0.008 mas; this is the smallest interferometrically measured stellar diameter to date, with an error of only 2%. By combining our measurement with the star's parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 +/- 0.06 R_Sun), effective temperature (7193 +/- 87 K, consistent with F0), luminosity (5.05 +/- 0.29 L_Sun), and the extent of the habitable zone (1.62 AU to 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 lambda 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 8799's mass and age considering two possibilities: 1.516 +0.038/-0.024 M_Sun and 33 +7/-13 Myr if the star is contracting toward the zero age main-sequence or 1.513 +0.023/-0.024 M_Sun and 90 +381/-50 Myr if it is expanding from it. This improved estimate of HR 8799's 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 habitable zone of this young star.Comment: Accepted for publication in ApJ; 37 pages, 6 tables, 13 figure

The ages of a-stars. I. Interferometric observations and age estimates for stars in the ursa major moving group

We have observed and spatially resolved a set of seven A-type stars in the nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners on the Center for High Angular Resolution Astronomy Array. At least four of these stars have large rotational velocities (v sin i ≳ 170 km s-1) and are expected to be oblate. These interferometric measurements, the stars observed photometric energy distributions, and v sin i values are used to computationally construct model oblate stars from which stellar properties (inclination, rotational velocity, and the radius and effective temperature as a function of latitude, etc.) are determined. The results are compared with MESA stellar evolution models to determine masses and ages. The value of this new technique is that it enables the estimation of the fundamental properties of rapidly rotating stars without the need to fully image the star. It can thus be applied to stars with sizes comparable to the interferometric resolution limit as opposed to those that are several times larger than the limit. Under the assumption of coevality, the spread in ages can be used as a test of both the prescription presented here and the MESA evolutionary code for rapidly rotating stars. With our validated technique, we combine these age estimates and determine the age of the moving group to be 414 ± 23 Myr, which is consistent with, but much more precise than previous estimates