J. Thomas Armstrong

Stellar Optical Interferometry in the 1990s

The unaided eye has an angular resolution of about 1 arcminute. From the invention of the telescope in the 17th century to the middle of the 1970s, astronomers improved on this resolution by two orders of magnitude by building bigger telescopes and putting them at good sites. Even at good sites, however, atmospheric turbulence limits the resolution at visible and infrared wavelengths to 1 arcsec or a little better. In the past 20 years, a further factor‐of‐ten improvement has come with two developments that deal with the atmosphere: “speckle interferometry,” in which the blurred image is frozen in a short exposure and the image is reconstructed from many exposures, and adaptive optics, in which the effects of the atmosphere are sensed, then corrected with a defbrmable mirror, before the image is recorded. (See Laird A. Thompsons article in PHYSICS TODAY, December 1994, page 24.)

Initial Results from the USNO Dispersed Fourier Transform Spectrograph

We have designed and constructed a ‘‘dispersed Fourier transform spectrometer’’ (dFTS), consisting of a conventionalFTSfollowedbyagratingspectrometer.Bycombiningthesetwodevices,wenegateasubstantialfraction of the sensitivity disadvantage of a conventional FTS for high-resolution, broadband, optical spectroscopy, while preserving many of the advantages inherent to interferometric spectrometers. In addition, we have implemented a simple and inexpensive laser metrology system, which enables very precise calibration of the interferometer wavelength scale. The fusion of interferometric and dispersive technologies with a laser metrology system yields an instrument well suited to stellar spectroscopy, velocimetry,and extrasolar planet detection, which is competitive with existing high-resolution, high-accuracy stellar spectrometers. In this paper we describe the design of our prototype dFTS,explain the algorithmwe use to efficiently reconstruct a broadbandspectrum from a sequence of narrowband interferograms, and present initial observations and resulting velocimetry of stellar targets. Subject headingg binaries: spectroscopic — instrumentation: interferometers — instrumentation: spectrographs — planetary systems — techniques: interferometric

Bootstrapping the NPOI: keeping long baselines in phase by tracking fringes on short baselines

The high-spatial-frequency fringes that contain detailed information about a stellar image are generally too weak to track. The Navy Prototype Optical Interferometer (NPOI) is the first interferometer to measure these fringes with the techniques of phase bootstrapping - the use of fringe tracking on two short baselines AB and BC to keep a longer baseline AC phased up - and wavelength bootstrapping - the use of fringe tracking at long wavelengths to keep the long baseline phased up at short wavelengths. We demonstrate the utility of phase and wavelength bootstrapping with the NPOI observations of stellar limb darkening of (beta) Cancri of Pauls et al. in these proceedings. The NPOI baselines and wavelength coverage were 19, 22, and 38 m and (lambda) 450 to (lambda) 850 nm. For (lambda) < 675 nm, the 37 m baseline samples the uv plane beyond the first null of the Airy disk.

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)

Navy Prototype Optical Interferometer observations of geosynchronous satellites

Using a 15.9  m baseline at the Navy Prototype Optical Interferometer (NPOI), we have successfully detected interferometric fringes in observations of the geosynchronous satellite (geosat) DirecTV-9S while it glinted on two nights in March 2009. The fringe visibilities can be fitted by a model consisting of two components, one resolved (≳3.7  m) and one unresolved (∼1.1  m). Both the length of the glint and the specular albedos are consistent with the notion that the glinting surfaces are not completely flat and scatter reflected sunlight into an opening angle of roughly 15°. Enhancements to the NPOI that would improve geosat observations include adding an infrared capability, which could extend the glint season, and adding larger, adaptive-optics equipped telescopes. Future work may test the feasibility of observing geosats with aperture-masked large telescopes and of developing an array of six to nine elements.

Resolving the effects of rotation in early type stars

We review the theory of rotating stars, first developed 80 years ago. Predictions include a specific relation between shape and angular velocity and between surface location and effective temperature and effective gravity. Seen at arbitrary orientation rapidly rotating stars will display ellipsoidal shapes and possibly quite asymmetric intensity distributions. The flattening due to rotation has recently been detected at PTI and VLTI. With the increasing baselines available in the visible and the implementation of closure phase measurements at the NPOI it is now possible to search for the surface brightness effects of rotation. Roche theory predicts only large scale deviations from the usual centro-symmetric limb-darkened models, ideal when the stellar disks are only coarsely imaged as now. We report here observations of Altair and Vega with the NPOI using baselines that detect fringes beyond the first Airy zero in both objects. Asymmetric, non-classical intensity distributions are detected. Both objects appear to be rotating at a large fraction of their breakup velocity. Vega is nearly pole on, accounting for its low apparent rotational velocity. Altairs inclination is intermediate, allowing high S/N detection of all the predicted features of a Roche spheroid. We describe how these objects will test this fundamental theory and how Vegas role as a standard will need reinterpretation.

Design of the long delay lines for the Navy Prototype Optical Interferometer

We have designed a method for introducing the large delays needed for the full 437 meter baseline imaging subarray of the Navy Prototype Optical Interferometer (NPOI). Long delay liens (LDLs) will introduce delay in discrete 29 meter increments for each of the six array elements. In conjunction with the 35 meters of delay from the continuously-variable fast delay lines, the LDLs will allow fringe tracking for all baselines at any position on the sky. We present the mechanical layout, alignment and vacuum design of the LDLs.

NPOI : recent progress and future prospects

The instrumental status of the Navy Prototype Optical Interferometer (NPOI) since the last SPIE meeting in 2006 is summarized, along with the results of the current science programs. The commissioning of new stations and plans for greatly increased telescope apertures are discussed, along with other instrumentation upgrades. Recent results in the areas of wide-angle astrometry, binary stars, physical modeling of the circumstellar disks of early-type stars, improvements in coherent averaging, and phase-reference imaging are also reviewed.

Navy Precision Optical Interferometer Observations of the Exoplanet Host κ Coronae Borealis and Their Implications for the Star's and Planet's Masses and Ages

We used the Navy Precision Optical Interferometer to measure the limb-darkened angular diameter of the exoplanet host star kappa CrB and obtained a value of 1.543 +/- 0.009 mas. We calculated its physical radius (5.06 +/- 0.04 R_Sun) and used photometric measurements from the literature with our diameter to determine kappa CrBs effective temperature (4788 +/- 17 K) and luminosity (12.13 +/- 0.09 L_Sun). We then placed the star on an H-R diagram to ascertain the stars age (3.42 +0.32/-0.25 Gyr) and mass (1.47 +/- 0.04 M_Sun) using a metallicity of [Fe/H] = +0.15. With this mass, we calculated the systems mass function with the orbital elements from a variety of sources, which produced a range of planetary masses: m_p sin i = 1.61 to 1.88 M_Jup. We also updated the extent of the habitable zone for the system using our new temperature.

Progress report on the construction of the Navy Prototype Optical Interferometer at the Lowell Observatory

The Navy Prototype Optical Interferometer (NPOI) at the Lowell Observatory near Flagstaff, Arizona is a tri-baseline stellar interferometer with specific design characteristics for astrometry and imaging. All major construction has been completed. Installation of scientific instrumentation began in July 1993 with first light expected during the Spring 1994. Here we present a description of the location, physical plan, and construction of the interferometer.