Fringe Visibility Estimators for the Palomar Testbed Interferometer

Visibility estimators and their performance are presented for use with the Palomar Testbed Interferometer (PTI). One operational mode of PTI is single-baseline visibility measurement using pathlength modulation with synchronous readout by a NICMOS-3 infrared array. Visibility is estimated from the fringe quadratures, either incoherently, or using source phase referencing to provide a longer coherent integration time. The visibility estimators differ those used with photon-counting detectors in order to account for biases attributable to detector offsets and read noise. The performance of these estimators is affected not only by photon noise, but also by the detector read noise and errors in estimating the bias corrections, which affect the incoherent and coherent estimators differently. Corrections for visibility loss in the coherent estimators using the measured tracking jitter are also presented.Comment: PASP in press (Jan 99). 13 Pages, no figure

The PRIMA fringe sensor unit

The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides fringe sensing for all observation modes, comprising off-axis fringe tracking, phase referenced imaging, and high-accuracy narrow-angle astrometry. It is installed at the Very Large Telescope Interferometer (VLTI) and successfully servoed the fringe tracking loop during the initial commissioning phase. Unique among interferometric beam combiners, the FSU uses spatial phase modulation in bulk optics to retrieve real-time estimates of fringe phase after spatial filtering. A R=20 spectrometer across the K-band makes the retrieval of the group delay signal possible. The FSU was integrated and aligned at the VLTI in summer 2008. It yields phase and group delay measurements at sampling rates up to 2 kHz, which are used to drive the fringe tracking control loop. During the first commissioning runs, the FSU was used to track the fringes of stars with K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT) and baselines of up to 96 m. Fringe tracking using two Very Large Telescope (VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and combining stellar light with two ATs, the FSU showed its ability to improve the VLTI sensitivity in K-band by more than one magnitude towards fainter objects, which is of fundamental importance to achieve the scientific objectives of PRIMA.Comment: 19 pages, 23 figures. minor changes and language editing. this version equals the published articl

Binary star astronomy with optical interferometry

The Mark III Interferometer on Mt. Wilson, a long-baseline optical interferometer, was in daily operation for more that seven years. During that time it achieved milliarcsecond angular resolution for binary star astronomy, with submilliarcsecond accuracy. For the first time many spectroscopic binaries have been resolved, including binaries in which the companion cannot be detected with spectroscopy. The high angular resolution means that the traditional gap between visual and spectroscopic binaries has been decreased by more than an order of magnitude. In order to confirm the performance of the Mark III Interferometer, this paper uses the results of astronomical observations, and compares the Mark III Interferometer with other high-resolution techniques, including astrometry, lunar occultation, photometry, speckle, and spectroscopy. Comparisons for a variety of binary stars among these techniques indicate that long baseline optical interferometry proves a reliable, fully automatic, daily accessible astronomical capability for achieving high resolution, high accuracy, high dynamic range, and high photometric measurement precision for the study of binary stars

New insights on the AU-scale circumstellar structure of FU Orionis

We report new near-infrared, long-baseline interferometric observations at the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and VLTI interferometers. This young stellar object has been observed on 42 nights over a period of 6 years from 1998 to 2003. We have obtained 287 independent measurements of the fringe visibility with 6 different baselines ranging from 20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane coverage, coupled with the published spectral energy distribution data, allows us to test the accretion disk scenario. We find that the most probable explanation for these observations is that FU Ori hosts an active accretion disk whose temperature law is consistent with standard models. We are able to constrain the geometry of the disk, including an inclination of 55 deg and a position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is detected in the data (at the two-sigma level) from the longest baselines, which we interpret as a possible disk hot-spot or companion. However, the oscillation in our best data set is best explained with an unresolved spot located at a projected distance of 10 AU at the 130 deg position angle and with a magnitude difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a companion of the central FU Ori system on an extremely eccentric orbit. We speculate that the close encounter of this putative companion and the central star could be the explanation of the initial photometric rise of the luminosity of this object

The Palomar Testbed Interferometer

The Palomar Testbed Interferometer (PTI) is a long-baseline infrared interferometer located at Palomar Observatory, California. It was built as a testbed for interferometric techniques applicable to the Keck Interferometer. First fringes were obtained in July 1995. PTI implements a dual-star architecture, tracking two stars simultaneously for phase referencing and narrow-angle astrometry. The three fixed 40-cm apertures can be combined pair-wise to provide baselines to 110 m. The interferometer actively tracks the white-light fringe using an array detector at 2.2 um and active delay lines with a range of +/- 38 m. Laser metrology of the delay lines allows for servo control, and laser metrology of the complete optical path enables narrow-angle astrometric measurements. The instrument is highly automated, using a multiprocessing computer system for instrument control and sequencing.Comment: ApJ in Press (Jan 99) Fig 1 available from http://huey.jpl.nasa.gov/~bode/ptiPicture.html, revised duging copy edi

Narrow-Angle and Wide-Angle Astrometry via Long Baseline Optical/Infrared Interferometers

Long baseline optical/infrared interferometers, such as the Mark III Stellar Interferometer on Mt. Wilson and the ASEPS-0 Testbed Interferometer on Palomar Mountain, California, have good capabilities for narrow-angle and wide-angle astrometry with very high precision. Using the Mark III Interferometer many spectroscopic binaries became “visual” for the first time. The measurement accuracy of angular separation is 0.2 mas, the smallest separation measured between two components is 2 mas, the maximum magnitude difference is 4 mag, and the smallest semimajor axis is 4 mas. Such high angular resolution and dynamic range have been used to determine stellar masses with precision of 2% and differential stellar luminosities to better than 0.05 mag for separations of less than 0.″2. For some binary stars, not only have the systems been resolved, but also the diameter of the primary component has been determined, yielding direct measurements of stellar effective temperature with high accuracy. For parallax determination, the precision is 1 mas or better and is unaffected by interstellar extinction. For wide-angle astrometry with the Mark III interferometer, the observation results yielded average formal 1σ errors for FK5 stars of about 10 mas. Presently a new infrared interferometer, the ASEPS-0 Testbed Interferometer on Palomar Mountain is under construction, and is being optimized to perform high accuracy narrow-angle astrometry using long baseline observations at 2.2 μm, with phase referencing for increased sensitivity. The goal is to demonstrate differential astrometric accuracies of 0.06–0.1 mas in order to allow for detection of extra-solar planets in the near future

The PHASES Differential Astrometry Data Archive. I. Measurements and Description

The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) monitored 51 sub-arcsecond binary systems to determine precision binary orbits, study the geometries of triple and quadruple star systems, and discover previously unknown faint astrometric companions as small as giant planets. PHASES measurements made with the Palomar Testbed Interferometer (PTI) from 2002 until PTI ceased normal operations in late 2008 are presented. Infrared differential photometry of several PHASES targets were measured with Keck Adaptive Optics and are presented.Comment: 33 pages emulateapj, Accepted to A

Keck Interferometer status and plans

Keck Interferometer is a NASA-funded project to combine the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 μm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot-Jupiters" by their direct emission. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Recent activity has included formal visibility mode commissioning, as well as science observations, and we briefly review some of the significant technical aspects and updates to the system. We have also completed laboratory development of the nuller. The nuller uses two modified Mach-Zehnder input nullers, a Michelson cross combiner, and a 10 μm array camera to produce background-limited null measurements. To provide required temporal stability for the nuller, the system incorporates end-to-end laser metrology with phase referencing from two 2.2 μm fringe trackers. The nuller recently completed its pre-ship review and is being installed on the summit. After nuller integration and test, the differential phase mode will be deployed, which will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control

Masses, luminosities, and orbital coplanarities of the µ Orionis quadruple-star system from phases differential astrometry

μ Orionis was identified by spectroscopic studies as a quadruple-star system. Seventeen high-precision differential astrometry measurements of μ Ori have been collected by the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES). These show both the motion of the long-period binary orbit and short-period perturbations superimposed on that caused by each of the components in the long-period system being themselves binaries. The new measurements enable the orientations of the long-period binary and short-period subsystems to be determined. Recent theoretical work predicts the distribution of relative inclinations between inner and outer orbits of hierarchical systems to peak near 40 and 140 degrees. The degree of coplanarity of this complex system is determined, and the angle between the planes of the A–B and Aa–Ab orbits is found to be 136.7 ± 8.3 degrees, near the predicted distribution peak at 140 degrees; this result is discussed in the context of the handful of systems with established mutual inclinations. The system distance and masses for each component are obtained from a combined fit of the PHASES astrometry and archival radial velocity observations. The component masses have relative precisions of 5% (component Aa), 15% (Ab), and 1.4% (each of Ba and Bb). The median size of the minor axes of the uncertainty ellipses for the new measurements is 20 micro-arcseconds (μas). Updated orbits for δ Equulei, κ Pegasi, and V819 Herculis are also presented

Stellar and Molecular Radii of a Mira Star: First Observations with the Keck Interferometer Grism

Using a new grism at the Keck Interferometer, we obtained spectrally dispersed (R ~ 230) interferometric measurements of the Mira star R Vir. These data show that the measured radius of the emission varies substantially from 2.0-2.4 microns. Simple models can reproduce these wavelength-dependent variations using extended molecular layers, which absorb stellar radiation and re-emit it at longer wavelengths. Because we observe spectral regions with and without substantial molecular opacity, we determine the stellar photospheric radius, uncontaminated by molecular emission. We infer that most of the molecular opacity arises at approximately twice the radius of the stellar photosphere.Comment: 12 pages, including 3 figures. Accepted by ApJ