Xiaopei Pan

Angular diameter measurements of stars

Angular diameters determined with the Mark III Optical Interferometer are presented for 12 stars at wavelengths of 450 and 800 nm. The uniform disk diameters resulting from fits to the visibility observations have rms residuals of order 1 percent for the 800 nm measurements and less than 3 percent for the 450 nm measurements. The improvement over previous observations with this instrument is due to improved data analysis and the use of a wider range of baseline lengths. An analysis of the calibration systematics for the Mark III Optical Interferometer is included. There is good agreement between these measurements and previously published data. The changes in uniform disk diameter between wavelengths of 450 and 800 nm agree with models of stellar atmospheres.

A distance of 133–137 parsecs to the Pleiades star cluster

Nearby ‘open’ clusters of stars (those that are not gravitationally bound) have played a crucial role in the development of stellar astronomy because, as a consequence of the stars having a common age, they provide excellent natural laboratories to test theoretical stellar models. Clusters also play a fundamental part in determining distance scales. The satellite Hipparcos surprisingly found that an extensively studied open cluster—the Pleiades (also known as the Seven Sisters)—had a distance of D = 118 ± 4 pc (refs 2, 3), about ten per cent smaller than the accepted value. The discrepancy generated a spirited debate because the implication was that either current stellar models were incorrect by a surprising amount or Hipparcos was giving incorrect distances. Here we report the orbital parameters of the bright double star Atlas in the Pleiades, using long-baseline optical/infrared interferometry. From the data we derive a firm lower bound of D > 127 pc, with the most likely range being 133 < D < 137 pc. Our result reaffirms the fidelity of current stellar models.

FU Orionis Resolved by Infrared Long-Baseline Interferometry at a 2 AU Scale

We present the first infrared interferometric observations of a young stellar object with a spatial projected resolution better than 2 AU. The observations were obtained with the Palomar Testbed Interferometer (PTI). FU Orionis exhibits a visibility of V2=0.72 ± 0.07 for a 103 ± 5 m-projected baseline at λ=2.2 μm. On the spatial scale probed by the PTI, the data are consistent with both a binary system scenario (a maximum magnitude difference of 2.7 ± 0.5 mag and the smallest separation of 0.35 ± 0.05 AU) and a standard luminous accretion disk model ( ~6 × 10−5 M☉ yr-1), where the thermal emission dominates the stellar scattering, and are inconsistent with a single stellar photosphere.

Determination of the visual orbit of the spectroscopic binary Alpha Andromedae with submilliarcsecond precision

The visual orbit of the spectroscopic binary Alpha And is determined independently of spectroscopic data using the Mark III Stellar Interferometer. Observations of Alpha And in 1988 and 1989 clearly demonstrate submilliarcsecond measurement precision at optical wavelengths. All of the orbital elements of Alpha And are calculated utilizing observations from the stellar interferometer only and are in excellent agreement with the spectroscopic results. However, three of these elements can only be obtained from interferometric data. Using both interferometric and spectroscopic observations, the definitive orbital elements are determined including angular semimajor axis, inclination, position angle of ascending node, longitude of periastron, period eccentricity, and epoch of periastron passage. In addition, the magnitude difference between the two components is measured, yielding delta-m = 1.82 +/- 0.04 mag at 800 nm and delta-m = 1.99 +/- 0.04 mag at 550 nm. Incorporating photometric observations, the color indices between 550 nm and 800 nm for the primary and the companion are determined as -0.11 +/- 0.03 mag +0.07 +/- 0.05 mag, respectively.

The orbit of Alpha Equulei measured with long-baseline optical interferometry - Component masses, spectral types, and evolutionary state

The apparent orbit of the double-lined spectroscopic binary Alpha Equulei was measured using observations, from June 13, 1989 to September 15, 1990, with the Mark III Optical Interferometer. The results, combined with the spectroscopic results of Rosvick and Scarfe (1991), were used to obtain estimates of the masses of the components, their absolute magnitudes, and the distance to the system. In addition, the magnitude differences between the components were determined at four wavelengths; these were combined with the colors reported by Stickland (1976) to derive colors for the two Alpha Equulei components and to estimate their spectral types.

The Visual Orbit of 64 Piscium

We report on the determination of the visual orbit of the double-lined spectroscopic binary system 64 Piscium with data obtained by the Palomar Testbed Interferometer in 1997 and 1998. 64 Psc is a nearly equal-mass double-lined binary system whose spectroscopic orbit is well known. We have estimated the visual orbit of 64 Psc from our interferometric visibility data. Our 64 Psc orbit is in good agreement with the spectroscopic results, and the physical parameters implied by a combined fit to our interferometric visibility data and radial velocity data of Duquennoy & Mayor result in precise component masses that agree well with their spectral type identifications. In particular, the orbital parallax of the system is determined to be 43.29 ± 0.46 mas, and masses of the two components are determined to be 1.223 ± 0.021 M☉ and 1.170 ± 0.018 M☉, respectively. Nadal et al. put forward arguments of temporal variability in some of the orbital elements of 64 Psc, presumably explained by an undetected component in the system. While our visibility data do not favor the Nadal temporal variability inference, neither is it definitive in excluding it. Consequently we have performed both high dynamic-range near-infrared imaging and spectroscopy of potential additional companions to the 64 Psc system. Our imaging and spectroscopic data do not support the conjecture of an additional component to 64 Psc, but we did identify a faint object with unusual red colors and spectra.

Apparent orbit of the spectroscopic binary Beta ARIETIS with the time Mark III Stellar Interferometer

The spectroscopic binary Beta Ari has been directly resolved with the Mark III Stellar Interferometer. Observations in 1988 were analyzed to determine the visual orbit of the system with the following results: eccentricity = 0.903 +/- 0.012, semimajor axis = 0.0361 +/- 0.0003 arcsec, inclination = 44.7 +/- 1.3 deg, longitude of periastron = 209.1 deg +/- 1.2 deg, position angle of ascending node = 79.1 deg +/- 0.8 deg. The measured magnitude difference between two components, Delta m = 2.63 +/- 0.22 at 800 nm, yields individual visual magnitudes of m(v1) = 2.70 +/- 0.02 and m(v2) = 5.80 +/- 0.20. Combined with data from spectroscopic observations, masses M1 = (2.34 +/- 0.10) solar masses, M2 = (1.34 +/- 0.07) solar masses, and geometrical parallax pi = 0.053 arcsec +/- 0.002 arcsec are derived. These results demonstrate that both components of Beta Ari agree well with the empirical mass-luminosity relation.

The Synergy of Direct Imaging and Astrometry for Orbit Determination of Exo-Earths

The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in the habitable zone (HZ) around a nearby star. Mass is one of the most important characteristics of a planet and can only be measured by observing the motion of the star around the planet-star center of gravity. The planets orbit can be measured either by imaging the planet at multiple epochs or by measuring the position of the star at multiple epochs by space-based astrometry. The measurement of an exoplanets orbit by direct imaging is complicated by a number of factors. One is the inner working angle (IWA). A space coronagraph or interferometer imaging an exo-Earth can separate the light from the planet from the light from the star only when the star-planet separation is larger than the IWA. Second, the apparent brightness of a planet depends on the orbital phase. A single image of a planet cannot tell us whether the planet is in the HZ or distinguish whether it is an exo-Earth or a Neptune-mass planet. Third is the confusion that may arise from the presence of multiple planets. With two images of a multiple planet system, it is not possible to assign a dot to a planet based only on the photometry and color of the planet. Finally, the planet-star contrast must exceed a certain minimum value in order for the planet to be detected. The planet may be unobservable even when it is outside the IWA, such as when the bright side of the planet is facing away from us in a crescent phase. In this paper we address the question: Can a prior astrometric mission that can identify which stars have Earth-like planets significantly improve the science yield of a mission to image exo-Earths? In the case of the Occulting Ozone Observatory, a small external occulter mission that cannot measure spectra, we find that the occulter mission could confirm the orbits of ~4 to ~5 times as many exo-Earths if an astrometric mission preceded it to identify which stars had such planets. In the case of an internal coronagraph we find that a survey of the nearest ~60 stars could be done with a telescope half the size if an astrometric mission had first identified the presence of Earth-like planets in the HZ and measured their orbital parameters.

ASEPS-0 Testbed Interferometer

The ASEPS-O Testbed Interferometer is a long-baseline infrared interferometer optimized for high-accuracy narrow-angle astrometry. It is being constructed by JPL for NASA as a testbed for the future Keck Interferometer to demonstrate the technology for the astrometric detection of exoplanets from the ground. Recent theoretical and experimental work has shown that extremely high accuracy narrow-angle astrometry, at the level of tens of microarcseconds in an hour of integration time, can be achieved with a long-baseline interferometer measuring closely-spaced pairs of stars. A system with performance close to these limits could conduct a comprehensive search for Jupiter- and Saturn-mass planets around stars of all spectral types, and for short-period Uranus-mass planets around nearby M and K stars. The key features of an instrument which can achieve this accuracy are long baselines to minimize atmospheric and photon-noise errors, a dual-star feed to route the light from two separate stars to two beam combiners, cophased operation using an infrared fringe detector to increase sensitivity in order to locate reference stars near a bright target, and laser metrology to monitor systematic errors. The ASEPS-O Testbed Interferometer will incorporate these features, with a nominal baseline of 100 m, 50- cm siderostats, and 40-cm telescopes at the input to the dual- star feeds. The fringe detectors will operate at 2.2 micrometers , using NICMOS-III arrays in a fast-readout mode controlling high-speed laser-monitored delay lines. Development of the interferometer is in progress, with installation at Palomar Mountain planned to begin in 1994.

The orbit of Phi Cygni measured with long-baseline optical interferometry - Component masses and absolute magnitudes

We present the orbit of the double-lined spectroscopic binary Φ Cygni, the distance to the system, and the masses and absolute magnitudes of its components. The apparent orbit is based on our observations with the Mark III Optical Interferometer, the distance (80.8±1.8 pc) and component masses (2.54±0.09 M ○ . and 2.44±0.08 M ○ ., in agreement with the previous determination by McAlister [AJ, 101, 2207 (1982)]) are based on the Mark III data combined with a reinterpretation of the spectroscopic data of Rach & Herbig [ApJ, 133, 143 (1961)], and the absolute V magnitudes (0 m .77±0 m .1 and 1 m .07±0 m .1) are derived from the distance, the cataloged integrated magnitude, and the magnitude difference from spectroscopic data