Close binary stars like spectroscopic binaries create a completely different environment than single stars for the
evolution of a protoplanetary disk. Dynamical interactions between one star and protoplanets in such systems provide
more challenges for theorists to model giant planet migration and formation of multiple planets. For habitable planets the
majority of host stars are in binary star systems. So far only a small amount of Jupiter-size planets have been discovered
in binary stars, whose minimum separations are 20 AU and the median value is about 1000 AU (because of difficulties in
radial velocity measurements). The SIM Lite mission, a space-based astrometric observatory, has a unique capability to
detect habitable planets in binary star systems. This work analyzed responses of the optical system to the field stop for
companion stars and demonstrated that SIM Lite can observe exoplanets in visual binaries with small angular
separations. In particular we investigated the issues for the search for terrestrial planets in P-type binary-planetary
systems, where the planets move around both stars in a relatively distant orbit.
It is a challenging task to find exoplanets because of the huge contrast between star and planets in mass and in
brightness. It is more challenging to determine the masses of exoplanets because it requires extremely high astrometric
accuracy. In particular detection of Earth-like planets needs sub-microarcsecond (µas) precision which is possible only
by narrow-angle astrometry via the SIM PlanetQuest mission. The narrow-angle observation mode of SIM PlanetQuest
requires several distant reference stars, which are close enough in the sky around the target, typically within one degree.
This paper provides statistical estimates of available reference stars for all candidate stars in searching for Earth-like
It is inevitable that some of reference stars will have binary components and planets. This paper describes the analysis
techniques and various error estimates for binary jitters and planet effects of reference stars. Because of the limited
number and duration of observations of SIM PlanetQuest, the Monte Carlo simulations indicate that certain long period
planets around reference stars may not be detected. Earth-like planets around target stars, however, can be detected
unambiguously. Finally, we demonstrate the current best estimates of instrument error, photon noise, reference stars
planet disturbance, stellar jitters, etc., and conclude that the orbits of Earth-like planets with sub-μas astrometric
signatures can be determined accurately by SIM PlanetQuest for nearby candidate stars.
The SIM PlanetQuest mission can provide microarcsecond (μas) accuracy for exoplanet searches and critical astrophysical research. SIM is the only mission which can measure angular wobbles caused by planets for determination of planetary masses. In order to reach μas accuracy the SIM instrument must be able to measure fringe parameters to the accuracy of picometers. It is necessary to investigate calibration techniques and to carefully analyze influences from ghost images in white light fringe measurements. This work will analyze focusing and tilt variations introduced by thermal changes in calibration processes. In particular the accuracy limits are presented for common short- and long-stroke experiments. A new, simple, practical calibration scheme is proposed and analyzed based on the SIM PlanetQuest's Micro-Arcsecond Metrology (MAM) testbed experiments.
The Space Interferometry Mission (SIM) will perform global astrometry (full sky), local wide-angle (15 degree) and narrow-angle (1 degree) observations to search extra-solar planets, and can calibrate stellar and galactic evolution theories. The astrometric accuracy of the SIM mission depends on spectral characteristics of the optics, detectors and targets. This paper will discuss the photometric throughput of the SIM instrument, and analyze effects of wavefront errors, optical mismatches and control biases as a function of wavelength. The color dependence models of the instrument optics including mirrors, lenses, field-stop and beam-splitter are presented. The performances of different detectors with a variety of coatings are compared. A model of the SIM fringe spectrometer is created. For early and late types of stars, brightness dependency errors are analyzed for different combinations of optics and detectors. Visibility loss due to imperfect optics is investigated in detail. Based on the models of instrument and estimated visibilities, the astrometric accuracies for various kinds of stars are evaluated.
It is important to emphasize that not only light sources, mirrors, lenses, field stop and detectors are all wavelength dependent, but also fringe visibility loss, wavefront error, optics control error, etc. are all a function of wavelengths. For the first time the estimate of SIM performance is based on spectral analysis of all factors above, rather than monochromatic approximations of detected fringes, or simply adopted constants. This paper summarizes the astrometric accuracies for a wide range of stars and various combinations of optical design and detector configurations. It has been verified that SIM has astrometric accuracy of about 4 μas for targets with different spectra.
The micro-arcsecond metrology testbed (MAM) provides a testing ground
for SIM to perform optical path difference measurements with picometer
(pm) precision. Because of imperfect optics and non-ideal laser
sources it is inevitable that the cyclic bias is one of the major
error sources for SIM. Many experiments have been conducted to
diagnose and to characterize cyclic bias in the laser gauges, and in
white light fringe detection. Our data analysis indicates that cyclic
bias in MAM has a predictable frequency and a relatively stable
amplitude. It has been proposed to use phase measurements at different
wavelengths to solve for the cyclic bias. The experiment results have
shown that the cyclic bias in SAVV are reduced from nm level to the level of hundred picometers. Besides the cyclic bias the effective
wavelengths of spectral channels have to be calibrated also. At
present, a new method using FFT technique and new metrology gauge
demonstrates that the wavelength determination has a precision of
10-4. The spectrometer in MAM is stable. The changes of
effective wavelengths in a few weeks is about one nanometer, or
less. Systematic biases above must be periodically calibrated.
We have developed a technique that allows SIM to measure
relative stellar positions with an accuracy of 1 micro-arcsecond
at any time during its 5-yr mission. Unlike SIM's standard
narrow-angle approach, Gridless Narrow Angle Astrometry (GNAA)
does not rely on the global reference frame of grid stars that
reaches full accuracy after 5 years. GNAA is simply the
application of traditional single-telescope narrow angle
techniques to SIM's narrow angle optical path delay measurements.
In GNAA, a set of reference stars and a target star are observed
at several baseline orientations. A linearized model uses delay
measurements to solve for star positions and baseline
orientations. A conformal transformation maps observations at
different epochs to a common reference frame. The technique works
on short period signals (P=days to months), allowing it to be
applied to many of the known extra-solar planets, intriguing
radio/X- ray binaries, and other periodic sources. The technique's
accuracy is limited in the long-term by false acceleration due to
a combination of reference star and target star proper motion. The
science capability, 1 micro-arcsecond astrometric precision, is
unique to SIM.
A fixed delay interferometer combined with a post-disperser is a new technique for high precision radial velocity (RV) measurements. The Doppler measurements are conducted by monitoring the stellar fringe phase shifts of the interferometer instead of absorption line centroid shifts as in the echelle. High Doppler sensitivity is achieved through optimizing the optical delay in the interferometer and measuring multiple fringes over a broadband. The broadband operation is achieved by using the post-disperser for dispersing fringes in different wavelengths.
Comparing to the state-of-the-art cross-dispersed echelle spectroscopy, this interferometer technique provides almost identical RV precision based on photon statistics. However, the interferometer method has a potential for lower systematic noise due to its simpler instrument response than the echelle. The interferometer can be optimized for higher throughput than the echelle. The interferometer approach also allows fringes to be recorded in one dispersion order instead of many cross-dispersed echelle orders. Therefore, this instrument opens up a great opportunity for multi-object observations to allow all sky surveys for extra-solar planets at moderate sized wide field telescopes.
Initial observations with a prototype at the Hobby-Eberly 9 m and Palomar 5 m telescopes demonstrate ~9 m/s Doppler RV precision with stellar fringe data recorded on a 1kx1k CCD detector (or 140 Å wavelength coverage), a S/N ~ 120 per pixel and a post-disperser spectral resolving power of R = 6,700. This precision is consistent with the photon noise limit. Future improvement in wavelength coverage and wavelength calibration can reduce the Doppler error to a few m/s or less.
The Palomar Testbed Interferometer (PTI) is an infrared, phase-tracking interferometer in operation at Palomar Mountain since July 1995. It was funded by NASA for the purpose of developing techniques and methodologies for doing narrowangle astrometry for the purpose of detecting extrasolar planets. The instrument employs active fringe trackingin the infrared (2.0-2.4 μm) to monitor fringe phase. It is a dual-star interferometer; it is able to measure fringes on two separate stars simultaneously. An end-to-end heterodyne laser metrology system is used to monitor the optical path length of the starlight. Recently completed engineering upgrades have improved the initial instrument performance. These upgrades are:extended wavelength coverage, a single mode fiber for spatial filtering, vacuum pipes to relay the beams, accelerometers on the siderostat mirrors and a new baseline. Results of recent astrometry data indicate the instrument is approaching the astrometric limit as set by the atmosphere.
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.
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.
For the first time, four spectroscopic binaries have been directly resolved with the Mark III Stellar Interferometer. Observations in 1988 and 1989 were analyzed, and visual orbits for four binaries have been determined. The semimajor axes for Beta Tri, Alpha Equ, Alpha And and Beta Ari are approximately 0.008 arcsec, 0.012 arcsec, 0.024 arcsec and 0.037 arcsec, respectively. The magnitude differences between two components are 0.5, 0.7, 1.8 and 2.6 mag, respectively. All of the orbital elements for Alpha And and Beta Ari were determined from interferometric data only, and agree well with spectroscopic observations. Predictions of relative position between the two components for these binaries are consistent with the measurements to less than 0.001 arcsec. Combined with data from spectroscopy, masses and distance for the double-lined spectroscopic binary Beta Ari are derived, and the results indicate that both components of Beta Ari agree well with the empirical mass-luminosity relation.