Space Interferometry Research Articles

Predicting the Optical Performance of the Space Interferometry Mission Using a Modeling, Testing, and Validation Methodology

This paper presents the modeling, testing, and validation methodologies developed to predict the optical performance of the Space Interferometry Mission (SIM) at the Jet Propulsion Laboratory (JPL). The modeling methodology combines structural, optical, and control system design within a common state space framework and incorporates reaction wheel assembly (RWA) disturbances to evaluate the end-to-end performance of the system requirements. The validation methodology uses the Micro-Precision Interferometer (MPI) testbed, which is a ground-based, representative hardware model of SIM. In this study, the integrated model of the MPI testbed was used to calculate the transfer functions from RWA input to optical performance output. The model-predicted transfer functions were compared with the MPI testbed measurements, and the accuracy of the integrated model was quantified using a metric that was based on output power of the transfer functions. The RWA disturbances were then propagated through the modeled and measured transfer functions to predict the optical performance of the MPI testbed. This method is called the “decoupled disturbance analysis” and relies on the “blocked” RWA disturbances, measured with the RWA hardmounted to a rigid surface. These predictions were compared with the actual (measured) optical performance of MPI, measured with the RWA mounted to MPI, to evaluate the accuracy of the decoupled disturbance analysis method. The results show that this method is not an accurate representation of the coupled boundary conditions that occurs when the RWA is mounted to the flexible MPI structure. In order to correct for the blocked RWA disturbance boundary conditions, the “coupled disturbance analysis” method was developed. This method uses “force filters” that depend on estimates of the interface accelerances of the RWA and the MPI structure to effectively transform the blocked RWA disturbance measurements into their corresponding “coupled” disturbances (the disturbances that would occur at the coupled RWA-MPI interface). Compared to the decoupled method, the coupled method more accurately predicts the system’s performance. Additionally, the RWA cross-spectral density terms were found to be influential in matching the performance predictions to the measured optical performance of MPI.

TheSpace Interferometry MissionAstrometric Grid Giant Star Survey. I. Stellar Parameters and Radial Velocity Variability

We present results from a campaign of multiple-epoch echelle spectroscopy of relatively faint (V = 9.5-13.5 mag) red giants observed as potential astrometric grid stars for the Space Interferometry Mission (SIM PlanetQuest). Data are analyzed for 775 stars selected from the Grid Giant Star Survey, spanning a wide range of effective temperatures (Teff), gravities, and metallicities. The spectra are used to determine these stellar parameters and to monitor radial velocity (RV) variability at the 100 m s-1 level. The degree of RV variation measured for 489 stars observed two or more times is explored as a function of the inferred stellar parameters. The percentage of RV-unstable stars is found to be very high—about two-thirds of our sample. It is found that the fraction of RV-stable red giants (at the 100 m s-1 level) is higher among stars with Teff ~ 4500 K, corresponding to the calibration-independent range of infrared colors 0.59 < (J - Ks)0 < 0.73. A higher percentage of RV-stable stars is found if additional constraints of surface gravity and metallicity ranges, 2.3 < log g < 3.2 and -0.5 < [Fe/H] < -0.1, respectively, are applied. Selection of stars based on only photometric values of effective temperature (4300 K < Teff < 4700 K) is a simple and effective way to increase the fraction of RV-stable stars. The optimal selection of RV-stable stars, especially in the case in which the Washington photometry is unavailable, can rely effectively on the 2MASS colors constraint 0.59 < (J - Ks)0 < 0.73. These results have important ramifications for the use of giant stars as astrometric references for the SIM PlanetQuest.

The Effects of Multiple Companions on the Efficiency of Space Interferometry Mission Planet Searches

ABSTRACTThe Space Interferometry Mission (SIM) is expected to make precise astrometric measurements that can be used to detect low‐mass planets around nearby stars. Since most nearby stars are members of multiple‐star systems, many of them will have a measurable acceleration due to their companion, which must be included when solving for astrometric parameters and searching for planetary perturbations. In addition, many of the stars with one radial velocity planet show indications of additional planets. Therefore, astrometric surveys such as SIM must be capable of detecting planets and measuring orbital parameters in systems with multiple stellar and/or planetary companions. We have conducted Monte Carlo simulations to investigate how the presence of multiple companions affects the sensitivity of an astrometric survey such as SIM. We find that the detection efficiency for planets in wide binary systems is relatively unaffected by the presence of a binary companion if the planetary orbital period is less than half the duration of the astrometric survey. For longer orbital periods, there are significant reductions in the sensitivity of an astrometric survey. In addition, we find that the signal required to detect a planet can be increased significantly due to the presence of an additional planet orbiting the same star. Fortunately, adding a modest number of precision radial velocity observations significantly improves the sensitivity for many multiple‐planet systems. Thus, the combination of radial velocity observations and astrometric observations by SIM will be particularly valuable for studying multiple‐planet systems.

The N2K Consortium. IV. New Temperatures and Metallicities for More than 100,000 FGK Dwarfs

We have created specialized target lists for radial velocity surveys that are biased toward stars that (1) possess planets and (2) are easiest to observe with current detection techniques. We use a procedure that uniformly estimates fundamental stellar properties of Tycho 2 stars, with errors, using spline functions of broadband photometry and proper motion found in Hipparcos/Tycho 2 and 2MASS. We provide estimates of Teff and distance for 2.4 × 106 Tycho 2 stars that lack trigonometric distances. For stars that appear to be FGK dwarfs, we also derive [Fe/H] and identify unresolved binary systems with mass ratios 1.25 < M1/M2 < 3.0. For FGK dwarfs with photometric error σV < 0.05, or V < 9, our temperature model gives a 1 σ error of σT = +58.7/ - 65.9 K and our metallicity model gives a 1 σ error of σ[Fe/H] = +0.13/ - 0.14 dex. The binarity model can be used to remove 70% of doubles with 1.25 < M1/M2 < 3.0 from a magnitude-limited sample of dwarfs at a cost of cutting 20% of the sample. Our estimates of distance and spectral type enable us to isolate 354,822 Tycho 2 dwarfs, 321,996 absent from Hipparcos, with giant contamination of 2.6% and 7.2%, respectively. Roughly 100,000 of these stars, not in Hipparcos, have sufficiently low photometric errors to retain 0.13-0.3 dex [Fe/H] accuracy and 80-100 K temperature accuracy (1 σ). Our metallicity estimates have been used to identify targets for N2K, a large-scale radial velocity search for hot jupiters, which has verified the errors presented here. The catalogs that we publish can be used to further large-scale studies of Galactic structure and chemical evolution and to provide potential reference stars for narrow-angle astrometry programs such as the Space Interferometry Mission and large-aperture optical interferometry.

On the Formation of Gas Giant Planets on Wide Orbits

A new suite of three dimensional radiative, gravitational hydrodynamical models is used to show that gas giant planets are unlikely to form by the disk instability mechanism at distances of ~100 AU to ~200 AU from young stars. A similar result seems to hold for the core accretion mechanism. These results appear to be consistent with the paucity of detections of gas giant planets on wide orbits by infrared imaging surveys, and also imply that if the object orbiting GQ Lupus is a gas giant planet, it most likely did not form at a separation of ~100 AU. Instead, a wide planet around GQ Lup must have undergone a close encounter with a third body that tossed the planet outward to its present distance from its protostar. If it exists, the third body may be detectable by NASA's Space Interferometry Mission.

Measuring Fundamental Galactic Parameters with Stellar Tidal Streams and SI M P l a n e tQ u e s t

Extended halo tidal streams from disrupting Milky Way satellites offer new opportunities for gauging fundamental Galactic parameters without challenging observations of the Galactic center. In the roughly spherical Galactic potential, tidal debris from a satellite system is largely confined to a single plane containing the Galactic center, so accurate distances to stars in the tidal stream can be used to gauge the Galactic center distance, R0, given reasonable projection of the stream orbital pole on the XGC axis. Alternatively, a tidal stream with orbital pole near the YGC axis, such as the Sagittarius stream, can be used to derive the speed of the local standard of rest (ΘLSR). Modest improvements in current astrometric catalogs might allow this measurement to be made, but NASA's Space Interferometry Mission (now SIM PlanetQuest) can definitively obtain both R0 and ΘLSR using tidal streams.

Probing the Shape of the Galactic Halo with Hypervelocity Stars

Precise proper-motion measurements (σμ ~ 10 μas yr-1) of the recently discovered hypervelocity star (HVS) SDSS J090745.0+024507 would yield significant constraints on the axis ratios and orientation of a triaxial model for the Galactic halo. Triaxiality of dark matter halos is predicted by cold dark matter models of galaxy formation and may be used to probe the nature of dark matter. However, unless the distance to this star is determined to better than 10%, these constraints suffer from one-dimensional degeneracies, which we quantify. We show how proper-motion measurements of several HVSs could simultaneously resolve the distance degeneracies of all such stars and produce a detailed picture of the triaxial halo. Additional HVSs may be found from radial velocity surveys or from parallax/proper-motion data derived from Gaia. High-precision proper-motion measurements of these stars using the Space Interferometry Mission (SIM PlanetQuest) would substantially tighten the constraints they yield on the Galactic potential.

Five New Multicomponent Planetary Systems

We report Doppler measurements for six nearby G- and K-type main-sequence stars that show multiple low-mass companions, at least one of which has planetary mass. One system has three planets, the fourth triple-planet system known around a normal star, and another has an extremely low minimum mass of 18 M_⊕. HD 128311 (K0 V) has two planets (one previously known) with minimum masses (M sin i) of 2.18M_J and 3.21M_J and orbital periods of 1.26 and 2.54 yr, suggesting a possible 2 : 1 resonance. For HD 108874 (G5 V), the velocities reveal two planets (one previously known) having minimum masses and periods of (M sin i_b = 1.36M_J, P_b = 1.08 yr) and (M sin i_c = 1.02M_J, P_c = 4.4 yr). HD 50499 (G1 V) has a planet with P = 6.8 yr and M sin i = 1.7M_J, and the velocity residuals exhibit a trend of -4.8 m s^(-1) yr^(-1), indicating a more distant companion with P > 10 yr and minimum mass of 2M_J. HD 37124 (G4 IV-V) has three planets, one having M sin i = 0.61M_J and P = 154.5 days, as previously known. We find two plausible triple-planet models that fit the data, both having a second planet near P = 840 days, with the more likely model having its third planet in a 6 yr orbit and the other one in a 29 day orbit. For HD 190360, we confirm the planet having P = 7.9 yr and M sin i = 1.5M_J as found by the Geneva team, but we find a distinctly noncircular orbit with e = 0.36 ± 0.03, rendering this not an analog of as had been reported. Our velocities also reveal a second planet with P = 17.1 days and M sin i = 18.1 M_⊕. HD 217107 (G8 IV) has a previously known hot Jupiter with M sin i = 1.4M_J and P = 7.13 days, and we confirm its high eccentricity, e = 0.13. The velocity residuals reveal an outer companion in an eccentric orbit, having minimum mass of M sin i > 2M_J, eccentricity e ~ 0.5, and a period P > 8 yr, implying a semimajor axis α > 4 AU and providing an opportunity for direct detection. We have obtained high-precision photometry of five of the six planetary host stars with three of the automated telescopes at Fairborn Observatory. We can rule out significant brightness variations in phase with the radial velocities in most cases, thus supporting planetary reflex motion as the cause of the velocity variations. Transits are ruled out to very shallow limits for HD 217107 and are also shown to be unlikely for the prospective inner planets of the HD 37124 and HD 108874 systems. HD 128311 is photometrically variable with an amplitude of 0.03 mag and a period of 11.53 days, which is much shorter than the orbital periods of its two planetary companions. This rotation period explains the origin of periodic velocity residuals to the two-planet model of this star. All of the planetary systems here would be further constrained with astrometry by the Space Interferometry Mission.

PEGASE: a DARWIN/TPF pathfinder

The space mission PEGASE, proposed to the CNES (Centre National d'Etudes Spa- tiales = French Space Agency) in the framework of its call for scientific proposals : flying missions, is a 2-aperture interferometer, composed by 3 free flying satellites (2 siderostats and 1 beam combiner), allowing baselines from 50 to 500 m in both nulling and visibility modes. With an angular resolution of a few mas and a spectral resolution of several tens in the spectral range 2.5-5 microns, PEGASE has several goals: - science : spectroscopy of hot jupiters (Pegasides) and brown dwarves, exploration of the inner part of protoplanetary disks - technology : validation in real space conditions of formation flying, nulling and visibility inter- ferometry concepts. PEGASE has been studied at a 0-level. In this paper, we summarize the scientific program and associated technological and mission trade-off coming from this 0-level study. We also discuss how PEGASE can be considered as a TPF/DARWIN pathfinder in an international roadmap towards more complex space interferometry missions such as DARWIN/TPF.

Design of a micro-satellite for precise formation flying demonstration

This paper presents a joint CNES-EADS Astrium contribution to the ESA's SMART-2 project. SMART-2, 2nd mission of the Satellite Missions for Advanced Research and Technology program, slated for launch in 2006, will test key technologies needed to develop two ambitious ESA missions: • LISA (Laser Interferometry Space Antenna), an ESA cornerstone mission dedicated to the detection and observation of gravitational waves; to be launched in 2011, • DARWIN, another ESA cornerstone mission dedicated to the search of Earth-like planets; to be launched in 2015. In Phase A study of this demonstrator, one of the options contemplated by ESA was considering two formation-flying satellites. In that sense, and in order to both reduce and share cost, CNES proposed with the technical support of EADS-Astrium, to build one of them from its Myriade micro-satellite product line, mainly used for LEO scientific applications. The study carried out has permitted to validate the concept of using a low-cost micro-satellite in a scientific interplanetary mission requiring not more than 10 μ m inter-satellite position accuracy!

Probing MACHOs toward the Galactic Bulge

If the massive compact halo object (MACHO) fraction of the Galactic dark halo is f ~ 20%, as suggested by some microlensing experiments, then about 1.2% of lensing events toward the Galactic bulge are due to MACHOs. For the 40% of these that lie nearby (Dl < 4 kpc), measurement of their distance Dl would distinguish them from bulge lenses, while measurement of their transverse velocity vl would distinguish them from disk lenses. Hence, it would be possible to identify about 0.5%(f/20%) of all events as due to MACHOs. I show that a planned experiment using the Space Interferometry Mission (SIM PlanetQuest) could thereby detect 1 or 2 such events. This is at the margin of what is required because of a small but nonnegligible background from spheroid stars.

A new design for a very low frequency spaceborne radio interferometer

The nontransparency and severe propagation effects of the terrestrial ionosphere make it impossible for Earth‐based instruments to study the universe at low radio frequencies. An exploration of the low‐frequency radio window with the resolution and sensitivity essential to meet the scientific objectives will necessarily require a dedicated satellite‐based interferometer operating at these frequencies. Such missions have been proposed in the literature for about the past 15 years. Today, the steady and impressive advances in technology and computing resources have brought us to the brink of a quantum jump in the performance and capabilities of such missions, increasing their scientific desirability manyfold. This paper presents the concept design which emerged from a study to investigate the feasibility of a low‐frequency satellite‐based interferometer operating in the frequency range 0.1–40 MHz titled Preparation for Radio Interferometry in Space. In place of trying to stretch and adapt the existing solutions to the requirements of very low frequency interferometry, this study attempts to tailor a design to meet its specific needs. The salient features of the design are an onboard correlator to reduce the data volumes to be transmitted to the Earth by about 2 orders of magnitude, use of three orthogonal dipoles in place of two to achieve better polarization characteristics, direct digitization of the entire radio frequency band of interest allowing broadband observations, an overlap in the observing frequency range with upcoming ground‐based instruments to aid in imaging and calibration, and an all‐sky imaging capability. The most constraining bottleneck for the present design is the large intraconstellation telemetry requirement. It is expected that technological solutions to meet this requirement will be found in the near future as other formation‐flying missions which share this requirement emerge.

Accuracy and Covariance Analysis of Global Astrometry with the Space Interferometry Mission

ABSTRACTConstruction of an all‐sky microarcsecond astrometric grid is an essential preliminary step toward global astrometry with the Space Interferometry Mission (SIM). The general setup of the global astrometry problem is given, and fast, robust algorithms to solve the problem are described. The results of extensive numerical simulations are used to outline the conditions necessary to achieve the performance goal of absolute astrometric accuracy. Rigorous computation of the global covariance matrix makes it possible to correctly interpret and utilize the SIM data. We formulate and resolve the new issue of the basic performance uncertainty of a single astrometric mission.

Design and evaluation of optimal reconfiguration maneuvers for separated space interferometry

Separated space interferometers require performing different type of maneuvers during their lifetime: resizing, rotating and slewing. They shall be conducted within a given period while maintaining a constrained formation. Moreover, the maneuvers and their implementation must be designed considering fuel expenditure optimization at formation level and its balancing between different satellites. This paper presents, based on the characterization of requirements and constraints, a set of algorithms for centralized optimal formation planning and coordination. These algorithms consider operational constraints (i.e. relative navigation) and are valid upon real mission conditions (i.e. perturbations and intersatellite link) and suitable for on-board implementation. They have been tuned, validated and benchmarked on a detailed formation flying functional simulator customized for the Formation Reconfiguration Mode of the ESA's Darwin mission, consisting of a formation of seven satellites around L2.

Can theLaser Interferometer Space AntennaResolve the Distance to the Large Magellanic Cloud?

The Laser Interferometer Space Antenna (LISA) is expected to detect N ~ 22 × 10±1 close white dwarf binaries in the Large Magellanic Cloud (LMC) through their gravitational radiation with signal-to-noise ratios greater than ~10 in observational durations of 3 yr or more. In addition to chirp mass, location on the sky, and other binary parameters, the distance to each binary is an independent parameter that can be extracted from an analysis of gravitational waves from these binaries. Using a sample of binaries, one can establish the mean distance to the LMC as well as the variance of this distance. Assuming no confusion noise at frequencies above 2 mHz, for data collected over 10 yr, LISA might determine the LMC distance to ~4.5(N/22)1/2(1 yr/Tobs)1/2% and the line-of-sight extent of the LMC to ~15(N/22)1/4%, relative to its distance, at the 1 σ confidence. For a 3 yr observational window, these estimates are degraded by a factor of ~2.5. In general, the estimates based on LISA are competitive with some of the proposed direct geometric techniques of measuring the LMC distance in the future with missions such as the Space Interferometry Mission and the Global Astrometric Interferometer for Astrophysics.

Fast time‐domain simulation for large‐order linear time‐invariant state space systems

AbstractTime‐domain simulation is essential for both analysis and design of complex systems. Unfortunately, high model fidelity leads to large system size and bandwidths, often causing excessive computation and memory saturation. In response we develop an efficient scheme for large‐order linear time‐invariant systems. First, the A matrix is block diagonalized. Then, subsystems of manageable dimensions and bandwidth are formed, allowing multiple sampling rates. Each subsystem is then discretized using a O(ns) scheme, where ns is the number of states. Subsequently, a sparse matrix O(ns) discrete‐time system solver is employed to compute the history of the state and output. Finally, the response of the original system is obtained by superposition. In practical engineering applications, closing feedback loops and cascading filters can hinder the efficient use of the simulation scheme. Solutions to these problems are addressed in the paper. The simulation scheme, implemented as a MATLAB function fastlsim, is benchmarked against the standard LTI system simulator lsim and is shown to be superior for medium to large systems. The algorithm scales close to O(n) for a set of benchmarked systems. Simulation of a high‐fidelity model of (ns ≈ 2200) the Space Interferometry Mission spacecraft illustrates real world application of the method. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Discovery of an Unbound Hypervelocity Star in the Milky Way Halo

We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a heliocentric radial velocity of 853 ± 12 km s-1, the largest velocity ever observed in the Milky Way halo. The star is either a hot blue horizontal-branch star or a B9 main-sequence star with a heliocentric distance of 39 or 71 kpc, respectively. Corrected for the solar reflex motion and to the local standard of rest, the Galactic rest-frame velocity is 709 km s-1. We suggest that this star is the first example of a hypervelocity star ejected from the Galactic center, as predicted by Hills and later discussed by Yu & Tremaine. The star's radial velocity vector points 174° from the Galactic center. The star has [Fe/H] ~ 0, consistent with a Galactic center origin, and a travel time of 80 Myr from the Galactic center, consistent with its stellar lifetime. If the star is indeed traveling from the Galactic center, it should have a proper motion of ~0.3 mas yr-1 observable with the Space Interferometry Mission or the Global Astrometric Interferometer for Astrophysics. Identifying additional hypervelocity stars throughout the halo will constrain the production rate history of hypervelocity stars at the Galactic center.

Apparent Stellar Wobble by a Planet in a Circumstellar Disk: Limitations on Planet Detection by Astrometry

Astrometric detection of a stellar wobble on the plane of the sky will provide us the next breakthrough in searching for extrasolar planets. The Space Interferometry Mission (SIM) is expected to achieve high-precision astrometry as accurate as 1 (mu)as, which is precise enough to discover a newborn Jupiter mass planet around a pre-main-sequence (PMS) star in the Taurus-Auriga star-forming region. PMS stars, however, have circumstellar disks that may be obstacles to the precise measurement of the stellar position.We present results on disk influences on the stellar wobble. The density waves excited by a planet move both the disk's mass center and the photocenter. The motion of the disk mass center induces an additional wobble of the stellar position, and the motion of the disk photocenter causes a contamination in the measurement of the stellar position. We show that the additional stellar motion dynamically caused by the disk's gravity is always negligible but that the contamination by the disk light can interfere with the precise measurement of the stellar position if the planet's mass is smaller than approximately 10MJ. The motion of the disk photocenter is sensitive to a slight change in the wave pattern and the disk properties. Measurements by interferometers are generally insensitive to extended sources such as disks. Because of this property, SIM will not suffer significant contamination by the disk light, even if the planet's mass is as small as 1M(sub J).

A Neptune‐Mass Planet Orbiting the Nearby M Dwarf GJ 436

We report precise Doppler measurements of GJ 436 (M2.5 V) obtained at Keck Observatory. The velocities reveal a planetary companion with orbital period of 2.644 days, eccentricity of 0.12 (consistent with zero), and velocity semiamplitude of K = 18.1 m s-1. The minimum mass (M sin i) for the planet is 0.067MJup = 1.2MNep = 21MEarth, making it the lowest mass exoplanet yet found around a main-sequence star and the first candidate in the Neptune-mass domain. GJ 436 (mass = 0.41 M☉) is only the second M dwarf found to harbor a planet, joining the two-planet system around GJ 876. The low mass of the planet raises questions about its constitution, with possible compositions of primarily H and He gas, ice/rock, or rock-dominated. The implied semimajor axis is a = 0.028 AU = 14 stellar radii, raising issues of planet formation, migration, and tidal coupling with the star. GJ 436 is more than 3 Gyr old, based on both kinematic and chromospheric diagnostics. The star exhibits no photometric variability on the 2.644 day Doppler period to a limiting amplitude of 0.0004 mag, supporting the planetary interpretation of the Doppler periodicity. Photometric transits of the planet across the star are ruled out for gas giant compositions and are also unlikely for solid compositions. As the third closest known planetary system, GJ 436 warrants follow-up observations by high-resolution optical and infrared imaging and by the Space Interferometry Mission.

Formation Pointing Dynamics of Tether-Connected Architecture for Space Interferometry

The baseline pointing dynamics, i.e., the attitude of the instantaneous plane of rotation, of a linear tethered interferometer with single baseline orbiting in an Earth trailing, heliocentric orbit is analyzed. Two tether arms provide a passive control of the spacecraft formation by keeping two light collectors and one combiner aligned while spinning about the boresight of the interferometer. The interferometer optical path delay variation under gravity gradient perturbation and unbalanced solar radiation torque is investigated analytically for a fixed-length dumbbell model. Analytical results are compared with results from numerical simulations of a visco-elastic massive tether model in order to assess the effect of the tether dynamics on the pointing stability of the formation. The study highlights the fundamental role of spin stabilization in counteracting the effect of external perturbation on the interferometer Optical Path Delay (OPD).