Precise Astrometric Measurements Research Articles

Klet Observatory – European Contribution to Detecting and Tracking of Near Earth Objects

Near Earth Object (NEO) research is an expanding field of astronomy. Is is important for solar system science and also for protecting human society from asteroid and comet hazard. A near-Earth object (NEO) can be defined as an asteroid or comet that has a possibility of making an approach to the Earth, or possibly even collide with it. The discovery rate of current NEO surveys reflects progressive improvement in a number of technical areas. An integral part of NEO discovery is astrometric follow-up fundamental for precise orbit computation and for the reasonable judging of future close encounters with the Earth including possible impact solutions. A wide international cooperation is fundamental for NEO research. The Klet Observatory (South Bohemia, Czech Republic) is aimed especially at the confirmation, early follow-up, long-arc follow-up and recovery of Near Earth Objects. It ranks among the world´s most prolific professional NEO follow-up programmes. The first NEO follow-up programme started at Klet in 1993 using 0.57-reflector equipped with a small CCD camera. A fundamental upgrade was made in 2002 when the 1.06-m KLENOT telescope was put into regular operation. The KLENOT Telescope is the largest telescope in Europe used exclusively for observations of minor planets (asteroids) and comets and full observing time is dedicated to the KLENOT team. Equipment, technology, software, observing strategy and results of both the Klet Observatory NEO Project between 1993-2010 and the first phase of the KLENOT Project from March 2002 to September 2008 are presented. They consist of thousands of precise astrometric measurements of Near Earth Objects and also three newly discovered Near Earth Asteroids. Klet Observatory NEO activities as well as our future plans fully reflect international strategies and cooperation in the field of NEO studies.

Solar-like oscillations in the G9.5 subgiantβAquilae

An interesting asteroseismic target is the G9.5 IV solar-like star beta Aql. This is an ideal target for asteroseismic investigations, because precise astrometric measurements are available from Hipparcos that greatly help in constraining the theoretical interpretation of the results. The star was observed during six nights in August 2009 by means of the high-resolution \'echelle spectrograph SARG operating with the TNG 3.58 m Italian telescope on the Canary Islands, exploiting the iodine cell technique. We present the result and the detailed analysis of high-precision radial velocity measurements, where the possibility of detecting time individual p-mode frequencies for the first and deriving their corresponding asymptotic values will be discussed. The time-series analysis carried out from \sim 800 collected spectra shows the typical p-mode frequency pattern with a maximum centered at 416 \muHz. In the frequency range 300 - 600 \muHz we identified for the first time six high S/N (\gtrsim 3.5) modes with l = 0,2 and 11 < n < 16 and three possible candidates for mixed modes (l = 1), although the p-mode identification for this type of star appears to be quite difficult owing to a substantial presence of avoided crossings. The large frequency separation and the surface term from the set of identified modes by means of the asymptotic relation were derived for the first time. Their values are \Delta \nu = 29.56 \pm 0.10 \muHz and \epsilon = 1.29 \pm 0.04, consistent with expectations. The most likely value for the small separation is \delta\nu_{02} = 2.55 \pm 0.71 \muHz.

KLENOT Project 2002–2008 contribution to NEO astrometric follow‐up

Abstract— Near‐Earth object (NEO) research plays an increasingly important role not only in solar system science but also in protecting our planetary environment as well as human society from the asteroid and comet hazard. Consequently, interest in detecting, tracking, cataloguing, and the physical characterizing of these bodies has steadily grown. The discovery rate of current NEO surveys reflects progressive improvement in a number of technical areas. An integral part of NEO discovery is astrometric follow‐up crucial for precise orbit computation and for the reasonable judging of future close encounters with the Earth, including possible impact solutions. The KLENOT Project of the Klet Observatory (South Bohemia, Czech Republic) is aimed especially at the confirmation, early follow‐up, long‐arc follow‐up, and recovery of near‐Earth objects. It ranks among the world's most prolific professional NEO follow‐up programs. The 1.06 m KLENOT telescope, put into regular operation in 2002, is the largest telescope in Europe used exclusively for observations of minor planets and comets, and full observing time is dedicated to the KLENOT team. In this paper, we present the equipment, technology, software, observing strategy, and results of the KLENOT Project obtained during its first phase from March 2002 to September 2008. The results consist of thousands of precise astrometric measurements of NEOs and also three newly discovered near‐Earth asteroids. Finally, we also discuss future plans reflecting also the role of astrometric follow‐up in connection with the modus operandi of the next generation surveys.

DETERMINING THE DISTANCE OF Cyg X-3 WITH ITS X-RAY DUST SCATTERING HALO

Using a cross-correlation method, we study the X-ray halo of Cyg X-3. Two components of dust distributions are needed to explain the time lags derived by the cross-correlation method. Assuming the distance as 1.7 kpc for Cygnus OB2 association (a richest OB association in the local Galaxy) and another uniform dust distribution, we get a distance of $7.2^{+0.3}_{-0.5}$ kpc (68$%$ confidence level) for Cyg X-3. When using the distance estimation of Cygnus OB2 as 1.38 kpc or 1.82 kpc, the inferred distance for Cyg X-3 is $3.4^{+0.2}_{-0.2}$ kpc or $9.3^{+0.6}_{-0.4}$ kpc respectively. The distance estimation uncertainty of Cyg X-3 is mainly related to the distance of the Cygnus OB2, which may be improved in the future with high precision astrometric measurements. The advantage of this method is that the result depends weakly on the photon energy, dust grain radius, scattering cross-section and so on.

Astrometric light-travel time signature of sources in nonlinear motion

Context. Very precise planned space astrometric missions and recent improvements in imaging capabilities require a detailed review of the assumptions of classical astrometric modeling. Aims. We show that Light-Travel Time must be taken into account in modeling the kinematics of astronomical objects in nonlinear motion, even at stellar distances. Methods. A closed expression to include Light-Travel Time in the current astrometric models with nonlinear motion is provided. Using a perturbative approach the expression of the Light-Travel Time signature is derived. We propose a practical form of the astrometric modelling to be applied in astrometric data reduction of sources at stellar distances(d > 1 pc). Results. We show that the Light-Travel Time signature is relevant at pas accuracy (or even at mas) depending on the time span of the astrometric measurements. We explain how information on the radial motion of a source can be obtained. Some estimates are provided for known nearby binary systems Conclusions. Given the obtained results, it is clear that this effect must be taken into account in interpreting precise astrometric measurements. The effect is particularly relevant in measurements performed by the planned astrometric space missions (GAIA, SIM, JASMINE, TPF/DARWIN). An objective criterion is provided to quickly evaluate whether the Light-Travel Time modeling is required for a given source or system.

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.

Choice of Observing Schedules for Astrometric Planet Searches

ABSTRACTThe Space Interferometry Mission (SIM) will make precise astrometric measurements that can be used to detect planets around nearby stars. Since observational time will be extremely valuable, it is important to consider how the choice of the observing schedule influences the efficiency of SIM planet searches. We have conducted Monte Carlo simulations of astrometric observations to understand the effects of different scheduling algorithms. We find that the efficiency of planet searches is relatively insensitive to the observing schedule for most reasonable observing schedules.

Potential Direct Single‐Star Mass Measurement

We analyze the light curve of the microlensing event OGLE-2003-BLG-175/MOA-2003-BLG-45 and show that it has two properties that, when combined with future high-resolution astrometry, could lead to a direct, accurate measurement of the lens mass. First, the light curve shows clear signs of distortion due to the Earth's accelerated motion, which yields a measurement of the projected Einstein radius E. Second, from precise astrometric measurements, we show that the blended light in the event is coincident with the microlensed source to within about 15 mas. This argues strongly that this blended light is the lens and hence opens the possibility of directly measuring the lens-source relative proper motion μrel and so the mass M = (c2/4G)μreltEE, where tE is the measured Einstein timescale. While the light-curve-based measurement of E is, by itself, severely degenerate, we show that this degeneracy can be completely resolved by measuring the direction of proper motion μrel.

Planet‐Finding Prospects for the Space Interferometry Mission

ABSTRACTThe Space Interferometry Mission (SIM) will make precise astrometric measurements that can be used to detect planets around nearby stars. We have simulated SIM observations and estimated the ability of SIM to detect planets with given masses and orbital periods and measured their orbital elements. We combine these findings with an estimate of the mass and period distribution of planets determined from radial velocity surveys to predict the number and characteristics of planets SIM would likely find. Our predictions are based on extrapolating the mass distribution of known extrasolar planets by up to a factor of ∼100. This extrapolation provides the best prediction we can make of the actual number of planets that SIM will detect and characterize, but may substantially over‐ or underestimate the frequency of Earth‐mass planets, especially if these form by a different mechanism than giant planets. We find that a SIM key project is likely to detect around one to five planets with masses ≤3 M⊕ (depending on mission parameters). SIM would measure masses and orbits with 30% accuracy for around zero to two of these planets, but is unlikely to measure orbits with 10% accuracy for more than one of them. SIM is likely to detect around five to 25 planets with mass less than 20 M⊕, measure masses and orbits with 30% accuracy for around two to 12 of these, and measure masses and orbits with 10% accuracy for around two to eight such planets. SIM is likely to find ∼25–160 planets of all masses, depending on the observing strategy and mission lifetime. Roughly 25%–65% of the planets detected by SIM have sufficiently large masses and short orbital periods that they can also be detected by radial velocity surveys. Radial velocity surveys could measure orbital parameters (not including inclination) for 30%–70% of the planets whose orbital parameters will be determined to within 30% by SIM.

Angular Radii of Stars via Microlensing

We outline a method by which the angular radii of giant and main-sequence stars located in the Galactic bulge can be measured to a few percent accuracy. The method combines comprehensive ground-based photometry of caustic-crossing bulge microlensing events, with a handful of precise (~10 μas) astrometric measurements of the lensed star during the event, to measure the angular radius of the source, θ*. Dense photometric coverage of one caustic crossing yields the crossing timescale Δt. Less frequent coverage of the entire event yields the Einstein timescale tE and the angle ϕ of source trajectory with respect to the caustic. The photometric light-curve solution predicts the motion of the source centroid up to an orientation on the sky and overall scale. A few precise astrometric measurements therefore yield θE, the angular Einstein ring radius. Then the angular radius of the source is obtained by θ* = θE(Δt/tE) sin ϕ. We argue that the parameters tE, Δt, ϕ, and θE, and therefore θ*, should all be measurable to a few percent accuracy for Galactic bulge giant stars using ground-based photometry from a network of small (1 m class) telescopes, combined with astrometric observations with a precision of ~10 μas to measure θE. We find that a factor of ~50 times fewer photons are required to measure θE to a given precision for binary lens events than for single-lens events. Adopting parameters appropriate to the Space Interferometry Mission (SIM), we find that ~7 minutes of SIM time is required to measure θE to ~5% accuracy for giant sources in the bulge. For main-sequence sources, θE can be measured to ~15% accuracy in ~1.4 hr. Thus, with access to a network of 1 m class telescopes, combined with 10 hr of SIM time, it should be possible to measure θ* to 5% for ~80 giant stars, or to 15% for roughly seven main-sequence stars. We also discuss methods by which the distances and spectral types of the source stars can be measured. A by-product of such a campaign is a significant sample of precise binary lens mass measurements.

Probing Titan's Atmosphere with the 1995 August Stellar Occultation

On 1995 August 21, Saturn's moon Titan occulted the 13th magnitude star GSC5254-00997. The predawn event was observed at 2.3 μm with the NASA 3.0-m Infrared Telescope Facility. We measured the average atmospheric scale height between latitudes 50°S and 67°S by fitting isothermal models to the immersion portion of the occultation lightcurve. Within the altitude range of 300 to 500 km, we find an isothermal scale height of Titan's sunset atmosphere of 55±9 km. Our derived temperature of 180±30 K is consistent with modeled temperature profiles derived from Voyager 1 IRIS observations (A. Coustenis and B. Bézard 1995, Icarus 115, 126–140) and the 28 Sagittarii occultation event (W. B. Hubbard et al. 1993, Astron. Astrophys. 269, 541–563). The atmospheric albedo asymmetry due to low level haze (Caldwell et al. 1992, Icarus 96, 1–9) is inferred from the difference in Titan's center-of-light (determined from precise astrometric measurements of images of Titan and the star before and after the occultation) and the actual center of Titan (determined from the occultation analysis). We find the southern hemisphere to be brighter than the northern hemisphere, a result consistent with Combes et al. (1997, Icarus 129, 482–497), Gibbard et al. (1999, Icarus 139, 189–201), and Smith et al. (1996 Icarus, 119, 336–349).

Proper-Motion Measurements with the VLA. I. Wide-Field Imaging and Pulse-gating Techniques

The pulsar velocity distribution provides information about the binary history of pulsar progenitors as well as asymmetries of the supernova events in which pulsars are born. Studies of local pulsars present a biased view of this distribution, since they preferentially select low velocity pulsars that have remained near their birthplaces in the Galactic plane. Using the VLA, we have studied the proper motions of a large sample of distant pulsars. These pulsars are generally faint, and the expected proper motions are small. In this paper, we describe the data analysis techniques that we have developed to allow precise astrometric measurements of faint sources with the VLA. These techniques include ``gating'' the VLA correlator to increase the signal-to-noise ratio of the pulsar by gathering data only during the pulse. Wide-field imaging techniques, including multiband imaging to account for bandwidth smearing, were used to allow identification of multiple in-beam background sources for astrometric calibration. We present the analysis of three pulsars, and demonstrate that astrometric accuracy of about ten milliarcseconds can be obtained for individual sources with our technique, allowing measurement of proper motions with errors of only a few milliarcseconds per year over our seven year baseline.

Astronomical Observations with Super High Spacial-Resolution and Precise Astrometric Measurements by New VLBI Technology

Astronomical Observations with Super High Spacial-Resolution and Precise Astrometric Measurements by New VLBI Technology

Astrometric Detection of a Low‐Mass Companion Orbiting the Star AB Doradus

We report submilliarcsecond--precise astrometric measurements for the late-type star AB Doradus via a combination of VLBI(very long baseline interferometry) and Hipparcos satellite data. Our astrometric analysis results in the precise determination of the kinematics of this star, that reveals an orbital motion readily explained as caused by the gravitational interaction with a low-mass companion.