Astrometric Observations Research Articles

Crowded-Field Astrometry with SIM PlanetQuest. II. An Improved Instrument Model

ABSTRACTIn a previous paper we described a method of estimating the single-measurement bias to be expected in astrometric observations of targets in crowded fields with the future Space Interferometry Mission PlanetQuest. That study was based on a simplified model of the instrument and the measurement process involving a single-pixel focal plane detector, an idealized spectrometer, and continuous sampling of the fringes during the delay scanning. In this paper we elaborate on this “instrument model” to include the following additional complications: spectral dispersion of the light with a thin prism, which turns the instrument camera into an objective prism spectrograph; a multiple-pixel detector in the camera focal plane; and binning of the fringe signal during scanning of the delay. The results obtained with this improved model differ in small but systematic ways from those obtained with the earlier simplified model. We conclude that it is the pixelation of the dispersed fringes on the focal plane detector that is responsible for the differences. The improved instrument model described here suggests additional ways of reducing certain kinds of confusion, and provides a better basis for the evaluation of instrumental effects in the future.

(42355) Typhon–Echidna: Scheduling observations for binary orbit determination

Abstract We describe a strategy for scheduling astrometric observations to minimize the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) Typhon–Echidna, revealing that Typhon and Echidna orbit one another with a period of 18.971 ± 0.006 days and a semimajor axis of 1628 ± 29 km , implying a system mass of ( 9.49 ± 0.52 ) × 10 17 kg . The eccentricity of the orbit is 0.526 ± 0.015 . Combined with a radiometric size determined from Spitzer Space Telescope data and the assumption that Typhon and Echidna both have the same albedo, we estimate that their radii are 76 −16 +14 and 42 −9 +8 km , respectively. These numbers give an average bulk density of only 0.44 −0.17 +0.44 g cm −3 , consistent with very low bulk densities recently reported for two other small transneptunian binaries.

A Measurement of Proper Motions of SiO Maser Sources in the Galactic Center with the VLBA

Abstract We report on the high-precision astrometric observations of maser sources around the Galactic Center in the SiO $J=1$-0 $v=1$ and 2 lines with the VLBA during 2001–2004. With phase-referencing interferometry referred to the radio continuum source Sgr A*, accurate positions of masers were obtained for three detected objects: IRS 10EE (7 epochs), IRS 15NE (2 epochs), and SiO 6(only 1 epoch). Because circumstellar masers of these objects were resolved into several components, proper motions for the maser sources were derived with several different methods. Combining our VLBA results with those of the previous VLA observations, we obtained the IRS 10EE proper motion of 76$\pm$3kms$^{-1}$ (at 8kpc) to the south relative to Sgr A*. Almost null proper motion of this star in the east–west direction results in a net transverse motion of the infrared reference frame of about 30$\pm$9kms$^{-1}$ to the west relative to Sgr A*. The proper-motion data also suggests that IRS 10EE is an astrometric binary with an unseen massive companion.

Astrometry of H2O Masers in Nearby Star-Forming Regions with VERA II SVS 13 in NGC 1333

Abstract We report on the results of multiepoch very long baseline interferometry (VLBI) observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H$_{2}$O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We carried out phase-referencing VLBI astrometry, and measured the annual parallax of the maser features in SVS 13 of 4.25$\pm$0.32mas, corresponding to a distance of 235$\pm$18pc from the Sun. Our result is consistent with a photometric distance of 220 pc, previously reported. Even though the maser features were detectable only for 6 months, the present result provides the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H$_{2}$O masers with LSR (local standard of rest) velocities of 7–8kms$^{-1}$ are most likely associated with VLA4A, which is a radio counterpart of SVS 13. It is currently difficult to attribute the observed proper motions of the maser features to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

Follow-up observations of binary ultra-cool dwarfs

Context. Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses have been measured to date, and the mass-luminosity relation still needs to be calibrated. Aims. We have monitored 14 very low mass multiple systems for several years to confirm their multiplicity and, for those with a short period, derive accurate orbital parameters and dynamical mass estimates. Methods. We have used high spatial resolution images obtained at the Paranal, Lick and HST observatories to obtain astrometric and photometric measurements of the multiple systems at several epochs. The targets have periods ranging from 5 to 200 years, and spectral types in the range M7.5–T5.5. Results. All of our 14 multiple systems are confirmed as common proper motion pairs. One system (2MASSW J0920122+351742) is not resolved in our new images, probably because the discovery images were taken near maximum elongation. Six systems have periods short enough to allow dynamical mass measurements within the next 15 to 20 years.We estimate that only 8% of the ultracool dwarfs in the solar neighborhood are binaries with separations large enough to be resolved, and yet periods short enough to derive astrometric orbital fits over a reasonable time frame with current instrumentation. A survey that doubles the number of ultracool dwarfs observed with high angular resolution is called for to discover enough binaries for a first attempt to derive the mass-luminosity relationship for very low-mass stars and brown dwarfs.

Testing the General Relativistic "No-Hair" Theorems Using the Galactic Center Black Hole Sagittarius A*

If a class of stars orbits the central black hole in our galaxy in short period (~ 0.1 year), high eccentricity (~ 0.9) orbits, they will experience precessions of their orbital planes induced by both relativistic frame-dragging and the quadrupolar gravity of the hole, at levels that could be as large as 10 microarcseconds per year, if the black hole is rotating faster than 1/2 of its maximum rotation rate. Astrometric observations of the orbits of at least two such stars can in principle lead to a determination of the angular momentum vector J of the black hole and its quadrupole moment Q_2. This could lead to a test of the general relativistic no-hair theorems, which demand that Q_2 = - J^2/M. Future high-precision adaptive infrared optics instruments make make such a fundamental test of the black-hole paradigm possible.

Photometric detection of pulsations in the stars of the wide binary WDS 12483-6708

Die Serie Communications in Asteroseismology (CoAst) stellt eine international gefragte Plattform für wissenschaftliche Publikationen der neuesten Forschungsergebnisse auf dem Gebiet der Asteroseismologie dar. Forschungsgegenstand der Asteroseismologie sind veränderliche und pulsierende Sterne. Neben den halbjährlich erscheinenden regulären Ausgaben mit Beiträgen aus der laufenden Forschung werden auch Sonderausgaben für Konferenzberichte bzw. Programmhandbücher herausgegeben. Volume 153 beinhaltet eine Vielzahl von Arbeiten mit Ergebnissen aus erdgebundenen und satellitengestüzten Beobachtungen pulsierender Sterne sowie über den Vergleich mit theoretischen Modellen. Erstmals berichtet auch das von der EU eingerichtete Forschungsnetzwerk HELAS - European Helio- and Asteroseismology Network - über neue Instrumente und Strukturen für den Daten- und Wissensaustausch unter Astronomen.

Astrometric observable and relativistic astrometric catalogues

We describe a way to compare current relativistic astrometric models accurate to the micro-arcsecond level from which an astrometric catalogue will be deduced. The observed stellar direction can be written as a function of several parts, linking the astrometric observables to the relativistic effects associated to the stellar kinematical properties and distances as seen inside the gravitational field of our Solar System, i.e. the so called relativistic astrometric parameters, providing a tool for comparing the RAMOD framework to the pM/pN approaches.

Absolute kinematics of radio source components in the complete S5 polar cap sample

We report on the first wide-field, high-precision astrometric analysis of the 13 extragalactic radio sources of the complete S5 polar cap sample at 15.4 GHz. We describe new algorithms developed to enable the use of differenced phase delays in wide-field astrometric observations and discuss the impact of using differenced phase delays on the precision of the wide-field astrometric analysis. From this global fit, we obtained estimates of the relative source positions with precisions ranging from 14 to 200 μas at 15.4 GHz, depending on the angular separation of the sources (from ~1.6 to ~20.8 degrees). These precisions are ~10 times higher than the achievable precisions using the phase-reference mapping technique.

White-light interferometry using a channeled spectrum 2 Calibration methods, numerical and experimental results

In the companion paper, [Appl. Opt. 46, 5853 (2007)] a highly accurate white light interference model was developed from just a few key parameters characterized in terms of various moments of the source and instrument transmission function. We develop and implement the end-to-end process of calibrating these moment parameters together with the differential dispersion of the instrument and applying them to the algorithms developed in the companion paper. The calibration procedure developed herein is based on first obtaining the standard monochromatic parameters at the pixel level: wavenumber, phase, intensity, and visibility parameters via a nonlinear least-squares procedure that exploits the structure of the model. The pixel level parameters are then combined to obtain the required "global" moment and dispersion parameters. The process is applied to both simulated scenarios of astrometric observations and to data from the microarcsecond metrology testbed (MAM), an interferometer testbed that has played a prominent role in the development of this technology.

The imaging performance of the SRC on Mars Express

The Mars Express spacecraft carries the pushbroom scanner high-resolution stereo camera (HRSC) and its added imaging subsystem super resolution channel (SRC). The SRC is equipped with its own optical system and a 1024×1024 framing sensor. SRC produces snapshots with 2.3 m ground pixel size from the nominal spacecraft pericenter height of 250 km, which are typically embedded in the central part of the large HRSC scenes. The salient features of the SRC are its light-weight optics, a reliable CCD detector, and high-speed read-out electronics. The quality and effective visibility of details in the SRC images unfortunately falls short of what has been expected. In cases where thermal balance cannot be reached, artifacts, such as blurring and “ghost features” are observed in the images. In addition, images show large numbers of blemish pixels and are plagued by electronic noise. As a consequence, we have developed various image improving algorithms, which are discussed in this paper. While results are encouraging, further studies of image restoration by dedicated processing appear worthwhile. The SRC has obtained more than 6940 images at the time of writing (1 September 2007), which often show fascinating details in surface morphology. SRC images are highly useful for a variety of applications in planetary geology, for studies of the Mars atmosphere, and for astrometric observations of the Martian satellites. This paper will give a full account of the design philosophy, technical concept, calibration, operation, integration with HRSC, and performance, as well as science accomplishments of the SRC.

Non-tidal vertical variations and the past star catalogs

AbstractTidal vertical variations have been removed in astrometric observations, but the non-tidal ones remain unknown. From the repeated observations of gravimetric networks in China, the non-tidal vertical variations at Beijing-Tangshan and West Yunnan are determined, which are of the order of 0.″2. Astrometric results, including past star catalogs, based on these observations should be corrected.

Phase referencing VLBI astrometry observation system: VERA

AbstractVERA aims at astrometric observations using phase referencing VLBI techniques, whose goal is a 10 micro arc-second accuracy for annual parallax measurements. VERA has four 20-m diameter VLBI radio telescopes in Japanese archipelago with the maximum baseline length of 2,300 km. They have the two-beam observing system, which makes simultaneous observations of two objects possible. This leads to very accurate phase referencing VLBI observations. An important science goal is to make a 3-dimensional map of the Galaxy and reveal its dynamics. In order to achieve this, VERA has the 22GHz and 43GHz bands for H2O and SiO maser objects, respectively. Maser objects are compact and suitable for astrometry observations. VERA's construction was started in 2000 and the array became operational in 2004. We have already measured annual parallaxes and proper motions of some galactic objects. In the future, VERA will collaborate with Korean and Chinese VLBI stations.

The RAMOD astrometric observable and the relativistic astrometric catalogs

AbstractWe describe a way to compare current relativistic astrometric models accurate to the micro-arcsecond level. The observed stellar direction can be written as a function of several parts, linking the astrometric observables to the relativistic effects associated to the stellar kinematical properties and distances as seen inside the gravitational field of our Solar System, i.e. the so called relativistic astrometric parameters, providing a tool for comparing the RAMOD framework to the pM/pN approaches.

Antarctic Project for astrometric observations

AbstractAstronomical Institute of Romanian Academy initiated this project in 2005. The results of researches related to the construction of mechanic systems and the proposed astrometric tasks will be finished in 2008. The module PROTEL is a result of several research groups and it will perform astrometric control observations remotely.

A strategy for implementing differential VLBI

AbstractBased on past astrometric and geodetic VLBI observations, a strategy for implementing differential VLBI (DVLBI) is developed by interpolating the non-geometric delay (NGD) at the target's direction using observations of several reference sources spreaded out within a circular ring centered on the target. In contrast to the ordinary approach, in our design the limitations in the angular distance are relaxed and the effects of observational uncertainties in reference sources are reduced. Analysis shows that in our design a precision of the NGD correction in S-band reaches about 1ns only. Our design can be adopted for observations of weak sources and in deep space exploration.

CCD astrometric observations of faint satellites and update of their orbits

AbstractThis paper reports on our observing campaign of faint satellites performed at the National Time Service Center and Sheshan station of SHAO from 1994 up to today. In the past few years due to benefit from using a large size CCD and the publication of the modern catalogues (UCAC2), a series of observations of faint satellites were obtained by us. Moreover the work of improving the orbit of Phoebe via numerical fit to the observations over a century is also presented.

Determination of the barycentric velocity of an astrometric satellite using its own observational data

AbstractThe problem of determination of the orbital velocity of an astrometric satellite from its own observational data is studied. It is well known that data processing of microarcsecond-level astrometric observations imposes very stringent requirements on the accuracy of the orbital velocity of satellite (a velocity correction of 1.45 mm/s implies an aberrational correction of 1 μas). Because of a number of degeneracies the orbital velocity cannot be fully restored from observations provided by the satellite. Seven constraints that must be applied on a velocity parameterization are discussed and formulated mathematically. It is shown what part of velocity can be recovered from astrometric data using a combined fit of both velocity parameters and astrometric parameters of the sources. Numerical simulations show that, with the seven constraints applied, the velocity and astrometric parameters can be reliably estimated from observational data. It is also argued that the idea to improve the velocity of an astrometric satellite from its own observational data is only useful if a priori information on orbital velocity justifies the applicability of the velocity constraints. The proposed model takes into account only translational motion of the satellite and ignores any satellite-specific parameters. Therefore, the results of this study are equally applicable to both scanning missions similar to Gaia, and pointing ones like SIM, provided that enough sources were observed sufficiently uniformly.

Solution of Earth orientation parameters in 20th century based on optical astrometry and new catalog EOC-3

AbstractA new star catalog has been derived recently, which is based on combination of Hipparcos/Tycho Catalogues with long lasting ground-based astrometric observations. In addition to ‘classical’ mean positions and proper motions of the stars, it contains also information on periodic motions of many stars that are due to double or multiple stellar systems. The catalog, called EOC-3, contains 4418 different objects, out of which 585 have significant periodic motions. This improved catalog is used as a reference frame in optical wavelength to derive a new series of Earth Orientation Parameters. To this end, almost five million observations of latitude/universal time variations made at 33 observatories are used. The new EOP series covers almost the entire 20th century (namely the interval 1899.7-1992.0).

Use of optical and radio astrometric observations of planets, satellites and spacecraft for ephemeris astronomy

AbstractDifferent types of radiometric observations of planets and spacecraft, as well as optical data used for constructing modern high-precision planet ephemerides are presented. New mass values for planets and asteroids are given. The IAA RAS EPM ephemerides (Ephemerides ofPlanets and theMoon) are the basis for the Russian “Astronomical Yearbook” and are used in the navigation program “GLONASS” and the cosmic program “Phobos-Grunt”.