Astrometric Positions Research Articles

Extragalactic Radio Source Selection for Use in Directly Linking Optical Astrometric Observations to the Radio Reference Frame

Future space-based optical astrometric satellite missions present the possibility of directly linking the radio and optical reference frames at the microarcsecond level. We have evaluated the current database of radio observations of the extragalactic objects that make up the International Celestial Reference Frame (ICRF) to determine the optimum candidates in terms of their radio properties for use as radio-optical frame tie sources. We use up-to-date radio astrometric and ancillary data to evaluate the sources in terms of their suitability for use by the Space Interferometry Mission (SIM), but the results apply equally well to any optical astrometric satellite or optical ground-based observations with sufficient brightness sensitivity to measure precise astrometric positions of extragalactic objects directly. Currently, the ICRF limits the accuracy of any radio-optical frame tie based on future optical astrometric satellite observations if the projected accuracies for these missions are realized. We have attempted to construct a suitable set of frame tie sources but found a significant deficit of candidate sources in the southern hemisphere. If the SIM optical frame is to be successfully tied to the ICRF, additional radio observations are necessary to overcome the limitations of the radio frame.

CCD astrometric observations of minor planets at the Belgrade observatory during 1999 (II)

76 precise astrometric positions of six observed minor planets (two NEO among them) are calculated. The SBIG ST-8 camera attached to Askania astrograph of Belgrade Observatory was used in the observations.

A photometric study of the contact binaries V523 Cas and TY UMa

Broad-band V and I photometry of the short-period W UMa-type contact binaries V523 Cas and TY UMa are reported. From the light curves, the system parameters have been determined and evidence is presented for extensive spot activity in both systems, with V523 Cas being the more active system. A compilation of all the available photoelectric and CCD-based timings of eclipse minima is made; the O–C diagrams show evidence for an increasing period in the case of V523 Cas and a decreasing period in TY UMa. In addition we derive a more accurate astrometric position for V523 Cas from our CCD images.

Astrometric photographic observations of Helene

Astrometric photographic positions of the Saturnian satellite Helene obtained from 22 plates taken in 6 nights are presented. The observations are distributed between 5 missions in 1985 and 1987. All positions are compared with those calculated by Jacobson (1999). The residuals have mean and standard deviation smaller than and respectively.

VLBA Observations of Radio Reference Frame Sources. III. Astrometric Suitability of an Additional 225 Sources

We present simultaneous dual-frequency Very Long Baseline Array 2 and 8 GHz observations of 225 of the 560 extragalactic sources for which positions were reported by Johnston and coworkers and which are now part of the International Celestial Reference Frame. These observations represent the third and final in a series of observations intended to obtain single-epoch images of the entire set of sources presented by Johnston and coworkers and, together with previously reported observations, bring the total number of sources observed to 389. As with previous papers in this series, we use these data to quantify the magnitude of the expected effect of intrinsic source on astrometric bandwidth synthesis Very Long Baseline Interferometry observations and to calculate a source structure for the observed sources. The index can be used as an estimate of the astrometric quality of the sources. Based on this indicator, correlations between the observed radio and the astrometric position accuracy and stability of the sources have been found. These correlations indicate that the more extended sources have larger position uncertainties and are less positionally stable than the more compact sources.

Virtual Impactors: Search and Destroy

If for an asteroid which has been observed only over a short arc and then lost there are orbits compatible with the observations resulting in collisions, recovery would be desirable to decide if it will actually impact. If recovery is essentially impractical, as is the case for many small asteroids in the 100- to 500-m-diameter range, the next best thing is to make sure that the lost asteroid is not on a collision course. We propose a method for achieving this guarantee, with an observational effort far smaller than the one required for recovery. The procedure involves the computation of an orbit that is compatible with the available observations and, by hypothesis, results in an impact at some later encounter; this we call a virtual impactor (VI). The collision at some future time is a strong constraint; thus the VI has a well determined orbit. We show that it is possible to compute for each given time of observation the skyprint of the VI, that is the set of astrometric positions compatible with an impact (or a near impact). The skyprint needs to be scanned by powerful enough telescopes to perform a negative observation; once this has been done for the skyprints of all VIs, collisions can be excluded even without recovery. We propose to apply this procedure to the case of the lost asteroid 1998 OX 4, for which we have found orbital solutions with impacts in the years 2014, 2038, 2044, and 2046. Suitable observing windows are found when the VI would be close to the Earth in 2001 and in 2003, and the corresponding skyprints are small enough to be covered with very few frames. This procedure might become more and more necessary in the future, as the number of discoveries of small potentially hazardous asteroids increases; we discuss the general principles and the validation procedures that should apply to such a VI removal campaign.

Optical Positions for a Sample of ICRF Sources

The aim of this paper is to present a large, coherent set of astrometric optical positions for ICRF sources in the Hipparcos reference frame. For this, we present positions for 315 ICRF sources measured in Digitized Sky Survey (DSS) images, with positions referred to the ACT, Hipparcos, and Tycho catalogs. The reductions were made relative to Hubble Space Telescope guide stars, which were previously corrected toward those same catalogs. The reductions errors are at 45 mas and even smaller for southern sources. The comparison between optical with ICRF radio positions shows that neither the residuals nor the internal errors exhibit any trend with magnitudes or with location on the DSS plates.

CCD astrometric observations of comets at the Belgrade observatory during 1998 - 2000 (I)

143 precise astrometric positions of seven comets were obtained with the CCD equipped Askania astrograph (13.5/160) of Belgrade Astronomical Observatory. The SBIG ST-8(1530 ? 1020 pixel array) camera was used in the observations.

Photographic positions for the first eight satellites of Saturn

Astrometric positions of the rst eight Saturnian satellites obtained from 138 photographic plates taken in 30 nights in the years 1982 to 1988 are presented. All positions are compared with those calculated by the theory TASS1.7 (Vienne & Duriez 1998). The observed minus calculated residuals give rise to standard deviations smaller than 0: 00 3.

CCD astrometric observations of Uranian satellites: 1995-1998

Astrometric positions of the five largest Uranian satellites from 750 CCD frames taken at the oppositions of 1995 through 1998 are presented. The images were obtained over 35 nights. Observed positions are compared with the calculated positions from GUST86. The standard deviations are better than for the four largest satellites and for Miranda.

2MASS Observations of the Large Magellanic Cloud

The Two Micron All Sky Survey (2MASS) has observed a portion of the Large Magellanic Cloud (LMC) at J (1.25 μm), H (1.65 μm), and Ks (2.17 μm), as part of its routine nightly operations with an automated 1.3-m telescope at CTIO, Chile. The camera observes the sky in the three channels simultaneously, using 256 × 256 HgCdTe detector arrays. The survey samples the sky in 6° x 8.′3 scans. The 2MASS Production Processing System provides final atlas images and source extractions with precise photometric calibration and astrometric positions. The survey's 10σ sensitivity is 15.8 mag at J, 15.1 at H, and 14.3 at Ks. 2MASS will ultimately detect ~107 point sources in the LMC and will support analyses of the ages, luminosity and mass functions, and metallicities of the red stellar populations and a census of AGB and carbon stars, as well as extinction maps, across the LMC. This work presents an initial analysis of ~20 square degrees.

2MASS observations of Wolf-Rayet stars and galaxies

The Two Micron All Sky Survey (2MASS) has observed a number of Wolf-Rayet stars and galaxies at J (1.25 μm), H (1.65 μm), and Ks (2.17 μm), as part of its systematic coverage of the entire sky with automated 1.3-m telescopes at Mt. Hopkins, AZ, and CTIO, Chile. The cameras observe the sky in the three channels simultaneously, using 256 x 256 HgCdTe-detector arrays. The survey samples the sky in 6° x 8.'3 tiles. The 2MASS Production Processing System transforms the raw binary survey data to final atlas images and source extractions with precise photometric calibration and astrometric positions. The survey's 10σ sensitivity is 15.8 mag at J, 15.1 at H, and 14.3 at Ks. In this poster we will present results from our analysis of several objects. We have observed WR stars both with and without associated ring nebulae. 2MASS imaging of the WR galaxies is far less affected by extinction, which can obscure much of the star formation activity. We will place these starburst dwarf galaxies in context with other galaxies, based on their near-IR colors and properties.

Digital Objective Prism Spectroscopy and Astrometry: Quantitative Methods and Application to High‐Velocity Stars

ABSTRACTWe describe methods for extracting quantitative spectra and astrometric positions from objective prism plates. In particular, we have studied 7161 stars in eight fields along the south Galactic meridian using the 6° + 4° objective prisms on the Curtis Schmidt telescope at Cerro Tololo Inter‐American Observatory. In this paper we concentrate on techniques for deriving radial velocities and astrometric positions from opposed dispersion plate pairs. For our data the typical accuracy in radial velocity is ∼8 km s−1. A catalog of 108 stars with high radial velocities is presented, which is complete (and unbiased in either kinematics or metallicity) to a limiting B magnitude of 12.5 in the eight fields studied. Only two of this sample of 108 stars have previously reported radial velocities.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

We present wide-field far-ultraviolet (FUV; 1300–1800 A) images of the Large and Small Magellanic Clouds (LMC, SMC). These data were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 (1990 December 1–10) and Astro-2 (1995 March 2–18) missions; the images provide an extensive FUV mosaic of the SMC and contain numerous regions in the LMC, covering a wide range of stellar densities and current star formation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OB associations are completely or partially included in the observed fields. FUV data can identify the hottest OB stars more easily than can optical photometry, and these stars dominate the ionizing flux, which is correlated to the observed Hα flux of the associated H II regions. Of the H II regions in the catalog of Davies, Elliott, & Meaburn (DEM), the UIT fields completely or partially include 102 DEM regions in the LMC and 74 DEM regions in the SMC. We present a catalog of FUV magnitudes derived from point-spread function photometry for 37,333 stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMC were presented recently by Cornett et al.), with a completeness limit of mUV ≈ 15 mag and a detection limit of mUV ≈ 17.5. The average uncertainty in the photometry is ~0.1 mag. The full catalog with astrometric positions, photometry, and other information is also available from publicly accessible astronomical data archives. We divide the catalog into field stars and stars that are in DEM regions. We analyze each of these two sets of stars independently, comparing the composite UV luminosity function of our data with UV magnitudes derived from stellar evolution and atmosphere models in order to derive the underlying stellar formation parameters. We find a most probable initial mass function (IMF) slope for the LMC field stars of Γ = -1.80 ± 0.09. The statistical significance of this single slope for the LMC field stars is extremely high, though we also find some evidence for a field star IMF slope of Γ ~ -1.4, roughly equal to the Salpeter slope. However, in the case of the stars in the DEM regions (the stars in all the regions were analyzed together as a single group), we find three IMF slopes of roughly equal likelihood: Γ = -1.0, -1.6, and -2.0. No typical age for the field stars is found in our data for time periods up to a continuous star formation age of 500 Myr, which is the maximum age consistent with the completeness limit magnitude of the catalog's luminosity function. The best age for the collection of cluster stars was found to be t0 = 3.4 ± 1.9 Myr; this is consistent with the age expected for a collection of OB stars from many different clusters.

Astrometric Observations of the Jovian Outer Satellites for 1990–1992

We have observed precise astrometric positions of the faint outer satellites of Jupiter J6–J12 with the Kiso 1.05 m Schmidt telescope. The observational period is from 1990 November through 1992 March. We discuss the postfit residuals after orbital improvement for our 1986–1990 observations and thereby emphasize the superiority of the intersatellite measures available only from wide-field Schmidt observations, which can cover all the satellites in a single plate.

Techniques for Schmidt Plate Reductions with Application to GSC1.2

Over the last few years there has been considerable progress in developing successful algorithms for obtaining astrometric quality positions from Schmidt plates which compensate for deficiencies of the polynomial approach; the sub-plate method (Taff 1989), the mask method (Taff et al. 1990), the collocation method (Bucciarelli et al. 1993) the filter method (Röser et al. 1995) and a filter weighting according to the method of infinitely overlapping circles (Morrison et al. 1996b.) However, none of these nor any other studies have investigated the magnitude dependence of the position estimates outside the magnitude range of standard reference catalogs. Often Schmidt plates cover the magnitude range from 6m to 19m (for the GSC it is 6m to 15m). Presently available reference catalogs, however, have a limiting magnitude of V ≈ 10m. Therefore, no magnitude dependent term for fainter stars can be reliably found by reducing the measurements based only on comparisons with reference stars.

145CCD astrometric positions of comets and minor planets: Observed during March-September 1998 at the Belgrade observatory

In this paper we present precise astrometric positions of four comets and four minor planets from the MPC critical list. All the observations were carried out with CCD equipped Askania astrograph (13.5/160). The SBIG ST-8 camera was used. Comet 1998 M5 (Stonehouse) occulted an anonymous star on July 9 and that phenomenon was observed successfully.

Progress in VLBI Stellar Astrometry for the NASA/Stanford Relativity Mission (Gravity Probe B)

AbstractThe NASA/Stanford Relativity Mission (Gravity Probe B) is to test the unverified “frame-dragging” prediction of general relativity through measurements of the precessions of orbiting gyroscopes. For mission accuracy goals to be met, the proper motion of a “guide star,” whose position will be used as an inertial reference, must be determined in an extragalactic reference frame with a standard error less than 0.5 mas/yr. We discuss our VLBI observations of the current guide-star candidates (radio stars HR 1099, HR 5110, and HR 8703) and our techniques for obtaining differential astrometric positions with the needed accuracy.

Precise CCD astrometric observations of minor planets and comets at the Belgrade observatory during 1997

73 precise astrometric positions of seven comets and two minor planets from MPC critical list - 1093 Freda and 4954 Eric - were obtained with the CCD equipped Askania astrograph (13.5/160) of Belgrade Astronomical Observatory. The SBIG ST-7 and ST-7 CCD cameras were used in the observations.

Observations of distant solar system bodies

AbstractMulticolour VRI photometry and astrometry of one Centaur and seven Edgeworth‐Kuiper objects were obtained. One object, 1994 JQ1 may be as red as 5145 Pholus, the reddest minor planet previously known. The Centaur 1995 DW2 has more moderate colour indices, similar to main‐belt asteroids. Seven R‐magnitudes were obtained for 1994 JS, 1995 FB21, and 1995 GY7. No light variation above the expected noise is evident, apart from a few outliers. A total of 47 astrometric positions were obtained for the eight objects. The four nights of observations with the ESO New Technology Telescope covered 0.52 square degrees. Two previosuly unknown object, 1995 FB21 and 1995 GY7, were discovered. We estimate the density of Edgeworth‐Kuiper objects brighter than R = 24 to 5.3 per square degree of sky near the ecliptic.