Extragalactic Reference Frame Research Articles

ASTROMETRY OFCASSINIWITH THE VLBA TO IMPROVE THE SATURN EPHEMERIS

Planetary ephemerides have been developed and improved over centuries. They are a fundamental tool for understanding solar system dynamics, and essential for planetary and small body mass determinations, occultation predictions, high-precision tests of general relativity, pulsar timing, and interplanetary spacecraft navigation. This paper presents recent results from a continuing program of high-precision astrometric very-long-baseline interferometry (VLBI) observations of the Cassini spacecraft orbiting Saturn, using the Very Long Baseline Array (VLBA). We have previously shown that VLBA measurements can be combined with spacecraft orbit determinations from Doppler and range tracking and VLBI links to the inertial extragalactic reference frame (ICRF) to provide the most accurate barycentric positions currently available for Saturn. Here we report an additional five years of VLBA observations along with improved phase reference source positions, resulting in an improvement in residuals with respect to the Jet Propulsion Laboratory's dynamical ephemeris.

Analytical Methods in Gravitational Microlensing

We discuss analytical results dealing with photometric and astrometric gravitational microlensing. The first two sections concern approximation methods that allow us to get solutions of the general lens equation near fold caustics and cusp points up to any prescribed accuracy. Two methods of finding approximate solutions near the fold are worked out. The results are applied to derive new corrections to total amplifications of critical source images. Analytic expressions are obtained in case of the Gaussian, power-law, and limb-darkening extended source models; here we present the first nonzero corrections to the well-known linear caustic approximation. Possibilities to distinguish different source models in observations are discussed on the basis of statistical simulations of microlensed light curves. In the next section, we discuss astrometric microlensing effects in various cases of extended sources and extended lenses, including a simple model of weak statistical microlensing by extended dark matter clumps. Random walks of a distant source image microlensed by stochastic masses are estimated. We note that the bulk motion of foreground stars induces a small apparent rotation of the extragalactic reference frame. Compact analytical relations describing the statistics of such motions are presented.

VLBI FOR GRAVITY PROBE B . V. PROPER MOTION AND PARALLAX OF THE GUIDE STAR, IM PEGASI

We present the principal astrometric results of the very-long-baseline interferometry (VLBI) program undertaken in support of the Gravity Probe B (GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and 2005. We discuss the statistical assumptions behind these results and our methods for estimating the systematic errors. We find the proper motion of IM Peg in an extragalactic reference frame closely related to the International Celestial Reference Frame 2 (ICRF2) to be -20.83 +- 0.03 +- 0.09 mas/yr in right ascension and -27.27 +- 0.03 +- 0.09 mas/yr in declination. For each component the first uncertainty is the statistical standard error and the second is the total standard error (SE) including plausible systematic errors. We also obtain a parallax of 10.37 +- 0.07 mas (distance: 96.4 +- 0.7 pc), for which there is no evidence of any significant contribution of systematic error. Our parameter estimates for the ~25-day-period orbital motion of the stellar radio emission have SEs corresponding to ~0.10 mas on the sky in each coordinate. The total SE of our estimate of IM Peg's proper motion is ~30% smaller than the accuracy goal set by the GP-B project before launch: 0.14 mas/yr for each coordinate of IM Peg's proper motion. Our results ensure that the uncertainty in IM Peg's proper motion makes only a very small contribution to the uncertainty of the GP-B relativity tests.

Astrophotometric variability of CFHT-LS Deep 2 QSOs

Context. The current conventional realization of the ICRS (International Celestial Reference System) is, in the radio wavelength, the International Celestial Reference Frame 2 (ICRF2). The individual positions of the defining sources have been found to have accuracies better than 1 milliarcsecond (mas). In 2012, the European astrometric satellite Gaia will be launched. This mission will provide an astrometric catalog of an estimated number of 500000 QSOs. The uncertainty in the coordinates is anticipated to be 200 microarcsecond (μas) for the magnitude = 20. If this were achieved, the ICRF and the Gaia related reference frame could be related with a μas accuracy. Aims. The goal of this work is both to measure the photometric variability of a set of quasars in a given field, and search wether this variability can be related to an astrometric instability characterized by a motion of the quasar photocenter. If this correlation existed for some given QSO, then it would be inadequate to materialize the Gaia extragalactic reference frame at the level of confidence required, i.e. the sub-milliarcsecond one. This should be an important result in the scope of the Gaia mission. Methods. We use QSO CCD images obtained over 4.5 years with the Canada France Hawai Telescope (CFHT) in the framework of the CFHT-Legacy Survey (CFHT-LS). The pictures were analysed with both the SExtractor software and customised codes to perform a photometric calibration together with an astrometric one. A total of 41 QSOs in the Deep 2 field were analysed. Magnitude variations during more than 50 months are given at three different bandwiths G, R, and I. Among the set above, 5 quasars were chosen to test the ties between the postion of their centroid and their magnitude variations. For one of these 5 QSOs, the proximity of a neighbouring star allows the comparison between the PSFs. Results. We clearly show significant photometric variations reaching sometimes more than one magnitude, for a good proportion of the 41 quasars in our sample. We show that these variations often occur within a few months, and that the correlation between the photometric curves in the three bands, G, R and I is obvious. As a second important result, we show that with a reasonably high probability, photometric variations for one quasar in our sample are accompagnied by substantial modification of its PSF.

Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame

The ICRF, currently based on the position of 717 extragalactic radio sources observed by VLBI, is the fundamental celestial reference frame adopted by the IAU in 1997. Within the next 10 years, the European space astrometry mission Gaia, to be launched by 2011, will permit determination of the extragalactic reference frame directly in the visible for the first time. Aligning these two frames with the highest accuracy will therefore be very important in the future for ensuring consistency between the measured radio and optical positions. This paper is aimed at evaluating the current astrometric suitability of the individual ICRF radio sources which are considered appropriate for the alignment with the future Gaia frame. To this purpose, we cross-identified the ICRF and the optical catalog Veron-Cetty and Veron (2006), to identify the optically-bright ICRF sources that will be positioned with the highest accuracy with Gaia. Then we investigated the astrometric suitability of these sources by examining their VLBI brightness distribution. We identified 243 candidate ICRF sources for the alignment with the Gaia frame (with an optical counterpart brighter than the apparent magnitude 18), but only 70 of these (10% of the ICRF sources) are found to have the necessary high astrometric quality (i.e. a brightness distribution that is compact enough) for this link. Additionally, it was found that the QSOs that will have the most accurate positions in the Gaia frame tend to have less-accurate VLBI positions, most probably because of their physical structures. Altogether, this indicates that identifying other high-quality VLBI radio sources suitable for the alignment with the future Gaia frame is mandatory.

CTIO 0.9m observations of ICRF optical counterparts

AbstractWe present astrometric results from 7 observing runs at the Cerro Tololo Interamerican Observatory (CTIO) 0.9m telescope of 197 extragalactic reference frame sources, selected from the original International Celestial Reference Frame (ICRF) catalog. This is part of the U.S. Naval Observatory (USNO) reference frame link program. Contemporaneous to the CTIO deep imaging, wide-field CCD data were taken with the USNO Twin Astrograph to provide accurate secondary reference stars in the 13 to 16 mag range. The optical positions are on the Hipparcos system (via Tycho-2 stars). The unweighted, mean RMS of positional differences ‘optical–radio’ for a single source is 28 and 25 mas for RA and Dec, respectively.

The Space Motion of the Globular Cluster NGC 6397

As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.

Near-infrared astrometry and photometry of Southern ICRF quasars

We present high quality astrometry, along with J, H, and Ks photometry, for 30 southern ICRF quasars observed with the ESO NTT/SOFI at La Silla. Our main purpose is to document the optical counterpart of ICRF quasars, by reporting their position in a stellar reference frame and their near-infrared photometry. Our investigation lies within the scope of extending the (radio) extragalactic reference frame into the optical range. An internal precision generally better than 10 mas was obtained in positioning the near-infrared counterpart of these quasars within their stellar surroundings. The actual astrometric accuracy is estimated from the departure between the observed and the radio (VLBI) positions of quasars, and is about 35 mas, mainly due to the statistical uncertainties of the stellar reference catalogue (UCAC2). The positional accuracy was improved to 25 mas for a subset of 14 quasars observed with extended fields of view, as obtained from mosaics of overlapping images. The 3-band photometry was achieved with a precision most often better than 0.04 mag, based on calibration with the 2MASS magnitudes. A spectral index α () was derived from the observed quasar fluxes in the near-infrared. With a mean value , this spectral index does not show any significant correlation with the radio spectral index.

Astrometric control of the inertiality of the Hipparcos catalog

Based on the most complete list of the results of an individual comparison of the proper motions for stars of various programs common to the Hipparcos catalog, each of which is an independent realization of the inertial reference frame with regard to stellar proper motions, we redetermined the vector ω of residual rotation of the ICRS system relative to the extragalactic reference frame. The equatorial components of this vector were found to be the following: ωx=+0.04±0.15 mas yr−1, ωy=+0.18±0.12 mas yr−1, and ωz=−0.35±0.09 mas yr−1.

The Location of the Core in M81

We report on VLBI observations of M81*, the northeast-southwest oriented nuclear core-jet source of the spiral galaxy M81, at five different frequencies between 1.7 and 14.8 GHz. By phase referencing to supernova 1993J we can accurately locate the emission region of M81* in the galaxy's reference frame. Although the emission region's size decreases with increasing frequency while the brightness peak moves to the southwest, the emission region seems sharply bounded to the southwest at all frequencies. We argue that the core must be located between the brightness peak at our highest frequency (14.8 GHz) and the sharp bound to the southwest. This narrowly constrains the location of the core, or the purported black hole in the center of the galaxy, to be within a region of ±0.2 mas or ±800 AU (at a distance of ~4 Mpc). This range includes the core position that we determined earlier by finding the most stationary point in the brightness distribution of M81* at only a single frequency. This independent constraint therefore strongly confirms our earlier core position. Our observations also confirm that M81* is a core-jet source, with a one-sided jet that extends to the northeast from the core, on average curved somewhat to the east, with a radio spectrum that is flat or inverted near the core and steep at the distant end. The brightness peak is unambiguously identified with the variable jet rather than the core, which indicates limitations in determining the proper motion of nearby galaxies and in refining the extragalactic reference frame.

Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic andHubble Space TelescopeWFPC2 Data

We have measured the absolute proper motion of the Fornax dwarf spheroidal galaxy from a combination of photographic plate material and Hubble Space Telescope Wide Field Planetary Camera 2 data that provide a time baseline of up to 50 yr. The extragalactic reference frame consists of eight QSO images and 48 galaxies. The absolute proper motion is μα cos δ = 0.59 ± 0.16 mas yr-1 and μδ = -0.15 ± 0.16 mas yr-1. The corresponding orbit of Fornax is polar, with an eccentricity of 0.27 and a radial period of 4.5 Gyr. Fornax's current location is near pericenter. The direction of the motion of Fornax supports the notion that Fornax belongs to the Fornax–Leo I–Leo II–Sculptor–Sextans stream as hypothesized by both Lynden-Bell and Majewski. According to our orbit determination, Fornax crossed the Magellanic plane ~190 Myr ago, a time that coincides with the termination of the star formation process in Fornax. We propose that ram pressure stripping due to the passage of Fornax through a gaseous medium denser than the typical intragalactic medium left behind from the LMC may have caused the end of star formation in Fornax. The excess, anomalous clouds within the south Galactic pole region of the Magellanic Stream, whose origin has long been debated in the literature as constituents of either the Magellanic Stream or of the extragalactic Sculptor group, are found to lie along the orbit of Fornax. We speculate that these clouds are stripped material from Fornax as the dwarf crossed the orbit of the Magellanic Clouds.

Aberration in proper motions

Approved space astrometric missions (SIM, GAIA) are aiming at a few microarcseconds per year precision in yearly proper motions and even less than a microarcsecond in the definition of an extragalactic reference frame. At such a level of accuracy, the curvature of stellar orbits around the center of the Galaxy cannot be neglected. The curvature of the Solar system barycentric motion induces a time-dependent component of the aberration, which has the properties of an apparent proper motion of the galaxies. This eect reaches 4as per year in some regions of the sky. In the case of stars, it is combined with a similar eect due to the curvature of the circular galactocentric orbit of the star, which may reach 60 as per year for a star situated at 500 parsecs from the center of the Galaxy, and much larger closer to it. The paper gives the proofs and the formulae permitting one to compute this aberration in proper motions. The conclusion is that, at this high level of accuracy, one should present the astrometric data in a galacto-centric rather than in a barycentric reference frame.

The SuperCOSMOS Sky Survey - III. Astrometry

In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the data base. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external data sets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho–2 reference catalogue, are accurate to better than ±0.2 arcsec at J,R∼19,18, rising to ±0.3 arcsec at J,R∼22,21, with positional-dependent systematic effects from bright to faint magnitudes at the ∼0.1-arcsec level. The proper motion measurements are shown to be accurate to typically ±10 mas yr−1 at J,R∼19,18, rising to ±50 mas yr−1 at J,R∼22,21, and are tied to zero using the extragalactic reference frame. We show that the zero-point errors in the proper motions are ≤1 mas yr−1 for R>17, and are no larger than ∼10 mas yr−1 for R<17 mas yr−1.

The SuperCOSMOS Sky Survey - III. Astrometry

In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the database. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external datasets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho-2 reference catalogue, are accurate to better than +/- 0.2 arcsec at J,R=19,18 rising to +/- 0.3 arcsec at J,R=22,21 with positional dependent systematic effects from bright to faint magnitudes at the +/- 0.1 arcsec level. The proper motion measurements are shown to be accurate to typically +/- 10 mas/yr at J,R=19,18 rising to +/- 50 mas/yr at J,R=22,21 and are tied to zero using the extragalactic reference frame. We show that the zeropoint errors in the proper motions are < 1 mas/yr for R > 17 and are no larger than 10 mas/yr for R < 17 mas/yr.

A Catalog of Faint Reference Stars in 398 Fields of Extragalactic Radio Reference Frame Sources

Positions of 89,422 stars in 398 fields of extragalactic reference frame sources have been determined using the Hamburg Zone Astrograph (northern hemisphere) and the US Naval Observatory Twin Astrograph, then stationed at the Black Birch Astrometric Observatory, New Zealand (southern hemisphere). Most stars are in the magnitude range 12 ≤ V ≤ 14, and the positions are accurate to ≈50 mas per coordinate at the epoch of observation, which ranges from the beginning of 1976 to the end of 1991. The catalog (ERLcat) is available on-line from USNO.

Proper Motion of the Compact, Nonthermal Radio Source in the Galactic Center, Sagittarius A*

Proper motions and radial velocities of luminous infrared stars in the galactic center have provided strong evidence for a dark mass of 2.5 million solar masses in the central 0.05 pc of the galaxy. The leading hypothesis for this mass is a black hole. High angular resolution measurements at radio wavelengths find a compact radio source, Sagittarius (Sgr) A*, that is either the faint glow from a small amount of material accreting onto the hole with low radiative efficiency or a miniature AGN core-jet system. We provide in this paper a full report on the first program that has measured the apparent proper motion of Sgr A* with respect to background extragalactic reference frame. Our current result is: mu_l,*=[-6.18+/-0.19] mas/y and mu_b,*=[-0.65+/-0.17] mas/y. The observations were obtained with the NRAO Very Large Array at 4.9 GHz over sixteen years. The proper motion of Sgr A* provides an estimate of its mass based on equipartition of kinetic energy between the hole and the surrounding stars. The measured motion is largest in galactic longitude. This component of the motion is consistent with the secular parallax that results from the rotation of the solar system about the center, which is a global measure of Oort's constants (A-B), with no additional peculiar motion of Sgr A*. The current uncertainty in Oort's galactic rotation constants limits the use of this component of the proper motion for a mass inference. In latitude we find a small, and weakly significant, peculiar motion of Sgr A*, -19+/-7 km/s after correction for the motion of the solar system with respect to the local standard of rest.

New Observations Testing the Adopted [ITAL]Hipparcos[/ITAL] Link to the International Celestial Reference Frame

This paper tests the Hipparcos link to the extragalactic reference frame by using new data obtained after the release of the catalog, thereby providing an external check on the accuracy of the established link process. The new data consist of 689 positions of Hipparcos stars determined in the International Celestial Reference Frame (ICRF) from CCD observations taken with the Flagstaff Astrometric Scanning Transit Telescope (FASTT). A comparison of the FASTT and Hipparcos catalog star positions observed in common finds that the axial rotations between the Hipparcos and ICRF reference frames are, respectively, (x, y, z) = (-2.2±3.3, -2.2±3.3, 3.3±2.9) (s.e.) mas at epoch 1996.5. All of these rotations are small and insignificant with respect to their errors (x and y are less than 1 σ detections whereas z is only a 1.1 σ detection). Hence, this paper finds that the Hipparcos link to the ICRF was well determined and is not seriously degrading with time.

CCD Positions for the Outer Planets in 1996–1997 Determined in the Extragalactic Reference Frame

This paper presents 316 new equatorial positions for the planets Uranus, Neptune, and Pluto obtained in 1996–1997 with the Flagstaff Astrometric Scanning Transit Telescope (FASTT). All the positions were determined in the extragalactic reference frame using CCD observations and are accurate to ~±02 in each coordinate. When these positions are compared with the latest JPL ephemerides (DE405), there is generally good agreement between the observed and calculated positions for these planets. In particular, the offset in the declination of Pluto (Δδ ~ 01) is no longer evident, and consequently, the DE405 is obviously an improvement over its predecessor, the DE403, in that respect. There appears to be a small offset, ~009, however, in the right ascensions of Uranus, which might be accidental or caused by errors in the FASTT observed or DE405 calculated ephemeris positions; future observations are planned to clarify this issue. With the release of the Hipparcos, Tycho, and ACT catalogs of extremely accurate reference star positions, it is now possible for telescopes, such as the FASTT, to determine the positions of planets and their satellites directly in the international celestial reference frame (ICRF) with greatly improved accuracy (~±006) in each coordinate. All future FASTT observations will be made in this manner, and the FASTT observing program will be expanded to include satellites of planets as well.

Astrometry of the Planetary-System Millisecond Pulsar B1257+12

AbstractWe present the VLBI determined position of the millisecond pulsar PSR B1257+12 which has been shown to have a planetary system (Wolszczan &amp; Frail 1992). The position determination for epoch June 20, 1992 is: αJ2000=13h0m3s.05005(5),δJ2000=+12°40’56″.7043(34). We indicate how the combination of such pulsar observations along with timing observations can be used to directly tie the solar-system dynamic reference frames and the extragalactic reference frame.

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.