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Hubble Space Telescope Fine Guidance Sensor 3 can generate sub-milliarcsecond precision parallaxes in eighteen months. We discuss the internal precision and external accuracy of our observations of Proxima Centauri and Barnard's Star. For some classes of targets Hubble Space Telescope will remain the parallax tool of choice for years to come. It can offer 0.5 mas precision. It will remain useful by satisfying urgent needs for quick results, by offering a 13 magnitude dynamic range, and by providing an unparalleled binary dissection capability.
The astrometric capability of the Hubble Space Telescope Planetary Camera (WF/PC1) is investigated, motivated by a study of the internal velocity distribution of globular clusters. The astrometric accuracy of the HST PC will be determined ultimately by 1) the accuracy to which the aberrated images can be ‘centered’, and 2) the accuracy to which the distortions across the PC field can be modeled. A series of overlapping exposures of two clusters, NGC 6752 and M15, are utilized to examine these issues.
We have made use of maximum-likelihood image reconstruction to address the first issue, with good success. Reconstruction improves both the detectability and precision of the image centers. A preliminary exploration of the second issue, that of modeling the distortion across the PC field, is also presented, using positions derived from the multiple overlapping exposures.
The HST Astrometry Science Team is using the Fine Guidance Sensors (FGS) in the Transfer Function (TF) Scan mode to search for binaries among the faint members of the Hyades cluster. To date (March 1994), nine binaries have been discovered among 24 stars examined. The closest pair (total V=13.5) has a separation of 0.051 arcsec; the faintest (sep=0.287 arcsec) has magnitudes V=15.0 and 16.5; neither object posed a challenge to the capabilities of FGS. For another pair, two observations 152 days apart show a 13 deg change in position angle, indicating rapid orbital motion. One decade should suffice to define the orbit with angular dimensions of sub-millisecond of arc accuracy.
Clearly, this work will soon permit mass determinations for low-luminosity members of the Hyades cluster. Moreover, information on the frequency of binaries will provide insight into the role of duplicity in star formation and in the dynamic evolution of the cluster. To be truly useful, a census of binaries in the Hyades (and other clusters) must ultimately reach cluster members fainter than those currently under investigation, requiring astrometry with sub-millisecond of arc accuracy at near-infrared wavelengths.
We briefly review the concept of double star measurement with HST Fine Guidance Sensors (FGS) in the Transfer Function (TF) Scan mode and give results for three calibration binaries observed with FGS3. Agreement among multiple observations indicates an astrometric precision of 1 millisecond of arc (mas) per observation. We compare measured angular separations with ephemeris values from orbits based entirely on speckle observations. This comparison shows that the accuracy of binary-star astrometry with FGS3 in the TF-Scan mode is 1 mas per observation. Multiple observations can be expected to produce relative positions of binary components at sub-millisecond of arc accuracy.
We have begun a program to establish a dynamical reference frame based on the motions of minor planets. The program will utilize observations from the Hubble Space Telescope, and will ultimately tie the HIPPARCOS reference system to a dynamical base. Thirty-four minor planets, 20 of which are suitable for observation with the Hubble Space Telescope, have been selected. Ground based observations, particularly crossing-point observations with long focus reflectors, have been initiated.
A computer program to simultaneously solve for the corrections of the orbits of the 34 minor planets including the crossing-point observations, was successfully run. The observations are treated by the method of W. H. Jeffreys. Using simulated data, solutions with and without crossing point observations demonstrate the value of those observations to produce a homogeneous and coherent set of results.
Various techniques have been used to evaluate and describe coordinate system irregularities. The basic techniques have developed from simple differences to differences in coordinate “bins” to full spherical and cylindrical harmonic treatments. We are undertaking a 15-year program of minor planet observation with the express purpose of finding basic parameters of the adopted fundamental system with respect to a dynamical reference frame. The program is expected to provide a few hundred observations per year of 34 minor planets selected for their distribution of physical and orbital characteristics. The ability of the program to contribute to our knowledge of the rotation of the fundamental system and systematic irregularities within the system will depend on the accuracies of the observations and the distribution of the observations over the orbits and over the celestial sphere. We are considering the use of splines as a method of evaluating systematic corrections to the extant fundamental system. The initial development of the formalism and prospects for evaluation are presented.
Since initial applications of strontium isotope human sourcing in the early 1990s, the use of the method has steadily increased in archaeology and in anthropology more broadly. Despite this trend, the collection of necessary baseline environmental data has not been standardized and sometimes does not occur at all. A thorough environmental sampling strategy will ensure that all the variability within a selected region is documented, which is a critical step to improving the accuracy of sourcing studies. Furthermore, shared strontium baseline data collections are needed to improve the intercomparability of datasets and results. This paper provides a case study from a semiarid region in northwestern New Mexico, USA, highlighting the need for a bottom-up approach to baseline data collection (from bedrock to animal) and describes the methods of pre-field planning and collecting, including rationales for what samples to collect for Sr isotope baseline data. The authors hope that this paper will lay a foundation for the implementation and standardization of Sr isotope baseline data collecting, which does not currently exist.
The program Pickles was developed as an aid for planning HST observations using the Space Telescope Science Institute's Guide Star Catalogue, which was generated from wide-field Schmidt plates. Pickles reads the catalogue from CD-ROM and then displays a one-degree square field. The HST focal plane apertures can then be displayed singly or in any combination which is at the choice of the observer (Fig. 1). The user can generate an aperture of a different type if need be. The stars can be displayed as open or filled circles with their relative sizes indicating their magnitude. Stars or other objects can be added and saved with the field.
Lunar laser ranging (LLR) (Dickey et al., 1994) consists of measuring changes in the round-trip travel time for a laser pulse traveling between a transmitter on the Earth and a reflector on the Moon. The lunar surface reflectors are still operating normally after almost three decades of use. The ranging data exhibit a rich spectrum of change due to many effects.
In accordance with action taken by the Commission at the 1932 meeting of the Union, the President has taken steps to ascertain the general opinion of computers and observers in regard to the co-operation of the Nautical Almanac offices in furnishing data for the equinox of 1900. The question under discussion is that of the adoption of the standard equinox to which observations and elements should be referred, e.g. 1900, 1950, etc. The opinion is being ascertained through a questionnaire, the results of which will be reported at the meeting. In this connection Commission 4 proposes discussion of the following resolution in co-operation with Commission 20:
“That, as from 1938 January 1, the equinox used for expressing the elements of cometary orbits and for cometary ephemerides shall be that of 1950.0. Further that, as from the same date, the equinox used in giving observed positions of comets shall be that of 1950.0, unless the observer, for good reasons, used some other equinox and expressly draws attention to the equinox used.”
At the first meeting of the newly formed Commission on Spectrophotometry, at Paris in 1935, a thorough discussion, aided by several reports, took place on the principles of this branch of astrophysics. So it will be sufficient now to treat only such special points of theory and practice as have won interest by researches of the last few years.
The partition of the subject matter of this report follows that of earlier reports. Drafts for individual sections or parts of such were contributed by C. Arpigny, L. Biermann, A. H. Delsemme, O. V. Dobrovolsky, L. Haser, G. Herzberg, B. J. Levin, Rh. Lüst, E. Roemer, V. Vanýsek, and K. Wurm. The overall editing was done by Mrs Rh. Lüst, V. Vanýsek, and the undersigned.
Before mentioning some of the more important developments it is noted that E. Roemer has continued to publish every few months short notes on comets in the Publications of the A.S.P. The annual reports in Quarterly Journal of the Royal Astronomical Society have been continued by B. G. Marsden and J. G. Porter.
Several collections of Russian papers have been published which give a survey of the work done in the U.S.S.R. The description of physical characteristics of comets which appeared from 1961— 1965 was continued by S. K. Vsehsvjatskij, who also completed a book on Nature and Origin of Comets and Meteoritic Matter.
In view of the considerable ground covered by the Commission at its Paris meetings and the fairly complete record of the activities of institutes and observatories, etc. published in the Minutes, it has not been deemed profitable by the president to call for further reports in advance of the Stockholm meeting. At the Paris meeting it was agreed that such reports be printed independently before each meeting of the Union and that reprints of or references to the published reports be sent to the president. It is hoped that all such reports if ready will be made available before the Stockholm meeting so that they may be summarized by the representatives in attendance or by the president and recorded in the Minutes. With reference to the pronouncement at the Paris meeting “that it is eminently desirable that more attention be given to the development of accurate general perturbations and mean elements on the basis of accurate osculating elements”, the president has visited the Planeten-Institut at Frankfurt and the Rechen-Institut at Berlin and has been in correspondence with the Leningrad Institute. From these sources particularly valuable material has been received.
The scope of this Report is to cover briefly the progress in the study of meteors and meteorites since the 1938 meeting of Commission 22. The Sub-Commission on the Zodiacal Light has been expanded to include also the Light of the Night Sky and is now presided over by M. Jean Dufay. The ever-increasing scientific interest in the problems of nocturnal radiations and the fascinating difficulties of interpretation promise an exciting future for research in these two observationally related fields. On the other hand, the present intensive interest in the earth’s upper atmosphere and in ionospheric research makes interpretation in any one field of research in Commission 22 even more dependent upon knowledge of results in the other fields. Hence it appears essential that Commission 22 and its sub-commission keep in close rapport and that a considerable overlapping of membership be maintained.
The current evidence concerning the nature of comet nuclei suggests that comets may be sizeable aggregations of interstellar grains. This is a progress report on an effort to find circumstances and processes whereby such aggregations might be formed in the solar system at distances far beyond the proto-planets during the early stages of solar-system development. Under investigation are interactions between the early solar “gale” and the surrounding interstellar gas and dust—the so-called “snowplow effect.” Compression of the gas and resultant motions of the dust coupled with the pressure radiation from the Sun and nearby new stars may, under certain idealized circumstances, produce a high enough concentration of dust for gravitational instability to occur in the dust, thereby producing km-sized coherent bodies. The likelihood or probability of actual comet formation by such processes remain to be determined.