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At the IAU XXVI General Assembly in 2006, the Division I decided to create the Working Group on Astrometry by Small Ground-Based Telescopes (WG-ASGBT). Its scientic goals are to foster the follow-up of small bodies detected by the large surveys including the NEOs; to set-up a dedicated observation network for the follow-up of objects which will be detected by Gaia; to contribute to the observation campaigns of the mutual events of natural satellites, stellar occultations, and binary asteroids; and to encourage teaching astrometry for the next generation. The present report gives the main activities carried out in these areas with small telescopes (diameter less than 2m).
A program to measure the night sky brightness has been in progress for some years in order to calibrate the extent of the night sky brightness surrounding the Van Vleck Observatory. Both the central intensity and the areal extent of the brightest sky caused by campus and city were repeatedly measured in order to gauge the extent of the problem. For this purpose, portable visual photometers were designed which have remained stable and usable for nearly a decade. They are now useful for the measurement of the effects of increases in urban growth and of the more flexible attitude of the campus administration toward excess campus lighting. The inability to define city populations which realistically model and predict the measured sky brightness is the largest source of uncertainty. The observatory is in the Northeastern Corridor where the observed brightness is the sum of the illumination from a number of overlapping city sources. Present light pollution studies have not correctly defined the model for the population of an urban area. The two paradigmatic definitions now available for use are shown to be flawed for light pollution comparisons. An algorithm, unlike any in present use, must be sought which distinguishes between individual core cities within metropolitan areas.
One of the continuing problems facing Commission 24 is its name. It has been known and reported for some years that the present title is not suitable for reasons that are all too obvious. This problem has continued during the last three years, despite the growing development and use of non-photographic methods and techniques.
Most of the recent meeting activity in which the Commission and its membership has been involved, has centered on its shared interests with Commission 8 and with the Working Group on reference frames formed at the last General Assembly. Since its inception at that time, C. A. Murray has been appointed to represent the Commission on that group.
The question of the title of Commission 24, obviously, offers a difficult problem as already mentioned in recent reports. Photographic Astrometry no longer describes the whole scope of the commission. This problem has continued during the last three years especially in view of the preparations for the astrometric tasks of the NASA Space Telescope and of the ESA satellite HIPPARCOS.
The precision obtained in equating a very precise linear dimension to a very small and imprecise angular dimension sets a limit on the precision of the distance scale of the Universe. Within the Solar System, errors in distances are of the order of one part in one hundred million, but beyond its limits only a single star, Barnard's star, has a distance known to better than one part in one hundred, and distances known to one part in twenty from parallaxes are limited to only a few hundred nearby stars. Yet most other distance methods and results must ultimately be calibrated against distances to nearby stars derived from the heliocentric parallax method and its observations and uncertainties.
Space motions are calculated for 145 dK2-M2 stars with radial velocities and with parallaxes and proper motions determined and published at the Van Vleck Observatory. The stars are divided into young and old disk components kinematically and also according to the age-sensitive CaII emission intensities. Rigorous solutions for the solar motion and velocity ellipsoid were calculated for each population group using three methods of weighting errors in parallax, proper motion and radial velocity. All methods show a mean motion of the young stars outward away from the galactic center of about ten km/sec when referred to the old stars. Details are presented by Upgren (1978). The conclusion appears to confirm an outward motion suggested by Kerr (1962) from 21-cm observations. It is reasonable to inquire whether a similar outward motion can be seen in the space motions of other stars. Unlike the Van Vleck parallaxes combined with the most rigorous of the weighting methods used here, earlier results may be too insensitive to measure such a motion. Nonetheless a search of some of the existing literature shows that a small outward motion is consistent with sources so far examined. Of the motion solutions listed by Delhaye (1965) only two involve space motions of stars covering most of the main sequence. For stars with velocity dispersions similar to ours, both studies show a small outward motion of the order of five km/sec (for the A dwarfs relative to the generally older F and G dwarfs). The A dwarfs brighter than 5m.5 (Eggen 1965) have a planar motion distribution very similar to our young dK-M stars and both groups possess a mean motion close to the basic and standard solar motions. These last are mostly based on young stars and the solar motion should be reexamined relative to stars of all ages. It might be worthwhile to redetermine the mean motions of stars for which age-dependent parameters are now available, such as the F-stars with uvby measures or the giants with DDO photometry, since the local standard of rest appears to be a function of stellar age. This study was supported by grant AST77-26554 of the National Science Foundation.
This paper reviews the main sequences defined by members of the Hyades cluster and by the field stars in the solar neighborhood. For this purpose, the discussion is limited primarily to the stars of the lower portions of the main sequence, especially those of spectral classes K and early M. There are two reasons for emphasis on the faint red dwarf stars. First, the value of a parallax depends on its size or, more accurately, on the error in parallax divided by the parallax itself. Large parallaxes of high precision occur in large numbers only for stars inhabiting the lower main sequence. Furthermore, brighter stars of earlier spectral classes are more likely to be influenced by evolutionary effects which may differ between the Hyades and field stars, and which are difficult to calibrate.
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