common, then enough could be studied and characterized to allow meaningful taxonomic classi
-
fication. POINTS could determine the prevalence and characteristics of planetary systems
(masses, orbital elements, and number of planets) as a function of spectral type, and it could
address the possible existence of planetary systems in stellar binaries and triples.
M. Global Reference Frame
Of considerable astrophysical importance is the definition of a superbly accurate global reference
frame (GRF). This is one of the essential responsibilities of
astrometry. Statistical position
errors of order 10 milliarcseconds (mas) at the mean epoch ~1940 characterize the current optical
frame, defined by the FK5 catalog.
[68]
Systematic errors exist at the ~0.1-0.2 arcsec level,
[69]
and
error due to proper motion errors accumulates at a rate of about 1
mas/yr. The HIPPARCOS
proper motion errors are approximately 2 mas/yr,
[70]
mainly due to the short mission lifetime.
Distortions in the HIPPARCOS GRF are expected to be ~2 mas and cannot be removed by link-
ing with the radio reference frame.
[71]
Hence, during the epoch of a POINTS mission, the best
optical reference frame positions will be accurate at about the ~40
mas level. Currently, the radio
reference frame (based on observations of quasars and
AGN), accurate to about 1 mas, and the
optical frame differ by ~20 mas.
[72]
Current reference frames are plagued by systematic error and
degradation due to proper motion uncertainties.
d:\points\proposal\sz1\scifull.lwp
POINTS Science
16:29 Wednesday, January 03, 1996
page 19 of 20
[72]
Ma, C., Shaffer, D.B., De Vegt, C., Johnston, K.J., and Russell, J.L., 1990, Astron J 99, 1284, "A Radio Optical
Reference Frame. I - Precise Radio Source Positions Determined by Mark III
VLBI - Observations from 1979 to
1988 and a Tie to the FK5"
[71]
Lattanzi, M.G., Bucciarelli, B., and Bernacca, P.L., 1990, Astrophys J Supp 73, 481, "On the Problem of the HIP-
PARCOS Reference Frame";
see also Kovalevsky, J., 1992, in ESA, Targets for Space-Based Interferometry, p. 9, "The Need of very Accurate
Optical Astrometry by Interferometric Techniques"
[70]
Kovalevsky, J., 1991, Astrophys. & Space Sci. 177, 457, "Objectives of Ground-Based Astrometry after HIPPAR-
COS"
[69]
J.F. Chandler, 1994, private communication;
Lindegren, L., Van Leeuwen, F., Petersen, C., Perryman, M.A.C., and Soderhjelm, S., 1992, Astron & Astrophys
258, 134, "Positions and Parallaxes from the Hipparcos Satellite: A First Attempt at a Global Astrometric Solu-
tion";
Carrasco, G., and Loyola, P, 1992, Astron & Astrophys Supp 95, 355, "Santiago Fundamental Catalogue - A Cata
-
logue of 1105 FK5 Stars (Equinox J2000.0)";
Morrison, L.V., Gibbs, P., Helmer, L., Fabricius, C., and Einicke, O., 1991, Astrophys. Space Sci. 177, 31, "Evi-
dence of Systematic Errors in FK5"
[68]
Fricke, W., Schwan, H., Lederle, T., Bastian, U., Bien, R., Burkhardt, G., Dumont, B., Hering, R., Jaehrling, R.,
and Jahreiss, H., 1988, Fifth Fundamental Catalogue (FK5). Part 1: Basic Fundamental Stars;
Fricke, W., Schwan, H., Corbin, T., Bastian, U., Bien, R., Cole, C., Jackson, E., Jaehrling, R., Jahreiss, H., and
Lederle, T., 1988, Fifth Fundamental Catalogue. Part 2: The FK5 Extension - New Fundamental Stars
greater than 90 percent detection probability exists for signatures between 0.01 and 0.05 Jove, and 100 percent
above 0.05 Jove, with no false detections. One Jove is defined as the astrometric signature of the Sun due to Jupi-
ter as seen from 10 pc.
[67]
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