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Astron. Astrophys. 344, 1014-1026 (1999)


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High-precision VLBI astrometry of radio-emitting stars

J.-F. Lestrade 1, R.A. Preston 2, D.L. Jones 2, R.B. Phillips 3, A.E.E. Rogers 3, M.A. Titus 3, M.J. Rioja 4 and D.C. Gabuzda 5

1 Observatoire de Paris-Meudon, URA-CNRS 1757, F-92195 Meudon Principal Cedex, France
2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
3 Haystack Observatory, Massachussets Institute of Technology, Route 40, Westford, Mass 01886, USA
4 Joint Institute for VLBI in Europe JIVE, Postbus 2, 7990 AA Dwingeloo, The Netherlands
5 Astro Space Center, Lebedev Physical Institute 53 Leninsky Prospekt, 117924 Moscow, Russia

Received 7 April 1998 / Accepted 5 November 1998

Abstract

Multiple-epoch phase-referenced VLBI observations of 11 radio-emitting stars have been conducted as part of an astrometric program to link the Hipparcos optical reference frame to the radio extragalactic reference frame. We present the VLBI positions, proper motions and trigonometric parallaxes from this program in the ICRF (International Celestial Reference Frame). These astrometric parameters are absolute because they are directly measured relative to the distant quasars used as VLBI phase reference calibrators. The mean astrometric precision achieved relative to the calibrators is 0.36 milliarcsecond and the highest precision is for the RS CVn close binary [FORMULA] CrB with formal uncertainties of 0.12 milliarcsecond for its relative position, 0.05 milliarcsecond for its annual proper motion and 0.10 milliarcsecond for its trigonometric parallax.

In addition to the Hipparcos link, these observations have provided several new results. The distance to the nearby Tau-Auriga star forming region is [FORMULA] pc, determined directly through the VLBI trigonometric parallax of the Pre-Main-Sequence star HD283447 of this region. The orthogonality of the 2 orbital planes in the ternary system Algol is supported by new astrometric evidences. The proper motions of HR5110, HR1099 and IM Peg, regarded as possible guide stars for the NASA Gravity Probe B space mission, have formal precisions of 0.16, 0.31 and 0.40 milliarcsecond per year, respectively, and the mission requirement is 0.15 milliarcsecond per year. The close binary UX Ari is the only star that exhibits an acceleration larger than [FORMULA] and the most plausible cause is the gravitational interaction of a third body. The distances of the stars HD199178, IM Peg and AR Lac were uncertain by as much as 50% before our observations and are now [FORMULA] pc, respectively. The two X-ray binaries in our program, LSI61303 and Cyg X1, exhibit larger than expected post-fit position residuals. The linear scale of the rms of these residuals is [FORMULA] cm, more than 10 times the stellar separations in these systems. For LSI 61303, this scale is consistent with the size of the free-free absorption region of the enshrouding material beyond which radio radiation can escape. This would argue for in situ acceleration of the energetic electrons responsible for the synchrotron emission detected by VLBI as proposed by Vestrand (1983). The space velocities of LSI 61303 and Cyg X1 determined with our VLBI proper motions, radial velocities and distances, are [FORMULA] and [FORMULA] km s-1, respectively, and are relatively high. This is expected if a neutron star or a black hole lurk in these systems.

Key words: techniques: interferometric – astrometry – reference systems – stars: binaries: general – stars: distances – radio continuum: stars

Correspondence to: lestrade@mesioa.obspm.fr

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999

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