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Astron. Astrophys. 339, 897-903 (1998)

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1. Introduction

In order to understand the nature of the central region of the Galaxy, it is crucial to have a good estimate of the mass distribution within the central parsecs of the dynamical center (Sgr A*). The most outstanding issue is that of the existence of a central massive black hole, with Sgr A* as the most likely candidate. On a larger scale, the evidence for a "bar", or a tri-axial bulge in our Galaxy accumulates. In the past decade many studies to probe the inner Galactic mass distribution have been performed, using line-of-sight velocities of gas and stars. However, because the exact type of orbits of the objects in these studies are unknown, the line-of-sight velocity information alone is of limited use. Therefore, the evidence for the existence of a massive black hole or a tri-axial bulge is heavily dependent on the assumptions made about the three-dimensional motions and the assumed potential. Genzel et al. (1994, 1997), Eckart & Genzel (1996), and Mezger et al. (1996) have reviewed the possibility of a black hole in the Galactic center (GC). Blitz et al. (1993) review the literature on the "bar" in the GC; another review is included in Morris & Serabyn (1996).

High-velocity stars ([FORMULA] 250 [FORMULA]) are an important constituent of the stars in the GC (Baud et al. 1975; Rieke & Rieke 1988; Van Langevelde et al. 1992a; and more recently Genzel et al. 1996). For example, Eckart & Genzel (1996) suggest that the stellar orbits in the inner parsec are isotropic in an axi-symmetric mass distribution. Hence, the high-velocity stars outline the tail of the stellar velocity distribution. On the other hand, Van Langevelde et al. (1992a) and Blommaert et al. (1998), argue that the high-velocity stars are merely bulge stars on highly elongated orbits that penetrate the center. Axi-symmetric mass distribution models are then insufficient to study the stellar dynamics of even the very center. In this case, the line-of-sight velocity dispersions - the velocity profiles - alone give a distorted picture.

Transverse motions
We also expect high-velocity stars in the transverse velocity domain. At the distance of the GC (8 kpc, Reid 1993), a typical stellar transverse velocity of 100 [FORMULA] corresponds to a proper motion of [FORMULA] 2.5 milli-arcsecond (mas) per year. With current technology, it is possible to measure proper motions in the GC, and to deduce the type of stellar orbits with a few years of observing. Recently, Eckart & Genzel (1996) measured the proper motions of stars using near-infrared images of late-type stars in the central 0.4 parsec of the GC. However, with infrared cameras, the field of view is limited to less than one arc-minute ([FORMULA] 2.5 parsec). We initiated a project to measure transverse velocities of OH/IR stars in the GC with VLBI. The OH/IR stars form a different sample of stars than the objects studied by Eckart & Genzel (1996) because they are located further out ([FORMULA] [FORMULA], or [FORMULA] 100 parsec) from Sgr A*. We assume that OH/IR stars are evolved, oxygen rich AGB stars, and refer to Iben & Renzini (1983) or Habing (1996) for further reading about AGB stars and their circumstellar envelopes.

Because of interstellar scattering in the direction of the GC, one cannot use the 1612 MHz OH masers in these OH/IR stars (Van Langevelde et al. 1992b; Frail et al. 1994). As scattering scales as [FORMULA], higher frequency observations are more favorable. Furthermore, it is well known that the circumstellar 43 GHz SiO masers (and 22 GHz H2O masers) are generated closer to the star than the OH masers. The SiO masers originate from the parts of the circumstellar shell that are very close to the star (a few stellar radii with a typical diameter of [FORMULA] 10 AU: Diamond et al. 1994; Miyoshi et al. 1995). Although the individual SiO maser spots are variable and are located around the star at a distance out to 5-10 AU, one can expect that the measured SiO maser position is within 10 AU of the star; i.e. the "average" spot position, which might be a blend of several of such spots, represents the stellar position within 1 mas. Also, the angular broadening due to scattering is less than 1 mas, altogether allowing a proper motion measurement of the underlying star accurate to about 30 [FORMULA] within 5 years.

Outline of this paper
We report on our efforts to obtain milli-arcsecond accurate positions for 10 SiO masers in OH/IR stars in the GC with the Very Long Baseline Array (VLBA). Because of the low elevation of the GC, the rapid phase fluctuations at 43 GHz and the low fluxes of the masers we used a special observing mode that included the phased Very Large Array (VLA) as described in Sect. 2. The resulting milli-arcsecond positions and accuracies achieved for 2 masers are given in Sect. 3. In Sect. 4. we finish with our conclusion and recommendations for future measurements.

This paper describes three sets of 43 GHz observations; the first, in May 1995, were made with the VLBA alone; the second and third were made in December 1995 and January 1996 and involved the VLBA and phased VLA. In May 1995 we also attempted to detect 22 GHz H2O masers in OH/IR stars. However, in the subsequent observations we restricted ourselves to the 43 GHz SiO masers.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998