# On a Source of Systematic Error in Absolute Measurement of Galactocentric Distance from Solving for the Stellar Orbit Around Sgr A*

## Igor' I. Nikiforov

(1) Sobolev Astronomical Institute, St. Petersburg State University, Universitetskij pr. 28, Staryj Peterhof, St. Petersburg 198504, Russia, nii@astro.spbu.ru

### Paper: Dynamics of Galaxies, Proceedings of the

**Abstract:**
Eisenhauer et al. (2003, 2005) derived absolute (geometrical) estimates
of the distance to the center of the Galaxy, R_{0}, from the star S2
orbit around Sgr A* on the assumption that the intrinsic velocity of
Sgr A* is negligible. This assumption produces the source of systematic
error in R_{0} value owing to a probable motion of Sgr A* relative to
the accepted velocity reference system which is arbitrary to some
extent. Eisenhauer et al. justify neglecting all three spatial
velocity components of Sgr A* mainly by low limits of Sgr A*'s proper
motion of 20-60 km/s. In this brief paper, a simple analysis in the
context of the Keplerian dynamics was used to demonstrate that neglect
of even low (perhaps, formal) radial velocity of Sgr A* leads to a
substantial systematic error in R_{0}: the same limits of 20-60 km/s
result in R_{0} errors of 1.3-5.6%, i.e., (0.1-0.45)*
(R_{0}/8) kpc, for current S2 velocities. Similar values for Sgr A*'s
tangential motion can multiply this systematic error in the case of S2
orbit by factor 1.5-1.9 in the limiting cases.

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