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

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

2008-03-06

arxiv.org/abs/0803.0825v1

Astronomy and Astrophysics

Astrophysics

6 pages, 0 figures. To be published in Dynamics of Galaxies, Proceedings of the International Conference held at Pulkovo Obser

Scientific paper

Eisenhauer et al. (2003, 2005) derived absolute (geometrical) estimates of the distanceto 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)$\times (R_0/8)$ 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 ${\approx}1.5$--1.9 in the limiting cases.

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