Stellar Orbits Near Sagittarius A*

A. Eckart(1), R. Genzel(2), T. Ott(2), R. Schödel(2)

Paper: MNRAS, in press

The SHARP/NTT stellar proper motion data now cover an interval from 1992 to 2000 and allow us to determine orbital accelerations for some of the most central stars. We confirm the stellar acceleration measurements obtained by Ghez et al. (2000) with NIRC at the Keck telescope. Our analysis differs in 3 main points from that of Ghez et al.: 1) We combine the high precision but shorter time scale NIRC/Keck data with the lower precision but longer time scale SHARP/NTT data set; 2) We statistically correct the observed accelerations for geometrical projection effects; 3) We exclude star S8 from the analysis of the amount and position of the central mass. From the combined SHARP/NTT and NIRC/Keck data sets we show that the stars S2, and most likely S1 and S8 as well, are on bound, fairly inclined (60^o < i < 80^o), and eccentric (0.4< e < 0.95) orbits around a central dark mass. The combination of both data sets results in a position of this central mass of 48⁺⁵⁴_-24 mas E and 18⁺⁴²_-61 mas S of the nominal radio position of Sgr A*. The mean statistically corrected enclosed mass derived from accelerations is M_acc=(5 +/- 3) * 10⁶M_o with current radial separations of S1 and S2 from SgrA* of about 8-10 mpc. This enclosed mass estimate is derived from individual stellar orbits as close to the massive black hole at the center of the Milky Way as currently possible. Although the uncertainties are large this estimate is fully consistent with the enclosed mass range of (2.6-3.3)*10⁶M_o derived by Genzel et al. (2000) from radial and/or proper motion velocities of a homogenized sample of sources. Star S8 was excluded from the analysis, since for the current proper motion velocity and radial separation from the center we find that the measured acceleration requires orbital motion around a compact object with a mass in excess of 3*10⁶M_o. The data suggest that this star either was or is subject to a close interaction with a different object or that its position measurements are influenced by the emission of a different cluster star. Therefore, we base the analysis of the enclosed mass solely on the available data for S1 and S2. We also discuss two late type stars with projected separations from Sgr A* of about 0.5'' and 1''. In addition to proper motions these stars have known radial velocities. Orbit calculations indicate that those stars are very likely at larger physical distances from the center and part of the larger scale central stellar cluster with a core radius of approximately 0.3 pc.