We present the results of 16 years of monitoring stellar orbits around the massive black hole in center of the Milky Way using high resolution near-infrared techniques. This work refines our previous analysis mainly by greatly improving the definition of the coordinate system, which reaches a longterm astrometric accuracy of ≈ 300 µas, and by investigating in detail the individual systematic error contributions. The combination of a long time baseline and the excellent astrometric accuracy of adaptive optics data allow us to determine orbits of 28 stars, including the star S2, which has completed a full revolution since our monitoring began. Our main results are: all stellar orbits are fit extremely well by a single point mass potential to within the astrometric uncertainties, which are now ≈ 6× better than in previous studies. The central object mass is (4.31 ± 0.06| stat ± 0.36| R0 ) × 10 6 M ⊙ where the fractional statistical error of 1.5% is nearly independent from R 0 and the main uncertainty is due to the uncertainty in R 0 . Our current best estimate for the distance to the Galactic Center is R 0 = 8.33 ± 0.35 kpc. The dominant errors in this value is systematic. The mass scales with distance as (3.95 ± 0.06) × 10 6 (R 0 /8 kpc) 2.19 M ⊙ . The orientations of orbital angular momenta for stars in the central arcsecond are random. We identify six of the stars with orbital solutions as late type stars, and six early-type stars as members of the clockwise rotating disk system, as was previously proposed. We constrain the extended dark mass enclosed between the pericenter and apocenter of S2 at less than 0.066, at the 99% confidence level, of the mass of Sgr A*. This is two orders of magnitudes larger than what one would expect from other theoretical and observational estimates.

Abstract. We present new calibrations of stellar parameters of O stars at solar metallicity taking non-LTE, wind, and lineblanketing effects into account. Gravities and absolute visual magnitudes are derived from results of recent spectroscopic analyses. Two types of effective temperature scales are derived: one from a compilation based on recent spectroscopic studies of a sample of massive stars -the "observational scale" -and the other from direct interpolations on a grid of non-LTE spherically extended line-blanketed models computed with the code CMFGEN (Hillier & Miller 1998) -the "theoretical scale". These T eff scales are then further used together with the grid of models to calibrate other parameters (bolometric correction, luminosity, radius, spectroscopic mass and ionising fluxes) as a function of spectral type and luminosity class. Compared to the earlier calibrations of Vacca et al. (1996) the main results are: -The effective temperature scales of dwarfs, giants and supergiants are cooler by 2000 to 8000 K, the theoretical scale being slightly cooler than the observational one. The reduction is the largest for the earliest spectral types and for supergiants. -Bolometric corrections as a function of T eff are reduced by 0.1 mag due to line blanketing which redistributes part of the UV flux in the optical range. For a given spectral type the reduction of BC is larger for early types and for supergiants. Typically BCs derived using the theoretical T eff scale are 0.40 to 0.60 mag lower than that of Vacca et al. (1996), whereas the differences using the observational T eff scale are somewhat smaller. -Luminosities are reduced by 0.20 to 0.35 dex for dwarfs, by ∼0.25 for all giants and by 0.25 to 0.35 dex for supergiants.The reduction is essentially the same for both T eff scales. It is independent of spectral type for giants and supergiants and is slightly larger for late type than for early type dwarfs. -Lyman continuum fluxes are reduced. Our theoretical values for the hydrogen ionising photon fluxes for dwarfs are 0.20 to 0.80 dex lower than those of Vacca et al. (1996), the difference being larger at late spectral types. For giants the reduction is of 0.25 to 0.55 dex, while for supergiants it is of 0.30 to 0.55 dex. Using the observational T eff scale leads to smaller reductions at late spectral types. The present results should represent a significant improvement over previous calibrations, given the detailed treatment of non-LTE line-blanketing in the expanding atmospheres of massive stars.

We report the definite spectroscopic identification of ≃ 40 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ≃ 80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick ( |h|/R ≃ 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R ≃ 1 ′′ ) and surface density profiles that scale as R −2 . We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the 'counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (ρ core 3 × 10 8 M ⊙ pc −3 ), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ≃ 6 ± 2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ≃ 1.5 × 10 4 M ⊙ . Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.