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.

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.

2. The nuclear star cluster ……………………………………………………. 7 2.1 The nuclear cluster of cool, old stars 2.2 The disk(s) of young O/WR-stars 2.3 The central S-star cluster and the distribution of B-stars 2.4 Is there an 'equilibrium' central stellar cusp ? 2.5 Stellar mass function 2.6 Chemical abundances 3. Interstellar matter …………………………………………………………. 35 3.1 Ionized gas in Sgr A West 3.2 Neutral gas 3.3 Dust and extinction toward the Galactic Center 3.4 Hot gas and high energy emission 4. Testing the black hole paradigm: is Sgr A* a massive black hole? ………. 43 4.1 Evidence for a central compact mass from gas motions 4.2 Evidence from stellar radial and proper motions 4.3 Constraints from stellar orbits 4.4 Very Long Baseline Interferometry of Sgr A* 4.5 Does Sgr A* have an event horizon? 4.6 Could Sgr A* be a binary ? 4.7 Alternatives to a black hole configuration 5. Mass distribution in the nuclear cluster …………………………………. 57 5.1 Outlier high velocity stars 5.2 Dark matter in the central parsec ? 5.3 Comparison to earlier statistical mass estimates 5.4 Does IRS 13E contain an intermediate mass black hole? 5.5 The distance to the Galactic Center 6. Paradox of youth: how did the young stars get into the central parsec? …. 6.1 Star formation history in the central parsecs 6.2 In situ star formation or in-spiral of clusters ? 6.3 Origin of B-stars in the central cusp: migration or scattering? 6.4 What powers the central parsec ? 7. Accretion and emission close to the central black hole ………………... 93 7.1 Steady emission from Sgr A* across the electromagnetic spectrum 7.2 Variable emission 7.3 Flares 7.4 Accretion onto the black hole 8. Concluding Remarks and Outlook …………………………………….. References …………………………………………………………….... _____________________________________________________________ , with β ranging from 1.5 to 2.3 in the analyses of Paumard et al. (2006), Lu et al. ( 2009) and Bartko et al. (2009Bartko et al. ( , 2010. The inner cutoff of the clockwise system is remarkably sharp. There are six O/WR-stars projected between 0.96" and 1.5" but none inside of 0.96".

We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the central 30 light days of the Galactic Center, taken with the new adaptive optics assisted, integral field spectrometer SINFONI on the ESO-VLT. To a limiting magnitude of K~16, 9 of 10 1 based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory, Chile 1 stars in the central 0.4", and 13 of 17 stars out to 0.7" from the central black hole have spectral properties of B0-B9, main sequence stars. Based on the 2.1127µm HeI line width all brighter early type stars have normal rotation velocities, similar to solar neighborhood stars.We combine the new radial velocities with SHARP/NACO astrometry to derive improved 3 d stellar orbits for six of these 'S'-stars in the central 0.5". Their orientations in space appear random. Their orbital planes are not co-aligned with those of the two disks of massive young stars 1-10" from SgrA*. We can thus exclude the hypothesis that the S-stars as a group inhabit the inner regions of these disks. They also cannot have been located/formed in these disks and then migrated inwards within their planes. From the combination of their normal rotation and random orbital orientations we conclude that the S-stars were most likely brought into the central light month by strong individual scattering events.The updated estimate of distance to the Galactic center from the S2 orbit fit is R o = 7.62 ± 0.32 kpc, resulting in a central mass value of 3.61 ± 0.32 x 10 6 M ⊙ .We happened to catch two smaller flaring events from SgrA* during our spectral observations. The 1.7-2.45µm spectral energy distributions of these flares are fit by a featureless, 'red' power law of spectral index α'=-4±1 (S ν~ν α' ). The observed spectral slope is in good agreement with synchrotron models in which the infrared emission 2 comes from accelerated non-thermal, high energy electrons in a radiative inefficient accretion flow in the central R~10 R s region.

Using 25 years of data from uninterrupted monitoring of stellar orbits in the Galactic Center, we present an update of the main results from this unique data set: A measurement of mass of and distance to Sgr A*. Our progress is not only due to the eight year increase in time base, but also due to the improved definition of the coordinate system. The star S2 continues to yield the best constraints on the mass of and distance to Sgr A*; the statistical errors of 0.13 × 10 6 M and 0.12 kpc have halved compared to the previous study. The S2 orbit fit is robust and does not need any prior information. Using coordinate system priors, also the star S1 yields tight constraints on mass and distance. For a combined orbit fit, we use 17 stars, which yields our current best estimates for mass and distance: M = 4.28 ± 0.10| stat. ± 0.21| sys × 10 6 M and R 0 = 8.32 ± 0.07| stat. ± 0.14| sys kpc. These numbers are in agreement with the recent determination of R 0 from the statistical cluster parallax. The positions of the mass, of the near-infrared flares from Sgr A* and of the radio source Sgr A* agree to within 1 mas. In total, we have determined orbits for 40 stars so far, a sample which consists of 32 stars with randomly oriented orbits and a thermal eccentricity distribution, plus eight stars for which we can explicitly show that they are members of the clockwise disk of young stars, and which have lower eccentricity orbits.