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.

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.

We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early and late type stars to m K ≥ 16, more than 2 magnitudes deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O-and B-stars reside in two strongly warped disks between 0.8" and 12" from SgrA*, as well as a central compact concentration (the S-star cluster) centered on SgrA*. The later type B stars (m K > 15) in the radial interval between 0.8" and 12" seem to be in a more isotropic distribution outside the disks. The observed dearth of late type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best fit power law of dN/dm ∝ m −0.45±0.3 .Since at least the WR/O-stars were formed in situ in a single star formation event ∼6 Myrs ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.8" and of the early-type stars at distances beyond 12" are compatible with a standard Salpeter/Kroupa IMF (best fit power law of dN/dm ∝ m −2.15±0.3 ).

The central parsec around the super-massive black hole in the Galactic Center hosts more than 100 young and massive stars. Outside the central cusp (R ∼ 1 ′′ ) the majority of these O and Wolf-Rayet (WR) stars reside in a main clockwise system, plus a second, less prominent disk or streamer system at large angles with respect to the main system. Here we present the results from new observations of the Galactic Center with the AO-assisted near-infrared imager NACO and the integral field spectrograph SINFONI on the ESO/VLT. These include the detection of 27 new reliably measured WR/O stars in the central 12" and improved measurements of 63 previously detected stars, with proper motion uncertainties reduced by a factor of four compared to our earlier work. Based on the sample of 90 well measured WR/O stars, we develop a detailed statistical analysis of their orbital properties and orientations. We show that half of the WR/O stars are compatible with being members of a clockwise rotating system. The rotation axis of this system shows a strong transition from the inner to the outer regions as a function of the projected distance from Sgr A*.-2 -The main clockwise system either is either a strongly warped single disk with a thickness of about 10 • , or consists of a series of streamers with significant radial variation in their orbital planes. 11 out of 61 clockwise moving stars have an angular separation of more than 30 • from the local angular momentum direction of the clockwise system. The mean eccentricity of the clockwise system is 0.36 ± 0.06. The distribution of the counter-clockwise WR/O star is not isotropic at the 98% confidence level. It is compatible with a coherent structure such as stellar filaments, streams, small clusters or possibly a disk in a dissolving state: 10 out of 29 counter-clockwise moving WR/O stars have an angular separation of more than 30 • from the local angular momentum direction of the counter-clockwise system. The observed disk warp and the steep surface density distribution favor in situ star formation in gaseous accretion disks as the origin of the young massive stars. projected distance [arcsec] 0 2 4 6 8 10 12 14 # stars 0 2 4 6 8 10 12 14 O/WR Stars >0 z J <0 z J projected distance [arcsec] 1 10 (max) z / J z average projected distance [arcsec]

Aims. We study a sample composed of 28 of the brightest stars in the Arches cluster. Our aim is to constrain their stellar and wind properties and to establish their nature and evolutionary status. Methods. We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT. Atmosphere models computed with the code CMFGEN are used to derive the effective temperatures, luminosities, stellar abundances, mass loss rates and wind terminal velocities. Results. We find that the stars in our sample are either H-rich WN7-9 stars (WN7-9h) or supergiants, two being classified as OIf + . All stars are 2-4 Myr old. There is marginal evidence for a younger age among the most massive stars. The WN7-9h stars reach luminosities as large as 2 × 10 6 L ⊙ , consistent with initial masses of ∼ 120 M ⊙ . They are still quite H-rich, but show both N enhancement and C depletion. They are thus identified as core H-burning objects showing products of the CNO equilibrium at their surface. Their progenitors are most likely supergiants of spectral types earlier than O4-6 and initial masses > 60 M ⊙ . Their winds follow a well defined modified wind momentum -luminosity relation (WLR): this is a strong indication that they are radiatively driven. Stellar abundances tend to favor a slightly super solar metallicity, at least for the lightest metals. We note however that the evolutionary models seem to under-predict the degree of N enrichment.

This paper reports measurements of Sgr A* made with NACO in L ′ -band (3.80 µm), Ks-band (2.12 µm) and H-band (1.66 µm) and with VISIR in N-band (11.88 µm) at the ESO VLT 1 , as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 4 April, 2007, a very bright flare was observed from Sgr A* simultaneously at L ′ -band and X-ray wavelengths. No emission was detected 1 The Very Large Telescope (VLT) at the European Southern Observatory (ESO) on Paranal, Chile: Program IDs 179.B-0261(A) and Program ID: 079.B-0929(A).using VISIR. The resulting SED has a blue slope (β > 0 for νL ν ∝ ν β , consistent with νL ν ∝ ν 0.4 ) between 12 micron and 3.8 micron.For the first time our high quality data allow a detailed comparison of infrared and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak) and prominent substructures in the 3.8 micron light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L ′ -band lightcurve we find that the flaring region must be no more than 1.2 R S in size.The high X-ray to infrared flux ratio, blue νL ν slope MIR to L ′ -band, and the soft νL ν spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.

Context. How star formation proceeds in the Galactic Center is a debated question. Addressing this question will help us understand the origin of the cluster of massive stars near the supermassive black hole, and more generally starburst phenomena in galactic nuclei.In that context, it is crucial to know the properties of young massive stars in the central parsec of the Galaxy. Aims. The main goal of this study is to derive the stellar and wind properties of the massive stars orbiting the supermassive black hole SgrA ⋆ in two counter-rotating disks. Methods. We use non-LTE atmosphere models including winds and line-blanketing to reproduce H and K band spectra of these stars obtained with SINFONI on the ESO/VLT. Results. The GC massive stars appear to be relatively similar to other Galactic stars. The currently known population of massive stars emit a total 6.0 × 10 50 s −1 (resp. 2.3 × 10 49 s −1 ) H (resp. He i) ionising photons. This is sufficient to produce the observed nebular emission and implies that, in contrast to previous claims, no peculiar stellar evolution is required in the Galactic Center. We find that most of the Ofpe/WN9 stars are less chemically evolved than initially thought. The properties of several WN8 stars are given, as well as two WN/C stars confirmed quantitatively to be stars in transition between the WN and WC phase. We propose the sequence (Ofpe/WN9 ⇋ LBV) → WN8 → WN/C for most of the observed GC stars. Quantitative comparison with stellar evolutionary tracks including rotation favour high mass loss rates in the Wolf-Rayet phase in these models. In the OB phase, these tracks nicely reproduce the average properties of bright supergiants in the Galactic Center.

Abstract. High spatial resolution observations in the 1 to 3.5 µm region of the Galactic Center source known historically as IRS 13 are presented. They include ground-based adaptive optics images in the H, Kp (2.12/0.4 µm) and L bands, HST-NICMOS data in filters between 1.1 and 2.2 µm, and integral field spectroscopic data from BEAR, an Imaging FTS, in the He  2.06 µm and the Brγ line regions. Analysis of all these data provides a completely new picture of the main component, IRS 13E, which appears as a cluster of seven individual stars within a projected diameter of ∼0.5 (0.02 pc). The brightest sources, 13E1, 13E2, 13E3 which is detected as a binary, and 13E4, are all massive stars of different type. The star 13E1 is a luminous, blue object, with no detected emission line. 13E2 and 13E4 are two hot, high-mass emission line stars, 13E2 being at the WR stage and 13E4 a massive O-type star. In contrast, 13E3A and B are extremely red objects, proposed as other examples of dusty WR stars, like IRS 21 (Tanner et al. 2002). All these sources have a common westward proper motion indicating they are bounded. Two other sources, detected after deconvolution of the AO images in the H and Kp bands, are also identified. One, that we call 13E5, is a red source similar to 13E3A and B, while the other one, 13E6, is probably a main sequence O star in front of the cluster. Considering this exceptional concentration of comoving massive hot stars, IRS 13E is proposed as the remaining core of a massive star cluster, which could harbor an intermediate-mass black hole (IMBH) (Portegies Zwart & McMillan 2002) of ∼1300 M . This detection plays in favor of a scenario, first suggested by Gerhard (2001), in which the helium stars and the other hot stars in the central parsec originate from the stripping of a massive cluster formed several tens of pc from the center. This cluster would have spiraled towards SgrA , and IRS 13E would be its remnant. Furthermore, IRS 13E might be the second black hole needed according to a model by Hansen & Milosavljević (2003) to drag massive main-sequence stars, in the required timescale, very close to the massive black hole. The detection of a discrete X-ray emission at the IRS 13 position (within the positional accuracy) is examined in this context.