[en] Two recent papers (Ghez et al. 2008; Gillessen et al. 2009) have estimated the mass of and the distance to the massive black hole (MBH) in the center of the Milky Way using stellar orbits. The two astrometric data sets are independent and yielded consistent results, even though the measured positions do not match when simply overplotting the two sets. In this Letter, we show that the two sets can be brought to excellent agreement with each other when we allow for a small offset in the definition of the reference frame of the two data sets. The required offsets in the coordinates and velocities of the origin of the reference frames are consistent with the uncertainties given in Ghez et al. The so-combined data set allows for a moderate improvement of the statistical errors of the mass of and the distance to Sgr A*, but the overall accuracies of these numbers are dominated by systematic errors and the long-term calibration of the reference frame. We obtain R₀ = 8.28 ± 0.15|_stat ± 0.29|_sys kpc and M_MBH = 4.30 ± 0.20|_stat ± 0.30|_sys x 10⁶ M_sun as best estimates from a multi-star fit.

[en] We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in 2013 April with NACO and SINFONI at the ESO VLT. The ''head'' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blueshifted side of the orbit, past pericenter approach. Ionized gas in the head is now stretched over more than 15,000 Schwarzschild radii R_S around the pericenter of the orbit, at ≈2000 R_S ≈ 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-γ luminosity of the head has been constant over the past nine years, to within ±25%, as have the line ratios Brackett-γ/Paschen-α and Brackett-γ/Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution due to hydrodynamical interactions with the hot gas around the black hole from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2

[en] The origin of the dense gas cloud G2 discovered in the Galactic Center is still a debated puzzle. G2 might be a diffuse cloud or the result of an outflow from an invisible star embedded in it. We present hydrodynamical simulations of the evolution of different spherically symmetric winds of a stellar object embedded in G2. We find that the interaction with the ambient medium and the extreme gravitational field of the supermassive black hole in the Galactic Center must be taken into account in such a source scenario. The thermal pressure of the hot and dense atmosphere confines the wind, while its ram pressure shapes it via stripping along the orbit, with the details depending on the wind parameters. Tidal forces squeeze the wind near pericenter, reducing it to a thin and elongated filament. We also find that in this scenario most of the Brγ luminosity is expected to come from the densest part of the wind, which has a highly filamentary structure with a low filling factor. For our assumed atmosphere, the observations can be best matched by a mass outflow rate of M-dot_w=8.8×10^-8 M_sun yr^-1 and a wind velocity of v_w = 50 km s^–1. These values are comparable with those of a young T Tauri star wind, as already suggested by Scoville and Burkert

[en] In early 2014, the fast-moving near-infrared source G2 reached its closest approach to the supermassive black hole Sgr A* in the Galactic center. We report on the evolution of the ionized gaseous component and the dusty component of G2 immediately after this event, revealed by new observations obtained in 2015 and 2016 with the SINFONI integral field spectrograph and the NACO imager at the ESO VLT. The spatially resolved dynamics of the Brγ line emission can be accounted for by the ballistic motion and tidal shearing of a test-particle cloud that has followed a highly eccentric Keplerian orbit around the black hole for the last 12 years. The non-detection of a drag force or any strong hydrodynamic interaction with the hot gas in the inner accretion zone limits the ambient density to less than a few ${10}^3{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$ at the distance of closest approach ($1500R_s$), assuming G2 is a spherical cloud moving through a stationary and homogeneous atmosphere. The dust continuum emission is unresolved in L′-band, but stays consistent with the location of the Brγ emission. The total luminosity of the Brγ and L′ emission has remained constant to within the measurement uncertainty. The nature and origin of G2 are likely related to that of the precursor source G1, since their orbital evolution is similar, though not identical. Both objects are also likely related to a trailing tail structure, which is continuously connected to G2 over a large range in position and radial velocity.

[en] The central pc around the super-massive black hole in the Galactic Center hosts a population of young and massive stars. The majority of these stars (outside the central 1') have been found to reside in disks. Here we develop a detailed statistical analysis method of the properties of these disks including a robust test of the significance of these disks versus an isotropic stellar population. We apply this method to the data set obtained with the AO assisted integral field spectrograph SINFONI on the ESO/VLT as of the end of 2007 and present preliminary results.

[en] We derive the extinction curve toward the Galactic center (GC) from 1 to 19 μm. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction-free flux reference is the 2 cm continuum emission observed by the Very Large Array. Toward the inner 14'' x 20'', we find an extinction of A_2.166μm = 2.62 ± 0.11, with a power-law slope of α = -2.11 ± 0.06 shortward of 2.8 μm, consistent with the average near-infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 μm. We find that it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance-dependent extinction with our distance-independent extinction, we derive the distance to the GC to be R₀ = 7.94 ± 0.65 kpc. Toward Sgr A* (r < 0.''5), we obtain A_H = 4.21 ± 0.10, A_Ks = 2.42 ± 0.10, and A_L' = 1.09 ± 0.13.

[en] 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 and distance to Sgr A*. Our progress is not only due to the eight-year increase in time base, but also 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 $0.13\times <!--\; ??\; -->{10}^6{M}_{\odot <!--\; ???\; -->}$ 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, the star S1 also 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: $M=4.28\pm <!--\; ??\; -->0.10{|}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}.}\pm <!--\; ??\; -->0.21{|}_{\mathrm{s}\mathrm{y}\mathrm{s}}\times <!--\; ??\; -->{10}^6{M}_{\odot <!--\; ???\; -->}$ and $R_0=8.32\pm <!--\; ??\; -->0.07{|}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}.}\pm <!--\; ??\; -->0.14{|}_{\mathrm{s}\mathrm{y}\mathrm{s}}\mathrm{k}\mathrm{p}\mathrm{c}$. 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 that we can explicitly show are members of the clockwise disk of young stars, and which have lower-eccentricity orbits.

[en] We obtain the basic properties of the nuclear cluster of the Milky Way. First, we investigate the structural properties by constructing a stellar density map of the central 1000″ using extinction-corrected old star counts from VISTA, WFC3/IR, and VLT/NACO data. We describe the data using two components. The inner, slightly flattened (axis ratio of $q=0.80\pm <!--\; ??\; -->0.04$) component is the nuclear cluster, while the outer component corresponds to the stellar component of the circumnuclear zone. For the nuclear cluster, we measure a half-light radius of $178\pm <!--\; ??\; -->{51}^{\mathrm{\prime <!--\; ???\; -->}\mathrm{\prime <!--\; ???\; -->}}\approx <!--\; ???\; -->7\pm <!--\; ??\; -->2$ pc and a luminosity of $M_{\mathrm{K}\mathrm{s}}=-<!--\; ???\; -->16.0\pm <!--\; ??\; -->0.5$. Second, we measure detailed dynamics out to 4 pc. We obtain 10,351 proper motions from AO data, and 2513 radial velocities from VLT/SINFONI data. We determine the cluster mass by means of isotropic spherical Jeans modeling. We fix the distance to the Galactic Center and the mass of the supermassive black hole. We model the cluster either with a constant M/L or with a power law. For the latter case, we obtain a slope of 1.18 ± 0.06. We get a cluster mass within 100″ of $M_{{100}^{\mathrm{\prime <!--\; ???\; -->}\mathrm{\prime <!--\; ???\; -->}}}=(6.09\pm <!--\; ??\; -->0.53{|}_{\mathrm{f}\mathrm{i}\mathrm{x}{R}_0}\pm <!--\; ??\; -->0.97{|}_{R_0})\times <!--\; ??\; -->{10}^6{M}_{\odot <!--\; ???\; -->}$ for both modeling approaches. A model which includes the observed flattening gives a 47% larger mass (see Chatzopoulos et al.). Our results slightly favor a core over a cusp in the mass profile. By minimizing the number of unbound stars within 8″, we obtain a distance of $R_0={8.53}_{-<!--\; ???\; -->0.15}^{+0.21}$ kpc when using an R₀ supermassive black hole mass relation from stellar orbits. Combining our results, we obtain $M/L=0.51\pm <!--\; ??\; -->0.12M_{\odot <!--\; ???\; -->}/L_{\odot <!--\; ???\; -->,\mathrm{K}\mathrm{s}}$, which is roughly consistent with a Chabrier IMF.

[en] We study the young S-stars within a distance of 0.04 pc from the supermassive black hole in the center of our Galaxy. Given how inhospitable the region is for star formation, their presence is more puzzling the younger we estimate their ages. In this study, we analyze the result of 12 years of high-resolution spectroscopy within the central arcsecond of the Galactic Center (GC). By co-adding between 55 and 105 hr of spectra we have obtained high signal-to-noise H- and K-band spectra of eight stars orbiting the central supermassive black hole. Using deep H-band spectra, we show that these stars must be high surface gravity (dwarf) stars. We compare these deep spectra to detailed model atmospheres and stellar evolution models to infer the stellar parameters. Our analysis reveals an effective temperature of 21,000–28,500 K, a rotational velocity of 60–170 km s⁻¹, and a surface gravity of 4.1–4.2. These parameters imply a spectral type of B0–B3V for these stars. The inferred masses lie within 8–14 $M_{\odot <!--\; ???\; -->}$. We derive an age of ${6.6}_{-<!--\; ???\; -->4.7}^{+3.4}$ Myr for the star S2, which is compatible with the age of the clockwise-rotating young stellar disk in the GC. We estimate the ages of all other studied S-stars to be less than 15 Myr, which is compatible with the age of S2 within the uncertainties. The relatively low ages for these S-stars favor a scenario in which the stars formed in a local disk rather than a field binary-disruption scenario that occurred over a longer period of time.

[en] We present new observations of the recently discovered gas cloud G2 currently falling toward the massive black hole in the Galactic Center. The new data confirm that G2 is on a highly elliptical orbit with a predicted pericenter passage mid-2013. The updated orbit has an even larger eccentricity of 0.966, an epoch of pericenter two months later than estimated before, and a nominal minimum distance of 2200 Schwarzschild radii only. The velocity gradient of G2 has developed further to 600 km s^–1 FWHM in summer 2012. We also detect the tail of similar total flux and on the same orbit as G2 along the trajectory at high significance. No hydrodynamic effects are detected yet, since the simple model of a tidally shearing gas cloud still describes the data very well. The flux of G2 has not changed by more than 10% between 2008 and 2012, disfavoring models where additional gas from a reservoir is released to the disrupting diffuse gas component.