We present new, more precise measurements of the mass and distance of our Galaxy's central supermassive black hole, Sgr A * . These results stem from a new analysis that more than doubles the time baseline for astrometry of faint stars orbiting Sgr A * , combining 2decades of speckle imaging and adaptive optics data. Specifically, we improve our analysis of the speckle images by using information about a star's orbit from the deep adaptive optics data (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) to inform the search for the star in the speckle years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). When this new analysis technique is combined with the first complete re-reduction of Keck Galactic Center speckle images using speckle holography, we are able to track the short-period star S0-38 (K-band magnitude=17, orbital period=19 yr) through the speckle years. We use the kinematic measurements from speckle holography and adaptive optics to estimate the orbits of S0-38 and S0-2 and thereby improve our constraints of the mass (M bh ) and distance (R o ) of Sgr A * : M bh = (4.02±0.16±0.04) ×10 6 M e and 7.86±0.14±0.04 kpc. The uncertainties in M bh and R o as determined by the combined orbital fit of S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, compared to an orbital fit of S0-2 alone and a factor of ∼2.5 compared to previous results from stellar orbits. This analysis also limits the extended dark mass within 0.01 pc to less than 0.13×10 6 M e at 99.7% confidence, a factor of 3 lower compared to prior work.

The nucleation process is crucial to many phase transitions, but its kinetics are difficult to predict and measure. We superheated and melted the interior of thermal-sensitive colloidal crystals and investigated by means of video microscopy the homogeneous melting at single-particle resolution. The observed nucleation precursor was local particle-exchange loops surrounded by particles with large displacement amplitudes rather than any defects. The critical size, incubation time, and shape and size evolutions of the nucleus were measured. They deviate from the classical nucleation theory under strong superheating, mainly because of the coalescence of nuclei. The superheat limit agrees with the measured Born and Lindemann instabilities. C rystal melting is of considerable fundamental and practical importance to science and technology. Yet, our understanding of it is still rather incomplete. Thermodynamics reveals to us the equilibrium phases, but the kinetics of phase transitions have proved difficult to predict (1-3). Crystals melt heterogeneously from surfaces or grain boundaries once they are heated to the melting point (3, 4). By suppressing surface melting (5-8), a single crystal can be superheated to temperatures above its melting point. This metastable state will eventually melt from the interior without any preferential sites. In such homogeneous melting, small liquid nuclei form spontaneously by thermal fluctuations. A spherical nucleus has the free energy (9) DG ¼ 4pr 2 g − 4 3 pr 3 nDm þ E strain ð1Þ where r is the radius, g is the surface tension, n is the number density of the nucleus, ∆m (>0) is the chemical potential difference between the super-heated crystal and liquid, E strain ¼ 4 3 pr 3 nDD is the misfit strain energy in the crystal caused by the volume change of the nucleus, and ∆D is the mean strain energy per particle from continuum elasticity and is not expected to depend on r. E strain is zero when the parent phase is fluid but is finite when the parent phase is solid (10). To minimize ∆G, small liquid nuclei tend to recrystallize rather than grow unless their size exceeds a critical value r* = 2g/[(∆m − ∆)n] corresponding to the barrier height of ∆G. The small length and time scales of the nu-cleation process preclude observation at the single-particle level in molecular systems. In contrast, micrometer-sized colloidal particles can serve as good model systems for phase transition studies because their thermal motions can be directly visualized and measured with video microscopy (11, 12). Crystallization (13), sublimation (14), and heterogeneous melting in polycrystals (4, 15) have been studied in colloids. We used thermal-sensitive N-isopropylacrylamide (NIPA) microgel colloidal spheres (4), whose effective diameter s linearly changes from 0.76 mm at 26.4°C to 0.67 mm at 30.6°C (10). The effective diameter is defined so that the melting volume fraction f m = 54.5%, which is the same as that of the hard spheres. By this definition, the measured freezing point f f = 49%, which is close to ...

We demonstrate that short-period stars orbiting around the supermassive black hole in our Galactic center can successfully be used to probe the gravitational theory in a strong regime. We use 19 years of observations of the two best measured short-period stars orbiting our Galactic center to constrain a hypothetical fifth force that arises in various scenarios motivated by the development of a unification theory or in some models of dark matter and dark energy. No deviation from general relativity is reported and the fifth force strength is restricted to an upper 95% confidence limit of jαj < 0.016 at a length scale of λ ¼ 150 astronomical units. We also derive a 95% confidence upper limit on a linear drift of the argument of periastron of the short-period star S0-2 of j _ ω S0-2 j < 1.6 × 10 −3 rad=yr, which can be used to constrain various gravitational and astrophysical theories. This analysis provides the first fully self-consistent test of the gravitational theory using orbital dynamic in a strong gravitational regime, that of a supermassive black hole. A sensitivity analysis for future measurements is also presented.

We report new observations of the Galactic Center source G2 from the W. M. Keck Observatory. G2 is a dusty red object associated with gas that shows tidal interactions as it nears closest approach with the Galaxy's central black hole. Our observations, conducted as G2 passed through periapse, were designed to test the proposal that G2 is a 3 earth mass gas cloud. Such a cloud should be tidally disrupted during periapse passage. The data were obtained using the Keck II laser guide star adaptive optics system (LGSAO) and the facility near-infrared camera (NIRC2) through the K' [2.1 µm] and L' [3.8 µm] broadband filters. Several results emerge from these observations: 1) G2 has survived its closest approach to the black hole as a compact, unresolved source at L'; 2) G2's L' brightness measurements are consistent with those over the last decade; 3) G2's motion continues to be consistent with a Keplerian model. These results rule out G2 as a pure gas cloud and imply that G2 has a central star. This star has a luminosity of ∼30 L ⊙ and is surrounded by a large (∼2.6 AU) optically thick dust shell. The differences between the L' and Br-γ observations can be understood with a model in which L' and Br-γ emission arises primarily from internal and external heating, respectively. We suggest that G2 is a binary star merger product and will ultimately appear similar to the B-stars that are tightly clustered around the black hole (the so-called S-star cluster).

We present new observations and analysis of G2 -the intriguing red emission-line object which is quickly approaching the Galaxy's central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes and include spectroscopy (R ∼ 3600) centered on the Hydrogen Brγ line as well as K ′ (2.1 µm) and L ′ (3.8 µm) imaging. Analysis of these observations shows the Br-γ line emission has a positional offset from the L ′ continuum. This offset is likely due to background source confusion at L ′ . We therefore present the first orbital solution derived from Br-γ line astrometry, which when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 ± 0.14), closer periastron (130 AU, 1900R s ), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L ′ astrometry. The new orbit casts doubt on previous associations of G2 and a low surface brightness "tail." It is shown that G2 has no K ′ counterpart down to K ′ ∼20 mag. G2's L ′ continuum and the Br-γ line-emission is unresolved in almost all epochs; however it is marginally extended in our highest quality Br-γ data set from 2006 and exhibits a clear velocity gradient at that time. While the observations altogether suggest that G2 has a gaseous component which is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object.