122 research outputs found

    Common Sense for Caring Organizations: Results from a Study of High-Performing Home Care Agencies and Nursing Homes

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    This study reports on results of a qualitative study of 21 high-performing Ohio nursing homes and home care agencies. The study focused on best practices for managing their direct care workforce to achieve high performance. The report includes the most prevalent practice themes as well as tips and management strategies

    The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole

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    Stars with short orbital periods at the center of our galaxy offer a powerful and unique probe of a supermassive black hole. Over the past 17 years, the W. M. Keck Observatory has been used to image the Galactic center at the highest angular resolution possible today. By adding to this data set and advancing methodologies, we have detected S0-102, a star orbiting our galaxy's supermassive black hole with a period of just 11.5 years. S0-102 doubles the number of stars with full phase coverage and periods less than 20 years. It thereby provides the opportunity with future measurements to resolve degeneracies in the parameters describing the central gravitational potential and to test Einstein's theory of General Relativity in an unexplored regime.Comment: Science, in press (published Oct 5, 2012). See Science Online for the Supplementary Material, or here: http://www.astro.ucla.edu/~ghezgroup/gc/research/S02_S0102_orbits.htm

    The Keplerian orbit of G2

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    We give an update of the observations and analysis of G2 - the gaseous red emission-line object that is on a very eccentric orbit around the Galaxy's central black hole and predicted to come within 2400 Rs in early 2014. During 2013, the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes were used to obtain three epochs of spectroscopy and imaging at the highest spatial resolution currently possible in the near-IR. The updated orbital solution derived from radial velocities in addition to Br-Gamma line astrometry is consistent with our earlier estimates. Strikingly, even ~6 months before pericenter passage there is no perceptible deviation from a Keplerian orbit. We furthermore show that a proposed "tail" of G2 is likely not associated with it but is rather an independent gas structure. We also show that G2 does not seem to be unique, since several red emission-line objects can be found in the central arcsecond. Taken together, it seems more likely that G2 is ultimately stellar in nature, although there is clearly gas associated with it.Comment: Proceedings of IAU Symposium #303, "The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus"; 2013 September 30 - October 4, Santa Fe New Mexico (USA

    An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis

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    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 two 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 - 2013) to inform the search for the star in the speckle years (1995 - 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 years) 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 (MbhM_{bh}) and distance (RoR_o) of Sgr A*: Mbh=4.02±0.16±0.04×106 M⊙M_{bh} = 4.02\pm0.16\pm0.04\times10^6~M_{\odot} and 7.86±0.14±0.047.86\pm0.14\pm0.04 kpc. The uncertainties in MbhM_{bh} and RoR_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 ∼\sim2.5 compared to previous results from stellar orbits. This analysis also limits the extended dark mass within 0.01 pc to less than 0.13×106 M⊙0.13\times10^{6}~M_{\odot} at 99.7% confidence, a factor of 3 lower compared to prior work.Comment: 56 pages, 14 figures, accepted to Ap

    Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?

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    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 (2006-2012) 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 Brγ 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 Brγ 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, 1600 R_s), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L' astrometry. It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L' continuum and the Brγ line emission appears unresolved in almost all epochs, which implies that the bulk of the emission resides in a compact region. The observations altogether suggest that while G2 has a gaseous component that 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

    Testing the gravitational theory with short-period stars around our Galactic Center

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    Motion of short-period stars orbiting the supermassive black hole in our Galactic Center has been monitored for more than 20 years. These observations are currently offering a new way to test the gravitational theory in an unexplored regime: in a strong gravitational field, around a supermassive black hole. In this proceeding, we present three results: (i) a constraint on a hypothetical fifth force obtained by using 19 years of observations of the two best measured short-period stars S0-2 and S0-38 ; (ii) an upper limit on the secular advance of the argument of the periastron for the star S0-2 ; (iii) a sensitivity analysis showing that the relativistic redshift of S0-2 will be measured after its closest approach to the black hole in 2018.Comment: 4 pages, 2 figures, proceedings of the 52nd Rencontres de Moriond, Gravitation Sessio
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