6,721 research outputs found

    The Position of Sgr A∗^* at the Galactic Center

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    The absolute position of the compact radio source at the dynamical center of the Galaxy, Sgr A∗^*, was known only to an accuracy of 0.2′′0.2'' in spite of its accurate location with respect to near-IR stellar sources to within 30 milliarcsecond (mas). To remedy this poor positional accuracy, we have selected 15 high-resolution, high-frequency VLA observations of Sgr A∗^* carried out in the last 13 years and determined the weighted average position with the average epoch 1992.4 to be at α\alpha, δ\delta[1950] = 17h42m17^{\rm h} 42^{\rm m} 29\dsec3076±0.0007\pm0.0007, −28∘59′18.484±0.014′′-28^\circ 59^\prime 18.484\pm0.014^{\prime\prime}, or α\alpha, δ\delta [2000] = 17h45m17^{\rm h} 45^{\rm m} 40\dsec0383±0.0007\pm0.0007, −29∘00′28.069±0.014′′-29^\circ 00^\prime 28.069\pm0.014^{\prime\prime} which agrees with earlier published values to within the 0.2′′0.2'' error bars of the earlier measurements. An accurate absolute position of Sgr A∗^* can be useful for its identification with sources at other wavelengths, particularly, in soft and hard X-rays with implications for the models of a massive black hole at the Galactic center.Comment: 11 pages, one figure and one table. ApJL (in press

    PT-symmetry broken by point-group symmetry

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    We discuss a PT-symmetric Hamiltonian with complex eigenvalues. It is based on the dimensionless Schr\"{o}dinger equation for a particle in a square box with the PT-symmetric potential V(x,y)=iaxyV(x,y)=iaxy. Perturbation theory clearly shows that some of the eigenvalues are complex for sufficiently small values of ∣a∣|a|. Point-group symmetry proves useful to guess if some of the eigenvalues may already be complex for all values of the coupling constant. We confirm those conclusions by means of an accurate numerical calculation based on the diagonalization method. On the other hand, the Schr\"odinger equation with the potential V(x,y)=iaxy2V(x,y)=iaxy^{2} exhibits real eigenvalues for sufficiently small values of ∣a∣|a|. Point group symmetry suggests that PT-symmetry may be broken in the former case and unbroken in the latter one

    Visualizing, rather than deriving Russell-Saunders terms : a classroom activity with quantum numbers

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    A one hour classroom activity is presented, aimed at consolidating the concepts of microstates and Russell-Saunders energy terms in transition metal atoms and coordination complexes. The unconventional approach, based on logic and intuition rather than rigorous mathematics, is designed to stimulate discussion and enhance familiarity with quantum numbers in classes of Chemistry undergraduate students

    Some Service Failures arising from Various Types of Corrosion

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    The Research Laboratory of the Development and Research Department of The Mond Nickel Company Limited, has been concerned is developing new heat and corrosion-resistant alloys to meet the demands of designers of gas-turbine chemical plant and electrical equipment.Premature service failures in these materials are not common, and when they do occur they can usually be traced to misuse or mal-treatment of the material during fabrication or appli-cation, or to some form of accidental contamination or damage. Important lessons have nevertheless been learned from post-mortem examinations of service failures, and the following five examples have been selected as of interest not only to works' metallurgists but also to designers, to whom these special materials offer new possibilities, and to fabricators, to whom they present certain problems

    Surveillance of healthcare-associated infection in hospitalised South African children: Which method performs best?

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    Background. In 2012, the South African (SA) National Department of Health mandated surveillance of healthcare-associated infection (HAI), but made no recommendations of appropriate surveillance methods.Methods. Prospective clinical HAI surveillance (the reference method) was conducted at Tygerberg Children’s Hospital, Cape Town, from 1 May to 31 October 2015. Performance of three surveillance methods (point prevalence surveys (PPSs), laboratory surveillance and tracking of antimicrobial prescriptions) was compared with the reference method using surveillance evaluation guidelines. Factors associated with failure to detect HAI were identified by logistic regression analysis.Results. The reference method detected 417 HAIs among 1 347 paediatric hospitalisations (HAI incidence of 31/1000 patient days; 95% confidence interval (CI) 28.2 - 34.2). Surveillance methods had variable sensitivity (S) and positive predictive value (PPV): PPS S = 24.9% (95% CI 21 - 29.3), PPV = 100%; laboratory surveillance S = 48.4% (95% CI 43.7 - 53.2), PPV = 55.2% (95% CI 50.1 - 60.2); and antimicrobial prescriptions S = 66.4% (95% CI 61.8 - 70.8%), PPV = 88.5% (95% CI 84.5 - 91.6). Combined laboratory-antimicrobial surveillance achieved superior HAI detection (S = 84.7% (95% CI 80.9 - 87.8%), PPV = 97% (95% CI 94.6 - 98.4%)). Factors associated with failure to detect HAI included patient transfer (odds ratio (OR) 2.0), single HAI event (OR 2.8), age category 1 - 5 years (OR 2.1) and hospitalisation in a general ward (OR 2.3).Conclusions. Repeated PPSs, laboratory surveillance and/or antimicrobial prescription tracking are feasible HAI surveillance methods for low-resource settings. Combined laboratory-antimicrobial surveillance achieved the best sensitivity and PPV. SA paediatric healthcare facilities should individualise HAI surveillance, selecting a method suited to available resources and practice context

    Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-like Objects near Sgr A*

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    We present radio images within 30′′'' of Sgr A* based on recent VLA observations at 34 GHz with 7.8 microJy sensitivity and resolution ∼88×46\sim88\times46 milliarcseconds (mas). We report 44 partially resolved compact sources clustered in two regions in the E arm of ionized gas that orbits Sgr A*. These sources have size scales ranging between ~50 and 200 mas (400 to 1600 AUs), and a bow-shock appearance facing the direction of Sgr A*. Unlike the bow-shock sources previously identified in the near-IR but associated with massive stars, these 34 GHz sources do not appear to have near-IR counterparts at 3.8 μ\mum. We interpret these sources as a candidate population of photoevaporative protoplanetary disks (proplyds) that are associated with newly formed low mass stars with mass loss rates ~10^{-7} - 10^{-6} solar mass per year and are located at the edge of a molecular cloud outlined by ionized gas. The disks are externally illuminated by strong Lyman continuum radiation from the ~100 OB and WR massive stars distributed within 10'' of Sgr A*. The presence of proplyds implies current in-situ star formation activity near Sgr A* and opens a window for the first time to study low mass star, planetary and brown dwarf formations near a supermassive black hole.Comment: 13 pages, 4 figures, ApJL (in press

    Compact Radio Sources within 30" of Sgr A*: Proper Motions, Stellar Winds and the Accretion Rate onto Sgr A*

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    Recent broad-band 34 and 44 GHz radio continuum observations of the Galactic center have revealed 41 massive stars identified with near-IR counterparts, as well as 44 proplyd candidates within 30" of Sgr A*. Radio observations obtained in 2011 and 2014 have been used to derive proper motions of eight young stars near Sgr A*. The accuracy of proper motion estimates based on near-IR observations by Lu et al. and Paumard et al. have been investigated by using their proper motions to predict the 2014 epoch positions of near-IR stars and comparing the predicted positions with those of radio counterparts in the 2014 radio observations. Predicted positions from Lu et al. show an rms scatter of 6 mas relative to the radio positions, while those from Paumard et al. show rms residuals of 20 mas, which is mainly due to uncertainties in the IR-based proper motions. Under the assumption of homogeneous ionized winds, we also determine the mass-loss rates of 11 radio stars, finding rates that are on average ∼\sim2 times smaller than those determined from model atmosphere calculations and near-IR data. Clumpiness of ionized winds would reduce the mass loss rate of WR and O stars by additional factors of 3 and 10, respectively. One important implication of this is a reduction in the expected mass accretion rate onto Sgr A* from stellar winds by nearly an order of magnitude to a value of few×10−7\times10^{-7} \msol\ yr−1^{-1}. Finally, we present the positions of 318 compact 34.5 GHz radio sources within 30\arcs\ of Sgr A*. At least 45 of these have stellar counterparts in the near-IR KsK_s (2.18 μ\mum) and L′L' (3.8μ\mum) bands.Comment: 30 pages, 4 figures, ApJ (in press

    ALMA and VLA Observations: Evidence for Ongoing Low-mass Star Formation near Sgr A*

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    Using the VLA, we recently detected a large number of protoplanetary disk (proplyd) candidates lying within a couple of light years of the massive black hole Sgr A*. The bow-shock appearance of proplyd candidates point toward the young massive stars located near Sgr A*. Similar to Orion proplyds, the strong UV radiation from the cluster of massive stars at the Galactic center is expected to photoevaporate and photoionize the circumstellar disks around young, low mass stars, thus allowing detection of the ionized outflows from the photoionized layer surrounding cool and dense gaseous disks. To confirm this picture, ALMA observations detect millimeter emission at 226 GHz from five proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We present the derived disk masses for four sources as a function of the assumed dust temperature. The mass of protoplanetary disks from cool dust emission ranges between 0.03 -- 0.05 solar mass. These estimates are consistent with the disk masses found in star forming sites in the Galaxy. These measurements show the presence of on-going star formation with the implication that gas clouds can survive near Sgr A* and the relative importance of high vs low-mass star formation in the strong tidal and radiation fields of the Galactic center.Comment: 13 pages, 3 figures, MNRAS (in press

    The Variability of Polarized Radiation from Sgr A*

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    Sgr A* is variable at radio and submillimeter wavelengths on hourly time scales showing time delays between the peaks of flare emission as well as linearly polarized emission at millimeter and sub-mm wavelengths. To determine the polarization characteristics of this variable source at radio frequencies, we present VLA observations of Sgr A* and report the detection of polarized emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz, respectively. The change in the time averaged polarization angle between 22 and 43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping (or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves of the degree of linearly polarized emission suggests a a correlation with the variability of the total intensity at 43 GHz. The available polarization data at radio and sub-mm wavelengths suggest that the rotation measure decreases with decreasing frequency. This frequency dependence, and observed changes in polarization angle during flare events, may be caused by the reduction in rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
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