The Expansion Center and Dynamical Age of the Galactic Supernova Remnant Cassiopeia A

We present proper motions for 21 bright main shell and 17 faint, higher-velocity, outer ejecta knots in the Cas A supernova remnant and use them to derive new estimates for the remnant's expansion center and age. Our study included 1951 - 1976 Palomar 5 m prime focus plates, 1988 - 1999 CCD images from the KPNO 4 m and MDM 2.4 m telescopes, and 1999 HST WFPC2 images. Measurable positions covered a 23 to 41 yr time span for most knots, with a few outer knots followed for almost 48 yr. We derive an expansion center of alpha = 23h 23m 27s.77 +- 0s.05, delta = 58d 48' 49".4 +- 0".4 (ICRS), with little difference between centers derived using outer or main shell knots. This position is 3.0 arcsec due north of that estimated by van den Bergh and Kamper (1983). It also lies 6.6 +- 1.5 arcsec almost due north (PA = 354 deg) of the remnant's recently-detected central X-ray point source, implying a transverse velocity for the X-ray point source of about 330 km/s at a distance of 3.4 kpc. Using the knots which lie out ahead of the remnant's forward blast wave, we estimate a knot convergent date of A.D. 1671.3 +- 0.9, assuming no deceleration. However, a deceleration of just approximately 1.6 km/(s yr) over a 300 yr time span would produce an explosion date A.D. 1680, consistent with the suspected sighting of the Cas A supernova by J. Flamsteed.Comment: Astron. Journal in press, sched. July 2001. AASTex5, 17 pages, 2 jpeg greyscale figures, 3 postscript figure

Shearfree perfect fluids with solenoidal magnetic curvature and a gamma-law equation of state

We show that shearfree perfect fluids obeying an equation of state p=(gamma -1) mu are non-rotating or non-expanding under the assumption that the spatial divergence of the magnetic part of the Weyl tensor is zero.Comment: 11 page

Rotating solenoidal perfect fluids of Petrov type D

We prove that aligned Petrov type D perfect fluids for which the vorticity vector is not orthogonal to the plane of repeated principal null directions and for which the magnetic part of the Weyl tensor with respect to the fluid velocity has vanishing divergence, are necessarily purely electric or locally rotationally symmetric. The LRS metrics are presented explicitly.Comment: 6 pages, no figure

Shear-free perfect fluids with a solenoidal electric curvature

We prove that the vorticity or the expansion vanishes for any shear-free perfect fluid solution of the Einstein field equations where the pressure satisfies a barotropic equation of state and the spatial divergence of the electric part of the Weyl tensor is zero.Comment: 9 page

Purely radiative perfect fluids

We study `purely radiative' (div E = div H = 0) and geodesic perfect fluids with non-constant pressure and show that the Bianchi class A perfect fluids can be uniquely characterized --modulo the class of purely electric and (pseudo-)spherically symmetric universes-- as those models for which the magnetic and electric part of the Weyl tensor and the shear are simultaneously diagonalizable. For the case of constant pressure the same conclusion holds provided one also assumes that the fluid is irrotational.Comment: 12 pages, minor grammatical change

Political market and regulatory uncertainty: Insights and implications for integrated strategy

Managers can craft effective integrated strategy by properly assessing regulatory uncertainty. Leveraging the existing political markets literature, we predict regulatory uncertainty from the novel interaction of demand and supply side rivalries across a range of political markets. We argue for two primary drivers of regulatory uncertainty: ideology-motivated interests opposed to the firm and a lack of competition for power among political actors supplying public policy. We align three, previously disparate dimensions of nonmarket strategy - profile level, coalition breadth, and pivotal target - to levels of regulatory uncertainty. Through this framework, we demonstrate how and when firms employ different nonmarket strategies. To illustrate variation in nonmarket strategy across levels of regulatory uncertainty, we analyze several market entry decisions of foreign firms operating in the global telecommunications sector

Near-Infrared Adaptive Optics Imaging of the Central Regions of Nearby Sc Galaxies: I. M33

Near-infrared images obtained with the Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) are used to investigate the stellar content within 18 arcsec of the center of the Local Group spiral galaxy M33. AGB stars with near-infrared spectral-energy distributions similar to those of giants in the solar neighborhood and Baade's Window are detected over most of the field. The bolometric luminosity function (LF) of these stars has a discontinuity near M_{bol} = -5.25, and comparisons with evolutionary tracks suggest that most of the AGB stars formed in a burst of star formation 1 - 3 Gyr in the past. The images are also used to investigate the integrated near-infrared photometric properties of the nucleus and the central light concentration. The nucleus is bluer than the central light concentration, in agreement with previous studies at visible wavelengths. The CO index of the central light concentration 0.5 arcsec from the galaxy center is 0.05, which corresponds to [Fe/H] = -1.2 for simple stellar systems. Hence, the central light concentration could not have formed from the chemically-enriched material that dominates the present-day inner disk of M33.Comment: 23 pages of text + 11 figures; to appear in A

Changing priorities in maternal and perinatal health in Gert Sibande District, South Africa

Gert Sibande District is a predominantly rural district in Mpumalanga Province, South Africa (SA), with a population of just over 1.1 million. It has a high prevalence of HIV infection and pregnancy-related hypertensive disease. In 2010 the district was one of the worst-performing health districts in SA, with a maternal mortality ratio of 328.0 per 100 000 births. Various programmes were introduced between 2010 and 2017 to address major causes of maternal and perinatal morbidity and mortality in the district. The focus has been on HIV-related morbidity, the direct obstetric causes of maternal and perinatal morbidity and mortality, and health systems strengthening. During the period 2010 - 2017, there was a steady decline in institutional maternal mortality with a drop of 71% in maternal deaths over a period of 6 years, from 328.0 per 100 000 births to 95.0. However, the ratio levelled off in 2016 and 2017, mainly as a result of a changing disease profile. The stillbirth rate showed a decline of 24.4% over a period of 8 years. With perseverance, rapid response and evidence-based strategies it was possible to more than halve the institutional maternal mortality ratio within 6 years. However, with the changing disease profile, conditions such as hypertensive disease in pregnancy should be prioritised and new strategies developed to further reduce maternal and perinatal mortality and morbidity

Silent universes with a cosmological constant

We study non-degenerate (Petrov type I) silent universes in the presence of a non-vanishing cosmological constant L. In contrast to the L=0 case, for which the orthogonally spatially homogeneous Bianchi type I metrics most likely are the only admissible metrics, solutions are shown to exist when L is positive. The general solution is presented for the case where one of the eigenvalues of the expansion tensor is 0.Comment: 11 pages; several typos corrected which were still present in CGQ version; minor change