When all thermonuclear sources of energy are exhausted a sufficiently heavy star will collapse. Unless fission due to rotation, the radiation of mass, or the blowing off of mass by radiation, reduce the star's mass to the order of that of the sun, this contraction will continue indefinitely. In the present paper we study the solutions of the gravitational field equations which describe this process. In I, general and qualitative arguments are given on the behavior of the metrical tensor as the contraction progresses: the radius of the star approaches asymptotically its gravitational radius; light from the surface of the star is progressively reddened, and can escape over a progressively narrower range of angles. In II, an analytic solution of the field equations confirming these general arguments is obtained for the case that the pressure within the star can be neglected. The total time of collapse for an observer comoving with the stellar matter is finite, and for this idealized case and typical stellar masses, of the order of a day; an external observer sees the star asymptotically shrinking to its gravitational radius.

On Continued Gravitational Contraction

We report on the design and performance of the Keck Cosmic Web Imager (KCWI), a general purpose optical integral field spectrograph that has been installed at the Nasmyth port of the 10 m Keck II telescope on Mauna Kea, HI. The novel design provides blue-optimized seeing-limited imaging from 350-560 nm with configurable spectral resolution from 1000 - 20000 in a field of view up to 20"x33". Selectable volume phase holographic (VPH) gratings and high performance dielectric, multilayer silver and enhanced aluminum coatings provide end-to-end peak efficiency in excess of 45% while accommodating the future addition of a red channel that will extend wavelength coverage to 1 micron. KCWI takes full advantage of the excellent seeing and dark sky above Mauna Kea with an available nod-and-shuffle observing mode. The instrument is optimized for observations of faint, diffuse objects such as the intergalactic medium or cosmic web. In this paper, a detailed description of the instrument design is provided with measured performance results from the laboratory test program and ten nights of on-sky commissioning during the spring of 2017. The KCWI team is lead by Caltech and JPL (project management, design and implementation) in partnership with the University of California at Santa Cruz (camera optical and mechanical design) and the W. M. Keck Observatory (observatory interfaces).

/pdf/the-keck-cosmic-web-imager-integral-field-spectrograph-5fdffuyivb.pdf

The Keck Cosmic Web Imager Integral Field Spectrograph

The European Pulsar Timing Array is a collaboration that has recently been formed between the five major radio observatories in Europe: Jodrell Bank, Effelsberg, Westerbork, Nancay and Sardinia. Together we work towards detecting gravitational waves. We combine the individual strengths of all the different observatories to obtain improved results. We give a short introduction on the partners, goals and instrumentation of this collaboration. Besides gravitational wave detection, the EPTA collaboration is sharing data to optimize timing on, for example, millisecond binary pulsars. We present some recent results of combining datasets of the four telescopes now in use for the EPTA.

The European Pulsar Timing Array

We investigate self-similar magnetohydrodynamic (MHD) processes in an isothermal self-gravitating fluid with a quasi-spherical symmetry and extend the envelope expansion with core collapse (EECC) solutions of Lou & Shen by incorporating a random magnetic field. Stagnation surfaces of EECC solutions that seperate core collapse and envelope expansion propagate at constant speeds either sub-magnetosonically or super-magnetosonically. Crossing the magnetosonic line twice analytically, there exists an infinite number of discrete magnetized EECC and ECCC solutions. In addition to the EECC shock solution which could change the central accretion rate, the magnetic field can also affect the core accretion rate. As the magnetic parameter $\lambda$ increases, the core accretion rate appropriate for the MHD EWCS becomes larger. Under the frozen-in approximation, magnetic fields in the envelope expansion portion would scale as $B\propto r^{-1}$, while in the core collapse portion they would scale as $B\propto r^{-1/2}$. We discuss several astrophysical applications of EECC similarity solutions to the formation process of proto-planetary nebulae connecting the AGB phase and the planetary nebula, to supernova remnants, collapse of magnetized molecular cloud, to $\textrm{H}\ {\textrm{\small{II}}}$ clouds surrounding massive OB stars and to a certain evolution phase of galaxy clusters.

Envelope Expansion with Core Collapse II. Quasi-Spherical Self-Similar Solutions for an Isothermal Magnetofluid

Re-examination of several key aspects of an earlier model analysis on cylinder hydrodynamics under self-gravity

\n We study the self-similar spherical hydrodynamics of γ = 4/3, with or without shocks, of general polytropic (GP) relativistically degenerate or hot matter in global mass reservoirs under the Paczynski–Wiita (PW) gravity that retains essential general relativistic (GR) effects for dynamic accretions on to growing Schwarzschild black holes (BHs). Various self-similar dynamic solutions are derived and constructed. Among others, one important solution type is the expansion-wave collapse solution (EWCS) with a central mass-growing BH event horizon in expansion. Such GP dynamic collapses are highly efficient for the rapid dynamic growth of supermassive and hypermassive BHs (SMBHs and HMBHs) in the Universe including the early Universe, in contrast to much slower disc mass accretion and BH merger processes. Particularly, the extra degree of freedom for the effective pressure of those relativistically degenerate or hot mass reservoirs makes it possible to construct EWCSs with even higher efficiency for BH growths. Pertinent astrophysical applications of our model also include dynamic core collapses of massive or very massive stars or supermassive stars and compact objects as well as supernova explosions. Under joint actions of effective pressure and the PW gravity, self-similar dynamic solutions with central expanding spherical voids or cavities with either sharp or smooth density transitions across their rims can emerge. Dynamic expansion shocks are also constructed and examined.

Dynamic spherical collapses towards growing black holes in relativistically degenerate or hot host mass reservoirs

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Dynamic spherical collapses towards growing black holes in relativistically degenerate or hot host mass reservoirs