Composite grains: Application to circumstellar dust

Using the discrete dipole approximation (DDA) we calculate the absorption efficiency of the composite grain, made up of a host silicate spheroid and inclusions of graphite, in the spectral region 5.0-25.0 μm. We study the absorption as a function of thevolume fraction of the inclusions. In particular, we study the variation in the 10.0 μm and 18.0 μm emission features with the volume fraction of the inclusions. Using the extinction efficiencies, of the composite grains we calculate the infrared fluxes at several dust temperatures and compare the model curves with the observed infrared emission curves (IRAS-LRS), obtained for circumstellar dust shells around oxygen rich M-type stars

Quasinormal mode characterization of evaporating mini black holes

According to recent theoretical developments, it might be possible to produce mini black holes in the high energy experiments in the LHC at CERN. We propose here a model based on the $n$-dimensional Vaidya metric in double null coordinates for these decaying black holes. The associated quasinormal modes are considered. It is shown that only in the very last instants of the evaporation process the stationary regime for the quasinormal modes is broken, implying specific power spectra for the perturbations around these mini black-holes. From scattered fields one could recover, in principle, the black hole parameters as well as the number of extra dimensions. The still mysterious final fate of such objects should not alter significantly our main conclusions.Comment: v4: 9 pages, 3 figures. Minor correction

A Spherically Symmetric Closed Universe as an Example of a 2D Dilatonic Model

We study the two-dimensional (2D) dilatonic model describing a massless scalar field minimally coupled to the spherically reduced Einstein-Hilbert gravity. The general solution of this model is given in the case when a Killing vector is present. When interpreted in four dimensions, the solution describes either a static or a homogeneous collision of incoming and outgoing null dust streams with spherical symmetry. The homogeneous Universe is closed.Comment: 5 pages, 2 figures, to appear in Physical Review

A radiating dyon solution

We give a non-static exact solution of the Einstein-Maxwell equations (with null fluid), which is a non-static magnetic charge generalization to the Bonnor-Vaidya solution and describes the gravitational and electromagnetic fields of a nonrotating massive radiating dyon. In addition, using the energy-momentum pseudotensors of Einstein and Landau and Lifshitz we obtain the energy, momentum, and power output of the radiating dyon and find that both prescriptions give the same result.Comment: 9 pages, LaTe

Entropy and Correlation Functions of a Driven Quantum Spin Chain

We present an exact solution for a quantum spin chain driven through its critical points. Our approach is based on a many-body generalization of the Landau-Zener transition theory, applied to fermionized spin Hamiltonian. The resulting nonequilibrium state of the system, while being a pure quantum state, has local properties of a mixed state characterized by finite entropy density associated with Kibble-Zurek defects. The entropy, as well as the finite spin correlation length, are functions of the rate of sweep through the critical point. We analyze the anisotropic XY spin 1/2 model evolved with a full many-body evolution operator. With the help of Toeplitz determinants calculus, we obtain an exact form of correlation functions. The properties of the evolved system undergo an abrupt change at a certain critical sweep rate, signaling formation of ordered domains. We link this phenomenon to the behavior of complex singularities of the Toeplitz generating function.Comment: 16 pgs, 7 fg