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• Ashkbiz Danehkar

Research Interests

photoionization modeling of planetary nebulae

A planetary nebula is ionized plasma clouds, produced from a hot central star of low to intermediate mass that is towards the end of its evolution. Its origin has been the subject of several investigations in recent years. It is a key point to understand the late evolution of low- and intermediate-mass stars and the physical mechanism of mass loss for stellar evolution. (Iben, 1995) Its properties of low-density dusty plasma may answer the remaining questions about the chemical composition of the interstellar medium and the chemical enrichment of our galaxy. (Parker et al., 2006)

There are two possible mechanisms for the formation of the planetary nebulae: single stars and binary central stars. The first mechanism involves a combination of magnetic fields and stellar rotation, which produces an axisymmetric mass-loss and expends stellar winds. (Blackman, 2004; Matt et al., 2004) However, we observe certain non-spherical shapes arguing for the binary scenario. (De Marco, 2006) Hence, planetary nebulae depict a remarkably diverse range of shapes, ranging from axisymmetric to aspherical forms.

In order to understand the nebular morphology, we need to determine the spectral-lines. It also involves the surface brightness of the nebula, since morphologies can be classified under the surface brightness to radius. (Stanghellini et al., 2002) The surface-brightness radius relation describes other nebular parameters such as the ionized mass and mean electron density. It is important to formulate a photoionization model, which can be applied to the surface-brightness radius relation. The spectral-lines can be then interpreted using the photoionization modeling analysis to match the nebular morphologies with observations.

Nonlinear phenomena in Plasma Physics

The propagation of acoustic nonlinear excitations in plasmas is particularly important, which lies at the intersection of plasma physics and nonlinear dynamics. We have observed the formation of solitary waves, which are associated with the mutual compensation between nonlinearity and dispersion. This topic is of particular interest since electron-accoutsic solitary waves often occur in space plasmas e.g. the Earth's bow shock, (Thomsen at al., 1983) the auroral magnetosphere, (Tokar & Gary, 1984) and the Broadband Electrostatic Noise. (Matsumoto at al., 1994)

I recently studied the electron-acoustic waves (EAWs) in the presence of suprathermal electrons background. (Danehkar et al., 2011) This investigation showed how the existence domains of EAWs and their characteristics depend on relevant intrinsic parameters, i.e. plasma composition, temperature, and superthermality index.

BRST Quantization in Quantum Field Theory

The BRST formalism presents the local gauge symmetry as being a replacement for the original gauge symmetry. The formalism was firstly introduced to the field theories by Becchi, Rouet, Stora, and Tyutin. The BRST formalism provided useful way of studying the perturbative renormalization, (Gomis & Weinberg, 1996) simultaneously local and rigid symmetries of a gauge-invariant theory, (Brandt at al., 1996) and the consistent interactions in gauge theories in terms of the deformation of the solution to the master equation. (Barnich & Henneaux, 1993)

I conducted research on the BRST couplings between a background field (BF) and dual formulation of linearized gravity. This investigation revealed that the dual formulation of linearized gravity is coupled to the topological BF model in five dimensions.

Covariant approach to general relativity

In a recent work, I investigated the dynamic equations governing the Weyl curvature by using the (3 + 1)-covariant approach to general relativity. In this approach, we rewrite equations governing relativistic fluid dynamics by using project vector and project symmetric traceless tensors instead of metrics. The results showed that the nonlocal interactions (tidal force and gravitational wave) are inconsistent without the magnetic part of the Weyl curvature.

In the theory of general relativity, one can split the Riemann curvature tensor into the Ricci tensor defined by the Einstein equation and the Weyl curvature tensor. Additionally, one can split the Weyl tensor into the electric part and the magnetic part, being due to some similarity to electrodynamical counterparts. (Thorne, 1980; Pirani, 1957) The Weyl curvature tensor describes the nonlocal long-range interactions as enabling gravitational act at a distance (tidal forces and gravitational waves). (Kofman & Pogosayn, 1995)