Ashkbiz Danehkar is a postdoctoral fellow at Harvard-Smithsonian Center for Astrophysics. He obtained his Ph.D. in Physics and Astronomy from Macquarie University. His research interests include theoretical physics, plasma physics and observational astronomy. Ashkbiz Danehkar has carried out research on observational astronomy at Macquarie University since 2010, and he has been involved with the studies of planetary nebulae: physical conditions, chemical abundances, kinematics and ionization structures. During his postgraduate studies in Plasma Physics at Queen's University Belfast, he studied linear and nonlinear structures in multi-species suprathermal plasmas using numerical methods in 2009. He was an early-stage researcher with the University of Craiova in 2008, involved in quantum field theory and constrained dynamics for theoretical physics.
Dr. Ashkbiz Danehkar
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, MS-70
Cambridge, MA 02138, USA
Office: B-306A, Phone: +1-617-955-0606
10 nights at Siding Spring Observatory
The Ultra-Fast Outflow of the Quasar PG1211+143 as Viewed by Time-Averaged Chandra Grating Spectroscopy
Danehkar, A., Nowak, M. A.,
Lee, J. C. and et al.
Fast, Low-ionization Emission Regions of the Planetary Nebula M2-42
Danehkar, A., Parker, Q. A.,
and Steffen, W.
Observations and three-dimensional ionization structure of the planetary nebula SuWt 2
Danehkar, A., Parker, Q. A.,
and Ercolano, B.
Electron-acoustic solitary waves in the presence of a suprathermal electron component
Danehkar, A., Saini, N. S.,
Hellberg, M. A. and Kourakis, I.
The centers of active galactic nuclei (AGNs) contain supermassive black holes, which actively grow by the accretion of surrounding material and drive near-relativistic outflows. Contemporary X-ray observations of AGNs in Seyfert galaxies at the low redshifts reveal the presemce of blueshifted absorption lines, which were interpreted as ionized gas outflows along the line of sight from accretion disks near supermassive black holes. Over half of Seyfert I AGNs were found to have blueshifted absorption lines attributed to ionized outflows in the range of several hundred km/sec commonly referred to as warm absorbers (WAs). A considerable fraction of AGNs showed ionized absorbing outflows with velocities upto of 0.1 to 0.4 times the speed of light, so called ultra-fast outflows (UFOs).
As a Postdoc Fellow at Smithsonian Astrophysical Observatory, Ashkbiz Danehkar has carried out X-ray data analysis and photoionization modeling of ionized absorbing outflows in Seyfert I AGNs since 2015. He studied time-averaged 433ks observations of the quasar PG 1211+143 in a Seyfert I galaxy taken with Chandra High Energy Transmission Grating Spectrometer. It is found that the velocity of X-ray ionized outflows reaches a mildly relativistic value of 0.06 times the speed of light, which is typical of UFOs in Seyfert 1 AGNs.
Planetary nebulae have an important role in Galactic chemical evolution by returning significant enriched material to the ISM. Observations of planetary nebulae are used to determine the elemental abundances of the interstellar medium present in our own and other galaxies. The chemical properties of a planetary nebula can be determined through plasma diagnostics and empirical analysis using key information carried by the nebular emission lines. The emission lines emitted by a planetary nebulae also carry important information about the kinematics features of the nebula. The kinematic analysis of planetary nebulae provides some insights into the stellar mass-loss process and the planetary nebule evolution.
A considerable fraction of central stars of planetary nebulae exhibit hydrogen-deficient fast-expanding atmospheres characterized by a large mass-loss rate, so called Wolf-Rayet type stars. What are less clear are the physical mechanisms and evolutionary paths that remove the hydrogen-rich outer layer from these degenerate cores, and transform it into a fast stellar wind. The main goal of Ashkbiz Danehkar's PhD thesis was to determine kinematic properties, physical conditions and chemical abundances for a sample of Galactic planetary nebulae with hydrogen-deficient Wolf-Rayet type central stars using integral field spectroscopic data collected at Siding Spring Observatory, which will provide clues about the origin and formation of their stellar atmospheres.
The morpho-kinematic studies of planetary nebulae (PN) provide valuable clues to the AGB mass-loss processes, the transition from the AGB to the PN phase, and the (pre-)PN evolution. During his PhD research, Ashkbiz Danehkar used IFU observations taken with the ANU 2.3-m Telescope at Siding Spring Observatory to conduct 3-D morpho-kinematic studies of planetary nebulae. He used spatially-resolved velocity distributions, together with the HST imaging, to determine their primary orientations and inclination of the planetary nebulae Hen 3-1333 and Hen 2-113. Ashkbiz Danehkar also utlized velocity-resolved channel maps of the Wolf-Rayet planetary nebula Th 2-A to develop a 3-D morpho-kinematic model with the modeling tool SHAPE, indicating that this apparently spherical nebula has indeed a pair of collimated bipolar outflows almost viewed pole-on. His empirical analysis of the collimated jets of of the planetary nebula M2-42 revealed that their features are typical of so called fast, low-ionization emission regions (FLIERs).
As a PhD student, Ashkbiz Danehkar carried out a series of 3-D photoionization modeling of planetary nebulae that are especially implemented with inhomogeneous non-spherical density grids and non-local thermodynamic equilibrium ionizing (NLTE) sources. He collected the data of the planetary nebula SuWt 2 using the Wide-Field Spectrograph (WiFeS/IFU) on the ANU 2.3-m Telescope at the Siding Spring Observatory in 2012. Ashkbiz Danehkar then utilized IFU observations to study the physical properties of the inner ring in the planetary nebula SuWt 2. This nebula has two A-type stars in a proven binary system at the centre, but their radiations are too soft to ionize the surrounding material. His photoionization modeling suggests the presence of an unseen hot ionizing source with an effective temperature of 150 kK. He also carried out 3-D photoionization modeling of the nebula Abell 48, constrained by an hydrogen-deficient expanding model atmosphere as the central ionizing source. The stellar parameters which corresponds to a relatively low-mass progenitor star rather than a massive Pop I star, are able to produce the observed nebular line fluxes.
Electron-acoustic waves are observed in space plasmas such as the Earth's magnetosphere where two distinct electron populations exist, namely cool and hot electrons. The observations of the auroral broadband electrostatic noise emissions revealed that the hot electron distribution often has a long-tailed suprathermal (non-Maxwellian) form. The main goal of his Master project was to investigate how various plasma parameters modify the electron-acoustic structures.
Over 2009 Ashkbiz Danehkar studied linear and nonlinear structures of electron-acoustic solitary waves in a collisionless and unmagnetized plasma consisting of cool inertial electrons, hot suprathermal electrons, and mobile ions. A linear dispersion relations was obtained for electron-acoustic waves, which depicts a strong dependence of the charge screening mechanism on excess suprathermality. A nonlinear (Sagdeev) pseudopotential technique was employed to investigate the existence of electron-acoustic solitary waves, and determine how their characteristics depend on various plasma parameters.
His results indicate that the thermal pressure deeply affects the electron-acoustic solitary waves. Only negative polarity waves were found to exist in the one-fluid model, which becomes narrower as deviation from the Maxwellian increases, while the wave amplitude at fixed soliton speed increases. However, for a constant value of the true Mach number, the amplitude decreases for increasing suprathermality. It is also found that the ion inertia has a trivial role in the supersonic domain, but it is important to support positive polarity waves in the subsonic domain.
Becchi, Rouet, Stora (1974,1975,1976), and Tyutin (1975) developed The BRST formalism to extend the gauge symmetry, which provides useful way of studying the consistent interactions in terms of the deformation to the solutions of the master equation. As the gauge symmetry can be made using a nilpotent derivation, the gauge action is invariant under the BRST symmetry. Replacing the gauge symmetry with the BRST symmetry introduces antifield, ghosts, and antighosts for each gauge variable. BRST cohomology was extended by the antifield formalism allowed to determine consistent interactions among the fields from coupling deformations of the master equation. Therefore, the BRST-antifield formalism presents an efficient tool to study the consistent interactions in gauge theory.
As an early-stage researcher in 2008, Ashkbiz Danehkar worked In a project on a dual linearized gravity coupled to a topological background (BF) field, which included courses on quantum field theory and constrained dynamics. During his time at University of Craiova, he learned about gauge and BRST symmtry in quantum field theory. Ashkbiz Danehkar studied BRST coupling between a dual formulation of linearized gravity and topological BF model, which could have some applications in cosmological models and dark energy.
In general relativity, the Riemann curvature tensor is split into the Ricci tensor and the Weyl curvature tensor. Moreover, the Weyl tensor can be can split into the electric part and the magnetic part. The electric part describes the tidal (Newtonian) force, while the magnetic part has no Newtonian analogy, the so-called gravitomagnetic field. General relativity predicts the presence of gravitomagnetic fields and gravitational waves. A binary system of massive compact objects can produce gravitational waves, while a rotating massive body such as supermassive black holes have been predicted to generate the gravitomagnetic field. The gravitomagnetic fields implies that the formation of the jets along angular momentum vector, which could be as a mechanism for the jet formation in quasars and AGNs.
Over 2008 Ashkbiz Danehkar worked as an early-stage researcher in a project on gravitational theories, which included courses on general relativity. He studied cosmological perturbations in general relativity using 1+3 and 1+1+2 covariant formalism and tetrad formalism in a relativistic cosmological model. The results show that cosmological models with only Newtonian field are inconsistent and obstruct sounding solutions. Therefore, both the electric and the magnetic part of the Weyl curvature are necessary for the nonlocal interaction of gravitation and the propagation of gravitational waves.