Ashkbiz Danehkar

Eureka Scientific | ESCI

Superwinds and superbubbles driven by mechanical feedback from super star clusters (SSCs) are common features in many star-forming galaxies. While the adiabatic fluid model can well describe the dynamics of superwinds, several observations of starburst galaxies revealed the presence of compact regions with suppressed superwinds and strongly radiati...

We present a detailed X-ray spectral study of the quasar PG 1211+143 based on Chandra High Energy Transmission Grating Spectrometer (HETGS) observations collected in a multi-wavelength campaign with UV data using the Hubble Space Telescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the Jansky Very Large Array (VLA). We constructed...

Wolf-Rayet ([WR]) and weak emission-line ($wels$) central stars of planetary nebulae (PNe) have hydrogen-deficient atmospheres, whose origins are not well understood. In the present study, we have conducted plasma diagnostics and abundance analysis of 18 Galactic PNe surrounding [WR] and $wels$ nuclei, using collisionally excited lines (CELs) and o...

RT Cru belongs to the rare class of hard X-ray emitting symbiotics, whose origin is not yet fully understood. In this work, we have conducted a detailed spectroscopic analysis of X-ray emission from RT Cru based on observations taken by the Chandra Observatory using the Low Energy Transmission Grating (LETG) on the High-Resolution Camera Spectromet...

Suprathermal electrons and inertial drifting electrons, so called electron beam, are crucial to the nonlinear dynamics of electrostatic solitary waves observed in several astrophysical plasmas. In this paper, the propagation of electron-acoustic solitary waves is investigated in a collisionless, unmagnetized plasma consisting of cool inertial backg...

The dust properties of the line-of-sight materials in neutron star low-mass X-ray binaries (LMXBs) can be probed by X-ray observations and laboratory experiments. We use a Markov chain Monte Carlo (MCMC) method to conduct a spectral analysis of Chandra ACIS-S/HETG archival data of a sample of LMXBs, including GX 5-1 and GX 13+1. Our MCMC-based anal...

The dust properties of the line-of-sight materials in neutron star low-mass X-ray binaries (LMXBs) can be probed by X-ray observations and laboratory experiments. We use a Markov chain Monte Carlo (MCMC) method to conduct a spectral analysis of Chandra ACIS-S/HETG archival data of a sample of LMXBs, including GX 5-1 and GX 13+1. Our MCMC-based anal...

Thermal energies deposited by OB stellar clusters in starburst galaxies lead to the formation of galactic superwinds. Multi-wavelength observations of starburst-driven superwinds pointed at complex thermal and ionization structures which cannot adequately be explained by simple adiabatic assumptions. In this study, we perform hydrodynamic simulatio...

Thermal energies deposited by OB stellar clusters in starburst galaxies lead to the formation of galactic superwinds. Multi-wavelength observations of starburst-driven superwinds pointed at complex thermal and ionization structures which cannot adequately be explained by simple adiabatic assumptions. In this study, we perform hydrodynamic simulatio...

Observations of some starburst-driven galactic superwinds suggest that strong radiative cooling could play a key role in the nature of feedback and the formation of stars and molecular gas in star-forming galaxies. These catastrophically cooling superwinds are not adequately described by adiabatic fluid models, but they can be reproduced by incorpo...

Observations of some starburst-driven galactic superwinds suggest that strong radiative cooling could play a key role in the nature of feedback and the formation of stars and molecular gas in star-forming galaxies. These catastrophically cooling superwinds are not adequately described by adiabatic fluid models, but they can be reproduced by incorpo...

ePoster (id. 1270) presented at the European Astronomical Society (EAS) Annual Meeting, Special Session SS16: Connecting stellar sources and the ionised ISM, Valencia, Spain, July 2022

Poster (P2b.406) presented at the 48th European Physical Society (EPS) Conference on Plasma Physics, Virtual, June 2022

Poster presented at Computational Astrophysics in the ngVLA Era: Synergistic Simulations, Theory, and Observations (ngVLA22), Flatiron Institute, New York, NY, USA, June 2022

The long-range gravitational terms associated with tidal forces, frame-dragging effects, and gravitational waves are described by the Weyl conformal tensor, the traceless part of the Riemann curvature that is not locally affected by the matter field. The Ricci and Bianchi identities provide a set of dynamical and kinematic equations governing the m...

The planetary nebula (PN) IC 4997 is one of a few rapidly evolving objects with variable brightness and nebular emission around a hydrogen-deficient star. In this study, we have determined the physical conditions and chemical abundances of this object using the collisionally excited lines (CELs) and optical recombination lines (ORLs) measured from...

The planetary nebula (PN) IC 4997 is one of a few rapidly evolving objects with variable brightness and nebular emission around a hydrogen-deficient star. In this study, we have determined the physical conditions and chemical abundances of this object using the collisionally excited lines (CELs) and optical recombination lines (ORLs) measured from...

The majority of planetary nebulae (PNs) show axisymmetric morphologies, whose causes are not well understood. In this work, we present spatially resolved kinematic observations of 14 Galactic PNs surrounding Wolf–Rayet ([WR]) and weak emission-line stars ( wels ), based on the H α and [N ii ] emission taken with the Wide Field Spectrograph on the A...

Poster (Id. S17.045) presented at the American Physical Society (APS) April Meeting, New York, NY, USA, April 2022

Integral field unit (IFU) spectroscopy of planetary nebulae (PNe) provides a plethora of information about their morphologies and ionization structures. An IFU survey of a sample of PNe around hydrogen-deficient stars has been conducted with the Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. In this paper, we present the Hα kinematic o...

Integral field unit (IFU) spectroscopy of planetary nebulae (PNe) provides a plethora of information about their morphologies and ionization structures. An IFU survey of a sample of PNe around hydrogen-deficient stars has been conducted with the Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. In this paper, we present the H$\alpha$ kine...

The majority of planetary nebulae (PNe) show axisymmetric morphologies, whose causes are not well understood. In this work, we present spatially resolved kinematic observations of 14 Galactic PNe surrounding Wolf-Rayet ([WR]) and weak emission-line stars ($wels$) based on the H$\alpha$ and [N II] emission taken with the Wide Field Spectrograph on t...

The planetary nebula (PN) Abell 48 (PN G029.0+00.4) is around a rare Wolf-Rayet [WN5] star whose stellar history is as yet unknown. Using the integral field observations of the Hαλ6563 and [N ii] λ6584 line emissions, we conducted a comprehensive spatio-kinematic analysis of this PN. A three-dimensional spatio-kinematic ionization model was develop...

The planetary nebula (PN) Abell 48 (PN G029.0+00.4) is around a rare Wolf-Rayet [WN5] star whose stellar history is as yet unknown. Using the integral field observations of the H$\alpha$ $\lambda$6563 and [N II] $\lambda$6584 line emissions, we conducted a comprehensive spatio-kinematic analysis of this PN. A three-dimensional spatio-kinematic ioni...

Talk presented at Evolved Stars and their Circumstellar Environments (Online Workshop), SOFIA Science Center, December 15, 2021

Poster presented at IAU Symposium 363: Neutron Star Astrophysics at the Crossroads: Magnetars and the Multimessenger Revolution (Remote Symposium), 29 Nov - 3 Dec 2021 https://astromeeting.gssi.it/

Wolf–Rayet ([WR]) and weak-emission-line ( wels ) central stars of planetary nebulae (PNs) have hydrogen-deficient atmospheres, whose origins are not well understood. In the present study, we have conducted plasma diagnostics and abundance analyses of 18 Galactic PNs surrounding [WR] and wels nuclei, using collisionally excited lines (CELs) and opt...

Poster (Id. JP11.003) presented at the 63rd Annual Meeting of the American Physical Society (APS) Division of Plasma Physics (DPP), Pittsburgh, PA, USA, November 2021 https://meetings.aps.org/Meeting/DPP21/Session/JP11.3 https://ui.adsabs.harvard.edu/abs/2021APS..DPPJP1003D/abstract

Talk presented at IAU Symposium 362 (IAUS362): Predictive Power of Computational Astrophysics as a Discovery Tool (Online Video-Conference), November 8, 2021 http://iaus362.astro.unistra.fr/IAUS362.html https://youtu.be/tXyB_lpLlPk

Starburst-driven galactic outflows in star-forming galaxies have been observed to contain complex thermal structures and emission line features that are difficult to explain by adiabatic fluid models and plasmas in photoionization equilibrium (PIE) and collisional ionization equilibrium (CIE). We previously performed hydrodynamic simulations of sta...

Superwinds and superbubbles driven by mechanical feedback from super star clusters (SSCs) are common features in many star-forming galaxies. While the adiabatic fluid model can well describe the dynamics of superwinds, several observations of starburst galaxies revealed the presence of compact regions with suppressed superwinds and strongly radiati...

Poster presented at the 1st Kavli Institute for Astronomy and Astrophysics (KIAA) Forum on Gas in Galaxies for Early Career Scientists (KooGiG-Junior Virtual Workshop), Kavli Institute for Astronomy and Astrophysics (KIAA) and the PKU-KIAA Innovation NSFC Group (PKING), November 2021 https://kiaa.pku.edu.cn/KooGig_junior21/Home.htm

In the holographic picture, the Brout-Englert-Higgs (BEH) mechanism in [Formula: see text]-dimensional Yang–Mills theories is conjectured to provide a Higgs-like mechanism for gravity in [Formula: see text] dimensions, resulting in massive (or massless) gravitons in IR (or UV) completions. Accordingly, one could imagine dual (magnetic-type) fields...

Poster presented at Star Formation: From Clouds to Discs (Session Jets, winds and molecular cloud feedback), Malahide, Ireland, 18-21 October 2021.

Talk presented at 4th Workshop on Numerical Modeling in MHD and Plasma Physics (MHD-PP; Virtual), 13 October 2021

Talk presented at Asymmetrical Post-Main-Sequence Nebulae 8 (APN8): the Shaping of Stellar Outflows (Virtual), October 8, 2021

Starburst-driven galactic outflows in star-forming galaxies have been observed to contain complex thermal structures and emission line features that are difficult to explain by adiabatic fluid models and plasmas in photoionization equilibrium (PIE) and collisional ionization equilibrium (CIE). We previously performed hydrodynamic simulations of sta...

Poster (No. 4P-B-05) presented at the 48th IEEE International Conference on Plasma Science (Virtual ICOPS 2021), Stateline, NV, USA, September 2021. Reference: A. Danehkar, IEEE International Conference on Plasma Science (ICOPS), 2021, https://doi.org/10.1109/ICOPS36761.2021.9588478

Interaction of a beam of electrons with the background electrons commonly occurs in high-energy astrophysical plasmas such as active galactic nuclei, supernovas, and pulsar environments, where positrons are also present. Previously, we have studied the properties of electrostatic solitary waves in two plasma models containing kappa-distributed elec...

In the holographic picture, the BEH mechanism in d-dimensional Yang-Mills theories is conjectured to provide a Higgs-like mechanism for gravity in d+1 dimensions, resulting in massive (or massless) gravitons in IR (or UV) completions. Accordingly, one could imagine dual (magnetic-type) fields of massive gravitons in the IR (low-energy) limit that a...

Talk presented at Chandra Data Science: Novel Methods in Computing and Statistics for X-ray Astronomy, Virtual Meeting, Chandra X-ray Center, USA, August 18, 2021

Talk presented at Challenges and Innovations in Computational Astrophysics - III (ChaICA-III; Virtual), 21 June 2021

Talk presented at American Physical Society (APS) April Meeting, Session Z09: Stars, White Dwarfs, & Thermonuclear Supernovae, Abstract id.Z09.006, Virtual Meeting, USA, April 20, 2021

Mechanical feedback from young massive stars in super star clusters contributes to the formation of superwinds and superbubbles in star-forming regions. We conduct hydrodynamic simulations using the non-equilibrium ionization package MAIHEM to explore how outflow velocity, mass-loading, metallicity, and ambient density can affect the occurrence of...

Talk presented at American Astronomical Society (AAS) Winter 237st Virtual Meeting, USA, January 2021

Talk presented at Challenges and Innovations in Computational Astrophysics - II (Virtual), November 2020

RT Cru belongs to the rare class of hard X-ray emitting symbiotics, whose origin is not yet fully understood. In this work, we have conducted a detailed spectroscopic analysis of X-ray emission from RT Cru based on observations taken by the Chandra Observatory using the Low Energy Transmission Grating (LETG) on the High-Resolution Camera Spectromet...

Talk presented at the 30th Midwest Relativity Meeting (Virtual), University of Notre Dame, IN, USA, October 2020

Local conformal symmetry introduces the conformal curvature (Weyl tensor) that gets split into its (gravito-) electric and magnetic (tensor) parts. Newtonian tidal forces are expected from the gravitoelectric field, whereas general-relativistic frame-dragging effects emerge from the gravitomagnetic field. The symmetric, traceless gravitoelectric an...

Local conformal symmetry introduces the conformal curvature (Weyl tensor) that gets split into its (gravito-) electric and magnetic (tensor) parts. Newtonian tidal forces are expected from the gravitoelectric field, whereas general-relativistic frame-dragging effects emerge from the gravitomagnetic field. The symmetric, traceless gravitoelectric an...

Talk at Galaxy Group Meeting, University of Michigan, MI, USA, November 2019

Talk at Extreme Astrophysics Group, University of Michigan, Michigan, MI, USA, October 2019

Talk presented at the 29th Midwest Relativity Meeting, Grand Valley State University, Grand Rapids, MI, USA, October 2019

Talk at Post-Doc Colloquium, University of Michigan, Ann Arbor, MI, USA, September 2019

Spectra emitted from ionized nebulae typically contain collisionally excited and recombination lines, which can be used to trace physical conditions and chemical abundances of the interstellar medium in our Galaxy and other galaxies. "AtomNeb" is a database containing atomic data stored in the Flexible Image Transport System (FITS) file format, inc...

Talk at Galaxy Group Meeting, University of Michigan, Ann Arbor, MI, USA, March 2019

We present the spectral analysis of Chandra/High Energy Transmission Grating Spectrometer (HETGS) and NuSTAR observations of the quasar PDS 456 from 2015, and XMM-Newton and NuSTAR archival data from 2013 to 2014, together with Chandra/HETGS data from 2003. We analyzed these three different epochs in a consistent way, looking for absorption feature...

We present the spectral analysis of Chandra/HETGS and NuSTAR observations of the quasar PDS 456 from 2015, and XMM-Newton and NuSTAR archival data from 2013-2014, together with Chandra/HETGS data from 2003. We analyzed these three different epochs in a consistent way, looking for absorption features corresponding to highly ionized blueshifted absor...

Over the past two decades, electric-magnetic duality has made significant progress in linearized gravity and higher spin gauge fields in arbitrary dimensions. By analogy with Maxwell theory, the Dirac quantization condition has been generalized to both the conserved electric-type and magnetic-type sources associated with gravitational fields and hi...

Over the past two decades, electric-magnetic duality has made significant progress in linearized gravity and higher spin gauge fields in arbitrary dimensions. By analogy with Maxwell theory, the Dirac quantization condition has been generalized to both the conserved electric-type and magnetic-type sources associated with gravitational fields and hi...

The emission lines emitted from gaseous nebulae carry valuable information about the physical conditions and chemical abundances of ionized gases in these objects, as well as the interstellar extinction. "proEQUIB" is a library containing several application programming interface (API) functions developed in the Interactive Data Language (IDL), whi...

Prompted by the H i Lyα absorption associated with the X-ray ultrafast outflow at -17,300 km s⁻¹ in the quasar PG 1211+143, we have searched archival UV spectra at the expected locations of H i Lyα absorption for a large sample of ultrafast outflows identified in XMM-Newton and Suzaku observations. Sixteen of the X-ray outflows have predicted H i L...

Prompted by the H I Ly$\alpha$ absorption associated with the X-ray ultra-fast outflow at -17,300 $\rm km~s^{-1}$ in the quasar PG~1211+143, we have searched archival UV spectra at the expected locations of H I Ly$\alpha$ absorption for a large sample of ultra-fast outflows identified in XMM-Newton and Suzaku observations. Sixteen of the X-ray outf...

The planetary nebula (PN) PB 8 around a [WN/WC]-hybrid central star is one of PNe with moderate abundance discrepancy factors (ADFs ~ 2-3), which could be an indication of a tiny fraction of metal-rich inclusions embedded in the nebula (bi-abundance). In this work, we have constructed photoionization models to reproduce the optical and infrared obs...

Poster presented at the American Astronomical Society Meeting No. 231, Washington, DC, USA, 2018

We observed the quasar PG1211+143 using the Cosmic Origins Spectrograph on the Hubble Space Telescope in April 2015 as part of a joint campaign with the Chandra X-ray Observatory and the Jansky Very Large Array. Our ultraviolet spectra cover the wavelength range 912-2100 A. We find a broad absorption feature (~1080 km/s) at an observed wavelength o...

The planetary nebula (PN) NGC 5189 around a Wolf-Rayet [WO] central star demonstrates one of the most remarkable complex morphologies among PNe with many multi-scale structures, showing evidence of multiple outbursts from an AGB progenitor. In this study we use multi-wavelength Hubble Space Telescope Wide Field Camera 3 (WFC3) observations to study...

We describe a program for the parallel implementation of multiple runs of XSTAR, a photoionization code that is used to predict the physical properties of an ionized gas from its emission and/or absorption lines. The parallelization program, called MPI_XSTAR, has been developed and implemented in the C++ language by using the Message Passing Interf...

The nonlinear propagation of electrostatic solitary waves is studied in a collisionless electron-positron pair plasma consisting of adiabatic cool electrons, mobile cool positrons (or electron holes), hot suprathermal electrons described by a $\kappa$ distribution, and stationary ions. The linear dispersion relation derived for electrostatic waves...

Talk given at CfA Postdoc Symposium, Harvard/CfA, Cambridge, USA, October 2017

We investigate the existence conditions and propagation properties of electron-acoustic solitary waves in a plasma consisting of an electron beam fluid, a cold electron fluid, and a hot suprathermal electron component modeled by a $\kappa$-distribution function. The Sagdeev pseudopotential method was used to investigate the occurrence of stationary...

Poster presented at "From Chandra to Lynx: Taking the Sharpest X-ray Vision Fainter and Farther" (CXO2LYNX), Harvard, Cambridge, MA, USA, 2017

Talk presented at the New England Regional Quasar and AGN Meeting (NERQUAM), Boston University, USA, May 2017

We present a brief review of the cohomological solutions of self-coupling interactions of the fields in the free Yang-Mills theory. All consistent interactions among the fields have been obtained using the antifield formalism through several order BRST deformations of the master equation. It is found that the coupling deformations halt exclusively...

Talk given at CfA Postdoc Symposium, Harvard/CfA, Cambridge, USA, October 2016

Lunch talk at High Energy Phenomena Seminar, Harvard-CfA, Cambridge, USA, September 2016

Spatially resolved observations of the planetary nebula M2-42 (PN G008.2-04.8) obtained with the Wide Field Spectrograph on the Australian National University 2.3 m telescope have revealed the remarkable features of bipolar collimated jets emerging from its main structure. Velocity-resolved channel maps derived from the [N II] $\lambda$6584 emissio...

We present a comprehensive set of spatially resolved, integral field spectroscopic mapping of the Wolf-Rayet planetary nebula Th 2-A, obtained using the Wide Field Spectrograph on the Australian National University 2.3-m telescope. Velocity-resolved H$\alpha$ channel maps with a resolution of 20 km s$^{-1}$ allow us to identify different kinematic...

Talk given at CfA Postdoc Symposium, Harvard University, Cambridge, USA, November 2015

We have performed integral field spectroscopy of the planetary nebulae Hen 3-1333 (PNG332.9−09.9) and Hen 2-113 (PNG321.0+03.9), which are unusual in exhibiting dual-dust chemistry and multipolar lobes but also ionized by late-type [WC 10] central stars. The spatially resolved velocity distributions of the Hα emission line were used to determine th...

We have used the Wide Field Spectrograph on the Australian National University 2.3-m telescope to perform the integral field spectroscopy for a sample of the Galactic planetary nebulae. The spatially resolved velocity distributions of the H$\alpha$ emission line were used to determine the kinematic features and nebular orientations. Our findings sh...

Talk given at 12th Asia-Pacific Regional IAU Meeting (APRIM), Daejeon, Korea, August 2014

We have carried out optical spectroscopic measurements of emission lines for a sample of Galactic planetary nebulae with Wolf-Rayet (WR) stars and weak emission-line stars (wels). The plasma diagnostics and elemental abundance analysis have been done using both collisionally excited lines (CELs) and optical recombination lines (ORLs). It is found t...

Poster presented at the 12th Asia-Pacific Regional IAU Meeting (APRIM), Daejeon, Korea, 2014