Browsing by Subject "Radiative transfer"

Now showing 1 - 9 of 9

Calculating the structure of protoplanetary disks within the first few AU using Pisco
(2011-12) Harrold, Samuel Thomas; Lacy, John Howard; Jaffe, Dan
The calculation of the physical conditions near the inner rim of a protoplanetary disk using the new computational model Pisco is described. Diagnostic plots illustrate solutions for disk structure, radiation field, chemical composition, and heating and cooling of the disk in a steady-state approximation for both disks with unsettled dust and with settled dust. Disks with unsettled dust are found to have hotter gas temperatures above the disk photosphere and a more pronounced temperature inversion at the disk photosphere. Recommendations are made for the development of Pisco. Pisco has the potential to explore what observed molecular emission can imply about disk structure.
The development of replicated optical integral field spectrographs and their application to the study of Lyman-alpha emission at moderate redshifts
(2015-08) Chonis, Taylor Steven; Hill, Gary J.; Finkelstein, Steven L; Gebhardt, Karl; Greene, Jenny E; Jaffe, Daniel T
In the upcoming era of extremely large ground-based astronomical telescopes, the design of wide-field spectroscopic survey instrumentation has become increasingly complex due to the linear growth of instrument pupil size with telescope diameter for a constant spectral resolving power. The upcoming Visible Integral field Replicable Unit Spectrograph (VIRUS), a baseline array of 150 copies of a simple integral field spectrograph that will be fed by 33,600 optical fibers on the upgraded Hobby-Eberly Telescope (HET) at McDonald Observatory, represents one of the first uses of large-scale replication to break the relationship between instrument pupil size and telescope diameter. By dividing the telescope's field of view between a large number of smaller and more manageable instruments, the total information grasp of a traditional monolithic survey spectrograph can be achieved at a fraction of the cost and engineering complexity. To highlight the power of this method, VIRUS will execute the HET Dark Energy Experiment (HETDEX) and survey ~420 square degrees of sky to an emission line flux limit of ~1e-17 erg/s/cm^2 to detect ~1e6 Lyman-alpha emitting galaxies (LAEs) as probes of large-scale structure at redshifts of 1.9
Integral field spectroscopy as a probe of galaxy evolution
(2011-08) Adams, Joshua Jesse; Gebhardt, Karl; Hill, Gary J.; Drory, Niv; Evans, Neal; Bromm, Volker
Optical spectroscopy and modeling are applied to four independent problems related to the structure and evolution of galaxies. The problems cover a broad range of look-back time and galaxy mass. Integral field spectroscopy with low surface brightness sensitivity is the tool employed to advance our understanding of the distribution, interplay, and evolution of the stars, dark matter, and gas. First, I review development and commissioning work done on the VIRUS-P instrument. I then present a large sample of galaxies over redshifts 1.9= 10E12 solar masses). Third, I study the dark matter halo profile in a nearby late-type dwarf galaxy in the context of the "core-cusp" controversy. N-body simulations predict such galaxies to have cuspy dark matter halos, while HI rotation curves and more recent hydrodynamical simulations indicate that such halos may instead be strongly cored. I measure the spatially resolved stellar velocity field and fit with two-integral Jeans models. A cuspy halo is preferred from the stellar kinematics. The mass models from stellar and gaseous kinematics disagree. The gas models assume circular motion in an infinitely thin disk which is likely unrealistic. The stellar kinematics presented are the first measurements of a collision-less tracer in such galaxies. Fourth, I attempt to measure diffuse H-alpha emission, fluoresced by the metagalactic UV background, in the outskirts of a nearby gas rich galaxy. I do not make a detection, but the deep flux limit over a large field-of-view places the most sensitive limit to-date on the UV background's photoionization rate of Gamma(z=0)<1.7x10E-14 1/s at 5 sigma certainty.
The kinetic Sunyaev-Zel’dovich effect as a probe of the physics of cosmic reionization : the effect of self-regulated reionization
(2014-12) Park, Hyunbae; Shapiro, Paul R.
We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the "self-regulation" of reionization: star formation in low-mass dwarf galaxies or minihalos is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (~10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization.
Monte Carlo radiation hydrodynamics in the super-Eddington regime
(2018-06-27) Tsang, Tsz Ho; Milosavljević, Miloš; Bromm, Volker; Jogee, Shardha; Evans, Neal J.; Davis, Shane
In this dissertation, we present three projects addressing the dynamical importance of radiation in turbulent media with super-Eddington flux. Examples of such media are massive star-forming environments and supermassive star atmospheres. While there are many theoretical models claiming the pivotal role of radiation in driving strong outflows and setting the star formation efficiency in the course of massive star formation, often they are based on ideal geometries and closure relations of the moment equations for radiation. To directly tackle the challenge of numerically modeling radiation-matter interactions in hydrodynamical simulations, we have adopted and tested a hybrid Monte Carlo radiation transport scheme. In the first project, with a standardized two-dimensional radiation-driven winds setup, we show that low-order methods tend to artificially reinforce the development of the low-density channels and underestimate the strength of radiation pressure. The accuracy of any numerical radiation transport scheme in producing truthful dynamics therefore depends on the validity of its underlying assumptions. In the second project, we carry out radiation hydrodynamical simulations of the formation of super star clusters in supersonically turbulent molecular clouds. The gas distribution is strongly inhomogeneous and that reduces the strength of radiation pressure in halting gas accretion compared to previous predictions. In the last project, unlike the inflow-outflow scenario of the first two, we aim to simulate the radiation hydrodynamics in quasi-hydrostatic media with extreme sensitivity of the opacity to density and temperature. We present the implementation and robustness test of the hybrid Monte Carlo estimators in preparation for direct simulations of the convective, radiation-dominated dynamics prevalent in the atmospheres of supermassive stars.
On explaining GRB prompt spectrum with photospheric emission model
(2018-05-07) Bhattacharya, Mukul; Kumar, Pawan; Paban, Sonia
In this thesis, we present our work towards understanding the radiation mechanism of Gamma Ray Bursts (GRBs) during prompt emission phase. We study the spectra of photospheric emission from highly relativistic gamma-ray burst outflows using a Monte Carlo (MC) code. We consider the Comptonization of photons with a fast cooled synchrotron spectrum in a relativistic jet with realistic photon to electron number ratio N [subscript γ] / N [subscript e] = 10⁵ using mono-energetic protons which interact with thermalised electrons through Coulomb interaction. The photons, electrons and protons are cooled adiabatically as the jet expands outwards. ...
Studying star formation at low and high redshift with integral field spectroscopy
(2011-05) Blanc, Guillermo; Gebhardt, Karl; Evans, Neal J.; Hill, Gary J.; Bromm, Volker; Gawiser, Eric
In this thesis I focus mainly in studying the process of star formation in both high redshift, and local star forming galaxies, by using an observational technique called integral field spectroscopy (IFS). Although these investigations are aimed at studying the star formation properties of these objects, throughout this work I will also discuss the geometric, kinematic, and chemical structures in the inter-stellar medium of these galaxies, which are intimately connected with the process of star formation itself. The studies presented here were conducted under the umbrella of two different projects. First, the HETDEX Pilot Survey for Emission Line Galaxies, where I have studied the properties of Ly-alpha emitting galaxies across the 2
The Lyman-alpha signature of the first galaxies
(2018-07-06) Smith, Aaron Robin; Bromm, Volker; Finkelstein, Steven; Gebhardt, Karl; Jogee, Shardha; Loeb, Abraham; Milosavljević, Miloš
This dissertation is presented as a collection of projects related to the first galaxies and supermassive black holes. The analysis centers around the Lyman α (Lyα) line of atomic hydrogen, which is an especially powerful probe of the high-redshift universe due to its characteristic strength and spectral properties. Several instruments and telescopes have recently been or will be targeting the Lyα line with significantly increased sensitivity across all redshifts. In light of such advances, we continue to be optimistic that when combined with other observational signatures, accurate Lyα modeling can reveal key physical properties about the intrinsic sources and external environments of these systems. The included works incorporate state-of-the-art radiation transport calculations within high resolution ab initio cosmological simulations or more idealized setups to understand the imprint of galaxy formation during the first billion years after the Big Bang. Each chapter is a self-contained project with the title corresponding to a peer-reviewed publication. The collection includes significant discussion of code development, analysis methods, specific applications, likely interpretations, and broad perspectives that will hopefully benefit future researchers in this field, as we observe and model the universe with ever increasing realism.
The three-dimensional structure and kinematics of protostellar envelopes
(2019-05-08) Yang, Yao-Lun; Offner, Stella; Evans, Neal J.; Green, Joel D.; Lee, Jeong-Eun; Jaffe, Daniel T.; Lacy, John H.; Bromm, Volker
Star formation is one of the key process that drives the evolution of galaxies and planetary systems. Understanding the formation and evolution of protostellar systems that would eventually form Sun-like stars not only provides insights on the origin of our solar system, but also constrains the properties of planetary systems similar to our solar system. The observations at far-infrared and submillimeter wavelengths using Herschel and ALMA, best characterize the youngest protostars, where disks are still small or not yet formed. Together with 3D radiative transfer calculations, we have begun to constrain the complex structure and kinematics of the protostellar envelopes, and eventually probe the initial conditions of disk formation. In this dissertation, I present a spectroscopy archive of young protostars observed by Herschel, the CDF archive, to characterize the observational signatures of embedded protostars, the stage soon after the formation of the protostars. The full spectra of protostars from 50–670 μm reveal a forest of molecular and atomic lines, probing the warm protostellar envelope heated by the central protostars and outflows. I characterize the spectral energy distributions of 27 embedded protostars, and analyze the correlations of the molecular and atomic emission. Moreover, I focus on the CO rotational emission from J[subscript up] = 4 to 43 to constrain the origins of CO emission. A statistical analysis shows that the fitted CO rotational temperatures form two distinct populations around 100 K and 350 K, consistent with the proposed origins of CO from entrained outflows and shocked gas. To quantitatively analyze the outflow morphology seen from the CO emission, I developed a method of using normalized azimuthal flux profile to quantify the bipolarity of the CO emission, indicating that the bipolarity extent decreases in higher-J CO transitions. With a well-calibrated dataset, I focus on the structure of the envelope surrounding the embedded protostars and the mass transfer occurred in the envelope. I use BHR 71 as the test-bed for modeling the structure of envelope and the infall kinematics constrained by the continuum spectral energy distribution consist of Herschel and Spitzer spectra and the recently-acquired ALMA observations, respectively. I successfully modeled the continuum SED of BHR 71 with a slowly rotating infalling envelope which shows an “inside-out” collapse. I simulate the synthetic observations with a 3D continuum radiative transfer pipeline using the publicly-available package, hyperion. The model includes not only the envelope but also bipolar cavities and a disk defined by the centrifugal radius of the envelope. I demonstrate a method for finding the best-fitting model by comparing with the SED as well as the photometric images from Herschel and Spitzer. The best-fitting model suggests an upper limit of 36000 years for the age since the collapse began and an inclination angle of 50°. […]