arXiv subjects

Christian D. Ott

Publications and source records attributed to Christian D. Ott.

At least 73 records · Page 4Linked to original sources

arXiv2009

A Model for Gravitational Wave Emission from Neutrino-Driven Core-Collapse Supernovae

Abridged: Using a suite of progenitor models, neutrino luminosities, and two- dimensional (2D) simulations, we investigate the matter gravitational-wave (GW) emission from postbounce phases of neutrino-driven core-collapse supernovae (CCSNe). The relevant phases are prompt and steady-state convection, the standing accretion shock instability (SASI), and asymmetric explosions. For the stages before explosion, we propose a model for the source of GW emission. Downdrafts of the postshock-convection/SASI region strike the protoneutron star "surface" with large speeds and are decelerated by buoyancy forces. We find that the GW amplitude is set by the magnitude of deceleration and, by extension, the downdraft's speed and the vigor of postshock-convective/SASI motions. However, the characteristic frequencies, which evolve from ~100 Hz after bounce to ~300-400 Hz, are primarily independent of these speeds, but are set by the deceleration timescale, which is in turn set by the buoyancy frequency at the lower boundary of postshock convection. Consequently, the GW characteristic frequencies are dependent upon a combination of core structure attributes, specifically the dense-matter equation of state (EOS) and details that determine the gradients at the boundary, including the accretion-rate history, the EOS at subnuclear densities, and neutrino transport. During explosion, the high frequency signal wanes and is replaced by a strong low frequency, ~10s of Hz, signal that reveals the general morphology of the explosion (i.e. prolate, oblate, or spherical). However, current and near-future GW detectors are sensitive to GW power at frequencies >50 Hz. Therefore, the signature of explosion will be the abrupt reduction of detectable GW emission.

astro-ph.SR↗

arXiv2009

The Gravitational Wave Signature of Core-Collapse Supernovae

We review the ensemble of anticipated gravitational-wave (GW) emission processes in stellar core collapse and postbounce core-collapse supernova evolution. We discuss recent progress in the modeling of these processes and summarize most recent GW signal estimates. In addition, we present new results on the GW emission from postbounce convective overturn and protoneutron star g-mode pulsations based on axisymmetric radiation-hydrodynamic calculations. Galactic core-collapse supernovae are very rare events, but within 3-5 Mpc from Earth, the rate jumps to 1 in ~2 years. Using the set of currently available theoretical gravitational waveforms, we compute upper-limit optimal signal-to-noise ratios based on current and advanced LIGO/GEO600/VIRGO noise curves for the recent SN 2008bk which exploded at ~3.9 Mpc. While initial LIGOs cannot detect GWs emitted by core-collapse events at such a distance, we find that advanced LIGO-class detectors could put significant upper limits on the GW emission strength for such events. We study the potential occurrence of the various GW emission processes in particular supernova explosion scenarios and argue that the GW signatures of neutrino-driven, magneto-rotational, and acoustically-driven core-collapse SNe may be mutually exclusive. We suggest that even initial LIGOs could distinguish these explosion mechanisms based on the detection (or non-detection) of GWs from a galactic core-collapse supernova.

astro-ph↗

arXiv2008

The gravitational wave burst signal from core collapse of rotating stars

We present results from detailed general relativistic simulations of stellar core collapse to a proto-neutron star, using two different microphysical nonzero-temperature nuclear equations of state as well as an approximate description of deleptonization during the collapse phase. Investigating a wide variety of rotation rates and profiles as well as masses of the progenitor stars and both equations of state, we confirm in this very general setup the recent finding that a generic gravitational wave burst signal is associated with core bounce, already known as type I in the literature. The previously suggested type II (or "multiple-bounce") waveform morphology does not occur. Despite this reduction to a single waveform type, we demonstrate that it is still possible to constrain the progenitor and postbounce rotation based on a combination of the maximum signal amplitude and the peak frequency of the emitted gravitational wave burst. Our models include to sufficient accuracy the currently known necessary physics for the collapse and bounce phase of core-collapse supernovae, yielding accurate and reliable gravitational wave signal templates for gravitational wave data analysis. In addition, we assess the possiblity of nonaxisymmetric instabilities in rotating nascent proto-neutron stars. We find strong evidence that in an iron core-collapse event the postbounce core cannot reach sufficiently rapid rotation to become subject to a classical bar-mode instability. However, many of our postbounce core models exhibit sufficiently rapid and differential rotation to become subject to the recently discovered dynamical instability at low rotation rates.

astro-ph↗

arXiv2008

2D Multi-Angle, Multi-Group Neutrino Radiation-Hydrodynamic Simulations of Postbounce Supernova Cores

We perform axisymmetric (2D) multi-angle, multi-group neutrino radiation-hydrodynamic calculations of the postbounce phase of core-collapse supernovae using a genuinely 2D discrete-ordinate (S_n) method. We follow the long-term postbounce evolution of the cores of one nonrotating and one rapidly-rotating 20-solar-mass stellar model for ~400 milliseconds from 160 ms to ~550 ms after bounce. We present a multi-D analysis of the multi-angle neutrino radiation fields and compare in detail with counterpart simulations carried out in the 2D multi-group flux-limited diffusion (MGFLD) approximation to neutrino transport. We find that 2D multi-angle transport is superior in capturing the global and local radiation-field variations associated with rotation-induced and SASI-induced aspherical hydrodynamic configurations. In the rotating model, multi-angle transport predicts much larger asymptotic neutrino flux asymmetries with pole to equator ratios of up to ~2.5, while MGFLD tends to sphericize the radiation fields already in the optically semi-transparent postshock regions. Along the poles, the multi-angle calculation predicts a dramatic enhancement of the neutrino heating by up to a factor of 3, which alters the postbounce evolution and results in greater polar shock radii and an earlier onset of the initially rotationally weakened SASI. In the nonrotating model, differences between multi-angle and MGFLD calculations remain small at early times when the postshock region does not depart significantly from spherical symmetry. At later times, however, the growing SASI leads to large-scale asymmetries and the multi-angle calculation predicts up to 30% higher average integral neutrino energy deposition rates than MGFLD.

astro-ph↗

arXiv2007

Maximum Entropy for Gravitational Wave Data Analysis: Inferring the Physical Parameters of Core-Collapse Supernovae

The gravitational wave signal arising from the collapsing iron core of a Type II supernova progenitor star carries with it the imprint of the progenitor's mass, rotation rate, degree of differential rotation, and the bounce depth. Here, we show how to infer the gravitational radiation waveform of a core collapse event from noisy observations in a network of two or more LIGO-like gravitational wave detectors and, from the recovered signal, constrain these source properties. Using these techniques, predictions from recent core collapse modeling efforts, and the LIGO performance during its S4 science run, we also show that gravitational wave observations by LIGO might have been sufficient to provide reasonable estimates of the progenitor mass, angular momentum and differential angular momentum, and depth of the core at bounce, for a rotating core collapse event at a distance of a few kpc.

astro-ph↗

arXiv2007

Cactus Framework: Black Holes to Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are intense narrowly-beamed flashes of gamma-rays of cosmological origin. They are among the most scientifically interesting astrophysical systems, and the riddle concerning their central engines and emission mechanisms is one of the most complex and challenging problems of astrophysics today. In this article we outline our petascale approach to the GRB problem and discuss the computational toolkits and numerical codes that are currently in use and that will be scaled up to run on emerging petaflop scale computing platforms in the near future. Petascale computing will require additional ingredients over conventional parallelism. We consider some of the challenges which will be caused by future petascale architectures, and discuss our plans for the future development of the Cactus framework and its applications to meet these challenges in order to profit from these new architectures.

cs.DC↗

arXiv2007

Generic Gravitational Wave Signals from the Collapse of Rotating Stellar Cores

We perform general relativistic simulations of stellar core collapse to a proto-neutron star, using a microphysical equation of state as well as an approximate description of deleptonization. We show that for a wide variety of rotation rates and profiles the gravitational wave burst signals from the core bounce are of a generic type, known as Type I in the literature. In our systematic study, using both general relativity and Newtonian gravity, we identify and individually quantify the micro- and macrophysical mechanisms leading to this result, i.e. the effects of rotation, the equation of state, and deleptonization. Such a generic type of signal templates will likely facilitate a more efficient search in current and future gravitational wave detectors of both interferometric and resonant type.

astro-ph↗

arXiv2007

Generic Gravitational Wave Signals from the Collapse of Rotating Stellar Cores: A Detailed Analysis

We present detailed results from performing general relativistic (GR) simulations of stellar core collapse to a proto-neutron star, using a microphysical equation of state (EoS) as well as an approximate description of deleptonization during the collapse phase. We show that for a wide variety of rotation rates and profiles the gravitational wave (GW) burst signals from the core bounce are of a generic type, already known as Type I in the literature. In addition, for most models the characteristic frequency of the GW burst signal lies in a narrow range around approximately 718 Hz. In our systematic study, using both GR and Newtonian gravity, we identify, individually quantify, and discuss in detail the micro- and macrophysical mechanisms leading to this result, i.e. the effects of rotation, the EoS, and deleptonization. We also discuss the detectability prospects of such GW burst signals by GW detectors, and infer that such a generic type of signal templates will likely facilitate a more efficient search in current and future detectors of both interferometric and resonant type.

astro-ph↗

arXiv2007

Simulations of Magnetically-Driven Supernova and Hypernova Explosions in the Context of Rapid Rotation

We present here the first 2D rotating, multi-group, radiation magnetohydrodynamics (RMHD) simulations of supernova core collapse, bounce, and explosion. In the context of rapid rotation, we focus on the dynamical effects of magnetic stresses and the creation and propagation of MHD jets. We find that a quasi-steady state can be quickly established after bounce, during which a well-collimated MHD jet is maintained by continuous pumping of power from the differentially rotating core. If the initial spin period of the progenitor core is $\sles$ 2 seconds, the free energy reservoir in the secularly evolving protoneutron star is adequate to power a supernova explosion, and may be enough for a hypernova. The jets are well collimated by the infalling material and magnetic hoop stresses, and maintain a small opening angle. We see evidence of sausage instabilities in the emerging jet stream. Neutrino heating is sub-dominant in the rapidly rotating models we explore, but can contribute 10$-$25% to the final explosion energy. Our simulations suggest that even in the case of modest or slow rotation, a supernova explosion might be followed by a secondary, weak MHD jet explosion, which, because of its weakness may to date have gone unnoticed in supernova debris. Furthermore, we suggest that the generation of a non-relativistic MHD precursor jet during the early protoneutron star/supernova phase is implicit in both the collapsar and "millisecond magnetar" models of GRBs. The multi-D, multi-group, rapidly rotating RMHD simulations we describe here are a start along the path towards more realistic simulations of the possible role of magnetic fields in some of Nature's most dramatic events.

astro-ph↗

arXiv2007

Rotating Collapse of Stellar Iron Cores in General Relativity

We present results from the first 2+1 and 3+1 simulations of the collapse of rotating stellar iron cores in general relativity employing a finite-temperature equation of state and an approximate treatment of deleptonization during collapse. We compare full 3+1 and conformally-flat spacetime evolution methods and find that the conformally-flat treatment is sufficiently accurate for the core-collapse supernova problem. We focus on the gravitational wave (GW) emission from rotating collapse, core bounce, and early postbounce phases. Our results indicate that the GW signature of these phases is much more generic than previously estimated. In addition, we track the growth of a nonaxisymmetric instability of dominant m = 1 character in two of our models that leads to prolonged narrow-band GW emission at ~ 920-930 Hz over several tens of milliseconds.

astro-ph↗

arXiv2007

Fragmentation of general relativistic quasi-toroidal polytropes

We investigate the role of rotational instabilities in the context of black hole formation in relativistic stars. In addition to the standard scenario - an axially symmetric dynamical instability forming a horizon at the star's center - the recently found low-$T/|W|$ instabilities are shown to lead to fragmentation and off-center horizon formation in differentially rotating stars. This process might be an alternative pathway to produce SMBHs from supermassive stars with inefficient angular momentum transport.

gr-qc↗

arXiv2006

Multi-Dimensional Explorations in Supernova Theory

In this paper, we bring together various of our published and unpublished findings from our recent 2D multi-group, flux-limited radiation hydrodynamic simulations of the collapse and explosion of the cores of massive stars. Aided by 2D and 3D graphical renditions, we motivate the acoustic mechanism of core-collapse supernova explosions and explain, as best we currently can, the phases and phenomena that attend this mechanism. Two major foci of our presentation are the outer shock instability and the inner core g-mode oscillations. The former sets the stage for the latter, which damp by the generation of sound. This sound propagates outward to energize the explosion and is relevant only if the core has not exploded earlier by some other means. Hence, it is a more delayed mechanism than the traditional neutrino mechanism that has been studied for the last twenty years since it was championed by Bethe and Wilson. We discuss protoneutron star convection, accretion-induced-collapse, gravitational wave emissions, pulsar kicks, the angular anisotropy of the neutrino emissions, a subset of numerical issues, and a new code we are designing that should supercede our current supernova code VULCAN/2D. Whatever ideas last from this current generation of numerical results, and whatever the eventual mechanism(s), we conclude that the breaking of spherical symmetry will survive as one of the crucial keys to the supernova puzzle.

astro-ph↗

arXiv2006

Non-axisymmetric instability and fragmentation of general relativistic quasi-toroidal stars

In a recent publication, we have demonstrated that differentially rotating stars admit new channels of black hole formation via fragmentation instabilities. Since a higher order instability of this kind could potentially transform a differentially rotating supermassive star into a multiple black hole system embedded in a massive accretion disk, we investigate the dependence of the instability on parameters of the equilibrium model. We find that many of the models constructed exhibit non-axisymmetric instabilities with corotation points, even for low values of T/|W|, which lead to a fission of the stars into one, two or three fragments, depending on the initial perturbation. At least in the models selected here, an m=1 mode becomes unstable at lower values of T/|W|, which would seem to favor a scenario where one black hole with a massive accretion disk forms. In this case, we have gained evidence that low values of compactness of the initial model can lead to a stabilization of the resulting fragment, thus preventing black hole formation in this scenario.

astro-ph↗

arXiv2006

A New Mechanism for Gravitational Wave Emission in Core-Collapse Supernovae

We present a new theory for the gravitational wave signatures of core-collapse supernovae. Previous studies identified axisymmetric rotating core collapse, core bounce, postbounce convection, and anisotropic neutrino emission as the primary processes and phases for the radiation of gravitational waves. Our results, which are based on axisymmetric, Newtonian radiation-hydrodynamics supernova simulations (Burrows et al. 2006), indicate that the dominant emission process of gravitational waves in core-collapse supernovae may be the oscillations of the protoneutron star core. The oscillations are predominantly of g-mode character, are excited hundreds of milliseconds after bounce, and typically last for several hundred milliseconds. Our results suggest that even nonrotating core-collapse supernovae should be visible to current LIGO-class detectors throughout the Galaxy, and depending on progenitor structure, possibly out to Megaparsec distances.

astro-ph↗

arXiv2006

The Spin Periods and Rotational Profiles of Neutron Stars at Birth

We present results from an extensive set of one- and two-dimensional radiation-hydrodynamic simulations of the supernova core collapse, bounce, and postbounce phases, and focus on the protoneutron star (PNS) spin periods and rotational profiles as a function of initial iron core angular velocity, degree of differential rotation, and progenitor mass. For the models considered, we find a roughly linear mapping between initial iron core rotation rate and PNS spin. The results indicate that the magnitude of the precollapse iron core angular velocities is the single most important factor in determining the PNS spin. Differences in progenitor mass and degree of differential rotation lead only to small variations in the PNS rotational period and profile. Based on our calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming angular momentum conservation, we estimate final neutron star rotation periods. We find periods of one millisecond and shorter for initial central iron core periods of below ~ 10 s. This is appreciably shorter than what previous studies have predicted and is in disagreement with current observational data from pulsar astronomy. After considering possible spindown mechanisms that could lead to longer periods we conclude that there is no mechanism that can robustly spin down a neutron star from ~ 1 ms periods to the "injection" periods of tens to hundreds of milliseconds observed for young pulsars. Our results indicate that, given current knowledge of the limitations of neutron star spindown mechanisms, precollapse iron cores must rotate with periods around 50-100 seconds to form neutron stars with periods generically near those inferred for the radio pulsar population.

astro-ph↗

arXiv2005

One-armed Spiral Instability in a Low T/|W| Postbounce Supernova Core

A three-dimensional, Newtonian hydrodynamic technique is used to follow the postbounce phase of a stellar core collapse event. For realistic initial data we have employed post core-bounce snapshots of the iron core of a 20 solar mass star. The models exhibit strong differential rotation but have centrally condensed density stratifications. We demonstrate for the first time that such postbounce cores are subject to a so-called low-T/|W| nonaxisymmetric instability and, in particular, can become dynamically unstable to an m=1 - dominated spiral mode at T/|W| ~ 0.08. We calculate the gravitational wave emission by the instability and find that the emitted waves may be detectable by current and future GW observatories from anywhere in the Milky Way.

astro-ph↗

arXiv2005

Black hole formation through fragmentation of toroidal polytropes

We investigate new paths to black hole formation by considering the general relativistic evolution of a differentially rotating polytrope with toroidal shape. We find that this polytrope is unstable to nonaxisymmetric modes, which leads to a fragmentation into self-gravitating, collapsing components. In the case of one such fragment, we apply a simplified adaptive mesh refinement technique to follow the evolution to the formation of an apparent horizon centered on the fragment. This is the first study of the one-armed instability in full general relativity.

gr-qc↗

arXiv2004

Rotating Core Collapse and Bipolar Supernova Explosions

We review some of the reasons for believing that the generic core-collapse supernova is neutrino-driven, not MHD-jet driven. We include a discussion of the possible role of rotation in supernova blast energetics and morphology, and speculate on the origin of Cas A's and SN1987A's ejecta fields. Two "explosive" phenomena may be associated with most core collapses, the neutrino-driven supernova itself and an underenergetic jet-like ejection that follows. The latter may be a magnetic wind that easily penetrates the debris created by the much more energetic supernova. We predict that many core-collapse supernova remnants should have sub-dominant jet-like features. In Cas A, we associate this sub-dominant collimated wind with the "jet/counter-jet" structure observed. We suggest that the actual Cas A explosion itself is at nearly right angles to this jet, along a rotation axis that coincides with the bulk of the ejecta, the iron lobes, and the putative direction of motion of the point source. It may be that when rotation becomes sufficiently rapid that the strong-neutrino-driven-supernova/weak-jet duality switches to a strong-MHD-jet scenario that might be associated with hypernovae, and in some cases GRBs. Finally, we present a calculation using a new 2D multi-group, flux-limited radiation/hydrodynamics code we have recently developed for the simulation of core-collapse supernovae. We discuss the rotation-induced anisotropy in the neutrino radiation field, neutrino heating, and the neutrino flux vectors and speculate on rotation's possible role in the supernova mechanism and in the overall supernova phenomenon.

astro-ph↗