Browsing by Subject "Supernovae"

Now showing 1 - 11 of 11

Collapsar accretion and the gamma-ray burst X-ray light curve
(2010-05) Lindner, Christopher Carl; Milosavljević, Miloš; Bromm, Volker; Kumar, Pawan; Sneden, Christopher A.; Wheeler, John C.
We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB [gamma]-ray and X-ray light curve, which lends support to the proposal by Kumar et al. (2008a,b) that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of [scientific symbols]. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate Ṁ [proportional to] t⁻²̇⁸, which resembles the L[subscript x] [proportional to] t⁻³ decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low-angular-momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ~ 20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 10¹⁰ K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt [gamma]-ray emission should have more slowly rotating envelopes.
Differential metal enrichment and dust synthesis in the aftermath of supernova explosions
(2016-08-12) Sluder, Alan; Milosavljević, Miloš; Bromm, Volker; Evans II, Neal J; Finkelstein, Steven L; Lazzati, Davide; Wheeler, John C
Emerging from the Big Bang, the Universe contained hydrogen, helium, and lithium. Heavier elements were synthesized in stars and stellar explosions. The first stars formed when the Universe was about a hundred million years old and exploded to scatter their newly-minted elements into space. Elements such as carbon and iron polluted the protogalactic gas clouds from which new stellar generations would form. Low mass stars dating from this early chemical enrichment have survived in the Galaxy as a fossil record of the young Universe. The formation of low-mass stars requires dust grains but dust synthesis is still incompletely understood. I investigate two questions central to the role of stellar explosions in the transformation of the Universe: how the measured variation in ancient stellar chemical abundance patterns relates to the complex hydrodynamics of stellar explosions---the supernovae, and how stellar explosions synthesize the dust that is necessary for low-mass star formation. First, I use high performance computing to model the aftermath of a supernova in an ancient cosmic dark matter halo. Tracing the transport of individual nucleosynthetically-enriched mass elements from their explosion site through the subsequent gravitational compression and collapse, I quantify how the complex hydrodynamics skews the chemical abundances in the collapsed gas from the explosion's gross elemental yields. I find that a portion of the measured variation in elemental abundance ratios in ancient low-mass stars can be ascribed to specific hydrodynamic effects. Second, I use chemical and kinetic calculations to study how, during the first three decades, gas-phase elements in supernova ejecta aggregate into solid dust grains. I compute the grains' chemical composition, size distribution, and mass yield. My model takes into account the effects of ejecta radioactivity, the lack of thermal coupling between gas and dust, and grain electric charge. The model incorporates non-equilibrium chemical reactions that produce atomic and molecular clusters, grain growth by accretion and coagulation, and grain destruction by evaporation and oxygen and noble gas ionic weathering. I find that a large fraction of refractory elements can be incorporated into grains with size distribution resembling that in the nearby supernova SN 1987A.
Discovery, observations and theory of over luminous supernovae and peculiar transients
(2010-05) Chatzopoulos, Emmanouil; Wheeler, J. Craig; Robinson, Edward L.; Kumar, Pawan; Milosavljevic, Milos; Vinko, Joszef
Modern wide-field imaging transient search projects led to the discovery of a new class of rare, over-luminous stellar explosions. Events like SN 2005ap (Quimby et al. 2007a), SN 2006gy (Quimby 2006; Smith et al. 2007), SN 2006tf (Quimby, Castro \& Mondol 2007; Quimby et al. 2007b; Smith et al. 2008), SN 2008am (Chatzopoulos et al. 2010), SN 2008es (Yuan et al. 2008; Gezari et al. 2008; Miller et al. 2008) SN 2008fz (Drake et al. 2009) and SN 2008iy (Miller et al. 2010) introduced us new evidence about stellar death, since traditional ideas about the mechanisms that can power these phenomena are found to be inadequate to explain the observed properties. The Texas Supernova Search Project (TSS; Quimby et al. 2005) and its successor, the ROTSE-Supernova Verification Project (RSVP; Yuan et al. 2007) discovered most of the above mentioned over-luminous supernovae (OLSNe) over the past five years of their operation. The advantage of this project is that it is essentially free of selection bias or the limits of a targeted search; the automated wide field (~3 square degrees) ROTSE-III telescopes (Akerlof et al. 2003), scan the whole sky blindly, looking for transients down to unfiltered magnitude ~19 mag and they do not focus on pre-selected galaxies. An estimated rate for these exceptionally luminous supernovae is ~ 2.6 10^{-7} events/Mpc^{3}/yr (Quimby et al. 2009a). Current and future transient search projects such as the SDSS-II Supernova Survey (Frieman et al. 2008),the Palomar Transient Factory (PTF; Law et al. 2009), SkyMapper (Schmidt et al. 2005), PanSTARRS (Chambers 2006) and Large Synoptic Survey Telescope (Tyson \& LSST collaboration 2002) are expected to increase the number of rare over-luminous (or, under-luminous) supernove and to discover new, unusual transients.
The hunt for the first supernovae : the source density and observability of pair instability supernovae from the first stars
(2012-05) Hummel, Jacob Alexander; Bromm, Volker
Theoretical models predict that some of the first stars ended their lives as extremely energetic pair-instability supernovae (PISNe). With energies approaching 10⁵³ ergs, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST), allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic halo mass function based approach, accounting for the effects of feedback from star formation on the PISN rate using cosmological simulations. We estimate an upper limit of ~0.2 PISNe per JWST field of view at any given time. Feedback can reduce this rate significantly, e.g., lowering it to as little as one PISN per 4000 JWST fields of view for the most pessimistic explosion models. We also find that the main obstacle to observing PISNe from the first stars is their scarcity, not their faintness; exposures longer than a few times 10⁴ s will do little to increase the number of PISNe found. Given this we suggest a mosaic style search strategy for detecting PISNe from the first stars. Even rather high redshift PISNe are unlikely to be missed by moderate exposures, and a large number of pointings will be required to ensure a detection.
Multidimensional multiscale dynamics of high-energy astrophysical flows
(2010-05) Couch, Sean Michael; Wheeler, J. Craig; Milosavljević, Miloš; Bromm, Volker; Hoeflich, Peter; Jaffe, Daniel; Kumar, Pawan
Astrophysical flows have an enormous dynamic range of relevant length scales. The physics occurring on the smallest scales often influences the physics of the largest scales, and vice versa. I present a detailed study of the multiscale and multidimensional behavior of three high-energy astrophysical flows: jet-driven supernovae, massive black hole accretion, and current-driven instabilities in gamma-ray burst external shocks. Both theory and observations of core-collapse supernovae indicate these events are not spherically-symmetric; however, the observations are often modeled assuming a spherically-symmetric explosion. I present an in-depth exploration of the effects of aspherical explosions on the observational characteristics of supernovae. This is accomplished in large part by high-resolution, multidimensional numerical simulations of jet-driven supernovae. The existence of supermassive black holes in the centers of most large galaxies is a well-established fact in observational astronomy. How such black holes came to be so massive, however, is not well established. In this work, I discuss the implications of radiative feedback and multidimensional behavior on black hole accretion. I show that the accretion rate is drastically reduced relative to the Eddington rate, making it unlikely that stellar mass black holes could grow to supermassive black holes in less than a Hubble time. Finally, I discuss a mechanism by which magnetic field strength could be enhanced behind a gamma-ray burst external shock. This mechanism relies on a current-driven instability that would cause reorganization of the pre-shock plasma into clumps. Once shocked, these clumps generate vorticity in the post-shock plasma and ultimately enhance the magnetic energy via a relativistic dynamo process.
A one dimensional model of convection in iron core collapse supernovae
(1998) Wang, Joseph Chen-yu, 1969-; Wheeler, J. Craig
Although convection is agreed to exist within the newly formed neutron stars in core collapse supernovae, its role remains unclear. Much of the uncertainty concerning the role of convection may be attributed to the fact that the main tools for the investigation of convection in these supernovae are multi-dimensional codes. While these codes provide insight into the role of convection in supernova, their computationally intensive nature makes it impractical to run these codes repeatedly under different physical assumptions and to explore the physics of the late time evolution of the iron core collapse. A one dimensional algorithm for modeling time-dependent convection was developed by treating the convectively unstable zones as two streams. Models using this algorithm were run using various parameters. A series of models were run including different types of neutrinos and different convective effects. Without convection, the shock stalled at a radius of 2 x 10⁷ cm. No qualitative differences that would aid the supernova explosion were observed when convection was included. In models containing all neutrino species, an entropy spike developed in the immediate post shock region. In contrast to interpretations which attribute this spike to radiative heating by neutrinos, the spike in the current models are believed to be caused by shock heating. Comparison of models containing different neutrino species suggests that a "heating dilemma" exists, in that for a shock evolving quasi-statically whose post-shock region is in hydrostatic equilibrium, increased post shock entropy is associated with a smaller shock stall radius. Therefore higher entropies behind the shock in these conditions correspond to model failure rather than to successful explosions. This correlation also appears to produce an inverse relationship between root mean square radiated neutrino energy and shock stall radius
Simulations of high-energy astrophysical phenomena
(2014-12) Lindner, Christopher Carl; Milosavljević, Miloš
Supercomputer technology has revolutionized our studies of the most energetic astrophysical phenomena. Here, I present my simulations of energetic outbursts of gamma rays and the explosions of massive stars, and my efforts to further the computational astrophysics frontier with the development of a radiation hydrodynamics code. First, I present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst (GRB) progenitor star, a "collapsar," onto the central compact object, which we assume is a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB [gamma]-ray and X-ray light curve, which lends support to the proposal by Kumar et al. (2008a,b) that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of ~ 0.2 [solar mass] s⁻¹. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied by a sudden and rapid power-law decline in the central accretion rate [mathematical formula], which resembles the L [subscript x] [is proportional to] t⁻³ decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low-angular-momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows, which is potentially suggestive of the "plateau"' phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase will be ~ 20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 10¹⁰ K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt [gamma]-ray emission should have more slowly rotating envelopes. Observational evidence suggests a link between long GRBs and Type Ic supernovae. I propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. I present spherically-symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies ~ 5 x 10⁵⁰ ergs, velocities ~ 3000 km s⁻¹, and unbound material masses > 5 [solar mass] are possible in a rapidly rotating 16 [solar mass] main sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications. In many high-energy astrophysical phenomena, the force of radiation pressure will have a direct effect on the hydrodynamics. Observing radiation is also the primary way we investigate our universe. With this in mind, I present my expansion of the FLASH hydrodynamics code, where I have implemented a gray, flux-limited diffusion (FLD) radiation hydrodynamics (RHD) solver. My solver utilizes the FLASH's diffusion packages that are powered by HYPRE. I have written a new, efficient radiation-matter coupling solver, which exactly integrates the equations for radiation-matter coupling and operates without any time step restrictions. I have also rewritten the unsplit hydrodynamics solver in FLASH to incorporate the changes in PPM characteristic tracing and the Riemann solver required to properly capture the radiation pressure force in regions that are not entirely optically thick. This has required the addition of a new Riemann solver to FLASH, similar to the Riemann solver in the CASTRO RHD code. I then present my validation tests of the code. This code will be made publicly available.
Superluminous supernovae : theory and observations
(2013-05) Chatzopoulos, Emmanouil; Wheeler, J. Craig
The discovery of superluminous supernovae in the past decade challenged our understanding of explosive stellar death. Subsequent extensive observations of superluminous supernova light curves and spectra has provided some insight for the nature of these events. We present observations of one of the most luminous self-interacting supernovae ever observed, the hydrogen-rich SN 2008am discovered by the Robotic Optical Transient Search Experiment Supernova Verification Project with the ROTSE-IIIb telescope located in the McDonald Observatory. We provide theoretical modeling of superluminous supernova light curves and fit the models to a number of observed events and similar transients in order to understand the mechanism that is responsible for the vast amounts of energy emitted by these explosions. The models we investigate include deposition of energy due to the radioactive decays of massive amounts of nickel-56, interaction of supernova ejecta with a dense circumstellar medium and magnetar spin-down. To probe the nature of superluminous supernovae progenitor stars we study the evolution of massive stars, including important effects such as rotation and magnetic fields, and perform multi-dimensional hydrodynamics simulations of the resulting explosions. The effects of rotational mixing are also studied in solar-type secondary stars in cataclysmic variable binary star systems in order to provide an explanation for some carbon-depleted examples of this class. We find that most superluminous supernovae can be explained by violent interaction of the SN ejecta with >1 Msun dense circumstellar shells ejected by the progenitor stars in the decades preceding the SN explosion.
The Texas supernova search
(2006) Quimby, Robert Michael; Wheeler, J. Craig
Supernovae (SNe) are popular tools to explore the cosmological expansion of the Universe owing to their bright peak magnitudes and reasonably high rates; however, even the relatively homogeneous Type Ia supernovae are not intrinsically perfect standard candles. Their absolute peak brightness must be established by corrections that have been largely empirical. Hundreds of SNe are now found every year, shrinking the statistical errors in the cosmological parameters, but most of these distant discoveries do little to further the physical understanding of SNe, which could illuminate unknown systematics. The Texas Supernova Search was not designed to discover the most SNe nor the most distant SNe, but it was instead created to amass a small collection of well-observed nearby SNe with detailed, multi-epoch spectral observations beginning at the earliest possible phases. For the past two years, I have pointed ROTSE-IIIb’s 1.85 X 1.85 degree field of view at nearby galaxy clusters and searched thousands of galaxies, covering hundreds of square degrees on the sky, for supernovae. With ToO time on the neighboring 9.2m Hobby-Eberly Telescope, the search has captured SNe spectra at some of the earliest phases ever. In this dissertation I describe the Texas Supernova Search and present the 30 supernovae, 11 novae, and 6 dwarf novae discovered in the first two years of the program.
Turbulent mixing of chemical elements in galaxies
(2008-05) Pan, Liubin; Scalo, John M.
Chemical elements synthesized in stars are released into the interstellar medium (ISM) from discrete and localized events such as supernova (SN) explosions and stellar winds. The efficiency of transport and mixing of the new nucleosynthesis products in the ISM determines the degree of chemical inhomogeneity in the galaxy, which is observable in objects of the same age, such as coeval stars and the ISM today. It also has implications for the transition from metal-poor to normal star formation in high-redshift galaxies. We develop a physical mixing model for chemical homogenization in the turbulent ISM of galaxies using modern theories and methods for passive scalar turbulence. A turbulent velocity field stretches, compresses and folds tracers into structures of smaller and smaller scales that can be homogenized faster by micro-scopic diffusivity, the only physical process that truly mixes. From a model that incorporates this physical process, an evolution equation for the probability distribution of the tracer concentration is derived. Including the processes of new metal release, infall of low metallicity gas and incorporation of metals into new stars in the equation, we establish a new approach to investigate chemical inhomogeneity in galaxies: a kinetic equation for the metallicity probability distribution function, containing all the 1-point statistical information of the metallicity fluctuations. Motivated by a recent interpretation of ultraviolet properties of high-redshift Lyman Break Galaxies, we apply this approach to study mixing of primordial gas in these galaxies and find that primordial gas can survive for ~ 100 Myr in the presence of continuous metal sources and turbulent mixing if the unlikely efficient mixing in SN shells is excluded. Recent observations show that the Galaxy has been extremely homogeneous during most of its history. In an attempt to understand the homogeneity using our approach, we find that standard chemical evolution models without infall give metallicity scatters consistent with observations while all the infall models produce scatters at least 5 times larger than observed. To avoid this discrepancy and to remain a valid solution to the G-dwarf problem, the main motivation for infall models, the infall gas is required to primarily consist of small clouds of size less than ~ 5 pc. Fluctuations in the carbon to oxygen abundance ratio are of astrobiological interest: regions with C>O are likely to be devoid of water, which is thought to be essential for life. A small degree of inhomogeneity in the ratio gives a finite probability for the existence of regions with C>O even when the average ratio is smaller than unity. As the mean C/O ratio increases, as supported by observations and theoretical models, the Galaxy will eventually make a transition from mostly oxygen-rich to mostly carbon-rich. To the extent that life requires liquid water, the formation of habitable planets would no longer be possible. Adopting a negative Galactic C/O radial gradient, the transition appears as an outward-moving dehydration wave from the inner regions of the Galaxy. Finally we examine the effect of turbulent stretching on nuclear flames in Type Ia Supernova (SN Ia) progenitors. Turbulent stretching exhibits strong intermittency at small scales where its probability distribution shows a broad tail, corresponding to intense but rare stretching events. These events have important implications for the flame burning state and thus for the deflagration to detonation transition (DDT) in SN Ia explosions. Current DDT models require a critical turbulent intensity or stretching over a flame region that is sufficiently large. We find that including local intermittent stretching in these models results in a shift toward larger transition densities at which the DDT occurs.
Use of near infrared spectra to probe the chemical structure of type Ia supernovae
(2006) Marion, George Howard; Wheeler, J. Craig; Höflich, Peter