1,720,988 research outputs found
The Detectability of Pair-Production Supernovae at z <~ 6
No full textNonrotating, zero-metallicity stars with initial masses 140<~M*<~260 Msolar are expected to end their lives as pair-production supernovae (PPSNe), in which an electron-positron pair-production instability triggers explosive nuclear burning. Interest in such stars has been rekindled by recent theoretical studies that suggest primordial molecular clouds preferentially form stars with these masses. Since metal enrichment is a local process, the resulting PPSNe could occur over a broad range of redshifts, in pockets of metal-free gas. Using the implicit hydrodynamics code KEPLER, we have calculated a set of PPSN light curves that addresses the theoretical uncertainties and allows us to assess observational strategies for finding these objects at intermediate redshifts. The peak luminosities of typical PPSNe are only slightly greater than those of Type Ia, but they remain bright much longer (~1 yr) and have hydrogen lines. Ongoing supernova searches may soon be able to limit the contribution of these very massive stars to <~1% of the total star formation rate density out to z~2, which already provides useful constraints for theoretical models. The planned Joint Dark Energy Mission satellite will be able to extend these limits out to z~6
Cosmological Impact Of Population III Binaries
Get PDFWe present the results of the stellar feedback from Population III (Pop III) binaries by employing improved, more realistic Pop III evolutionary stellar models. To facilitate a meaningful comparison, we consider a fixed mass of 60 M-circle dot incorporated in Pop III stars, either contained in a single star, or split up in binary stars of 30 M-circle dot each or an asymmetric case of one 45 and one 15 M-circle dot star. Whereas the sizes of the resulting H II regions are comparable across all cases, the He III regions around binary stars are significantly smaller than that of the single star. Consequently, the He+ 1640 angstrom recombination line is expected to become much weaker. Supernova (SN) feedback exhibits great variety due to the uncertainty in possible explosion pathways. If at least one of the component stars dies as a hypernova about 10 times more energetic than conventional core-collapse SNe, the gas inside the host minihalo is effectively blown out, chemically enriching the intergalactic medium (IGM) to an average metallicity of 10(-4)-10(-3) Z(circle dot), out to similar to 2 kpc. The single star, however, is more likely to collapse into a black hole, accompanied by at most very weak explosions. The effectiveness of early chemical enrichment would thus be significantly reduced, in contrast to. the lower mass binary stars, where at least one component is likely to contribute to heavy element production and dispersal. Important new feedback physics is also introduced if close binaries can form high-mass X-ray binaries, leading to the pre-heating and -ionization of the IGM beyond the extent of the stellar H II regions.IAU-Gruber FellowshipStanwood Johnston FellowshipKITP Graduate FellowshipDOE HEP Program DE-SC0010676NSF AST 0909129, AST-1009928, AST-1109394, PHY02-16783NASA Theory Program NNX14AH34GNASA NNX09AJ33GARC Future Fellowship FT120100363Monash University Larkins FellowshipDOE DE-GF02-87ER40328, DE-FC02-09ER41618Astronom
Detecting primordial stars
No full textWe study the detectability of primordial metal-free stars, both through direct searches for their emission, as well as searches for the resulting supernovae. We show that enrichment is a local process that takes place over an extended redshift range. While the duration of the transition from a metal-free to an enriched universe depends on several unknown factors, in all models late-forming metal-free stars are found in 107.5 108.0Msun objects, which are just large enough to cool, but small enough to not be clustered near areas of previous star formation. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyman-α emitters. If metal-free stars have masses 140Msun ≲ M* ≲ 260Msun, they are expected to end their lives as pair-production supernovae (PPSNe), in which an electron positron pair-production instability triggers explosive nuclear burning. Using the implicit hydrodynamics code KEPLER, we calculate a set of PPSNe light curves that allows us to assess observational strategies for finding these objects. The peak luminosities of typical PPSNe are only slightly greater than those of Type Ia supernovae, but they remain bright much longer ( ̃1 year) and have hydrogen lines. Ongoing supernova searches may soon be able to place stringent limits on the fraction of very massive stars out to z ≈ 2. The planned Joint Dark Energy Mission satellite will be able to extend these limits out to z ≈ 6
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Turbulence Modeling in Core-Collapse Supernovae with Machine Learning
Get PDFChaotic fluid motion, known at the small scales as turbulence, can significantly alter the large-scale evolution of astrophysical events. For example, the growth and impact of convection to produce a successful core-collapse supernova (CCSN) depend upon the evolution of turbulence. The ideal way to investigate it in such an environment would be with high-resolution direct numerical simulations (DNS) that resolve the turbulent energy cascade down to some small scale where the energy could be safely assumed to dissipate as heat. Unfortunately, given the high Reynolds number and a large range of spatial scales, this is well beyond the current state-of-the-art computational 3D CCSN models. Since turbulence cannot be properly simulated in 1D or 2D, a subgrid-scale model (SGS) is needed to capture the unresolved 3D physics. Simple analytical SGS models often lack accuracy, though, and complex ones are difficult to tune, resulting in limited generalizability based on initial conditions. Given the recent successes in turbulence SGS modeling with Machine Learning (ML) in adjacent fields, this thesis develops an ML algorithm to analyze current simulations and study the features of turbulence in CCSN. To demonstrate its efficacy, we test our ML approach on modeling dynamic 3D HD & MHD turbulence, integrating the former into a 1D code to study the role of one specific feature of turbulence (the effective turbulent pressure) on the fate of CCSN simulations. Furthermore, our ML tools can be used to study the broader effects of turbulence, explore other ML architectures, and be integrated in the outside 1- and multi-dimensional CCSN codes. The ML framework (Sapsan) and its implementation into a 1-dimensional code (COLLAPSO1D) are open-sourced and available for public use
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The Evolution and Explosion of Massive Stars
Get PDFHow a massive star ends its life depends upon how that life has been lived - the rotation, mass and composition it was born with, mass loss and exchange, and the complex convective and nuclear burning episodes it experienced along the way. In the end, the presupernova stellar core has a density structure that can be characterized by its "compactness" - essentially how fast the density declines outside the iron core. The likelihood that a massive star explodes, by any means, is sensitive to this compactness. It turns out, perhaps surprisingly, that the compactness is not a monotonic function of the star's birth mass, and, in some mass regions, whether the star explodes or not is almost random. Here the stellar physics underlying the development of compactness is explored for a fine grid of masses across a broad range of masses (9 - 120 solar masses). Using the model set generated, and with collaborators, the resulting explosions are explored assuming a neutrino-powered mechanism. Full isotopic nucleosynthesis, light curves, and remnant masses are calculated and found to be in good agreement with observations. Neglecting rotation, most stars above 20 Msun do not explode - though there are exceptions. In a related study, based on the same model set, the upper bound to the most luminous supernovae is explored considering various energy sources. The brightest possible supernova is found to be a rotationally powered explosion of a stripped core
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Signatures of the Late Time Core-Collapse Supernova Environment
Get PDFThe hot and dense proto-neutron star (PNS) born subsequent to core-collapse in a type II supernova explosion is an intense source of neutrinos of all flavors. It emits the 3-5 x 10^53 ergs of gravitational binding energy gained during collapse as neutrino radiation on a time scale of tens of seconds as it contracts, becomes increasingly neutron-rich and cools. While the supernova explosion mechanism and associated accretion of material is expected to influence the neutrino emission at early time (i.e. t <~ 1 s post bounce) the late time neutrino signal is shaped by the properties of the PNS, such as the nuclear equation of state (EoS), neutrino opacities in dense matter, and other microphysical properties that affect the cooling timescale by influencing either neutrino diffusion or convection. Detection of significant numbers of late time supernova neutrinos will provide a direct window into the properties of nuclear matter and neutron stars, if the neutrino signal can be modeled accurately. The average emitted neutrino energies also strongly affect nucleosynthesis in the neutrino driven wind, neutrino induced nucleosynthesis further out in the star, and the patterns of neutrino oscillations outside of the PNS.This thesis examines a number of aspects of this environment. First, the equations of spherically symmetric general relativistic radiation hydrodynamics are discussed, a new code for calculating neutrino transport in PNSs is described, and first results from this code are presented. It is found that the NDW is neutron rich for at least a few seconds, in contrast to other recent work. This change in the expected wind electron fraction is traced to the correct treatment of the nucleon dispersion relations in an interacting medium and turns out to be influenced by the sub-nuclear density symmetry energy. Late time convection in PNSs is also studied. It is found that the density dependence of the symmetry energy may affect the duration of convective activity, which is imprinted in the neutrino luminosity evolution. The second part of the thesis focuses on the neutrino driven wind (NDW) which is blown from the surface of the PNS. Time-dependent hydrodynamic calculations of the NDW are presented, which include accurate weak interaction physics coupled to a full nuclear reaction network. Using two published models of PNS neutrino luminosities,predictions of the contribution of the NDW to the integrated nucleosynthetic yield of the entire supernova are made. For the neutrino luminosity histories considered, it is found no r-process occurs in the most basic wind scenario because the NDW entropy is too low, the dynamical timescale is too long, and the wind electron fraction is too high. It is possible that the wind produces the N = 50 closed shell isotopes, but this depends on the neutrino luminosities employed. The effect of a secondary heating source on the wind is then considered. The general characteristics of a secondary heating source required to produce r-process nucleosynthesis are discussed. Then gravitoacoustic power excited either by convection or g-mode oscillations of the PNS is considered as a possible source of this heating. It is found that this a viable mechanism for increasing the wind entropy and decreasing the the dynamical timescale to values that are favorable for the r-process, when the neutrino spectra found in the first part of the thesis are assumed
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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The Effects of Small-Scale Mixing Processes on Supernova Progenitors
Get PDFMinor mixing processes—any fluid processes that mix material or transport heat other than convection or other large-scale flows—play a critical role in stellar evolution. These processes have been invoked through phenomenological models in order to explain away many issues in stellar evolution, such as the blue-red supergiant ratio problem and the progenitor problem of SN 1987A. We discuss our incompressible numerical simulations of one such process, thermohaline convection. We developed a semi-analytic formulation of the thermal and compositional transport of this process, which has been tested by other groups and implemented into MESA and the Toulouse–Geneva Evolution code. We also discuss compressible simulations of overshooting convection by Brummell et al. (2002) and the incompressible simulations of semi- convection by Wood et al. (2013), from which we developed one-dimensional models. We present some recent results of the effects of these models on a 15 solar mass star using the stellar evolution code KEPLER. These simulations put stricter limits on the extent of overshooting convection and the efficiency of semi-convection
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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