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Search Results: 1 - 10 of 197017 matches for " Brian W. O'Shea "
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Local and Global Radiative Feedback from Population III Star Formation
Brian W. O'Shea,Daniel J. Whalen
Physics , 2010, DOI: 10.1063/1.3518840
Abstract: We present an overview of recent work that focuses on understanding the radiative feedback processes that are potentially important during Population III star formation. Specifically, we examine the effect of the Lyman-Werner (photodissociating) background on the early stages of primordial star formation, which serves to delay the onset of star formation in a given halo but never suppresses it entirely. We also examine the effect that both photodissociating and ionizing radiation in I-fronts from nearby stellar systems have on the formation of primordial protostellar clouds. Depending on the strength of the incoming radiation field and the central density of the halos, Pop III star formation can be suppressed, unaffected, or even enhanced. Understanding these and other effects is crucial to modeling Population III star formation and to building the earliest generations of galaxies in the Universe.
Galaxy Clusters at the Edge: Temperature, Entropy, and Gas Dynamics at the Virial Radius
Jack O. Burns,Samuel W. Skillman,Brian W. O'Shea
Physics , 2010, DOI: 10.1088/0004-637X/721/2/1105
Abstract: Recently, Suzaku has produced temperature and entropy profiles, along with profiles of gas density, gas fraction, and mass, for multiple galaxy clusters out to ~r_200 (~= virial radius). In this paper, we compare these novel X-ray observations with results from N-body + hydrodynamic adaptive mesh refinement cosmological simulations using the Enzo code. There is excellent agreement in the temperature, density, and entropy profiles between a sample of 27 mostly substructure-free massive clusters in the simulated volume and the observed clusters. This supports our previous contention that clusters have "universal" outer temperature profiles. Furthermore, it appears that the simplest adiabatic gas physics used in these Enzo simulations is adequate to model the outer regions of these clusters without other mechanisms (e.g., non-gravitational heating, cooling, magnetic fields, or cosmic rays). However, the outskirts of these clusters are not in hydrostatic equilibrium. There is significant bulk flow and turbulence in the outer intracluster medium created by accretion from filaments. Thus, the gas is not fully supported by thermal pressure. The implications for mass estimation from X-ray data are discussed.
The Formation of Population III Binaries from Cosmological Initial Conditions
Matthew J. Turk,Tom Abel,Brian W. O'Shea
Physics , 2009, DOI: 10.1126/science.1173540
Abstract: Previous high resolution cosmological simulations predict the first stars to appear in the early universe to be very massive and to form in isolation. Here we discuss a cosmological simulation in which the central 50 solar mass clump breaks up into two cores, having a mass ratio of two to one, with one fragment collapsing to densities of 10^{-8} g/cc. The second fragment, at a distance of 800 astronomical units, is also optically thick to its own cooling radiation from molecular hydrogen lines, but is still able to cool via collision-induced emission. The two dense peaks will continue to accrete from the surrounding cold gas reservoir over a period of 10^5 years and will likely form a binary star system.
Protostellar Feedback Processes and the Mass of the First Stars
Jonathan C. Tan,Britton D. Smith,Brian W. O'Shea
Physics , 2010, DOI: 10.1063/1.3518887
Abstract: We review theoretical models of Population III.1 star formation, focusing on the protostellar feedback processes that are expected to terminate accretion and thus set the mass of these stars. We discuss how dark matter annihilation may modify this standard feedback scenario. Then, under the assumption that dark matter annihilation is unimportant, we predict the mass of stars forming in 12 cosmological minihalos produced in independent numerical simulations. This allows us to make a simple estimate of the Pop III.1 initial mass function and how it may evolve with redshift.
From F=ma to Flying Squirrels: Curricular Change in an Introductory Physics Course
Brian W. O'Shea,Laura Terry,Walter Benenson
Physics , 2013,
Abstract: We present outcomes from curricular changes made to an introductory calculus-based physics course whose audience is primarily life science majors, the majority of whom plan to pursue post-baccalaureate studies in medical and scientific fields. During the 2011-12 academic year, we implemented a "Physics of the life sciences" curriculum centered on a draft textbook that takes a novel approach to teaching physics to life science majors. In addition, substantial revisions were made to the homework and hands-on components of the course to emphasize the relationship between physics and the life sciences and to help the students to learn to apply physical intuition to life science-oriented problems. Student learning and attitudinal outcomes were assessed both quantitatively, using standard physics education research instruments, and qualitatively, using student surveys and a series of post-semester interviews. Students experienced high conceptual learning gains, comparable to other active learning-based physics courses. Qualitatively, a substantial fraction of interviewed students reported an increased interest in physics relative to the beginning of the semester. Furthermore, more than half of students self-reported that they could now relate physics topics to their majors and future careers, with interviewed subjects demonstrating a high level of ability to come up with examples of how physics affects living organisms and how it helped them to better understand content presented in courses in their major.
Fragmentation in Dusty Low-Metallicity Star Forming Halos
Gregory R. Meece,Britton D. Smith,Brian W. O'Shea
Physics , 2013, DOI: 10.1088/0004-637X/783/2/75
Abstract: The first stars in the universe, termed Population III, are thought to have been very massive compared to the stars that form in the present epoch. As feedback from the first generation of stars altered the contents of the interstellar medium, the universe switched to a low-mass mode of star formation, which continues in the high metallicity stars formed in the present era. Several studies have investigated the transition between metal-free and metal-enriched star formation, with tentative evidence being found for a metallicity threshold near 10^-3.5 Z_sun due to atomic and molecular transitions and another threshold near 10^-5.5 Z_sun due to dust. In this work, we simulate the formation of stars in idealized low-metallicity halos using the AMR code Enzo. We conduct several simulations of 10^6 M_sun and 10^7 M_sun halos in which the metal content, initial rotation, and degree of turbulence are varied in order to study the effect of these properties on gas fragmentation over a range of densities. We find tentative support for the idea of a critical metallicity, but the effect of varying metallicity on the gas we observe is not as dramatic as what has been reported in earlier studies. We find no clear relation between the initial spin or the initial level of turbulence in the halo and the final properties of the gas contained therein. Additionally, we find that the degree to which the Jeans length is refined, the initial density profile of the gas, and the inclusion of deuterium chemistry each have a significant effect on the evolution and fragmentation of the gas in the halo - in particular, we find that at least 64 grid cells are needed to cover the Jeans length in order to properly resolve the fragmentation.
Tracing the Evolution of High Redshift Galaxies Using Stellar Abundances
Brian D. Crosby,Brian W. O'Shea,Timothy C. Beers,Jason Tumlinson
Physics , 2013,
Abstract: This paper presents the first results from a model for chemical evolution that can be applied to N-body cosmological simulations and quantitatively compared to measured stellar abundances from large astronomical surveys. This model convolves the chemical yield sets from a range of stellar nucleosynthesis calculations (including AGB stars, Type Ia and II supernovae, and stellar wind models) with a user-specified stellar initial mass function (IMF) and metallicity to calculate the time-dependent chemical evolution model for a "simple stellar population" of uniform metallicity and formation time. These simple stellar population models are combined with a semi-analytic model for galaxy formation and evolution that uses merger trees from N-body cosmological simulations to track several $\alpha$- and iron-peak elements for the stellar and multiphase interstellar medium components of several thousand galaxies in the early ($z \geq 6$) universe. The simulated galaxy population is then quantitatively compared to two complementary datasets of abundances in the Milky Way stellar halo, and is capable of reproducing many of the observed abundance trends. The observed abundance ratio distributions are qualitatively well matched by our model, and the observational data is best reproduced with a Chabrier IMF, a chemically-enriched star formation efficiency of $0.2$, and a redshift of reionization of $7$.
Population III Star Formation In Large Cosmological Simulations I. Halo Temporal and Physical Environment
Brian D. Crosby,Brian W. O'Shea,Britton D. Smith,Matthew J. Turk,Oliver Hahn
Physics , 2013, DOI: 10.1088/0004-637X/773/2/108
Abstract: We present a semi-analytic, computationally inexpensive model to identify halos capable of forming a Population III star in cosmological simulations across a wide range of times and environments. This allows for a much more complete and representative set of Population III star forming halos to be constructed, which will lead to Population III star formation simulations that more accurately reflect the diversity of Population III stars, both in time and halo mass. This model shows that Population III and chemically enriched stars coexist beyond the formation of the first generation of stars in a cosmological simulation until at least z~10, and likely beyond, though Population III stars form at rates that are 4-6 orders of magnitude lower than chemically enriched stars by z=10. A catalog of more than 40,000 candidate Population III forming halos were identified, with formation times temporally ranging from z=30 to z=10, and ranging in mass from 2.3x10^5 M_sun to 1.2x10^10 M_sun. At early times, the environment that Population III stars form in is very similar to that of halos hosting chemically enriched star formation. At later times Population III stars are found to form in low-density regions that are not yet chemically polluted due to a lack of previous star formation in the area. Population III star forming halos become increasingly spatially isolated from one another at later times, and are generally closer to halos hosting chemically enriched star formation than to another halo hosting Population III star formation by z~10.
Cosmological Simulations of Isotropic Conduction in Galaxy Clusters
Britton D. Smith,Brian W. O'Shea,G. Mark Voit,David Ventimiglia,Samuel W. Skillman
Physics , 2013, DOI: 10.1088/0004-637X/778/2/152
Abstract: Simulations of galaxy clusters have a difficult time reproducing the radial gas-property gradients and red central galaxies observed to exist in the cores of galaxy clusters. Thermal conduction has been suggested as a mechanism that can help bring simulations of cluster cores into better alignment with observations by stabilizing the feedback processes that regulate gas cooling, but this idea has not yet been well tested with cosmological numerical simulations. Here we present cosmological simulations of ten galaxy clusters performed with five different levels of isotropic Spitzer conduction, which alters both the cores and outskirts of clusters, but not dramatically. In the cores, conduction flattens central temperature gradients, making them nearly isothermal and slightly lowering the central density but failing to prevent a cooling catastrophe there. Conduction has little effect on temperature gradients outside of cluster cores because outward conductive heat flow tends to inflate the outer parts of the intracluster medium (ICM) instead of raising its temperature. In general, conduction tends reduce temperature inhomogeneity in the ICM, but our simulations indicate that those homogenizing effects would be extremely difficult to observe in ~5 keV clusters. Outside the virial radius, our conduction implementation lowers the gas densities and temperatures because it reduces the Mach numbers of accretion shocks. We conclude that despite the numerous small ways in which conduction alters the structure of galaxy clusters, none of these effects are significant enough to make the efficiency of conduction easily measurable unless its effects are more pronounced in clusters hotter than those we have simulated.
The Santa Fe Light Cone Simulation Project: II. The Prospects for Direct Detection of the WHIM with SZE Surveys
Eric J. Hallman,Brian W. O'Shea,Britton D. Smith,Jack O. Burns,Michael L. Norman
Physics , 2009, DOI: 10.1088/0004-637X/698/2/1795
Abstract: Detection of the Warm-Hot Intergalactic Medium (WHIM) using Sunyaev-Zeldovich effect (SZE) surveys is an intriguing possibility, and one that may allow observers to quantify the amount of "missing baryons" in the WHIM phase. We estimate the necessary sensitivity for detecting low density WHIM gas with the South Pole Telescope (SPT) and Planck Surveyor for a synthetic 100 square degree sky survey. This survey is generated from a very large, high dynamic range adaptive mesh refinement cosmological simulation performed with the Enzo code. We find that for a modest increase in the SPT survey sensitivity (a factor of 2-4), the WHIM gas makes a detectable contribution to the integrated sky signal. For a Planck-like satellite, similar detections are possible with a more significant increase in sensitivity (a factor of 8-10). We point out that for the WHIM gas, the kinematic SZE signal can sometimes dominate the thermal SZE where the thermal SZE decrement is maximal (150 GHz), and that using the combination of the two increases the chance of WHIM detection using SZE surveys. However, we find no evidence of unique features in the thermal SZE angular power spectrum that may aid in its detection. Interestingly, there are differences in the power spectrum of the kinematic SZE, which may not allow us to detect the WHIM directly, but could be an important contaminant in cosmological analyses of the kSZE-derived velocity field. Corrections derived from numerical simulations may be necessary to account for this contamination.
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