Effects of Varying the Three-Body Molecular Hydrogen Formation Rate in Primordial Star Formation

The transformation of atomic hydrogen to molecular hydrogen through three-body reactions is a crucial stage in the collapse of primordial, metal-free halos, where the first generation of stars (Population III stars) in the Universe are formed. However, in the published literature, the rate coefficient for this reaction is uncertain by nearly an order of magnitude. We report on the results of both adaptive mesh refinement (AMR) and smoothed particle hydrodynamics (SPH) simulations of the collapse of metal-free halos as a function of the value of this rate coefficient. For each simulation method, we have simulated a single halo three times, using three different values of the rate coefficient. We find that while variation between halo realizations may be greater than that caused by the three-body rate coefficient being used, both the accretion physics onto Population III protostars as well as the long-term stability of the disk and any potential fragmentation may depend strongly on this rate coefficient.Comment: 29 pages, 7 figures; Accepted for publication in The Astrophysical Journa

Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts

Ionization fronts, the sharp radiation fronts behind which H/He ionizing photons from massive stars and galaxies propagate through space, were ubiquitous in the universe from its earliest times. The cosmic dark ages ended with the formation of the first primeval stars and galaxies a few hundred Myr after the Big Bang. Numerical simulations suggest that stars in this era were very massive, 25 - 500 solar masses, with H II regions of up to 30,000 light-years in diameter. We present three-dimensional radiation hydrodynamical calculations that reveal that the I-fronts of the first stars and galaxies were prone to violent instabilities, enhancing the escape of UV photons into the early intergalactic medium (IGM) and forming clumpy media in which supernovae later exploded. The enrichment of such clumps with metals by the first supernovae may have led to the prompt formation of a second generation of low-mass stars, profoundly transforming the nature of the first protogalaxies. Cosmological radiation hydrodynamics is unique because ionizing photons coupled strongly to both gas flows and primordial chemistry at early epochs, introducing a hierarchy of disparate characteristic timescales whose relative magnitudes can vary greatly throughout a given calculation. We describe the adaptive multistep integration scheme we have developed for the self-consistent transport of both cosmological and galactic ionization fronts.Comment: 6 pages, 4 figures, accepted for proceedings of HEDLA2010, Caltech, March 15 - 18, 201

Observing the First Stars and Black Holes

The high sensitivity of JWST will open a new window on the end of the cosmological dark ages. Small stellar clusters, with a stellar mass of several 10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun should be directly detectable out to redshift z=10, and individual supernovae (SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible beyond this redshift. Dense primordial gas, in the process of collapsing from large scales to form protogalaxies, may also be possible to image through diffuse recombination line emission, possibly even before stars or BHs are formed. In this article, I discuss the key physical processes that are expected to have determined the sizes of the first star-clusters and black holes, and the prospect of studying these objects by direct detections with JWST and with other instruments. The direct light emitted by the very first stellar clusters and intermediate-mass black holes at z>10 will likely fall below JWST's detection threshold. However, JWST could reveal a decline at the faint-end of the high-redshift luminosity function, and thereby shed light on radiative and other feedback effects that operate at these early epochs. JWST will also have the sensitivity to detect individual SNe from beyond z=10. In a dedicated survey lasting for several weeks, thousands of SNe could be detected at z>6, with a redshift distribution extending to the formation of the very first stars at z>15. Using these SNe as tracers may be the only method to map out the earliest stages of the cosmic star-formation history. Finally, we point out that studying the earliest objects at high redshift will also offer a new window on the primordial power spectrum, on 100 times smaller scales than probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and Concurrent Facilities", Astrophysics & Space Science Library, Eds. H. Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008

Physical Processes in Star Formation

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00693-8.Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.Peer reviewedFinal Accepted Versio

Using the physical properties of artefacts to manage through‐life knowledge flows in the built environment: an initial exploration

Effective through‐life management of built facilities requires effective through‐life knowledge management to support it. The KIM (Immortal Information and Through‐Life Knowledge Management) project attempted to develop such an approach, based on a dichotomy of knowledge and information. Knowledge is conceived in terms of communities of practice. An initial philosophical analysis demonstrates deficiencies in this conception. Drawing inspiration from production theory, a tripartite analysis is offered, suggesting that knowledge flows consist of: social practices, information and physical properties. Literature on physical properties from design studies, production management and ethnomethodology is briefly reviewed to demonstrate the information bearing functions of physical properties. Fieldwork conforming to the unique adequacy requirement of methods was carried out on construction sites, in hotel and hospital facilities during the use stage of their life cycles. Safety barriers on construction sites were found to have informational properties beyond their function as a physical barrier. The quality of information delivered by wayfinding signs was found to depend upon both the physical placement of the signs in relation to the surrounding environment and the physical layout of the sign itself. It was found that social practices are institutionalized to repair the knowledge flow when the physically instantiated wayfinding system breaks down. Finally, through the investigation of practices surrounding emergency resuscitation equipment, it was found that if the physical properties of information are not designed to mesh with the work practices of the setting, this will lead to a breakdown in the knowledge flow. It is suggested that elements of knowledge management, ethnomethodology, production management and design studies might be integrated to form the basis of a hybrid discipline

Re-ionizing the universe without stars

Recent observations show that the measured rates of star formation in the early universe are insufficient to produce re-ionization, and therefore, another source of ionizing photons is required. In this Letter, we examine the possibility that these can be supplied by the fast accretion shocks formed around the cores of the most massive haloes (10.5 < log M/M (aS (TM))< 12) on spatial scales of order 1 kpc. We model the detailed physics of these fast accretion shocks, and apply these to a simple 1-D spherical hydrodynamic accretion model for baryonic infall in dark matter halos with an Einasto density distribution. The escape of UV photons from these halos is delayed by the time taken to reach the critical accretion shock velocity for escape of UV photons; 220 km s(-1), and by the time it takes for these photons to ionize the surrounding baryonic matter in the accretion flow. Assuming that in the universe at large the baryonic matter tracks the dark matter, we can estimate the epoch of re-ionization in the case that accretion shocks act alone as the source of UV photons. We find that 50% of the volume (and 5-8% of the mass) of the universe can be ionized by z similar to 7-8. The UV production rate has an uncertainty of a factor of about 5 due to uncertainties in the cosmological parameters controlling the development of large scale structure. Because our mechanism is a steeply rising function of decreasing redshift, this uncertainty translates to a re-ionization redshift uncertainty of less than +/- 0.5. We also find that, even without including the UV photon production of stars, re-ionization is essentially complete by z similar to 5.8. Thus, fast accretion shocks can provide an important additional source of ionizing photons in the early universe.Peer reviewe

Abordagem sistêmica do gerenciamento pelas diretrizes: conceituação e aplicação A systematic approach to policy deployment: concept and application

Este artigo apresenta um modelo teórico do Gerenciamento pelas Diretrizes (GD), organizando suas principais características sob uma ótica conceitual, explorando os seus aspectos orgânicos. Inicialmente, é estabelecido o seu processo característico para, em seguida, serem definidos quatro elementos que o complementam: (1) sistema de priorização; (2) dispositivos estruturais; (3) dispositivos conciliadores; e (4) sistema de informação. A partir desta referência e por meio da pesquisa-ação é articulada sua implementação numa empresa de serviços que atua como distribuidor de cerveja, atendendo uma grande região geográfica. Desta intervenção, surgem novas características que contribuem para o aprimoramento da proposta.<br>This article presents the theoretical model of policy deployment, exploring and organizing its organic features. At first, the featuring process is established, afterwards four complementary elements are defined: the priority system, the structural devices, the conciliatory devices and the information system. The test is done by research method, "research by intervention in one reality". The intervention was promoted in a beer retailer that operates covering a wide geographic area. New findings were gotten therefore leading condition to improve the model proposed