This review describes the use of low-energy collisionally activated dissociation (CAD) with both tandem quadrupole and ion-trap mass spectrometry toward structural characterization of ...glycerophospholipids (GPLs), including classes of glycerophosphocholine, glycerophosphoethanolamine, glycerophosphoserine, glycerophosphoglycerol glycerophosphoinositol and glycerophosphatidic acid, as well as their lyso-, plasmanyl-, and plasmenylphospholipid subclasses. The mechanisms underlying the fragmentation processes leading to structural characterization of GPLs in various ion forms desorbed by electrospray ionization in the positive-ion and negative-ion modes are also discussed. The tandem mass spectrometric approaches afford the identification of the polar head group, the fatty acid substituents and the location of the radyl groups on the glycerol backbone of all the GPLs.
Previous high-resolution cosmological simulations predicted that the first stars to appear in the early universe were very massive and formed in isolation. Here, we discuss a cosmological simulation ...in which the central 50 Mmiddle dot in circle (where Mmiddle dot in circle is the mass of the Sun) clump breaks up into two cores having a mass ratio of two to one, with one fragment collapsing to densities of 10⁻⁸ grams per cubic centimeter. The second fragment, at a distance of approximately 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 approximately 10⁵ years and will likely form a binary star system.
We present the grackle chemistry and cooling library for astrophysical simulations and models. grackle provides a treatment of non-equilibrium primordial chemistry and cooling for H, D and He ...species, including H2 formation on dust grains; tabulated primordial and metal cooling; multiple ultraviolet background models; and support for radiation transfer and arbitrary heat sources. The library has an easily implementable interface for simulation codes written in c, c++ and fortran as well as a python interface with added convenience functions for semi-analytical models. As an open-source project, grackle provides a community resource for accessing and disseminating astrochemical data and numerical methods. We present the full details of the core functionality, the simulation and python interfaces, testing infrastructure, performance and range of applicability. grackle is a fully open-source project and new contributions are welcome.
By definition, Population III stars are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas phase, leading to large characteristic masses. Population II ...stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10 super(-6)-10 super(-3.5) Z sub(middot in circle). We present an adaptive mesh refinement radiation hydrodynamics simulation that follows the transition from Population III to Population II star formation. The maximum spatial resolution of 1 comoving parsec allows for individual molecular clouds to be well resolved and their stellar associations to be studied in detail. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair-instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5% to nearly the cosmic fraction in halos with merger histories rich in halos above 10 super(7) M sub(middot in circle). A single pair-instability supernova is sufficient to enrich the host halo to a metallicity floor of 10 super(-3) Z sub(middot in circle) and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Ly alpha systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t-Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling.
ABSTRACT
Massive stars provide feedback that shapes the interstellar medium of galaxies at all redshifts and their resulting stellar populations. Here we present three adaptive mesh refinement ...radiation hydrodynamics simulations that illustrate the impact of momentum transfer from ionizing radiation to the absorbing gas on star formation in high‐redshift dwarf galaxies. Momentum transfer is calculated by solving the radiative transfer equation with a ray‐tracing algorithm that is adaptive in spatial and angular coordinates. We find that momentum input partially affects star formation by increasing the turbulent support to a three‐dimensional rms velocity equal to the circular velocity of early haloes. Compared to a calculation that neglects radiation pressure, the star formation rate is decreased by a factor of 5 to 1.8 × 10−2 M⊙ yr−1 in a dwarf galaxy with a dark matter and stellar mass of 2.0 × 108 and 4.5 × 105 M⊙, respectively, when radiation pressure is included. Its mean metallicity of 10−2.1 Z⊙ is consistent with the observed dwarf galaxy luminosity–metallicity relation. However, one may naively expect from the calculation without radiation pressure that the central region of the galaxy overcools and produces a compact, metal‐rich stellar population with an average metallicity of 0.3 Z⊙, indicative of an incorrect physical recipe. In addition to photoheating in H ii regions, radiation pressure further drives dense gas from star‐forming regions, so supernova feedback occurs in a warmer and more diffuse medium, launching metal‐rich outflows. Capturing this aspect and a temporal separation between the start of radiative and supernova feedback are numerically important in the modelling of galaxies to avoid the ‘overcooling problem’. We estimate that dust in early low‐mass galaxies is unlikely to aid in momentum transfer from radiation to the gas.
To define a quantitative stratification algorithm for the risk of early-onset sepsis (EOS) in newborns ≥ 34 weeks' gestation.
We conducted a retrospective nested case-control study that used split ...validation. Data collected on each infant included sepsis risk at birth based on objective maternal factors, demographics, specific clinical milestones, and vital signs during the first 24 hours after birth. Using a combination of recursive partitioning and logistic regression, we developed a risk classification scheme for EOS on the derivation dataset. This scheme was then applied to the validation dataset.
Using a base population of 608,014 live births ≥ 34 weeks' gestation at 14 hospitals between 1993 and 2007, we identified all 350 EOS cases <72 hours of age and frequency matched them by hospital and year of birth to 1063 controls. Using maternal and neonatal data, we defined a risk stratification scheme that divided the neonatal population into 3 groups: treat empirically (4.1% of all live births, 60.8% of all EOS cases, sepsis incidence of 8.4/1000 live births), observe and evaluate (11.1% of births, 23.4% of cases, 1.2/1000), and continued observation (84.8% of births, 15.7% of cases, incidence 0.11/1000).
It is possible to combine objective maternal data with evolving objective neonatal clinical findings to define more efficient strategies for the evaluation and treatment of EOS in term and late preterm infants. Judicious application of our scheme could result in decreased antibiotic treatment in 80,000 to 240,000 US newborns each year.
Cosmological hydrodynamical simulations of galaxy evolution are increasingly able to produce realistic galaxies, but the largest hurdle remaining is in constructing subgrid models that accurately ...describe the behaviour of stellar feedback. As an alternate way to test and calibrate such models, we propose to focus on the circumgalactic medium (CGM). To do so, we generate a suite of adaptive mesh refinement simulations for a Milky-Way-massed galaxy run to z = 0, systematically varying the feedback implementation. We then post-process the simulation data to compute the absorbing column density for a wide range of common atomic absorbers throughout the galactic halo, including H i, Mg ii, Si ii, Si iii, Si iv, C iv, N v, O vi and O vii. The radial profiles of these atomic column densities are compared against several quasar absorption line studies to determine if one feedback prescription is favoured. We find that although our models match some of the observations (specifically those ions with lower ionization strengths), it is particularly difficult to match O vi observations. There is some indication that the models with increased feedback intensity are better matches. We demonstrate that sufficient metals exist in these haloes to reproduce the observed column density distribution in principle, but the simulated CGM lacks significant multiphase substructure and is generally too hot. Furthermore, we demonstrate the failings of inflow-only models (without energetic feedback) at populating the CGM with adequate metals to match observations even in the presence of multiphase structure. Additionally, we briefly investigate the evolution of the CGM from z = 3 to present. Overall, we find that quasar absorption line observations of the gas around galaxies provide a new and important constraint on feedback models.
MAGNETIC FIELDS IN POPULATION III STAR FORMATION TURK, Matthew J; OISHI, Jeffrey S; ABEL, Tom ...
Astrophys.J.745:154,2012,
02/2012, Letnik:
745, Številka:
2
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
We study the buildup of magnetic fields during the formation of Population III star-forming regions by conducting cosmological simulations from realistic initial conditions and varying the Jeans ...resolution. To investigate this in detail, we start simulations from identical initial conditions, mandating 16, 32, and 64 zones per Jeans length, and study the variation in their magnetic field amplification. We find that, while compression results in some amplification, turbulent velocity fluctuations driven by the collapse can further amplify an initially weak seed field via dynamo action, provided there is sufficient numerical resolution to capture vortical motions (we find this requirement to be 64 zones per Jeans length, slightly larger than but consistent with previous work run with more idealized collapse scenarios). We explore saturation of amplification of the magnetic field, which could potentially become dynamically important in subsequent, fully resolved calculations. We have also identified a relatively surprising phenomenon that is purely hydrodynamic: the higher-resolved simulations possess substantially different characteristics, including higher infall velocity, increased temperatures inside 1000 AU, and decreased molecular hydrogen content in the innermost region. Furthermore, we find that disk formation is suppressed in higher-resolution calculations, at least at the times that we can follow the calculation. We discuss the effect this may have on the buildup of disks over the accretion history of the first clump to form as well as the potential for gravitational instabilities to develop and induce fragmentation.
Satellite-derived high-resolution precipitation products (HRPP) have been developed to address the needs of the user community and are now available with 0.25° × 0.25° (or less) subdaily resolutions. ...This paper evaluates a number of commonly available satellite-derived HRPPs covering northwest Europe over a 6-yr period. Precipitation products include the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA), the Climate Prediction Center (CPC) morphing (CMORPH) technique, the CPC merged microwave technique, the Naval Research Laboratory (NRL) blended technique, and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) technique. In addition, the Geosynchronous Operational Environmental Satellite (GOES) precipitation index (GPI) and the European Centre for Medium-Range Weather Forecasting (ECMWF) operational forecast model products are included for comparison. Surface reference data from the European radar network is used as ground truth, supported by the Global Precipitation Climatology Centre (GPCC) precipitation gauge analysis and gauge data over the United Kingdom. Measures of correlation, bias ratio, probability of detection, and false alarm ratio are used to evaluate the products. Results show that satellite products generally exhibit a seasonal cycle in correlation, bias ratio, probability of detection, and false alarm ratio, with poorer statistics during the winter. The ECMWF model also shows a seasonal cycle in the correlation, although the results are poorer during the summer, while the bias ratio, probability of detection, and false alarm ratio are consistent through all seasons. Importantly, all the satellite HRPPs underestimate precipitation over northwest Europe in all seasons.
Numerous cosmological hydrodynamic studies have addressed the formation of galaxies. Here we choose to study the first stages of galaxy formation, including nonequilibrium atomic primordial gas ...cooling, gravity, and hydrodynamics. Using initial conditions appropriate for the concordance cosmological model of structure formation, we perform two adaptive mesh refinement simulations of image10 super(8) M sub(image) galaxies at high redshift. The calculations resolve the Jeans length at all times with more than 16 cells and capture over 14 orders of magnitude in length scales. In both cases, the dense, 10 super(5) solar mass, one parsec central regions are found to contract rapidly and have turbulent Mach numbers up to 4. Despite the ever decreasing Jeans length of the isothermal gas, we only find one site of fragmentation during the collapse. However, rotational secular bar instabilities transport angular momentum outward in the central parsec as the gas continues to collapse and lead to multiple nested unstable fragments with decreasing masses down to sub-Jupiter mass scales. Although these numerical experiments neglect star formation and feedback, they clearly highlight the physics of turbulence in gravitationally collapsing gas. The angular momentum segregation seen in our calculations plays an important role in theories that form supermassive black holes from gaseous collapse.
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