The ability of primordial gas to cool in protogalactic haloes exposed to ultraviolet (UV) radiation is critically dependent on the self-shielding of H2. We perform radiative transfer calculations of ...LW line photons, post-processing outputs from three-dimensional adaptive mesh refinement simulations of haloes with T
vir≳ 104 K at z ∼ 10. We calculate the optically thick photodissociation rate numerically, including the effects of density, temperature and velocity gradients in the gas, as well as line overlap and shielding of H2 by H i, over a large number of sightlines. In low-density regions (n ≲ 104 cm−3) the dissociation rates exceed those obtained using most previous approximations by more than an order of magnitude; the correction is smaller at higher densities. We trace the origin of the deviations primarily to inaccuracies of (i) the most common fitting formula for the suppression of the dissociation rate, from Draine and Bertoldi and (ii) estimates for the effective shielding column density from local properties of the gas. The combined effects of gas temperature and velocity gradients are comparatively less important, typically altering the spherically averaged rate only by a factor of ≲2. We present a simple modification to the Draine & Bertoldi fitting formula for the optically thick rate which improves agreement with our numerical results to within ∼15 per cent, and can be adopted in future simulations. We find that estimates for the effective shielding column can be improved by using the local Sobolev length. Our correction to the H2 self-shielding reduces the critical LW flux to suppress H2 cooling in T
vir≳ 104 K haloes by an order of magnitude; this increases the number of such haloes in which supermassive (M ∼ 105 M⊙) black holes may have formed.
It has been shown that HD molecules can form efficiently in metal-free gas collapsing into massive protogalactic haloes at high redshift. The resulting radiative cooling by HD can lower the gas ...temperature to that of the cosmic microwave background (CMB), T
CMB= 2.7(1 +z) K, significantly below the temperature of a few ×100 K achievable via H2 cooling alone, and thus reduce the masses of the first generation of stars. Here we consider the suppression of HD cooling by ultraviolet (UV) irradiation in the Lyman-Werner (LW) bands. We include photodissociation of both H2 and HD, and explicitly compute the self-shielding and shielding of both molecules by neutral hydrogen, H i, as well as the shielding of HD by H2. We use a simplified dynamical collapse model, and follow the chemical and thermal evolution of the gas, in the presence of a UV background. We find that a LW flux of J
crit,HD≈ 10−22 erg cm−2 sr−1 s−1 Hz−1 is able to suppress HD cooling and thus prevent collapsing primordial gas from reaching temperatures below ∼100 K. The main reason for the lack of HD cooling for J > J
crit,HD is the partial photodissociation of H2, which prevents the gas from reaching sufficiently low temperatures (T < 150 K) for HD to become the dominant coolant; direct HD photodissociation is unimportant except for a narrow range of fluxes and column densities. Since the prevention of HD cooling requires only partial H2 photodissociation, the critical flux J
crit,HD is modest, and is below the UV background required to re-ionize the Universe at z∼ 10-20. We conclude that HD cooling can reduce the masses of typical stars only in rare haloes forming well before the epoch of re-ionization.
The NOvA experiment has seen a 4.4σ signal of ν¯e appearance in a 2 GeV ν¯μ beam at a distance of 810 km. Using 12.33×1020 protons on target delivered to the Fermilab NuMI neutrino beamline, the ...experiment recorded 27 ν¯μ→ν¯e candidates with a background of 10.3 and 102 ν¯μ→ν¯μ candidates. This new antineutrino data are combined with neutrino data to measure the parameters |Δm322|=2.48−0.06+0.11×10−3 eV2/c4 and sin2θ23 in the ranges from (0.53–0.60) and (0.45–0.48) in the normal neutrino mass hierarchy. The data exclude most values near δCP=π/2 for the inverted mass hierarchy by more than 3σ and favor the normal neutrino mass hierarchy by 1.9σ and θ23 values in the upper octant by 1.6σ.
ABSTRACT
It is commonly believed that the earliest stages of star formation in the Universe were self‐regulated by global radiation backgrounds – either by the ultraviolet (UV) Lyman–Werner (LW) ...photons emitted by the first stars (directly photodissociating H2), or by the X‐rays produced by accretion on to the black hole (BH) remnants of these stars (heating the gas but catalysing H2 formation). Recent studies have suggested that a significant fraction of the first stars may have had low masses (a few M⊙). Such stars do not leave BH remnants and they have softer spectra, with copious infrared (IR) radiation at photon energies ∼1 eV. Similar to LW and X‐ray photons, these photons have a mean‐free path comparable to the Hubble distance, building up an early IR background. Here we show that if soft‐spectrum stars, with masses of a few M⊙, contributed ≳0.3 per cent of the UV background (or their mass fraction exceeded ∼80 per cent), then their IR radiation dominated radiative feedback in the early Universe. The feedback is different from the UV feedback from high‐mass stars, and occurs through the photodetachment of H− ions, necessary for efficient H2 formation. Nevertheless, we find that the baryon fraction which must be incorporated into low‐mass stars in order to suppress H2 cooling is only a factor of a few higher than for high‐mass stars.
Ultrasonic additive manufacturing (UAM) is a solid state manufacturing process that combines additive joining of thin metal tapes and subtractive computer numerical control milling operations to ...generate near-net shape metallic parts. We conducted a design of experiments study with the goal to optimize UAM process parameters for aluminum 6061. Weld force, weld speed, amplitude, and temperature were varied based on a Taguchi L18 experimental design matrix and tested for mechanical strength using a shear test and a comparative push-pin test. Statistical methods including analysis of variance (ANOVA), mean effects plots, and interaction effects plots were conducted to determine optimal process parameters. Results indicate that weld amplitudes of 32.76 µm and weld speeds of 84.6 mm/s yield maximum mechanical strength while temperature and force are statistically insignificant for the parameter levels tested. Annealing of cold-worked foil stock produces a 13% strength increase for UAM samples over homogeneous annealed material.
Two different nuclear-medium effects are isolated using a low three-momentum transfer subsample of neutrino-carbon scattering data from the MINERvA neutrino experiment. The observed hadronic energy ...in charged-current ν_{μ} interactions is combined with muon kinematics to permit separation of the quasielastic and Δ(1232) resonance processes. First, we observe a small cross section at very low energy transfer that matches the expected screening effect of long-range nucleon correlations. Second, additions to the event rate in the kinematic region between the quasielastic and Δ resonance processes are needed to describe the data. The data in this kinematic region also have an enhanced population of multiproton final states. Contributions predicted for scattering from a nucleon pair have both properties; the model tested in this analysis is a significant improvement but does not fully describe the data. We present the results as a double-differential cross section to enable further investigation of nuclear models. Improved description of the effects of the nuclear environment are required by current and future neutrino oscillation experiments.
The two-detector design of the NOvA neutrino oscillation experiment, in which two functionally identical detectors are exposed to an intense neutrino beam, aids in canceling leading order effects of ...cross-section uncertainties. However, limited knowledge of neutrino interaction cross sections still gives rise to some of the largest systematic uncertainties in current oscillation measurements. We show contemporary models of neutrino interactions to be discrepant with data from NOvA, consistent with discrepancies seen in other experiments. Adjustments to neutrino interaction models in GENIE are presented, creating an effective model that improves agreement with our data. We also describe systematic uncertainties on these models, including uncertainties on multi-nucleon interactions from a newly developed procedure using NOvA near detector data.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Burkholderia pseudomallei (Bp), the agent of melioidosis, causes disease ranging from acute and rapidly fatal to protracted and chronic. Bp is highly infectious by aerosol, can cause severe disease ...with nonspecific symptoms, and is naturally resistant to multiple antibiotics. However, no vaccine exists. Unlike many Bp strains, which exhibit random variability in traits such as colony morphology, Bp strain MSHR5848 exhibited two distinct and relatively stable colony morphologies on sheep blood agar plates: a smooth, glossy, pale yellow colony and a flat, rough, white colony. Passage of the two variants, designated "Smooth" and "Rough", under standard laboratory conditions produced cultures composed of > 99.9% of the single corresponding type; however, both could switch to the other type at different frequencies when incubated in certain nutritionally stringent or stressful growth conditions. These MSHR5848 derivatives were extensively characterized to identify variant-associated differences. Microscopic and colony morphology differences on six differential media were observed and only the Rough variant metabolized sugars in selective agar. Antimicrobial susceptibilities and lipopolysaccharide (LPS) features were characterized and phenotype microarray profiles revealed distinct metabolic and susceptibility disparities between the variants. Results using the phenotype microarray system narrowed the 1,920 substrates to a subset which differentiated the two variants. Smooth grew more rapidly in vitro than Rough, yet the latter exhibited a nearly 10-fold lower lethal dose for mice than Smooth. Finally, the Smooth variant was phagocytosed and replicated to a greater extent and was more cytotoxic than Rough in macrophages. In contrast, multiple locus sequence type (MLST) analysis, ribotyping, and whole genome sequence analysis demonstrated the variants' genetic conservation; only a single consistent genetic difference between the two was identified for further study. These distinct differences shown by two variants of a Bp strain will be leveraged to better understand the mechanism of Bp phenotypic variability and to possibly identify in vitro markers of infection.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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