202 research outputs found
Radiative transfer simulations of cosmic reionization I: methodology and initial results
We present a new hybrid code for large volume, high resolution simulations of
cosmic reionization, which utilizes a N-body algorithm for dark matter,
physically motivated prescriptions for baryons and star formation, and an
adaptive ray tracing algorithm for radiative transfer of ionizing photons. Two
test simulations each with 3 billion particles and 400 million rays in a 50
Mpc/h box have been run to give initial results. Halos are resolved down to
virial temperatures of 10^4 K for the redshift range of interest in order to
robustly model star formation and clumping factors. This is essential to
correctly account for ionization and recombination processes. We find that the
halos and sources are strongly biased with respect to the underlying dark
matter, re-enforcing the requirement of large simulation boxes to minimize
cosmic variance and to obtain a qualitatively correct picture of reionization.
We model the stellar initial mass function (IMF), by following the spatially
dependent gas metallicity evolution, and distinguish between the first
generation, Population III (PopIII) stars and the second generation, Population
II (PopII) stars. The PopIII stars with a top-heavy IMF produce an order of
magnitude more ionizing photons at high redshifts z>10, resulting in a more
extended reionization. In our simulations, complete overlap of HII regions
occurrs at z~6.5 and the computed mass and volume weighted residual HI
fractions at 5<z<6.5 are both in good agreement with high redshift quasar
absorption measurements from SDSS. The values for the Thomson optical depth are
consistent within 1-sigma of the current best-fit value from third-year WMAP.Comment: 15 pages, 13 figures; accepted by ApJ; higher resolution figures can
be found at http://www.astro.princeton.edu/~htrac/reionization.htm
Approaching the dynamics of hot nucleons in supernovae
All recent numerical simulations agree that stars in the main sequence mass
range of 9-40 solar masses do not produce a prompt hydrodynamic ejection of the
outer layers after core collapse and bounce. Rather they suggest that stellar
core collapse and supernova explosion are dynamically distinct astrophysical
events, separated by an unspectacular accretion phase of at least ~40 ms
duration. As long as the neutrinospheres remain convectively stable, the
explosion dynamics is determined by the neutrons, protons, electrons and
neutrinos in the layer of impact-heated matter piling up on the protoneutron
star. The crucial role of neutrino transport in this regime has been emphasized
in many previous investigations. Here, we search for efficient means to address
the role of magnetic fields and fluid instabilities in stellar core collapse
and the postbounce phase.Comment: 4 pages, contribution to Nuclei in the Cosmos VIII, Jul. 19-23,
submitted to Nucl. Phys.
Muscleblind-like 3 deficit results in a spectrum of age-associated pathologies observed in myotonic dystrophy.
Myotonic dystrophy type I (DM1) exhibits distinctive disease specific phenotypes and the accelerated onset of a spectrum of age-associated pathologies. In DM1, dominant effects of expanded CUG repeats result in part from the inactivation of the muscleblind-like (MBNL) proteins. To test the role of MBNL3, we deleted Mbnl3 exon 2 (Mbnl3(ΔE2)) in mice and examined the onset of age-associated diseases over 4 to 13 months of age. Accelerated onset of glucose intolerance with elevated insulin levels, cardiac systole deficits, left ventricle hypertrophy, a predictor of a later onset of heart failure and the development of subcapsular and cortical cataracts is observed in Mbnl3(ΔE2) mice. Retention of embryonic splice isoforms in adult organs, a prominent defect in DM1, is not observed in multiple RNAs including the Insulin Receptor (Insr), Cardiac Troponin T (Tnnt2), Lim Domain Binding 3 (Ldb3) RNAs in Mbnl3(ΔE2) mice. Although rare DM1-like splice errors underlying the observed phenotypes cannot be excluded, our data in conjunction with the reported absence of alternative splice errors in embryonic muscles of a similar Mbnl3(ΔE2) mouse by RNA-seq studies, suggest that mechanisms distinct from the adult retention of embryonic splice patterns may make important contributions to the onset of age-associated pathologies in DM1
Verification of the Parallel Pin-Wise Core Simulator pCTF/PARCSv3.2 in Operational Control Rod Drop Transient Scenarios
This is an Accepted Manuscript of an article published by Taylor & Francis in Nuclear Science and Engineering on 2017, available online: https://www.tandfonline.com/doi/full/10.1080/00295639.2017.1320892[EN] Thanks to advances in computer technology, it is feasible to obtain detailed reactor core descriptions for safety analysis of the light water reactor (LWR), in order to represent realistically the fuel elements design, as is the case for three-dimensional coupled simulations for local neutron kinetics and thermal hydraulics. This scenario requires an efficient thermal-hydraulic code that can produce a response in a reasonable time for large-scale, detailed models. In two-fluid codes, such as the thermal-hydraulic subchannel code COBRA-TF, the time restriction is even more important, since the set of equations to be solved is more complex. We have developed a message passing interface parallel version of COBRA-TF, called pCTF. The parallel code is based on a cell-oriented domain decomposition approach, and performs well in models that consist of many cells. The Jacobian matrix is computed in parallel, with each processor in charge of calculating the coefficients related to a subset of the cells. Furthermore, the resulting system of linear equations is also solved in parallel, by exploiting solvers and preconditioners from PETSc. The goal of this study is to demonstrate the capability of the recently developed pCTF/PARCS coupled code to simulate large cores with a pin-by-pin level of detail in an acceptable computational time, using for this purpose two control rod drop operational transients that took place in the core of a three-loop pressurized water reactor. As a result, the main safety parameters of the core hot channel have been calculated by the coupled code in a pin level of detail, obtaining best estimate results for this transient.This work has been partially supported by the Universitat Politecnica de Valencia under Projects COBRA_PAR (PAID-05-11-2810) and OpenNUC (PAID-05-12), and by the Spanish Ministerio de Economia y Competitividad under Projects SLEPc-HS (TIN2016-75985-P) and NUC-MULTPHYS (ENE2012-34585).Ramos Peinado, E.; Roman Moltó, JE.; Abarca Giménez, A.; Miró Herrero, R.; Bermejo, JA.; Ortego, A.; Posada-Barral, JM. (2017). Verification of the Parallel Pin-Wise Core Simulator pCTF/PARCSv3.2 in Operational Control Rod Drop Transient Scenarios. Nuclear Science and Engineering. 187(3):254-267. https://doi.org/10.1080/00295639.2017.1320892S2542671873Cuervo, D., Avramova, M., Ivanov, K., & Miró, R. (2006). Evaluation and enhancement of COBRA-TF efficiency for LWR calculations. Annals of Nuclear Energy, 33(9), 837-847. doi:10.1016/j.anucene.2006.03.011Ramos, E., Roman, J. E., Abarca, A., Miró, R., & Bermejo, J. A. (2016). Control rod drop transient analysis with the coupled parallel code pCTF-PARCSv2.7. Annals of Nuclear Energy, 87, 308-317. doi:10.1016/j.anucene.2015.09.016T. DOWNAR et al. “PARCS v2.7 U.S. NRC Core Neutronics Simulator: User Manual” (2006).T. DOWNAR et al. “PARCS v2.7 U.S. NRC Core Neutronics Simulator: Theory Manual” (2006)
Dark Energy, scalar-curvature couplings and a critical acceleration scale
We study the effects of coupling a cosmologically rolling scalar field to
higher order curvature terms. We show that when the strong coupling scale of
the theory is on the 10^{-3}-10^{-1}eV range, the model passes all experimental
bounds on the existence of fifth forces even if the field has a mass of the
order of the Hubble scale in vacuum and non-suppressed couplings to SM fields.
The reason is that the coupling to certain curvature invariant acts as an
effective mass that grows in regions of large curvature. This prevents the
field from rolling down its potential near sources and makes its effects on
fifth-force search experiments performed in the laboratory to be observable
only at the sub-mm scale. We obtain the static spherically symmetric solutions
of the theory and show that a long-range force appears but it is turned on only
below a fixed Newtonian acceleration scale of the order of the Hubble constant.
We comment on the possibility of using this feature of the model to alleviate
the CDM small scale crisis and on its possible relation to MOND.Comment: 12 pages, 2 figure
Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?
The classical radiometer equation is commonly used to calculate the
detectability of the 21cm emission by diffuse cosmic hydrogen at high
redshifts. However, the classical description is only valid in the regime where
the occupation number of the photons in phase space is much larger than unity
and they collectively behave as a classical electromagnetic field. At redshifts
z<20, the spin temperature of the intergalactic gas is dictated by the
radiation from galaxies and the brightness temperature of the emitting gas is
in the range of mK, independently from the existence of the cosmic microwave
background. In regions where the observed brightness temperature of the 21cm
signal is smaller than the observed photon energy, of 68/(1+z) mK, the
occupation number of the signal photons is smaller than unity. Neverethless,
the radiometer equation can still be used in this regime because the weak
signal is accompanied by a flood of foreground photons with a high occupation
number (involving the synchrotron Galactic emission and the cosmic microwave
background). As the signal photons are not individually distinguishable, the
combined signal+foreground population of photons has a high occupation number,
thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA
Growth and anisotropy of ionization fronts near high redshift quasars in the MassiveBlack simulation
We use radiative transfer to study the growth of ionized regions around the
brightest, z=8 quasars in a large cosmological hydrodynamic simulation that
includes black hole growth and feedback (the MassiveBlack simulation). We find
that in the presence of the quasar s the comoving HII bubble radii reach 10
Mpc/h after 20 My while with the stellar component alone the HII bubbles are
smaller by at least an order of magnitude. Our calculations show that several
features are not captured within an analytical growth model of Stromgren
spheres. The X-ray photons from hard quasar spectra drive a smooth transition
from fully neutral to partially neutral in the ionization front. However the
transition from partially neutral to fully ionized is significantly more
complex. We measure the distance to the edge of bubbles as a function of angle
and use the standard deviation of these distances as a diagnostic of the
isotropy of ionized regions. We find that the overlapping of nearby ionized
regions from clustered halos not only increases the anisotropy, but also is the
main mechanism which allows the outer radius to grow. We therefore predict that
quasar ionized bubbles at this early stage in the reionization process should
be both significantly larger and more irregularly shaped than bubbles around
star forming galaxies. Before the star formation rate increases and the
Universe fully reionizes, quasar bubbles will form the most striking and
recognizable features in 21cm maps.Comment: 11 pages, 10 figures. Updated after referee repor
Ultradeep Infrared Array Camera Observations of sub-L* z~7 and z~8 Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity Galaxies to the Stellar Mass Density and Reionization
We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame
optical) fluxes of 14 newly WFC3/IR-detected z=7 z_{850}-dropout galaxies and 5
z=8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allow
accurate removal of contaminating foreground light, enabling reliable flux
measurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to
[3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection
for the z_{850}-dropouts and an upper limit for the Y_{105}-dropouts. We
construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes
and fit stellar population synthesis models to derive mean redshifts, stellar
masses, and ages. For the z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1},
(U-V)_{rest}=0.4, reddening A_V=0, stellar mass M*=1.2^{+0.3}_{-0.6} x 10^9
M_sun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, and
SSFR=1.7Gyr^{-1} are similar to values reported for luminous z=7 galaxies,
indicating the galaxies are smaller but not younger. The sub-L* galaxies
observed here contribute significantly to the stellar mass density and under
favorable conditions may have provided enough photons for sustained
reionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropouts
have stellar masses that are uncertain by 1.5 dex due to the near-complete
reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the
stellar mass density to M_{UV,AB} < -18 declines from
rho*(z=7)=3.7^{+1.0}_{-1.8} x 10^6 M_sun Mpc^{-3} to rho*(z=8) < 8 x 10^5 M_sun
Mpc^{-3}, following (1+z)^{-6} over 3<z<8. Lower masses at z=8 would signify
more dramatic evolution, which can be established with deeper IRAC
observations, long before the arrival of the James Webb Space Telescope.Comment: 6 pages, 3 figures, 2 tables, emulateapj, accepted for publication in
ApJ
- …