Sandwich-type zeolite intergrowths with MFI and the novel extralarge pore IDM-1 as ordered end-members

Stacking faults are two-dimensional planar defects frequently arising in zeolites, modifying their properties and potentially affecting their performance in catalysis and separation applications. In classical zeolite intergrowths, a topologically unique zeolite layer may often pile up after some spatial transformation (lateral translation, rotation, and/or reflection) that may occur in different amounts or directions with about similar probabilities, leading to a difficult to control disorder. Here, we present a new kind of zeolite intergrowth that requires an additional topologically distinct layer rather than a spatial transformation of a unique one. Stacking of the so-called pentasil layers produces the well-known medium pore zeolite MFI. Intercalation in strict alternation of a topologically distinct second layer sandwiched between pentasil layers expands the structure to produce the new extra-large pore IDM-1. Stacking disorder modulates the structural expansion along the stacking direction. The disordered materials have been studied by simulation of the X-ray diffraction patterns using the program DIFFaX and by Cs-corrected high-resolution electron microscopy. We show that disorder does not occur at random but in extended domains and can be controlled all the way from MFI to IDM-1 by just varying the concentration of the synthesis mixture

Variación espacio-temporal de la cobertura de coral del norte del Sistema Arrecifal Mesoamericano, Península de Yucatán, México

Evaluating the response of coral assemblages to different disturbances is important because variations in species composition may have consequences for ecosystem functioning due to their different functional roles in coral reefs. This study evaluates changes in diversity, structure and composition of coral assemblages of the coral reefs of two national parks in the northern sector of the Mesoamerican Barrier Reef System spanning the period from 2006 to 2012, just after the impact of two hurricanes in the area. Coral assemblages in the Cancún National Park included fewer species and lower live coral coverage ( < 15%) than those recorded in Cozumel. In the Cancún National Park, the species with the highest coral cover was Porites astreoides (more than 40% relative cover), and no significant temporal changes were observed in live coral cover and species composition. On the other hand, in the Cozumel National Park the dominant species were Agaricia agaricites, Siderastrea siderea and Porites astreoides, and the coral reefs showed an increase in live coral cover from 16% in 2006 to 29% in 2012. The dynamics of coral assemblages differed between the two parks: while there is an apparent stability in the current composition of the Cancún reefs, the Cozumel reefs show an increase in the abundance of the aforementioned dominant species. However, it is possible that the population characteristics of the species that dominate the coral assemblages in both national parks, such as those of fast population growth and of small colony size, do not entirely fulfill the main function of accretion and habitat heterogeneity, and more research is therefore needed to test this hypothesis.Evaluar la respuesta de las comunidades de coral a diferentes perturbaciones es importante ya que las variaciones en la composición de las especies pueden tener consecuencias en el funcionamiento del ecosistema, debido a los diferentes roles funcionales que cada especie tiene dentro de él. En este estudio se evaluaron los cambios en la diversidad, estructura y composición de las comunidades en los arrecifes de dos Parques Nacionales ubicados en el sector norte del Sistema Arrecifal Mesoamericano, durante el periodo 2006-2012, justo después del impacto de dos huracanes en la zona. En el Parque Nacional Cancún se registraron pocas especies de coral y una cobertura de coral vivo baja ( < 15%) sin cambios temporales significativos; la especie más dominante en este parque fue Porites astreoides con más del 40% de cobertura relativa. Por otro lado, los arrecifes de coral del Parque Nacional de Cozumel mostraron un incremento en la cobertura de coral del 16% en 2006 a 29% en 2012; las especies dominantes, y que incrementaron su cobertura en este periodo, fueron Agaricia agaricites, Siderastrea siderea y P. astreoides. Aunque la dinámica de las comunidades de coral fue diferente en ambos parques, es posible que las características poblacionales de las especies que dominan las comunidades de coral en todos los arrecifes no cumplan por completo con las funciones principales de acreción y heterogeneidad de hábitat; sin embargo se necesita más investigación para poder evaluar esta hipótesis

High-redshift post-reionization cosmology with 21cm intensity mapping

We investigate the possibility of performing cosmological studies in the redshift range 2.5<z<5 through suitable extensions of existing and upcoming radio-telescopes like CHIME, HIRAX and FAST. We use the Fisher matrix technique to forecast the bounds that those instruments can place on the growth rate, the BAO distance scale parameters, the sum of the neutrino masses and the number of relativistic degrees of freedom at decoupling, Neff. We point out that quantities that depend on the amplitude of the 21cm power spectrum, like f\u3c38, are completely degenerate with \u3a9HI and bHI, and propose several strategies to independently constrain them through cross-correlations with other probes. Assuming 5% priors on \u3a9HI and bHI, kmax=0.2 h Mpc-1 and the primary beam wedge, we find that a HIRAX extension can constrain, within bins of \u394 z=0.1: 1) the value of f\u3c38 at 4%, 2) the value of DA and H at 1%. In combination with data from Euclid-like galaxy surveys and CMB S4, the sum of the neutrino masses can be constrained with an error equal to 23 meV (1\u3c3), while Neff can be constrained within 0.02 (1\u3c3). We derive similar constraints for the extensions of the other instruments. We study in detail the dependence of our results on the instrument, amplitude of the HI bias, the foreground wedge coverage, the nonlinear scale used in the analysis, uncertainties in the theoretical modeling and the priors on bHI and \u3a9HI. We conclude that 21cm intensity mapping surveys operating in this redshift range can provide extremely competitive constraints on key cosmological parameters

Cosmological Hydrodynamic Simulations with Suppressed Variance in the Ly alpha Forest Power Spectrum

We test a method to reduce unwanted sample variance when predicting Lyα forest power spectra from cosmological hydrodynamical simulations. Sample variance arises due to sparse sampling of modes on large scales and propagates to small scales through nonlinear gravitational evolution. To tackle this, we generate initial conditions in which the density perturbation amplitudes are fixed to the ensemble average power spectrum—and are generated in pairs with exactly opposite phases. We run 50 such simulations (25 pairs) and compare their performance against 50 standard simulations by measuring the Lyα 1D and 3D power spectra at redshifts z = 2, 3, and 4. Both ensembles use periodic boxes of $40\,{h}^{-1}\mathrm{Mpc}$ containing 5123 particles each of dark matter and gas. As a typical example of improvement, for wavenumbers $k=0.25\,h{\mathrm{Mpc}}^{-1}$ at z = 3, we find estimates of the 1D and 3D power spectra converge 34 and 12 times faster in a paired–fixed ensemble compared with a standard ensemble. We conclude that, by reducing the computational time required to achieve fixed accuracy on predicted power spectra, the method frees up resources for exploration of varying thermal and cosmological parameters—ultimately allowing the improved precision and accuracy of statistical inference

Constraining cosmology with machine learning and galaxy clustering: the CAMELS-SAM suite

As the next generation of large galaxy surveys come online, it is becoming increasingly important to develop and understand the machine learning tools that analyze big astronomical data. Neural networks are powerful and capable of probing deep patterns in data, but must be trained carefully on large and representative data sets. We developed and generated a new `hump' of the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project: CAMELS-SAM, encompassing one thousand dark-matter only simulations of (100 $h^{-1}$ cMpc)$^3$ with different cosmological parameters ($\Omega_m$ and $\sigma_8$) and run through the Santa Cruz semi-analytic model for galaxy formation over a broad range of astrophysical parameters. As a proof-of-concept for the power of this vast suite of simulated galaxies in a large volume and broad parameter space, we probe the power of simple clustering summary statistics to marginalize over astrophysics and constrain cosmology using neural networks. We use the two-point correlation function, count-in-cells, and the Void Probability Function, and probe non-linear and linear scales across $0.68<$ R $<27\ h^{-1}$ cMpc. Our cosmological constraints cluster around 3-8$\%$ error on $\Omega_{\text{M}}$ and $\sigma_8$, and we explore the effect of various galaxy selections, galaxy sampling, and choice of clustering statistics on these constraints. We additionally explore how these clustering statistics constrain and inform key stellar and galactic feedback parameters in the Santa Cruz SAM. CAMELS-SAM has been publicly released alongside the rest of CAMELS, and offers great potential to many applications of machine learning in astrophysics: https://camels-sam.readthedocs.io.Comment: 40 pages, 22 figures (11 made of subfigures

Signatures of photon and axion-like particle mixing in the gamma-ray burst jet

Photons couple to Axion-Like Particles (ALPs) or more generally to any pseudo Nambu-Goldstone boson in the presence of an external electromagnetic field. Mixing between photons and ALPs in the strong magnetic field of a Gamma-Ray Burst (GRB) jet during the prompt emission phase can leave observable imprints on the gamma-ray polarization and spectrum. Mixing in the intergalactic medium is not expected to modify these signatures for ALP mass > 10^(-14) eV and/or for < nG magnetic field. We show that the depletion of photons due to conversion to ALPs changes the linear degree of polarization from the values predicted by the synchrotron model of gamma ray emission. We also show that when the magnetic field orientation in the propagation region is perpendicular to the field orientation in the production region, the observed synchrotron spectrum becomes steeper than the theoretical prediction and as detected in a sizable fraction of GRB sample. Detection of the correlated polarization and spectral signatures from these steep-spectrum GRBs by gamma-ray polarimeters can be a very powerful probe to discover ALPs. Measurement of gamma-ray polarization from GRBs in general, with high statistics, can also be useful to search for ALPs.Comment: 17 pages, 3 figures. Accepted for publication in JCAP with minor change