H2 distribution during formation of multiphase molecular clouds

H2 is the simplest and the most abundant molecule in the ISM, and its formation precedes the formation of other molecules. Understanding the dynamical influence of the environment and the interplay between the thermal processes related to the formation and destruction of H2 and the structure of the cloud is mandatory to understand correctly the observations of H2. We perform high resolution MHD colliding flow simulations with the AMR code RAMSES in which the physics of H2 has been included. We compare the simulation results with various observations including the column densities of excited rotational levels. Due to a combination of thermal pressure, ram pressure and gravity, the clouds produced at the converging point of HI streams are highly inhomogeneous. H2 molecules quickly form in relatively dense clumps and spread into the diffuse interclump gas. This in particular leads to the existence of significant abundances of H2 in the diffuse and warm gas that lies in between clumps. Simulations and observations show similar trends, specially for the HI-to-H2 transition. The abundances of excited rotational levels, calculated at equilibrium in the simulations are very similar to the observed abundances inferred from FUSE results. This is a direct consequence of the presence of the H2 enriched diffuse and warm gas. Our simulations show that H2 rapidly forms in the dense clumps and, due to the complex structure of molecular clouds, quickly spreads at lower densities. Consequently a significant fraction of warm H2 exists in the low density gas. This warm H2 leads to column densities of excited rotational levels close to the observed ones likely revealing the complex intermix between the warm and the cold gas in molecular clouds. This suggests that the 2-phase structure of molecular clouds is an essential ingredient to fully understand molecular hydrogen in these objects.Comment: 16 pages, 19 figures. Accepted for publication in A&

A fast tree-based method for estimating column densities in adaptive mesh refinement codes: Influence of UV radiation field on the structure of molecular clouds

International audienceContext. Ultraviolet radiation plays a crucial role in molecular clouds. Radiation and matter are tightly coupled and their interplay influences the physical and chemical properties of gas. In particular, modeling the radiation propagation requires calculating column densities, which can be numerically expensive in high-resolution multidimensional simulations. Aims. Developing fast methods for estimating column densities is mandatory if we are interested in the dynamical influence of the radiative transfer. In particular, we focus on the effect of the UV screening on the dynamics and on the statistical properties of molecular clouds.Methods. We have developed a tree-based method for a fast estimate of column densities, implemented in the adaptive mesh refinement code RAMSES. We performed numerical simulations using this method in order to analyze the influence of the screening on the clump formation.Results. We find that the accuracy for the extinction of the tree-based method is better than 10%, while the relative error for the column density can be much more. We describe the implementation of a method based on precalculating the geometrical terms that noticeably reduces the calculation time. To study the influence of the screening on the statistical properties of molecular clouds we present the probability distribution function of gas and the associated temperature per density bin and the mass spectra for different density thresholds.Conclusions. The tree-based method is fast and accurate enough to be used during numerical simulations since no communication is needed between CPUs when using a fully threaded tree. It is then suitable to parallel computing. We show that the screening for far UV radiation mainly affects the dense gas, thereby favoring low temperatures and affecting the fragmentation. We show that when we include the screening, more structures are formed with higher densities in comparison to the case that does not include this effect. We interpret this as the result of the shielding effect of dust, which protects the interiors of clumps from the incoming radiation, thus diminishing the temperature and changing locally the Jeans mass

Propuesta de un modelo de suspensión hindlimb para generar atrofia muscular en las extremidades posteriores de la rata

61 p.La suspensión Hindlimb, es el modelo de atrofia muscular inducido por descarga más utilizado, propuesto en el año 2002 por Morey-Holton & Globus. El propósito de este estudio es valorar los efectos estructurales, del modelo de atrofia por desuso propuesto, en los músculos sóleo y EDL de ratas Sprague-Dawley.Se sometió a 12 ratas, 6 ratas en suspensión Hindlimb y 6 ratas del grupo control durante 14 días, finalizado este periodo se extrajeron los músculos sóleo y EDL de la pata trasera izquierda, para ser analizados histológicamente. Los resultados revelan que ocurre una disminución de AST estadísticamente significativa en el grupo Hindlimb (P 0,05) no fue significativo. En conclusión el modelo de suspensión propuesto genera atrofia muscular demostrada por la disminución del área de sección transversal y la disminución de la relación peso músculo/peso rata. No existen diferencias significativas en la comparación del AST en un músculo rápido y en un músculo lento. Por lo tanto ambos músculos se atrofian con el modelo propuesto.La relación peso músculo/ peso rata es menor en el grupo suspendido en comparación con el grupo control y la variación del peso de las ratas Sprague-Dawley del grupo Control y grupo Hindlimb durante los 14 días de experimentación no fue estadísticamente significativo, por lo que los cambios generados en los músculos sóleo y EDL fueron producidos por la atrofia muscula

Efecto de la agricultura de conservación y convencional en la captura de carbono, en los municipios de Yalaguina y Estelí, en el año 2019

El proyecto de investigación se realizó en tres comunidades del municipio de Yalaguina, departamento de Madriz y dos comunidades del municipio de Estelí. Con el objetivo de evaluar los porcentajes de carbono orgánico, aportado por los sistemas de agricultura convencional y de conservación. Mediante la realización de esta investigación se estableció un diseño experimental en parcelas de conservación y convencional. Para calcular el peso fresco se tomó un tamaño muestral de 3 réplicas por cada sustrato en la parte alta, media y baja de cada parcela, utilizando así, el método de cuadrante en parcelas con 5 años de manejo, para calcular el peso seco se hizo secado de muestras de biomasa y muestras de suelo a una temperatura de 105ºC por un período de 24 horas y así diferenciar del peso fresco, una vez ya secas las muestras de suelo se extrajeron 10g para ser colocadas a la mufla a una temperatura de 450ºC por un período de 2 horas, esto con el objetivo de sacar el porcentaje de carbono nitrógeno concentrado en el suelo. Se encontró un efecto significativo (p = 0.0317) del factor comunidad en función de la producción media de biomasa total (fitomasa de maíz y necromasa) según el manejo que se le da por comunidad, obteniendo mayor producción en la comunidad de La Libertad, esto es debido al seguimiento del buen manejo que hacen los productores en las parcelas de esa comunidad, también debido a que los periodos de cosecha son más seguidos que en las otras comunidades

Physical conditions for dust grain alignment in Class 0 protostellar cores II. The role of the radiation field in models aligning/disrupting dust grains

The polarized dust emission observed in Class 0 protostellar cores at high angular resolution with ALMA has raised several concerns about the grain alignment conditions in these regions. We aim to study the role of the radiation field on the grain alignment mechanisms occurring in the interior (<1000 au) of Class 0 protostars. We produce synthetic observations of the polarized dust emission from a MHD model of protostellar formation, using the POLARIS dust radiative transfer tool, which includes dust alignment with Radiative Torques Alignment (RATs). We test how the polarized dust emission from the model core depends on the irradiation conditions in the protostellar envelope, by varying the radiation due to accretion luminosity propagating from the central protostellar embryo throughout the envelope. The level of grain alignment efficiency obtained in the radiative transfer models is then compared to (sub-) millimeter ALMA dust polarization observations of Class 0 protostars. Our radiative transfer calculations have a central irradiation that reproduces the protostellar luminosities typically observed towards low- to intermediate-mass protostars, as well as super-paramagnetic grains, and grains >10 micron, which are required to bring the dust grain alignment efficiencies of the synthetic observations up to observed levels. Our radiative transfer calculations show that irradiation plays an important role in the mechanisms that dictate the size range of aligned grains in Class 0 protostars. Regions of the envelope that are preferentially irradiated harbor strong polarized dust emission but can be affected by the rotational disruption of dust grains. Episodes of high luminosity could affect grain alignment and trigger grain disruption mechanisms. [abridged

H2 formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations

Molecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H formation in a variety of interstellar environments.VW’s research is funded by an ERC Starting Grant (3DICE, grant agreement 336474). GV acknowledges financial support from the National Science Foundation’s Astronomy & Astrophysics Division (Grants No. 1311958 and 1615897). LH acknowledges support from ERC Consolidator Grant GRANN (grant agreement no. 648551). GN acknowledges support from the Swedish Research Council. VW, FD and SM acknowledge the CNRS program ”Physique et Chimie du Milieu Interstellaire” (PCMI) co-funded bythe Centre National d’Etudes Spatiales (CNES). SDP acknowledges funding from STFC, UK. V.V acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (MagneticYSOS project, grant agreement No 679937)

Impact du transfert radiatif et de la chimie sur la formation des nuages moléculaires

The interstellar medium (ISM) is a highly complex system. It corresponds to an intermediate scale between stars and galaxies. The interstellar gas is present throughout the galaxy, filling the volume between stars. A wide variety of coupled processes, such as gravity, magnetic fields, turbulence and chemistry, participate in its evolution, making the modeling of the ISM a challenging problem. A correct description of the ISM requires a good treatment of the magnetohydrodynamics (MHD) equations, gravity, thermal balance, and chemical evolution within the molecular clouds.This thesis work aims at a better understanding of the formation and evolution of molecular clouds, specially how they become "molecular", paying particular attention to the transition HI-to-H2. We have performed ideal MHD simulations of the formation of molecular clouds and the formation of molecular hydrogen under the influence of gravity and turbulence, using accurate estimates for the shielding effects from dust and the self-shielding for H2, calculated with a Tree-based method, able to provide fast estimates of column densities.We find that H2 is formed faster than predicted by the usual estimates due to local density enhancements created by the gas turbulent motions. Molecular hydrogen, formed at higher densities, could then migrate toward low density warmer regions.Total H2 column densities show that the HI-to-H2 transition occurs at total column densities of a few 10^20 cm−2. We have calculated the populations of rotational levels of H2 at thermal equilibrium, and integrated along several lines of sight. These two results reproduce quite well the values observed by Copernicus and FUSE, suggesting that the observed transition and the excited populations could arise as a consequence of the multi-phase structure of molecular clouds. As H2 formation is prior to further molecule formation, warm H2 could possibly allow the development of a warm chemistry, and eventually explain some aspects of the molecular richness observed in the ISM.Le milieu interstellaire (MIS) est un système extrêmement complexe. Il correspond à une échelle intermédiaire entre les étoiles et les galaxies. Le gaz interstellaire est présent dans toute la galaxie, remplissant l’espace entre les étoiles. Une grande diversité de processus couplés, comme la gravité, le champs magnétiques, la turbulence et la chimie, participe à son évolution, faisant de la modélisation du MIS un problème ardu. Une description correcte du MIS nécessite un bon traitement des équations de la magnetohydrodynamique (MHD), de la gravité, du bilan thermique et de l’évolution chimique à l’intérieur du nuage moléculaire.L’objectif de ce travail de thèse est une meilleure compréhension de la formation et de l’évolution des nuages moléculaires, et plus particulièrement de la transition du gaz atomique en gaz moléculaire. Nous avons réalisé des simulations numériques de la formation des nuages moléculaires et de la formation de l’hydrogène moléculaire sous l’influence de la gravité et de la turbulence MHD, en utilisant des estimations précises de l’écrantage par les poussières et de l’auto-écrantage par la molécule H2. Ceci a été calculé grâce à une méthode en arbre, à même de fournir une rapide estimation des densités de colonne.Nous avons trouvé que l’hydrogène moléculaire se forme plus rapidement que prévu par les estimations classiques du fait de l’augmentation de densité locale provoquée par les fluctuations turbulentes du gaz. L’hydrogène moléculaire, formé à des densités plus élevées, peut alors migrer vers les régions plus chaudes et moins denses.Les densités de colonne totale d’hydrogène moléculaire montrent que la transition HI-H2 se produit à des densités de colonne de quelques 10^20 cm−2. Nous avons calculé les populations des niveaux rotationnels de H2 à l’équilibre thermique et intégré le long de plusieurs lignes de visée. Ces résultats reproduisent bien les valeurs observées par Copernicus et FUSE, suggérant que la transition observée et les populations excitées pourraient être une conséquence de la structure multi-phasique des nuages moléculaires. Comme la formation de H2 précède la formation des autres molécules, le H2 chaud pourrait permettre le développement d’espèces endothermiques et éventuellement expliquer certains aspects de la richesse moléculaire observée dans l’ISM

Diseño y evaluación de un modelo de negocios e-commerce en marca de moda de ropa y calzado de la mujer moderna Dimensión Azul, Grupo Colgram S.A.

Tesis (Ingeniero Industrial)Esta tesis aborda la oportunidad que presenta el E-commerce como herramienta de apoyo a las ventas para una empresa Chilena, Dimensión Azul del Grupo Colgram. Dimensión Azul es una marca de ropa y calzado de Moda que busca entregar un producto de mayor calidad a sus clientes, está enfocada a la mujer moderna y Cosmopolita, esta marca fue fundada en 1989 y busca día a día su incorporación y posicionamiento al mercado chileno con el apoyo del grupo COLGRAM S.A

Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration

Context. High-resolution millimeter and submillimeter (mm and submm) polarization observations have opened a new era in the understanding of how magnetic fields are organized in star forming regions, unveiling an intricate interplay between the magnetic fields and the gas in protostellar cores. However, to assess the role of the magnetic field in the process of solar-type star formation, it is important to understand to what extent the polarized dust emission is a good tracer of the magnetic field in the youngest protostellar objects. Aims. In this paper, we present a thorough investigation of the fidelity and limitations of using dust polarized emission to map the magnetic field topologies in low-mass protostars. Methods. To assess the importance of these effects, we performed an analysis of magnetic field properties in 27 realizations of magnetohydrodynamics (MHD) models following the evolution of physical properties in star-forming cores. Assuming a uniform population of dust grains the sizes of which follow the standard MRN size distribution, we analyzed the synthetic polarized dust emission maps produced when these grains align with the local B-field because of radiative torques (B-RATs). Results. We find that mm and submm polarized dust emission is a robust tracer of the magnetic field topologies in inner protostellar envelopes and is successful at capturing the details of the magnetic field spatial distribution down to radii ~100 au. Measurements of the line-of-sight-averaged magnetic field line orientation using the polarized dust emission are precise to <15° (typical of the error on polarization angles obtained with observations from large mm polarimetric facilities such as ALMA) in about 75%–95% of the independent lines of sight that pass through protostellar envelopes. Large discrepancies between the integrated B-field mean orientation and the orientation reconstructed from the polarized dust emission are mostly observed in (i) lines of sight where the magnetic field is highly disorganized and (ii) those that probe large column densities. Our analysis shows that the high opacity of the thermal dust emission and low polarization fractions could be used to avoid using the small fraction of measurements affected by large errors