Milieu interstellaire

Résumé

300px|vignette|La répartition d'hydrogène ionisé dans des régions du milieu interstellaire, vue depuis l'hémisphère nord de la Terre. En astronomie, le milieu interstellaire (en anglais, interstellar medium ou ISM) est la matière qui, dans une galaxie, remplit l'espace entre les étoiles et se fond dans le milieu intergalactique environnant. Il est un mélange de gaz (ionisés, atomiques et moléculaires), de rayons cosmiques et de poussières. L'énergie qui occupe le même volume, sous forme de rayonnement électromagnétique, correspond au champ de rayonnement interstellaire. Les étoiles se forment au sein des régions les plus denses du milieu (les nuages moléculaires) et approvisionnent le milieu en matière et en énergie au moyen de nébuleuses planétaires, vents solaires, supernova et leur extinction finale. Cette interaction entre les étoiles et le milieu interstellaire lui-même aide à définir la vitesse à laquelle une galaxie épuise sa réserve gazeuse, et donc sa durée de formation d'ét

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This thesis is anchored in the research of better understanding of the chemical evolution of galaxies. Many dwarf spheroidal galaxies (dSph) are orbiting the Milky Way displaying various star formation histories. The formation of these dwarf galaxies and the drivers of their evolution are still not well understood. We studied in detail one of them, Sextans, a low mass dSph composed of an old population. 90% of Sextans stars were formed 11-14 Gyr ago (Lee et al. 2009). Battaglia et al. (2011) show the presence of a radial metallicity gradient in Sextans considering observation based on intermediate resolution spectroscopy until the tidal radius. For a projected radius R < 0.8 deg stars cover the whole range of [Fe/H], while for R > 0.8 deg all stars are metal-poor [Fe/H] < -2.2 dex. Due to its distance and its low surface brightness high resolution spectra are required to obtain accurate abundances. Up to recently only 14 Sextans stars have been observed in high resolution spectroscopy (Shetrone et al. 2001; Aoki et al. 2009; Tafelmeyer et al. 2010; Honda et al. 2011) but in majority they were focused only on extremely metal-poor stars. In order to understand the chemical evolution of Sextans we needed to study a statistically significant sample of stars covering a larger metallicity range. This has been done in this work based on the largest high resolution spectroscopic sample of 87 red giant branch stars ever obtained in such low mass dwarf spheroidal galaxies. Our sample of stars located in the center of Sextans displays a large metallicity distribution ranging from -1.0 dex to -3.5 dex. We derived abundances of 10 elements : 3 alpha-elements (Mg, Ca and Ti), 5 iron-peak elements (Sc, Cr,Mn, Co and Ni) and 2 neutron-capture elements (Ba and Eu). Combining the results we had the first picture of the chemical evolution of Sextans. From [alpha/Fe] distribution reflecting the ratio between supernovae [SNe II/SNe Ia], we found that the domination of SNeIa in chemical evolution occurred around [Fe/H] = -2.0 dex. A lower knee than for Fornax and Sculptor suggests that star formation was less efficient in Sextans in agreement with their respective estimated mass. The absence of intrinsic scatter in [alpha/Fe] showed that in the center of Sextans the interstellar medium has been homogeneously enriched through star formation. Some hypothesis like tidal interactions with the Milky Way could explain this abundance homogeneity in the center of the galaxy and the metallicity gradient toward the outskirts. Abundances in iron-peak elements and neutron-capture elements give new observational constraints for a better understanding of the various nucleosynthesis sources of these elements.

EPFL2015

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LABORATORY DETECTION OF HOCN AND TENTATIVE IDENTIFICATION IN Sgr B2

Sandra Bruenken

The rotational spectrum of cyanic acid, HOCN, has been detected in the centimeter-wave band in a molecular beam by Fourier transform microwave spectroscopy and in the millimeter-wave band by conventional spectroscopy in a low-pressure laboratory discharge. Spectroscopic constants, including the nitrogen hyperfine coupling constant, derived from 31 a-type transitions between 21 and 360 GHz with J up to 17 and K-a

2009

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Atomic-Step Enriched Ruthenium-Iridium Nanocrystals Anchored Homogeneously on MOF-Derived Support for Efficient and Stable Oxygen Evolution in Acidic and Neutral Media

Junjie Li, Bo Li, Yue Li, Lei Liu, Vasiliki Tileli

Achieving an efficient and stable oxygen evolution reaction (OER) in an acidic or neutral medium is of paramount importance for hydrogen production via proton exchange membrane water electrolysis (PEM-WE). Supported iridium-based nanoparticles (NPs) are the state-of-the-art OER catalysts for PEM-WE, but the nonhomogeneous dispersion of these NPs on the support together with their nonuniform sizes usually leads to catalyst migration and agglomeration under strongly corrosive and oxidative OER conditions, eventually causing the loss of active surface area and/or catalytic species and thereby the degradation of OER performance. Here, we design a catalyst comprising surface atomic-step enriched ruthenium-iridium (RuIr) nanocrystals homogeneously dispersed on a metal organic framework (MOF) derived carbon support (RuIr@CoNC), which shows outstanding catalytic performance for OER with high mass activities of 2041, 970 and 205 A g(RuIr)(-1) at an overpotential of 300 mV and can sustain continuous OER electrolysis up to 40, 45, and 90 h at 10 mA cm(-2) with minimal degradation in 0.5 M H2SO4 (pH = 0.3), 0.05 M H2SO4 (pH = 1), and PBS (pH = 7.2) electrolytes, respectively. Comprehensive experimental studies and density functional theory (DFT) calculations reveal that the good performance of RuIr@CoNC can be attributed, on one hand, to the presence of abundant atomic steps that maximize the exposure of catalytically active sites and lower the limiting potential of the rate-determining step of OER and, on the other hand, to the strong interaction between RuIr nanocrystals and the CoNC support that endows homogeneous dispersion and firm immobilization of RuIr catalysts on CoNC. The RuIr@CoNC catalysts also show outstanding performance in a single-cell PEM electrolyzer, and their large-quantity synthesis is demonstrated.

AMER CHEMICAL SOC2021

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Atomic-Step Enriched Ruthenium-Iridium Nanocrystals Anchored Homogeneously on MOF-Derived Support for Efficient and Stable Oxygen Evolution in Acidic and Neutral Media

Junjie Li, Bo Li, Yue Li, Lei Liu, Vasiliki Tileli

AMER CHEMICAL SOC2021

EPFL2015