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Concept

Naissance des étoiles

Résumé

La naissance des étoiles ou formation stellaire, voire stellogénèse ou stellogonie, est un domaine de recherche en astrophysique, qui consiste en l'étude des modes de formation des étoiles et des systèmes planétaires. Les étoiles en formation sont fréquemment appelées « étoiles jeunes ». Selon le scénario actuellement admis, confirmé par l'observation, les étoiles se forment en groupe à partir de la contraction gravitationnelle d'une nébuleuse, un nuage de gaz et de poussière, qui se fragmente en plusieurs cœurs protostellaires. Ceux-ci se contractent en leur centre en formant une étoile, tandis que la matière en périphérie se retrouve sous forme d'une enveloppe et d'un disque d'accrétion. Ce dernier disparaît généralement avec le temps, mais, entre-temps, des planètes peuvent s'y former. Problématique La formation stellaire est un domaine qui suscite l'intérêt, non seulement en raison des phénomènes complexes et mystérieux qui s'y déroulent comme l'accrétion et l'éjectio

Source officielle

https://fr.wikipedia.org/wiki/Naissance_des_%C3%A9toiles

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La Voie lactée, aussi nommée la Galaxie (avec une majuscule), est une galaxie spirale barrée qui comprend entre 200 et d'étoiles, et sans doute plus de de planètes. Elle abrite le Système solaire e

Le Système solaire (avec majuscule), ou système solaire (sans majuscule), est le système planétaire du Soleil, auquel appartient la Terre. Il est composé de cette étoile et des objets célestes gravi

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The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

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

Chargement

The Agora High-Resolution Galaxy Simulations Comparison Project. Ii. Isolated Disk Test

Hongyu Li, Alessandro Lupi, Yves Revaz

Using an isolated Milky Way-mass galaxy simulation, we compare results from nine state-of-the-art gravitohydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e. g., radiative cooling and UV background by the standardized package GRACKLE) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly formed stellar clump mass functions show more significant variation (difference by up to a factor of similar to 3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low-density region, and between more diffusive and less diffusive schemes in the high-density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

Iop Publishing Ltd2016

Chargement

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We present a suite of high-resolution cosmological zoom-in simulations to z = 4 of a 10(12) M (circle dot) halo at z = 0, obtained using seven contemporary astrophysical simulation codes (Art-I, Enzo, Ramses, Changa, Gadget-3, Gear, and Gizmo) widely used in the numerical galaxy formation community. The physics prescriptions for gas cooling and heating and star formation are the same as the ones used in our previous Assembling Galaxies of Resolved Anatomy (AGORA) disk comparison but now account for the effects of cosmological processes such as the expansion of the universe, intergalactic gas inflow, and the cosmic ultraviolet background radiation emitted by massive stars and quasars. In this work, we introduce the most careful comparison yet of galaxy formation simulations run by different code groups, together with a series of four calibration steps each of which is designed to reduce the number of tunable simulation parameters adopted in the final run. In the first two steps, we methodically calibrate the gas physics, such as cooling and heating, in simulations without star formation. In the third step, we seek agreement on the total stellar mass produced with the common star formation prescription used in the AGORA disk comparison, in stellar-feedback-free simulations. In the last calibration step, we activate stellar feedback, where each code group is asked to set the feedback prescription to as close to the most widely used one in its code community as possible, while aiming for convergence in the stellar mass at z = 4 to the values predicted by semiempirical models. After all the participating code groups successfully complete the calibration steps, we achieve a suite of cosmological simulations with similar mass assembly histories down to z = 4. With numerical accuracy that resolves the internal structure of a target halo (less than or similar to 100 physical pc at z = 4), we find that the codes overall agree well with one another, e.g., in gas and stellar properties, but also show differences, e.g., in circumgalactic medium (CGM) properties. We argue that, if adequately tested in accordance with our proposed calibration steps and common parameters, high-resolution cosmological zoom-in simulations can have robust and reproducible results. New code groups are invited to join and enrich this comparison by generating equivalent models or to test the code's compatibility on their own, by adopting the common initial conditions, the common easy-to-implement physics package, and the proposed calibration steps. Further analyses of the zoom-in simulations presented here will be presented in forthcoming reports from the AGORA Collaboration, including studies of the CGM, simulations by additional codes, and results at lower redshift.

IOP PUBLISHING LTD2021

Chargement

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

EPFL2015

The Agora High-Resolution Galaxy Simulations Comparison Project. Ii. Isolated Disk Test

Hongyu Li, Alessandro Lupi, Yves Revaz

Iop Publishing Ltd2016

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IOP PUBLISHING LTD2021