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Title: The effects of galactic fountains on the chemical evolution of galaxies
Authors: Spitoni, Emanuele
Keywords: ISM: jets and outflowsGalaxy: disk - abundances - evolution - open cluster and associationsstars: supernovae
Issue Date: 5-Mar-2010
Publisher: Università degli studi di Trieste
Abstract: In this thesis we study the effect of galactic fountains, namely gas and flows from the disk of galaxies produced by multiple supernova explosions, on the chemical evolution of galaxies. Sequential supernova explosions create a superbubble, whereas the swept up interstellar medium is concentrated in a supershell which can break out a stratified medium, producing bipolar outflows. The gas of the supershells can fragment into clouds which eventually fall toward the disk producing so-called galactic fountains. Many works in literature have dealt with superbubble expansion in stratified media. However, very few papers in the past have taken into account the chemical evolution of the superbubble and how the supershell get polluted from the metals produced by supernova explosions. With this thesis for the first time the effect of galactic fountains we consider in a detailed chemical evolution model for the Milky Way. In the first part of our work we study the expansion law and chemical enrichment of a supershell powered by the energetic feedback of a typical Galactic OB association at various galactocentric radii. We follow the orbits of the fragments created when the supershell breaks out and we compare their kinetic and chemical properties with the available observations of high - and intermediate - velocity clouds. We use the Kompaneets (1960) approximation for the evolution of the superbubble driven by sequential supernova explosions and we compute the abundances of oxygen and iron residing in the thin cold supershell. Due to Rayleigh-Taylor instabilities we assume that supershells are fragmented and we follow the orbit of the clouds either ballistically or by means of a hybrid model considering viscous interaction between the clouds and the extra-planar gas. We find that if the initial metallicity is solar, the pollution from the dying stars of the OB association has a negligible effect on the chemical composition of the clouds. The maximum height reached by the clouds above the plane seldom exceeds 2 kpc and when averaging over different throwing angles, the landing coordinate differs from the throwing coordinate by only 1 kpc. Therefore, it is unlikely that galactic fountains can affect abundance gradients on large scales. The range of heights and [O/Fe] ratios spanned by our clouds suggest that the high velocity clouds cannot have a Galactic origin, whereas intermediate velocity clouds have kinematic properties similar to our predicted clouds but have observed overabundances of the [O/Fe] ratios that can be reproduced only with initial metallicities which are too low compared to those of the Galaxy disk. Even if it is unlikely that galactic fountains can affect abundance gradients on large scales, they can still affect the chemical enrichment of the interstellar medium (ISM) because of the time-delay due to the non-negligible time taken by fountains to orbit around and fall back into the Galaxy. This implies a delay in the mixing of metals in ISM which conflicts with the instantaneous mixing approximation usually assumed in all models in literature. We test whether relaxing this approximation in a detailed chemical evolution model can improve or worsen the agreement with observations. To do that, we investigate two possible causes for relaxing of the instantaneous mixing: i) the ``galactic fountain time delay effect'' and ii) the ``metal cooling time delay effect''. We find that the effect of galactic fountains is negligible if an average time delay of 0.1 Gyr, as suggested by our model, is assumed. Longer time delays produce differences in the results but they are not realistic. The metal cooling time delays produce strong effects on the evolution of the chemical abundances only if we adopt stellar yields depending on metallicity. If, instead, the yields computed for the solar chemical composition are adopted, negligible effects are produced, as in the case of the galactic fountain delay. The relaxation of the IMA by means of the galactic fountain model, where the delay is considered only for massive stars and only in the disk, does not affect the chemical evolution results. The combination of metal dependent yields and time delay in the chemical enrichment from all stars starting from the halo phase, instead, produces results at variance with observations.
Description: 2008/2009
NBN: urn:nbn:it:units-7517
Appears in Collections:Scienze fisiche

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