Dust modeling and evolutionary implications

From the beginning of the 20th entury astronomers started to move their studies toward that parti ular omponent whi h ould be responsible of the reddening and the extin tion of stars radiation. They agreed that this omponent must be represented by a population of mi ros opi parti les named interstellar dust, that it must be mixed with the interstellar gas. Later on, in the se ond half of the entury, dust was re ognized to play very many important roles in the Milky Way and other galaxies. These ndings hanged the astronomer's view of the interstellar spa e, that is now fully onsidered as an a tive element in gala ti evolution. For this reason to a hieve a more reliable understanding of the nature of interstellar dust is ne essary to take into a ount the response of dust to the physi al onditions of the regions in whi h dust grains are embedded. Over the years astronomers used the variation of extin tion respe t to wavelength, the interstellar extin tion urve, to study osmi dust. In this thesis I elaborate a physi al and hemi al model of dust, to investigate the remarkable variety of gala ti interstellar extin tion urves. In Chapter 1 I introdu e the roles of the interstellar dust, its pro essing in the interstellar medium, and the tools to address the problem of its nature. In Chapter 2 I present the exploited model in a simpli ed version, and an appli ation to some pe uliar lines of sight in our galaxy. In Chapter 3 the omplete version of the model is put forward, and, together with its simpli ed version, applied to a large sample of Milky Way interstellar extin tion urves. All di erent avours of observed extin tion urves, ranging from the average gala ti extin tion urve to very pe uliar pro les, an be des ribed by su h a model. I show that a mixture of ore mantle sili ate/ arbon grains together with a relatively small number (54 spe ies in four harge states) of poly y li aromati hydro arbons an reprodu e the features of the extin tion urve in the ultraviolet, dismissing an old obje tion to the ontribution of poly y li aromati hydro arbons to the interstellar extin tion urve. In Chapter 4 I ompare an evolutionary model of the physi al properties of arbona eous grain mantles with their determination through the tting pro edure outlined in Chapter 3. The results of su h omparison demonstrates, that in the framework of the adopted dust model, the whole sample of gala ti extin tion urves are in striking agreement with the proposed evolutionary s enario, requiring physi al onditions fully onsistent with standard values of density, temperature, radiation eld intensity, and average age of di use interstellar louds. The results of this thesis show that the present model is able to re on ile the great variety of observed interstellar extin tion urves within an uni ed evolutionary s enario, without making drasti hanges to stru tural properties of dust grains.