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Mottini, Marta (2006): Gauging the Universe: the Effect of the Metallicity on the Cepheid Period-Luminosity Relation. Dissertation, LMU München: Faculty of Physics
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Abstract

The aim of this thesis is to assess the effect of the metallicity on the Cepheid Period-Luminosity (PL) relation. The novelty of the approach adopted in this project consists in the homogeneous analysis of a large sample of Cepheids (72) observed in three galaxies (the Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud), spanning a factor of ten in metallicity. This allows us to explore the effect of the metallicity on the PL relation in a wide range and to study the gas enrichment histories of three different galaxies. To fulfil this goal, firstly, we have selected a sample of Cepheids for which distances and accurate photometry are available in the literature and we have collected high-resolution, high signal-to-noise spectra of these stars, using the highly advanced facilities of the European Southern Observatory in Chile. Secondly, we have directly measured iron and alpha-elements (O, Na, Mg, Al, Si, Ca, Ti) abundances of our sample from these spectra. We have compared our iron abundances with studies on Galactic and Magellanic Cepheids and found a good agreement for the average values and for the individual stars in common. We have then made a broader comparison with results for the Magellanic Clouds from the analysis of F and K non-variable supergiants (they have ages and temperatures similar to Cepheid stars) and of B stars, which are progenitors of Cepheids, and found a good agreement. Cepheids do not show any peculiar differences with these two other population of stars, this indicate that, during this evolutionary stage, there are no changes of the original iron content of the gas from which they were formed. We have then studied the trends of the individual alpha-elements abundance ratios relative to iron as a function of the iron content of our programme star. We can draw some preliminary conclusion considering oxygen, silicon and calcium as the most reliable indicators among the alpha-elements we have analysed. The trends of the abundance ratios of O, Si and Ca are in fairly good agreement with observational studies on Cepheids and on different kinds of stellar populations in the Galaxy and the Magellanic Clouds. The elemental abundances we have determined were used to investigate the effect of metallicity on the PL relation in the V and K bands, in order to check if there is a change of the effect as wavelength increases. We note different behaviours in the two bands. The metallicity has an effect in the V band in the sense that metal-rich Cepheids are fainter than metal-poor ones, while it does not have any effects in the K band. Thus, to safely measure the distances of galaxies, one can use the PL relation in the infrared bands (namely K), so as to minimise the effect of the metallicity. Using the K band has the additional advantage of reducing the effects of the interstellar extinction to the level of other systematic and random errors.