Gauging the Universe: the Effect of the Metallicity on the Cepheid Period-Luminosity Relation

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.