Classical Cepheids are variable stars with essential importance from both astrophysical and cosmological point of view. These are massive giant- or supergiant stars that are located within the Instability Strip on the Hertzsprung-Russell diagram (HRD), and they exhibit stable, periodic
light variation. This variability is due to radial pulsation excited and governed by the astrophysical process called kappa-mechanism.
The importance of pulsating stars, especially Cepheids, is given by the fact that we can reveal details on the internal structure and evolution of these stars by studying their oscillations. In cosmology they are mainly used as distance indicators, because the existence of the period-luminosity
relation discovered 100 years ago make them useful for measuring distances on extragalactic (i.e. several megaparsecs) scale.
A small subgroup of Classical Cepheids contain stars that pulsate in two excited modes simultaneously. These are called double-mode, or beat Cepheids. Modeling these stars dates back to the 1990s, and it was recognized quite early that the period ratio (P 1 /P 0 ) depends strongly on other physical parameters like effective temperature, mass, luminosity and metallicity. Nevertheless, checking the theoretical predictions with observations was limited for a while, because, for example, having metallicity information for such stars requires high resolution, high signal-to-noise measurements that are feasible only with modern state-of-the-art instruments.
By the beginning of the 21st century only about two dozens of double-mode Cepheids were known within the Milky Way. On the other hand, hundreds of such stars were discovered in the Large- and Small Magellanic Cloud by the MACHO and OGLE sky surveys. Unfortunately, due to instrumental constraints, only the Milky Way (galactic-) Cepheids could be studied by high-resolution spectroscopy to measure their metallicities directly. This is why new methods that attempt to derive physical parameters, like metallicity, solely from photometry gained more attention by now.
My PhD dissertation focuses on studies related to these problems. The aim of my work is to reveal new details on the metallicity dependence of the pulsational properties of beat Cepheids.
Galactic beat Cepheids are so bright that their metallicities can be measured directly with high-resolution spectroscopy using big telescopes. In 2004, our foreign collaborators obtained such measurements for 17 beat Cepheids using the FEROS spectrograph attached to the 2.2 m telescope of the European Southern Observatory (ESO) at La Silla, Chile. 4 of our program stars have been
studied before only with photometry. After reducing and analysing these data, my work focused on quantifying the correlation between the period ratio and metallicity that was revealed by theoretical studies earlier.
As an application of my results, I examined the double-mode Cepheids in the Large- and Small Magellanic Cloud attempting to correlate their metallicities with the Fourier parameters of their light curves. After collecting the existing photometric data from literature, First, I derived the Fourier-amplitude ratios and phase differences of the light curves that characterize the shape of
the light curves. After that I looked for correlations between these Fourier parameters and the metallicities obtained from period ratios using the relation that I determined for the galactic beat Cepheids.
The new scientific results I found are summarized as follows.
Using new high-resolution spectroscopic data I derived the following physical parameters for 17 Galactic double-mode Cepheids: effective temperature (T eff ), surface gravity (log g) and metallicity expressed as the relative abundance of iron to hydrogen, [Fe/H] = log(N Fe /N H ) − log(N Fe /N H ) ⊙ . These were the first spectroscopic metallicities for 4 of the program stars that were studied before only photometrically. Years later, using more recent metallicity measurements collected from literature, I derived homogenized metallicities for all the program stars by bringing them onto a consistent metallicity scale reducing the systematic offsets plaguing the early metallicity measurements.
Based on the new spectroscopic data mentioned above I quantified and improved the period - period ratio - metallicity (log P - P 1 /P 0 - [Fe/H]) relation that is valid for beat Cepheids pulsating in the fundamental mode and the first overtone simultaneously. By extrapolating the relation toward lower metallicities I estimated the metallicities of double-mode Cepheids in the Large- and Small Magellanic Clouds..
