Spectroscopic and physical characterization of the Galactic O-type stars targeted by the IACOB & OWN surveys

O stars represent a fundamental phase in the zoo of massive stars. They ionize the material around them and flood their environment with processed material via strong stellar winds and supernova explosions. On top of the mass-loss, factors such as rotation and multiplicity (as well as the interplay between them) have become key processes to understand their evolution. A better understanding of these complex objects requires better empirical characterization of their physical properties, something that can only be obtained through extensive studies of large spectroscopic datasets. This has been the main purpose of this thesis, a thorough empirical characterization --mainly through quantitative spectroscopy but also taking into account information about photometry and parallaxes-- of a sample of 415 Galactic O-type stars (i.e. representing a sample 5-10 times larger than those considered in previous similar studies). A crucial part of the work has been the compilation of an (almost) homogeneous database of optical spectra of Galactic O stars. The bulk of the observations come from the extensive spectroscopic databases gathered in the framework of the IACOB and OWN surveys. In total, we have used 2900 spectra of 415 Galactic O-type stars, covering a range in spectral types from O2 to O9.7 and all luminosity classes. In a first step, the multi-epoch character of the compiled spectroscopic dataset has allowed us to perform a spectroscopic variability assessment of the full sample of stars. Then, we have proceed with a detailed spectroscopic analysis of those (285) stars in the sample not identified as double-line spectroscopic binaries. To cope with the large amount of targets, we make use of two semi-automated (but supervised) tools for quantitative spectroscopic analysis developed in the framework of the IACOB project. First, a combined Fourier transform and profile fitting method is used to derive vsini and the amount of macroturbulence broadening affecting the line-profiles using the sc iacob-broad tool. Then, in order to derive the stellar atmosphere and wind parameters, we perform a detailed quantitative spectroscopic analysis using sc iacob-gbat, a semi-automatized tool based on chi-squared optimization with a vast grid of fastwind models. The program calculates in a fast, objective, and reproducible way, values and uncertainties for effective temperature, gravity, helium abundance, microturbulence, and wind-strength. As part of this work, we have initially selected and analyzed a benchmark subsample (namely, the grid of O-type standard stars for spectral classification) to establish a step-by-step protocol to be followed in the analysis of the complete combined IACOB+OWN sample. This sample has been also used to assess the level of agreement between our results and those provided by more traditional "by-eye" techniques, and to construct new linear temperature and gravity calibrations as a function of spectral type and luminosity class for Galactic O-type stars. We then extend our investigation to the whole sample of Galactic O-type stars surveyed by IACOB and OWN. We use the results of the spectroscopic analysis to provide a global empirical overview of the various investigated parameters in several diagnostic diagrams, as well as to investigate some topics of interest in the field of massive stars. In particular, we introduce a distance-independent version of the wind-momentum luminosity relationship to evaluate our results with respect to previous theoretical and empirical studies. We also assess the apparent offset between the theoretical and empirical zero-age-main-sequence for stars more massive than 32 solar masses. Last, we study the distribution and evolution of the spin rate of O-type stars, and compare the extracted empirical information with predictions by state-of-the-art models of single and binary star evolution. In the last chapter we perform a thorough assessment of the available information about parallaxes for our complete sample of Galactic O-type stars. We evaluate, in terms of quantity and accuracy, the improvement from the Hipparcos-Tycho catalog to the recent second data release from the Gaia mission. Then, we benefit from the parallaxes provided by Gaia-DR2 to derive the absolute magnitudes, radii, luminosities, and spectroscopic masses of our benchmark subsample: the grid of O-type standard stars for spectral classification