Multiplicity properties and spectral classification of B-type stars

Massive stars are usually called cosmic engines due to their well-known importance at different scales, from shaping their surroundings to even cosmological implications. Binarity is one of the main ingredients in the evolution of massive stars as it has been demonstrated by the high binary fraction of OB-type stars found by several studiesin the Galaxy and the Magellanic Clouds. Binarity can also play a crucial role onother physical properties of massive stars such as rotation and chemical evolution. Furthermore, massive binaries might end their lives as double-compact objects, which are progenitors of gravitational waves. To understand the connection between binarity and the evolution and final fate of massive stars, it is imperative to characterise large and homogeneous samples of massive binaries. In particular, the distributions of their orbital parameters are important inputsfor evolutionary and population-synthesis models used to investigate stellar populations and the chemical evolution of galaxies. Among massive stars, B-type stars are the most numerous representatives and the spectral range between B0 and B3 dominates the rate of core-collapse supernovae. Here I present a detailed study of different aspects of B-type stars in this important spectral range. Firstly, I describe my work for the Galactic O-type Stars Spectroscopic Survey (GOSSS) in Chapter 2. In line with previous instalments of the GOSSS series, a new atlas of galactic B-type giants and supergiants and a new grid of standard stars for spectral classification are presented. Taking one step further, I have developed a quantitative approach to spectral classification of B-type stars, based on the equivalent widths ofspectral lines that have been used historically for this purpose. I propose a classification procedure that uses in first instance the quantitative criteria, and can then use thestandard stars defined in this work for complicated cases. In Chapter 3, I present the B-type Binaries Characterisation (BBC) programme, the largest multi-epoch, spectroscopic study of early B-type binaries in the 30 Doradus region of the Large Magellanic Cloud. In the context of this programme, I havedetermined orbital solutions for a sample of 88 B-type Binaries and the distributions oftheir orbital parameters that are contrasted to other samples of massive stars. Interesting similarities are found when comparing the period distribution of the BBC stars witha sample of O-type stars from the same region of 30 Doradus. This evidence, added to the similar multiplicity fractions of B- and O-type stars, argues in favour of similar formations mechanisms for the complete range of core-collapse-supernova progenitors, but the intrinsic distributions need further study to draw more definite conclusions. The fourth Chapter of this thesis delves into the search of compact companions to B-type stars. Taking advantage of the numerous BBC sample and their derived orbitalproperties, I used the K-P diagram (semi-amplitude velocity vs. orbital period) in Chapter 3 to identify stars with large velocities that could be caused by the presence of massive companions that remained hidden in the spectra of the system. The K-P diagram is explored further in Chapter 4, and strategies are discussed to limit the inclusion of false-positives among the candidates to binaries with compact companions. I have used spectral classifications to estimate primary masses and derive the mass of the secondaries. The best candidates (those with high mass ratios) are discussed further. Finally, Chapter 5 presents a summary of the results presented in this thesis, and describes the possible lines of research that could follow the work developed during my PhD.