Author:

Abstract:

B-type stars have temperatures ranging between 10,000 and 30,000 K and masses between around 3 and 15 solar masses. This places them both in the regime of intermediate mass stars, but also of massive stars, both of which have markedly different stellar structures, evolutionary paths, and final products. Whilst intermediate mass stars are more prevalent in the Universe, it is the massive stars (born with greater than 8 solar masses) that dominate the energy budget of star-forming galaxies, have strong stellar winds, highly ionising radiation, and have a significant impact on their surroundings via their energetic deaths as supernovae. A characteristic of massive stars is that the majority of them appear to be in binary or higher order multiple systems, which can drastically alter the evolutionary path of the member stars if they are close enough to interact, and can produce gravitational wave events like binary black hole mergers. This PhD thesis provides observational constraints on the multiplicity of B-type stars in the Galaxy. The first step in this is the analysis of multi-epoch VLT/FLAMES optical spectroscopy of 80 B-type stars in the Galactic open cluster NGC 6231. We monitor the radial velocities (RV) of these stars. For the stars with statistically significant variation in their RVs, we search for periodicity in these variations and fit orbital solutions to the measured RVs. We constrain an observed spectroscopic binary fraction of 33±5% for these stars, and an intrinsic binary fraction of 52±8% when correcting for observational biases. 15 single-lined spectroscopic binaries (SB1s) and 5 double-lined spectroscopic binaries (SB2s) were identified in the sample, and consequently orbital parameter distributions of the B-type binaries were obtained. The measured binary fraction suggests that B-type stars are less frequently found in binary systems than their higher-mass O-type counterparts, possibly suggesting that the frequency of binaries has a mass dependence. The retrieved orbital properties generally resemble those of B- and O-type stars in and out of the Galaxy. The next step in the study of the B-type binaries of NGC 6231 in this thesis is the analysis of the identified SB1s in the sample. The second part of the thesis aims to characterise the hidden companions to the SB1s. Fourier spectral disentangling is applied to the spectra of the SB1s to extract the putative signatures of faint companions, so that newly disentangled SB2s are identified. In the case for which a faint companion can not be extracted from the spectra, we perform atmospheric and evolutionary fitting and use the binary mass function of each SB2 obtained in the first study to determine the viability of each target of harbouring a compact object companion, i.e. a black hole (BH) or neutron star (NS). Seven of the 15 original SB1s were found to be SB2 systems with mass ratios down to 0.1. For the remaining eight targets, four are candidates for harbouring NSes and BHs given their estimated companion masses. Two of these targets, HD 152200 and CXOU J165421.3-415536, have companion mass ranges of 1 and 3.5 Msun, which lies within the estimated mass range for Galactic NSes. The other two, V* V946 Sco and CD-41 11038, have estimated companion mass ranges between 2.5 to 8 solar masses and 1.6 to 26 solar masses respectively, which lies within the estimated mass range for Galactic BHs. However, unambiguous confirmation of these systems as X-ray quiet compact object harbouring binaries will only result from photometric and interferometric follow-up observations of these objects. The final step in this thesis was to analyse the entire B-type star sample of NGC 6231, with the aim to homogeneously derive the spin distribution of the sample and compare it with previously derived distributions from various clusters and OB associations in the Milky Way and the Magellanic Clouds. This way, the impact of age and metallicity on the spin distribution of B-type stars can be studied. We performed atmospheric fitting on the sample, constraining the stellar parameters including the projected rotational velocity. The retrieved physical parameters are in tension with the expected age of the cluster, but the constrained projected rotational velocities are deemed reliable. The spin distribution of the B-type stars in NGC 6231 is broadly consistent with other B-star populations in the Large and Small Magellanic Clouds, and late-type B-type stars tend to rotate closer to their critical ('break-up') velocities than earlier-type B-type stars. B-type stars in binaries also tend to rotate slower than their single star counterparts.