A census of Herbig Ae/Be stars: new candidates and analysis from a Gaia perspective

Herbig Ae/Be stars are pre-main sequence sources, canonically defined as having masses 2 Msun < M < 10 Msun, which are key to understanding the differences in formation mechanisms between low- and high-mass stars. The study of the general properties of these objects is hampered by the lack of a well-defined, homogeneous sample, and because few and mostly serendipitously discovered sources are known. As a consequence, many open problems involving high-mass star formation suffer from biases and lack of completeness. I study the general properties of the 252 known and proposed Herbig Ae/Be stars with parallaxes from Gaia DR2 at the start of this doctoral thesis. High-mass stars were found to have a much smaller infrared excess and optical variability compared to lower-mass stars, with the break at around 7 Msun. Different or differently acting (dust-) disc-dispersal mechanisms are proposed for this break. The variability indicator developed in this dissertation shows that approximately 25% of all Herbig Ae/Be stars are strongly variable. Evidence is provided to support the hypothesis that this variability is in most cases due to asymmetric dusty disc structures seen edge-on. Using that well-characterised sample of Herbig Ae/Be stars as a training sample for a bespoke machine learning algorithm, a homogeneous and well-defined catalogue of 8470 new pre-main sequence candidates was obtained. In parallel, a catalogue of 693 new classical Be candidates was produced. At least 1361 sources are potentially new Herbig Ae/Be stars according to their position in the Hertzsprung-Russell diagram. This increases the number of known objects of the class by an order of magnitude. In addition, I discuss the results of independent spectroscopic observations conducted for a selection of 145 new Herbig Ae/Be and 14 new classical Be stars. These independent observations further confirm the quality and the accuracy of the classification. I conclude with an analysis of the general properties of the new catalogues that validates the results and conclusions obtained for the set of previously known Herbig Ae/Be stars.