Identification and classification of H alpha-excess candidates in the northern Galactic Plane.

The key-role of studying H alpha emission from astronomical sources has been well established in the past decades. Its relevance is set by the impressively wide range of H alpha-emitting populations, the inspection of which can significantly contribute to understanding our Universe in terms of its evolution and composition. Therefore, finding and classifying new emission-line sources has become a fundamental task in modern astronomy, fueled by the availability of a constantly increasing number of wide field surveys. State-of-the-art techniques to identify H alpha emission line sources in narrow-band photometric surveys consist of searching for H alpha excess with reference to nearby objects in the sky (position-based selection). However, while this approach usually yields very few spurious detections, it may fail to select intrinsically faint and/or rare H alpha-excess sources. In order to try and obtain a more complete representation of the heterogeneous emission line populations, I develop a technique to find outliers relative to nearby objects in the colour-magnitude diagram (CMD-based selection). By leveraging astrometric and photomeric information from Gaia, and by combining position-based and CMD-based selections, I build an updated catalogue of H alpha-excess candidates from data obtained in the INT Photometric H alpha Survey of the Northern Galactic Plane (IPHAS). Out of 7,474,835 inspected targets, 28,496 H alpha-excess candidates are identified. I further perform spectroscopic follow-up observations and classification of 114 objects from this catalogue, that enable to test the novel selection method. Out of the 69 spectroscopically confirmed H alpha emitters in the sample, 14 are identified only by the CMD-based selection, and would be thus missed by the classic position-based technique. In addition, I explore one of the possible applications of the CMD-based selection. More specifically, I describe how accreting white dwarf candidates can be identified among the H alpha-excess sources, with the introduction of information regarding their X-ray emission and variability amplitude. A specific application of these criteria is provided by the target HBHA 4204-09. Spectroscopically confirmed as a cataclysmic variable, this source is part of the H alpha-excess candidates that also show a significantly large variability amplitude. The analysis of its light curve supports its further classification as a nova-like variable. Moreover, I show how the CMD-based selection can be augmented with the introduction of additional colours. Especially with this latter improvement, the CMD-based technique can be used to support the pre-selection and classification of emission line sources in large spectroscopic surveys.