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A naive Bayes classifier for identifying Class II YSOs has been constructed and applied to a region of the Northern Galactic Plane containing 8 million sources with good quality Gaia EDR3 parallaxes. The classifier uses the five features: Gaia G -band variability, WISE mid-infrared excess, UKIDSS and 2MASS near-infrared excess, IGAPS Hα excess, and overluminosity with respect to the main sequence. A list of candidate Class II YSOs is obtained by choosing a posterior threshold appropriate to the task at hand, balancing the competing demands of completeness and purity. At a threshold posterior greater than 0.5, our classifier identifies 6504 candidate Class II YSOs. At this threshold, we find a false positive rate around 0.02 per cent and a true positive rate of approximately 87 per cent for identifying Class II YSOs. The ROC curve rises rapidly to almost one with an area under the curve around 0.998 or better, indicating the classifier is efficient at identifying candidate Class II YSOs. Our map of these candidates shows what are potentially three previously undiscovered clusters or associations. When comparing our results to published catalogues from other young star classifiers, we find between one quarter and three quarters of high probability candidates are unique to each classifier, telling us no single classifier is finding all young stars. [ABSTRACT FROM AUTHOR]

Faint, hidden contaminants in the point spread functions (PSFs) of stars cause shifts to their measured positions. Wilson & Naylor showed failing to account for these shifts can lead to a drastic decrease in the number of returned catalogue matches in crowded fields. Here, we highlight the effect these perturbations have on cross-matching, for matches between Gaia DR2 and Wide-field Infrared Survey Explorer (WISE) stars in a crowded Galactic plane region. Applying the uncertainties as quoted to Gaussian-based astrometric uncertainty functions (AUFs) can lead, in dense Galactic fields, to only matching 55 per cent of the counterparts. We describe the construction of empirical descriptions for AUFs, building on the cross-matching method of Wilson & Naylor, utilizing the magnitudes of both catalogues to discriminate between true and false counterparts. We apply the improved cross-matching method to the Galactic plane | b | ≤ 10. We provide the most likely counterpart matches and their respective probabilities. We also analyse several cases to verify the robustness of the results, highlighting some important caveats and considerations. Finally, we discuss the effect that PSF resolution has by comparing the intra-catalogue nearest neighbour separation distributions of a sample of likely contaminated WISE objects and their corresponding Spitzer counterpart. We show that some WISE contaminants are resolved in Spitzer, with smaller intra-catalogue separations. We have highlighted the effect contaminant stars have on WISE, but it is important for all photometric catalogues, playing an important role in the next generation of surveys, such as the Large Synoptic Survey Telescope. [ABSTRACT FROM AUTHOR]

A fundamental process in astrophysics is the matching of two photometric catalogues. It is crucial that the correct objects be paired, and that their photometry does not suffer from any spurious additional flux. We compare the positions of sources in Wide-field Infrared Survey Explorer (WISE), INT Photometric Hα Survey, Two Micron All Sky Survey and AAVSO Photometric All Sky Survey with Gaia Data Release 1 astrometric positions. We find that the separations are described by a combination of a Gaussian distribution, wider than naively assumed based on their quoted uncertainties, and a large wing, which some authors ascribe to proper motions. We show that this is caused by flux contamination from blended stars not treated separately. We provide linear fits between the quoted Gaussian uncertainty and the core fit to the separation distributions. We show that at least one in three of the stars in the faint half of a given catalogue will suffer from flux contamination above the 1 per cent level when the density of catalogue objects per point spread function area is above approximately 0.005. This has important implications for the creation of composite catalogues. It is important for any closest neighbour matches as there will be a given fraction of matches that are flux contaminated, while some matches will be missed due to significant astrometric perturbation by faint contaminants. In the case of probability-based matching, this contamination affects the probability density function of matches as a function of distance. This effect results in up to 50 per cent fewer counterparts being returned as matches, assuming Gaussian astrometric uncertainties for WISE–Gaia matching in crowded Galactic plane regions, compared with a closest neighbour match. [ABSTRACT FROM AUTHOR]