The Gaia/IPHAS and Gaia/KIS value-added catalogues

We present a sub-arcsecond crossmatch of Gaia DR2 against the INT Photometric H alpha Survey of the Northern Galactic Plane Data Release 2 (IPHAS DR2) and the Kepler-INT Survey (KIS). The resulting value-added catalogues (VACs) provide additional precise photometry to the Gaia photometry (r, i, and H alpha for IPHAS, with additional U and g for KIS). In building the catalogue, proper motions given in Gaia DR2 are wound back to match the epochs of IPHAS DR2, thus ensuring high proper motion objects are appropriately crossmatched. The catalogues contain 7927 224 and 791 071 sources for IPHAS and KIS, respectively. The requirement of > 5 sigma parallax detection for every included source means that distances out to 1-1.5 kpc are well covered. We define two additional parameters for each catalogued object: (i) f(c), a magnitude-dependent tracer of the quality of the Gaia astrometric fit; (ii) f(FP), the false-positive rate for parallax measurements determined from astrometric fits of a given quality at a given magnitude. Selection cuts based on these parameters can be used to clean colour-magnitude and colour-colour diagrams in a controlled and justified manner. We provide both full and light versions of the VAC, with VAC-light containing only objects that represent our recommended trade-off between purity and completeness. Uses of the catalogues include the identification of new variable stars in the matched data sets, and more complete identification of H alpha-excess emission objects due to separation of high-luminosity stars from the main sequence.