The Photometric Selection of M-dwarfs using Gaia, WISE and 2MASS photometry

We present criteria for the photometric selection of M-dwarfs using all-sky photometry, with a view to identifying M-dwarf candidates for inclusion in the input catalogues of upcoming all-sky surveys, including TESS and FunnelWeb. The criteria are based on Gaia, WISE and 2MASS all-sky photometry, and deliberately do not rely on astrometric information. In the lead-up to the availability of truly distance-limited samples following the release of Gaia DR2, this approach has the significant benefit of delivering a sample unbiased with regard to space velocity. Our criteria were developed by using Galaxia synthetic galaxy model predictions to evaluate both M-dwarf completeness and false-positive detections (i.e. non-M-dwarf contamination rates). In addition to the previously known sensitivity of J-H colour for giant-dwarf discrimination at cool temperatures, we find the WISE W1-W2 colour is also effective at discriminating M-dwarfs from cool giants. We have derived two sets of Gaia G > 14.5 criteria - a "high-completeness" set that contains 78,340 stars, of which 30.7-44.4% are expected to be M-dwarfs and contains 99.3% of the total number of expected M-dwarfs; and a "low-contamination" set that prioritises the stars most likely to be M-dwarfs at a cost of a reduction in completeness. This subset contains 40,505 stars and is expected to be comprised of 58.7-64.1% M-dwarfs, with a completeness of 98%. Comparison of the high-completeness set with the TESS Input Catalogue has identified 234 stars not currently in that catalogue, which preliminary analysis suggests could be useful M-dwarf targets for TESS. We also compared the criteria to selection via absolute magnitude and a combination of both methods. We found that colour selection in combination with an absolute magnitude limit provides the most effective way of selecting M-dwarfs en masse.