Assessing distances and consistency of kinematics in Gaia/TGAS

Abstract

We apply the statistical methods by Schoenrich, Binney & Asplund to assess the quality of distances and kinematics in the RAVE-TGAS and LAMOST-TGAS samples of Solar neighbourhood stars. These methods yield a nominal distance accuracy of 1-2%. Other than common tests on parallax accuracy, they directly test distance estimations including the effects of distance priors. We show how to construct these priors including the survey selection functions (SSFs) directly from the data. We demonstrate that neglecting the SSFs causes severe distance biases. Due to the decline of the SSFs in distance, the simple 1/parallax estimate only mildly underestimates distances. We test the accuracy of measured line-of-sight velocities (v_los) by binning the samples in the nominal v_los uncertainties. We find: a) the LAMOST v_los have a ~ -5 km/s offset; b) the average LAMOST measurement error for v_los is ~7 km/s, significantly smaller than, and nearly uncorrelated with the nominal LAMOST estimates. The RAVE sample shows either a moderate distance underestimate, or an unaccounted source of v_los dispersion (e_v) from measurement errors and binary stars. For a subsample of suspected binary stars in RAVE, our methods indicate significant distance underestimates. Separating a sample in metallicity or kinematics to select thick-disc/halo stars, discriminates between distance bias and e_v. For LAMOST, this separation yields consistency with pure v_los measurement errors. We find an anomaly near longitude l~(300+/-60)deg and distance s~(0.32+/-0.03)kpc on both sides of the galactic plane, which could be explained by either a localised distance error or a breathing mode.