GaiaUnlimited: The old stellar disc of the Milky Way as traced by the Red Clump

Abstract: We present an exploration of the Milky Way's structural parameters using an all-sky sample of RC giants to map the stellar density from the inner to the outer parts of the Galactic disc. These evolved giants are considered to be standard candles due to their low intrinsic variance in their absolute luminosities, allowing us to estimate their distances with reasonable confidence. We exploit all-sky photometry from the AllWISE mid-infrared survey and the Gaia survey, along with astrometry from Gaia Data Release 3 and recent 3D extinction maps, to develop a probabilistic scheme in order to select with high confidence \rc{}-like stars. Our curated catalogue contains about 10 million sources, for which we estimate photometric distances based on the WISE $W1$ photometry. We then derive the selection function for our sample, which is the combined selection function of sources with both \gaia{} and \allwise{} photometry. Using the distances and accounting for the full selection function of our observables, we are able to fit a two-disc, multi-parameter model to constrain the scale height (\hz{}), scale-length (\rd{}), flaring, and the relative mass ratios of the two disc components. We illustrate and verify our methodology using mock catalogues of \rc{} stars. We find that the \rc{} population is best described by a flared thin disc with scale length \rd{}=$3.56\pm0.32$ kpc and scale height at the Sun of \hzsun{}=$0.17\pm0.01$ kpc, and a shorter and thicker disc with \rd{}=$2.59\pm0.11$ kpc, \hzsun{}=$0.45\pm0.11$ kpc, with no flare. The thicker disc constitutes 64\% of the \rc{} stellar mass beyond 3 kpc, while the thin disk shows evidence of being warped beyond 9 kpc from the Galactic center. The residuals between the predicted number density of RC stars from our axisymmetric model and the measured counts show possible evidence of a two-armed spiral perturbation in the disc of the Milky Way.