Selection function of the dataset. Panel a) shows the continuous selection function used in this work and defined in Eq. (2); panel b) shows the ratio between spectroscopically observed and astrometric member stars for a 15×15 gird in the (R, G) plane. Panel c) shows the selection function integrated over all magnitudes (Eq. (3)) as a solid line and the ratio between observed and member stars for 100 different grids of binning in R.