\[Figure2\] which are slightly inconsistent with the model, we find that a majority of them has higher contamination (green and orange dots). A more rigorous cut in TOPCAT would have eliminated them, but it serves as an illustration how well our general selection method using the $k$-neighbourhood works. On the other hand, there are 5 stars with higher contamination in the trailing tail in Fig. \[Figure2\] that are quite consistent with the model (left panel) and they are those farthest away from the cluster centre. At the same time these are the stars that are most distant from the Sun (up to 300 pc). The restrictions from Sec. \[prep\] applied to the full Gaia DR2 data set to get an astrometrically and photometrically clean sample creates some incompleteness here, especially at the faint end of the stellar sample. Note that incompleteness in signal leads to higher contamination by noise.

The internal contamination by field stars is given by summing up the values of $p_{cont}$. We sub-divided the PrS stars into 3 subgroups: a) 725 stars within the tidal radius $r_t$ of 10.77 pc which is derived below, b) 279 stars between 1 and 2 tidal radii, and c) 389 stars 