Median stream properties computed from the proper motion and CMD filtered GD-1 stars, and the same for a toy model for the stream track. Panel (a): sky positions of candidate GD-1 stars. Lines show a fourth-order polynomial fit to the median stream track offset by ±0.°75; the enclosed region is adopted as the nominal stream region. Panels (b) and (c): sky positions of simulated star particles from mock stream models of the GD-1 stream in which the progenitor is currently disrupting (toy model 1, panel (b)) or the progenitor fully disrupted 500 Myr ago and results in the observed under-density at ϕ₁ ∼ −20° (toy model 2, panel (c)). Panel (d): background-subtracted surface-density estimates computed in successive, overlapping 3° windows for both GD-1 data and the model stream. Panel (e): median proper motion along the stream computed in the same bins as the previous panel (not solar reflex corrected). The two toy models have consistent median proper motions (overlapping dashed lines). Panel (f): same as panel (d), but for proper motion in stream latitude.