Searching for the GD-1 Stream Progenitor in Gaia DR2 with Direct N-body
  Simulations
  We perform a large suite of direct N-body simulations aimed at revealing the
location of the progenitor, or its remnant, of the GD-1 stream. Data from
\gaia\ DR2 reveals the GD-1 stream extends over $\approx 100^\circ$, allowing
us to determine the stream's leading and trailing ends. Our models suggest the
length of the stream is consistent with a dynamical age of between 2-3 Gyr and
the exact length, width and location of the GD-1 stream correspond to the
stream's progenitor being located between $-30^\circ < \phi_{1,\mathrm{pro}} <
-45^{\circ}$ in the standard GD-1 coordinate system. The model stream density
profiles reveal that intact progenitors leave a strong over-density,
recently-dissolved progenitors appear as gaps in the stream as escaped stars
continue to move away from the remnant progenitor's location, and
long-dissolved progenitors leave no observational signature on the remaining
stream. Comparing our models to the GD-1 stream yields two possible scenarios
for its progenitor's history: a) the progenitor reached dissolution
approximately 500 Myr ago during the cluster's previous perigalactic pass and
is both located at and responsible for the observed gap at $\phi_1=-40^{\circ}$
or b) the progenitor reached dissolution over 2.5 Gyr ago, the fully-dissolved
remnant is at $-30^\circ < \phi_1 < -45^{\circ}$, and an observational
signature of its location no longer exists. That the dissolved progenitor is in
the range $-30^\circ < \phi_1 < -45^{\circ}$ implies that density fluctuations
outside of this range, e.g., a deep gap at $\phi_1 \approx -20^\circ$, are
likely produced by compact baryonic or dark-matter perturbers.
