  Stars that approach a supermassive black hole (SMBH) too closely can be
disrupted by the tidal gravitational field of the SMBH. The resulting debris
forms a tidal stream orbiting the SMBH which can collide with itself due to
relativistic apsidal precession. These self-collisions dissipate energy,
causing the stream to circularize. We perform kinematic simulations of these
stream self-collisions to estimate the efficiency of this circularization as a
function of SMBH mass $M_\bullet$ and penetration factor $\beta$, the ratio of
the tidal radius to the pericenter distance. We uncover two distinct regimes
depending on whether the time $t_c$ at which the most tightly bound debris
circularizes is greater or less than the time $t_{\rm fb}$ at which the mass
fallback rate peaks. The bolometric light curve of energy dissipated in the
stream self-collisions has a single peak at $t > t_{\rm fb}$ in the slow
circularization regime ($t_c > t_{\rm fb}$), but two peaks (one at $t < t_{\rm
fb}$ and a second at $t_{\rm fb}$) in the fast circularization regime ($t_c <
t_{\rm fb}$). Tidal streams will circularize in the slow (fast) regime for
apsidal precession angles less (greater) than 0.2 radians which occur for
$\beta \lesssim (\gtrsim) (M_\bullet/10^6M_\odot)^{-2/3}$. The observation of
prominent double peaks in bolometric TDE light curves near the transition
between these two regimes would strongly support our model of tidal-stream
kinematics.
