The persistence of pancakes and the revival of self-gravity in tidal
  disruption events
  The destruction of a star by the tides of a supermassive black hole (SMBH)
powers a bright accretion flare, and the theoretical modeling of such tidal
disruption events (TDEs) can provide a direct means of inferring SMBH
properties from observations. Previously it has been shown that TDEs with
$\beta = r_{\rm t}/r_{\rm p} = 1$, where $r_{\rm t}$ is the tidal disruption
radius and $r_{\rm p}$ is the pericenter distance of the star, form an in-plane
caustic, or ``pancake,'' where the tidally disrupted debris is compressed into
a one-dimensional line within the orbital plane of the star. Here we show that
this result applies generally to all TDEs for which the star is fully
disrupted, i.e., that satisfy $\beta \gtrsim 1$. We show that the location of
this caustic is always outside of the tidal disruption radius of the star and
the compression of the gas near the caustic is at most mildly supersonic, which
results in an adiabatic increase in the gas density above the tidal density of
the black hole. As such, this in-plane pancake revitalizes the influence of
self-gravity even for large $\beta$, in agreement with recent simulations. This
finding suggests that for all TDEs in which the star is fully disrupted,
self-gravity is revived post-pericenter, keeps the stream of debris narrowly
confined in its transverse directions, and renders the debris prone to
gravitational instability.
