A cubic polynomial $f$ with a periodic Siegel disk containing an eventual
image of a critical point is said to be a \emph{Siegel capture polynomial}. If
the Siegel disk is invariant, we call $f$ a \emph{IS-capture polynomial} (or
just an IS-capture; IS stands for Invariant Siegel). We study the location of
IS-capture polynomials in the parameter space of all cubic polynomials and show
that any IS-capture is on the boundary of a unique hyperbolic component
determined by the rational lamination of the map. We also relate IS-captures to
the cubic Principal Hyperbolic Domain and its closure (by definition, the
\emph{cubic Principal Hyperbolic Domain} consists of cubic hyperbolic
polynomials with Jordan curve Julia sets) and prove that, in the slice of cubic
polynomials given by a fixed multiplier at one of the fixed points, the closure
of the cubic principal hyperbolic domain might possibly only have bounded
complementary domains $U$ such that (1) critical points of $f\in U$ are
distinct and belong to $J(f)$, and (2) $J(f)$ has positive Lebesgue measure and
carries an invariant line field.