  This investigation delves into the ringdown signals produced by semiclassical
stars, which are ultra-compact, regular solutions of the Einstein equations
incorporating stress-energy contributions from quantum vacuum polarization.
These stars exhibit an approximately Schwarzschild exterior and an interior
composed of a constant-density classical fluid and a cloud of vacuum
polarization. By adjusting their compactness and density, we can alter the
internal structure of these stars without modifying the exterior. This
adaptability enables us to examine the sensitivity of the ringdown signal to
the innermost regions of the emitting object and to compare it with similar
geometries that differ substantially only at the core. Our results indicate
that echo signals are intrinsically linked to the presence of stable light
rings and can be very sensitive to the internal structure of the emitting
object. This point was previously overlooked, either due to the imposition of
reflective boundary conditions at the stellar surface or due to the assumption
of low curvature interior geometries. Specifically, for stellar-sized
semiclassical stars, we find that the interior travel time is sufficiently
prolonged to render the echoes effectively unobservable. These findings
underscore the potential efficacy of ultra-compact objects as black hole
mimickers and emphasize that any phenomenological constraints on such objects
necessitate a detailed understanding of their specific properties and core
structure.
