  We study the response of the structure of an asymptotic giant branch (AGB)
star to the accretion of a brown dwarf or planet in its interior. In
particular, we examine the case in which the brown dwarf spirals-in, and the
accreted matter is deposited at the base of the convective envelope and in the
thin radiative shell surrounding the hydrogen burning shell. In our spherically
symmetric simulations, we explore the effects of different accretion rates and
we follow two scenarios in which the amounts of injected mass are equal to
$\sim 0.01$ and $\sim 0.1 M_\odot$. The calculations show that for high
accretion rates ($\dot M_{acc} = 10^{-4} M_\odot yr^{-1}$), the considerable
release of accretion energy produces a substantial expansion of the star and
gives rise to hot bottom burning at the base of the convective envelope. For
somewhat lower accretion rates ($\dot M_{acc} = 10^{-5} M_\odot yr^{-1}$), the
accretion luminosity represents only a small fraction of the stellar
luminosity, and as a result of the increase in mass (and concomitantly of the
gravitational force), the star contracts. Our simulations also indicate that
the triggering of thermal pulses is delayed (accelerated) if mass is injected
at a slower (faster) rate. We analyze the effects of this accretion process on
the surface chemical abundances and show that chemical modifications are mainly
the result of deposition of fresh material rather than of active
nucleosynthesis. Finally, we suggest that the accretion of brown dwarfs and
planets can induce the ejection of shells around giant stars, increase their
surface lithium abundance and lead to significant spin-up. The combination of
these features is frequently observed among G and K giant stars.
