The accretion of brown dwarfs and planets by giant stars - I. Asymptotic giant branch stars

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 partic ular, we examine the case in which the brown dwarf spirals-in, and the accr eted matter is deposited at the base of the convective envelope and in the thin radiative shell surrounding the hydrogen burning shell. In our spheric ally symmetric simulations, we explore the effects of different accretion r ates and we follow two scenarios in which the amounts of injected mass are equal to similar to 0.01 and similar to 0.1 M-.. The calculations show that for high accretion rates ((M) over dot(acc) = 10(-4) M-. yr(-1)), the cons iderable release of accretion energy produces a substantial expansion of th e star and gives rise to hot bottom burning at the base of the convective e nvelope. For somewhat lower accretion rates ((M) over dot(acc) = 10(-5) M-. yr(-1)), the accretion luminosity represents only a small fraction of the stellar luminosity, and as a result of the increase in mass land concomitan tly of the gravitational force), the star contracts. Our simulations also i ndicate that the triggering of thermal pulses is delayed (accelerated) if m ass is injected at a slower (faster) rate. We analyse the effects of this a ccretion 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 b rown dwarfs and planets can induce the ejection of shells around giant star s, increase their surface lithium abundance and lead to significant spin-up . The combination of these features is frequently observed among G and K gi ant stars.