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 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 ~ 0.01 and ~ 0.1 M_solar. The calculations show that for high accretion rates (solarM_acc= 10^-4 M_solaryr ^-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 (solar M_acc= 10^-5 M_solar 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 analyse 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.