The Accretion of Brown Dwarfs and Planets by Giant Stars -- I. AGB 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 $\sim 0.01$ 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.Comment: 11 pages, 9 Postscript figures, to be published in the MNRAS. see also http://www-laog.obs.ujf-grenoble.fr/~sies

The Swallowing of Planets by Giant Stars

We present simulations of the accretion of a massive planet or brown dwarf by an AGB star. In our scenario, close planets will be engulfed by the star, spiral-in and be dissipated in the ``accretion region'' located at the bottom of the convective envelope of the star. The deposition of mass and chemical elements in this region will release a large amount of energy that will produce a significant expansion of the star. For high accretion rates, hot bottom burning is also activated. Finally, we present some observational signatures of the accretion of a planet/brown dwarf and we propose that this process may be responsible for the IR excess and high lithium abundance observed in 4-8% of single G and K giants.Comment: 4 pages, 1 figure, to appear in "Unsolved Problems in Stellar Evolution", ed. M. Livio, Cambridge University Press, in pres

The accretion of planets and brown dwarfs by giant stars -- II. solar mass stars on the red giant branch

This paper extends our previous study of planet/brown dwarf accretion by giant stars to solar mass stars located on the red giant branch. The model assumes that the planet is dissipated at the bottom of the convective envelope of the giant star. The giant's evolution is then followed in detail. We analyze the effects of different accretion rates and different initial conditions. The computations indicate that the accretion process is accompanied by a substantial expansion of the star, and in the case of high accretion rates, hot bottom burning can be activated. The possible observational signatures that accompany the engulfing of a planet are also extensively investigated. They include : the ejection of a shell and a subsequent phase of IR emission, an increase in the 7Li surface abundance and a potential stellar metallicity enrichment, spin-up of the star due to the deposition of orbital angular momentum, the possible generation of magnetic fields and a related X-ray activity due to the development of shear at the base of the convective envelope, and the effects on the morphology of the horizontal branch in globular clusters. We propose that the IR excess and high Li abundance observed in 4-8% of the G and K giants originate from the accretion of a giant planet, a brown dwarf or a very low-mass star.Comment: Accepted for publication in MNRAS, 15 pages, 10 Postscript figures. Also available at http://www-laog.obs.ujf-grenoble.fr/~sies

On the Formation and Evolution of Common Envelope Systems

We discuss the formation of a common envelope system following dynamically unstable mass transfer in a close binary, and the subsequent dynamical evolution and final fate of the envelope. We base our discussion on new three-dimensional SPH calculations that we have performed for a close binary system containing a $4\,M_\odot$ red giant with a $0.7\,M_\odot$ main-sequence star companion. The initial parameters are chosen to model the formation of a system resembling V~471~Tau, a typical progenitor of a cataclysmic variable binary. In our highest-resolution calculation, we find evidence for a corotating region of gas around the central binary. This is in agreement with the theoretical model proposed by Meyer \& Meyer-Hofmeister (1979) for the evolution of common envelope systems, in which this central corotating region is coupled to the envelope through viscous angular momentum transport only. We also find evidence that the envelope is convectively unstable, in which case the viscous dissipation time could be as short as $\sim100$ dynamical times, leading to rapid ejection of the envelope. For V~471~Tau, our results, and the observed parameters of the system, are entirely consistent with rapid envelope ejection on a timescale $\sim1\,$yr and an efficiency parameter $\alpha_{CE}\simeq1$.Comment: uses AAS latex macros v4, 36 pages with figures, submitted to ApJ, complete postscript also available at http://ensor.mit.edu/~rasio/paper