What Giants Do When They Are Tickled: Energy Considerations In The Common Envelope Interaction

Details What Giants Do When They Are Tickled: Energy Considerations In The Common Envelope Interaction What Giants Do When They Are Tickled: Energy Considerations In The Common Envelope Interaction

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21538503d&link_type=abstract

American Astronomical Society, AAS Meeting #215, #385.03; Bulletin of the American Astronomical Society, Vol. 42, p.600

Astronomy and Astrophysics

Astronomy

Scientific paper

The common envelope interaction (CE) takes place when an evolving giant fills its Roche Lobe transferring mass onto a close companion. In some cases this can lead to the companion's own expansion and the filling of its own Roche Lobe. In this event, the giant will engulf the companion. There follows a relatively brief period of in-spiral during which the companion donates orbital energy to the envelope of the giant. This can result in the complete ejection of the giant envelope. In this case the core of the giant and the companion emerge as a new close binary. The CE interaction is the cause of all evolved close binaries from the least massive to the most massive and might be involved with the production of Type Ia supernovae.
Simulations of the CE interaction and observed post-CE systems are used to determine the efficiency of the CE ejection, also called alpha. Alpha has been notoriously difficult to determine, partly because it is possibly variable. In this contribution we redefine alpha, showing how past definitions have contributed confusion. We also use simulations and observations from the literature to demonstrate how alpha is inversely proportional to the ratio of the companion mass to the primary mass at the time of the CE. This result is surprising because it implies that lower mass companions eject the envelope more efficiently. It also appears that lighter companions have alpha values larger than one, revealing a secondary energy source that helped the envelope ejection. As in past work, we identify this source as thermal energy of the envelope, unleashed when the interaction time-scale is longer than the dynamical time-scale of the envelope. Finally, we use the Virial Theorem to parametrize this additional energy source.
This research was funded by NSF grant 0607111.

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