A circumbinary disc in the final stages of common envelope and the core-degenerate scenario for Type Ia supernovae

Abstract

We study the final stages of the common envelope (CE) evolution and find that a substantial fraction of the ejected mass does not reach the escape velocity. To reach this conclusion we use a self‐similar solution under simplifying assumptions. Most of the gravitational energy of a companion white dwarf (WD) is released in the envelope of a massive asymptotic giant branch (AGB) or the red giant branch (RGB) star in a very short time. This rapid energy release forms a blast wave in the envelope. We follow the blast wave propagation from the centre of the AGB outwards, and show that ∼1–10 per cent of the ejected envelope remains bound to the remnant binary system. We suggest that due to angular momentum conservation and further interaction with the binary system, the fall‐back material forms a circumbinary disc around the post‐AGB Core and the companion WD. The interaction of the circumbinary disc with the binary system will reduce the orbital separation much more than expected of the dynamical phase (where the envelope is ejected) of the CE alone. The smaller orbital separation favours a merger at the end of the CE phase or a short time after, while the core is still hot. This is another channel for the formation of a massive WD with super‐Chandrasekhar mass that might explode as a Type Ia supernova. We term this the core‐degenerate (CD) scenario.

The jet feedback mechanism (JFM) in stars, galaxies and clusters

“…It is possible that in some cases at late times the mass inner to the orbit of the core and the companion is low, but mass in the circumbinary disk is large. Kashi & Soker (2011) suggest that the interaction of the circumbinary disc with the binary system might reduce the orbital separation much more than expected during the preceding dynamical phase, when part of the envelope was ejected and the circumbinary disk was formed. This situation resembles that of planet-migration in protoplanetary disks.…”

Planet formation from the ejecta of common envelopes

“…While both Soker (1992Soker ( , 2004 and Sandquist et al (1998) considered the envelope before the ejection, De Marco et al (2011) propose that the part of the ejected envelope that remains gravitationally bound to the system may fall back and form a circumbinary disk. The implications of such a disk for the final fate of the star and for type Ia supernovae have been explored by Kashi & Soker (2011).…”

“…One of them is typically a giant, for instance in an AGB phase (Herwig 2005), while the companion can be a white dwarf or a mainsequence star. In this phase, the orbital period decreases due to gravitational drag and tidal interactions (e.g., Iben & Livio 1993;Kashi & Soker 2011), while both the energy and angular momentum of the companion are injected into the envelope. A recent review of the CE evolution was provided by Ivanova et al (2013).…”

“…The latter may correspond to an upper limit, since their calculation did not consider the potential rotation of the envelope. An analytical assessment by Kashi & Soker (2011), on the other hand, indicates that 1−10% of the envelope mass is retained. Their model yields a bound fraction of 5% for their fiducial case, a red giant with 5 M , and a larger fraction of ∼10% for a system like NN Ser.…”

“…We therefore expect that the most efficient energy deposition occurs on scales close to the core, where the timescales are shortest, while hydrodynamical simulations may find that the complete process takes longer, because some of the energy is deposited at earlier stages. Since a large portion of the energy is thus deposited in a short time close to the central core, we adopt the instantaneous injection approximation developed by Kashi & Soker (2011) in this manuscript. A consideration of the enthalpy indeed shows that the material close to the core can be evacuated on short timescales, thus favoring the survival of low-mass companions by preventing a merger with the core (Ivanova & Chaichenets 2011).…”