Noam Soker

Common envelope evolution: where we stand and how we can move forward

This work aims to present our current best physical understanding of common-envelope evolution (CEE). We highlight areas of consensus and disagreement, and stress ideas which should point the way forward for progress in this important but long-standing and largely unconquered problem. Unusually for CEE-related work, we mostly try to avoid relying on results from population synthesis or observations, in order to avoid potentially being misled by previous misunderstandings. As far as possible we debate all the relevant issues starting from physics alone, all the way from the evolution of the binary system immediately before CEE begins to the processes which might occur just after the ejection of the envelope. In particular, we include extensive discussion about the energy sources and sinks operating in CEE, and hence examine the foundations of the standard energy formalism. Special attention is also given to comparing the results of hydrodynamic simulations from different groups and to discussing the potential effect of initial conditions on the differences in the outcomes. We compare current numerical techniques for the problem of CEE and also whether more appropriate tools could and should be produced (including new formulations of computational hydrodynamics, and attempts to include 3D processes within 1D codes). Finally we explore new ways to link CEE with observations. We compare previous simulations of CEE to the recent outburst from V1309 Sco, and discuss to what extent post-common-envelope binaries and nebulae can provide information, e.g. from binary eccentricities, which is not currently being fully exploited.

The common envelope phase in the evolution of binary stars

The common envelope phase in the evolution of binary systems is examined. Three parameters are identified which characterize the efficiency of energy deposition, the importance of three-dimensional effects, and the efficiency of spin-up of the envelope. It is demonstrated that the efficiency of deposition of orbital energy into envelope ejection can be quite low. It is found that significant spin-up of the envelope can be expected to occur in relatively early stages, when the spiralling-in occurs inside evolved supergiant envelopes. In normal giants spin-up can occur only in the final stages of the spiralling-in process. The results of a simplified three-dimensional numerical calculation of the common envelope phase are presented, and the implications of the results for the formation of planetary nebulae with binary nuclei, double white dwarf systems, and FK Com stars are discussed. 52 references.

The Formation of Very Narrow Waist Bipolar Planetary Nebulae

We discuss the interaction of the slow wind blown by an asymptotic giant branch (AGB) star with a collimated fast wind (CFW) blown by its main-sequence or white dwarf companion, at orbital separations in the range of several AU a 200 AU. The CFW results from accretion of the AGB wind into an accretion disk around the companion. The fast wind is collimated by the accretion disk. We argue that such systems are the progenitors of bipolar planetary nebulae and bipolar symbiotic nebulae with a very narrow equatorial waist between the two polar lobes. The CFW wind will form two lobes along the symmetry axis and will further compress the slow wind near the equatorial plane, leading to the formation of a dense slowly expanding ring. Therefore, contrary to the common claim that a dense equatorial ring collimates the bipolar flow, we argue that in the progenitors of very narrow waist bipolar planetary nebulae, the CFW, through its interaction with the slow wind, forms the dense equatorial ring. Only later in the evolution, and after the CFW and slow wind cease, does the mass-losing star leave the AGB and blow a second, more spherical, fast wind. At this stage the flow structure becomes the one that is commonly assumed for bipolar planetary nebulae, i.e., collimation of the fast wind by the dense equatorial material. However, this results in the broadening of the waist in the equatorial plane and cannot by itself account for the presence of very narrow waists or jets. We conduct a population synthesis study of the formation of planetary nebulae in wide binary systems which quantitatively supports the proposed model. The population synthesis code follows the evolution of both stars and their arbitrarily eccentric orbit, including mass loss via stellar winds, for 5 ? 104 primordial binaries. We show the number of expected systems that blow a CFW is in accord with the number found from observations, to within the many uncertainties involved. Overall, we find that ~5% of all planetary nebulae are bipolars with very narrow waists. Our population synthesis not only supports the CFW model but more generally supports the binary model for the formation of bipolar planetary nebulae.

Properties that Cannot Be Explained by the Progenitors of Planetary Nebulae

I classify a large number of planetary nebulae (458) according to the process that caused their progenitors to blow axisymmetrical winds. The classification is based primarily on the morphologies of the different planetary nebulae, assuming that binary companions, stellar or substellar, are necessary in order to have axisymmetrical mass loss on the asymptotic giant branch. I propose four evolutionary classes, according to the binary-model hypothesis: (1) Progenitors of planetary nebula that did not interact with any companion. These amount to ~10% of all planetary nebulae. (2) Progenitors that interact with stellar companions that avoided a common envelope, 11+2−3% of all nebulae. (3) Progenitors that interact with stellar companions via a common envelope phase, 23+11−5% of all nebulae. (4) Progenitors that interact with substellar (i.e., planets and brown dwarfs) companions via a common envelope phase, 56+5−8% of all nebulae. In order to define and build the different classes, I start with clarifying some relevant terms and processes related to binary evolution. I then discuss kinematical and morphological properties of planetary nebulae that appear to require the interaction of the planetary nebula progenitors and/or their winds with companions, stellar or substellar.

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

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.

Main-Sequence Stellar Eruption Model for V838 Monocerotis

We propose that the energy source of the outburst of V838 Mon and similar objects is an accretion event, i.e., gravitational energy rather than thermonuclear runaway. We show that the merger of two main-sequence stars, of masses M1 1.5 M☉ and M2 0.1-0.5 M☉, can account for the luminosity, large radius, and low effective temperature of V838 Mon and similar objects. By varying the masses and types of the merging stars, and by considering slowly expanding, rather than hydrostatic, envelopes, this model can account for a large range in luminosities and radii of such outburst events.

Theory of local thermal instability in spherical systems

The gasdynamical properties of local thermal instability in optically thin astrophysical plasmas as it occurs in spherical accretion and winds is investigated. In a medium characterized by both thermal and hydrostatic equilibrium, if the cooling function is not an explicit function of position and does not display isentropic thermal instability, then isobaric thermal instability by the Field criterion is present if and only if convective instability is present by the Schwarzschild criterion. In this case, thermal overstability cannot occur. Convective instability by the Schwarzschild criterion will also occur in accretion flows locally dominated by external heating or in marginally unbound, radiatively cooling outflows. A very general Lagrangian equation for the development of nonradial thermal instability in flows with spherical symmetry is derived and is solved analytically in certain regimes. The results are applied to cluster X-ray cooling flows.

Binary Progenitor Models for Bipolar Planetary Nebulae

We propose an explanation for the positive correlation of bipolar planetary nebulae with massive progenitors in the paradigm of binary system progenitors. We list 10 critical observations, and argue that single-star models for the formation of bipolar planetary nebulae encounter difficulties complying with these observations. On the other hand, binary system progenitors can naturally explain these key observations, and in addition explain the rich varieties of structures possessed by bipolar planetary nebulae. Based on three of the critical observations, and on previous works by Corradi and Schwarz and by Morris, we postulate that the progenitors of bipolar planetary nebulae are binary stellar systems in which the secondary diverts a substantial fraction of the mass lost by the asymptotic giant branch (AGB) primary, but the systems avoid the common envelope phase for a large fraction of the interaction time. The positive correlation of bipolar planetary nebulae with massive progenitors, M 2 M☉, is attributed to the larger ratios of red giant branch (RGB) to AGB radii which low-mass stars attain, compared with massive stars. These larger radii on the RGB cause most stellar binary companions, which potentially could have formed bipolar planetary nebulae if the primary had been on the AGB, to interact with low-mass primaries already on the RGB. This scenario predicts that the central stars of most bipolar planetary nebulae are in binary systems having orbital periods in the range of a few days to few times 10 yr.

The Role of Magnetic Fields in Cluster Cooling Flows

We show that the magnetic field may become very important in the inner regions of cluster cooling flows. The magnetic pressure becomes comparable to the thermal pressure within a radius of typically 1 – 10 kpc. Within this region, reconnection of the magnetic field becomes efficient and magnetic energy is released at a rate of 1041 – 1043 erg s-1. Buoyancy is ineffective in transporting the magnetic field out of the cooling flow.

Explaining the Type Ia supernova PTF 11kx with a violent prompt merger scenario

We argue that the multiple shells of circumstellar material (CSM) and the supernovae (SN) ejecta interaction with the CSM starting 59 days after the explosion of the Type Ia SN (SN Ia) PTF 11kx, are best described by a violent prompt merger. In this prompt merger scenario the common envelope (CE) phase is terminated by a merger of a WD companion with the hot core of a massive asymptotic giant (AGB) star. In most cases the WD is disrupted and accreted onto the more massive core. However, in the rare cases where the merger takes place when the WD is denser than the core, the core will be disrupted and accreted onto the cooler WD. In such cases the explosion might occur with no appreciable delay, i.e., months to years after the termination of the CE phase. This, we propose, might be the evolutionary route that could lead to the explosion of PTF 11kx. This scenario can account for the very massive CSM within � 1000 AU of the exploding PTF 11kx star, for the presence of hydrogen, and for the presence of shells in the CSM.