This work aims to present our current best physical understanding of common-envelope evolution (CEE). We highlight areas of consensus and disagree- ment, 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.

We use three-dimensional hydrodynamical simulations to study the rapid infall phase of the common envelope interaction of a red giant branch star of mass equal to 0.88 M and a companion star of mass ranging from 0.9 down to 0.1 M . We first compare the results obtained using two different numerical techniques with different resolutions, and find overall very good agreement. We then compare the outcomes of those simulations with observed systems thought to have gone through a common envelope. The simulations fail to reproduce those systems in the sense that most of the envelope of the donor remains bound at the end of the simulations and the final orbital separations between the donor's remnant and the companion, ranging from 26.8 down to 5.9 R , are larger than the ones observed. We suggest that this discrepancy vouches for recombination playing an essential role in the ejection of the envelope and/or significant shrinkage of the orbit happening in the subsequent phase.

We have undertaken quantitative analysis of four LMC and SMC O4-9.7 extreme supergiants using far-ultraviolet FUSE , ultraviolet IUE /HST and optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing (Hillier & Miller 1998) are used to analyse wind and photospheric spectral features simultaneously. Using Hα to constrain Ṁ , He i-ii photospheric lines reveal stellar temperatures which are systematically (5-7.5kK) and substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions, since derived temperatures generally yield consistent fits across the entire λλ912-7000 Å observed spectral range. In particular, we are able to resolve the UV-optical temperature discrepancy identified for AzV 232 (O7 Iaf + ) in the SMC by Fullerton et al. (2000).The temperature and abundance sensitivity of far-UV, UV and optical lines is discussed. 'Of' classification criteria are directly linked to (strong) nitrogen enrichment (via N iii λ4097) and (weak) carbon depletion (via C iii λλ4647-51), providing evidence for mixing of unprocessed and CNO processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via O ii lines in the O9.7 supergiant for which it is also found to be somewhat depleted. Unfortunately, He/H is very difficult to determine in individual O supergiants, due to uncertainties in microturbulence and the atmospheric scale height. The effect of wind clumping is also investigated, for which P v λλ1118-28 potentially provides a useful diagnostic in O-star winds, unless phosphorus can be independently demonstrated to be underabundant relative to other heavy elements. Revised stellar properties affect existing calibrations of (i) Lyman continuum

We present a quantitative classification scheme for carbon and oxygen sequence Wolf-Rayet stars. Our scheme uses new high-quality optical AAT and INT observations of 20 stars for which we provide narrow-band photometry and estimates of interstellar reddenings. In increasing order of excitation, our spectral classes range from WC11 to WC4 for WolfRayet stars with a dominant carbon line visual spectrum, and subsequently from WO4 to WO1 for those with predominantly oxygen lines. We refine existing WC and WO schemes to incorporate stars with higher and lower excitation spectral features. Both massive stars and central stars of planetary nebulae (CSPNe) can be classified with the unified system. We have found no criterion that cleanly separates spectra of the two types of star, including elemental abundances (C/O or C/He). However, CSPNe show a wider range of line strength and width than massive stars in the same ionization subclass. Systematically lower FWHM(C iv X5808) values are observed from WO-type CSPNe than from massive WO stars.For WC4-11 stars, our primary diagnostic is the equivalent width or line flux ratio C Iv XX5801-12/C iii X5696. We extend the use of this as the principal criterion throughout the WC sequence, with few reclassifications necessary relative to Smith, Shara & Moffat. For WO stars, C iii is absent and our new criteria, using primarily oxygen lines, take over smoothly. We define subclasses W04-1, using 0 vi XX3811-34/O v X5590 as our primary diagnostic. The continuation in spectral sequence from WC to WO is used to indicate that the sequence is a result primarily of excitation effects, rather than significant abundance differences.Our scheme allows us to confirm that massive stars and CSPNe are differently distributed over the subclasses. Around 3/5 of massive WC stars lie within the range WC5-8, while -1/5 of CSPNe are found within these spectral types. Stars within both the highest (WO 1) and lowest (WC10-11) excitation spectral classes are unique to CSPNe. A WC classification for the hot R CrB star V348 Sgr is excluded (previously [WC12]) since both C iii X5696 and C iv X5808 are absent in its optical spectrum. Additional criteria allow us to distinguish between WC-type, 'weak emission line' CSPNe, and 0 stars, allowing us to reclassify the central star of IRAS 21282+5050 (previously [WC11]) as an 0 star.

The α formalism is a common way to parametrize the common envelope interaction between a giant star and a more compact companion. The α parameter describes the fraction of orbital energy released by the companion that is available to eject the giant star's envelope. By using new, detailed stellar evolutionary calculations, we derive a user-friendly prescription for the λ parameter and an improved approximation for the envelope binding energy, thus revising the α equation. We then determine α both from simulations and from observations in a self-consistent manner. By using our own stellar structure models as well as population considerations to reconstruct the primary's parameters at the time of the common envelope interaction, we gain a deeper understanding of the uncertainties. We find that systems with very low values of q (the ratio of the companion's mass to the mass of the primary at the time of the common envelope interaction) have higher values of α. A fit to the data suggests that lower-mass companions are left at comparable or larger orbital separations to more massive companions. We conjecture that lower-mass companions take longer than a stellar dynamical time to spiral into the giant's core, and that this is key to allowing the giant to use its own thermal energy to help unbind its envelope. As a result, although systems with light companions might not have enough orbital energy to unbind the common envelope, they might stimulate a stellar reaction that results in the common envelope ejection.

Planetary nebulae (PNe) are circumstellar gas ejected during an intense mass-losing phase in the the lives of asymptotic giant branch stars. PNe have a stunning variety of shapes, most of which are not spherically symmetric. The debate over what makes and shapes the circumstellar gas of these evolved, intermediate mass stars has raged for two decades. Today the community is reaching a consensus that single stars cannot trivially manufacture PNe and impart to them non spherical shapes and that a binary companion, possibly even a sub-stellar one, might be needed in a majority of cases. This theoretical conjecture has however not been tested observationally. In this review we discuss the problem both from the theoretical and observational standpoints, explaining the obstacles that stand in the way of a clean observational test and ways to ameliorate the situation. We also discuss indirect tests of this hypothesis and its implications for stellar and galactic astrophysics.

We present hydrodynamic simulations of the common envelope binary interaction between a giant star and a compact companion carried out with the adaptive mesh refinement code enzo and the smooth particle hydrodynamics code phantom. These simulations mimic the parameters of one of the simulations by Passy et al., but assess the impact of a larger, more realistic initial orbital separation on the simulation outcome. We conclude that for both codes the post-common envelope separation is somewhat larger and the amount of unbound mass slightly greater when the initial separation is wide enough that the giant does not yet overflow or just overflows its Roche lobe. phantom has been adapted to the common envelope problem here for the first time and a full comparison with enzo is presented, including an investigation of convergence as well as energy and angular momentum conservation. We also set our simulations in the context of past simulations. This comparison reveals that it is the expansion of the giant before rapid in-spiral and not spinning up of the star that causes a larger final separation. We also suggest that the large range in unbound mass for different simulations is difficult to explain and may have something to do with simulations that are not fully converged.

We report the serendipitous detection of the planetary nebula NGC 5315 by the Chandra X-ray Observatory. The Chandra imaging spectroscopy results indicate that the X-rays from this PN, which harbors a Wolf-Rayet (WR) central star, emanate from a T X ∼ 2.5 × 10 6 K plasma generated via the same windwind collisions that have cleared a compact (∼ 8000 AU radius) central cavity within the nebula. The inferred X-ray luminosity of NGC 5315 is ∼ 2.5 × 10 32 erg s −1 (0.3-2.0 keV), placing this object among the most luminous such "hot bubble" X-ray sources yet detected within PNe. With the X-ray detection of NGC 5315, objects with WR-type central stars now constitute a clear majority -2of known examples of diffuse X-ray sources among PNe; all such "hot bubble" PN X-ray sources display well-defined, quasi-continuous optical rims. We therefore assert that X-ray-luminous hot bubbles are characteristic of young PNe with large central star wind kinetic energies and closed bubble morphologies. However, the evidence at hand also suggests that processes such as wind and bubble temporal evolution, as well as heat conduction and/or mixing of hot bubble and nebular gas, ultimately govern the luminosity and temperature of superheated plasma within PNe.