Stars are made of hydrogen and “not-hydrogen”, or so a senior colleague once told me. Indeed, when their hydrogen fuel runs out, the stars become very large, much like said senior colleague. Single stars then eject their nuclear-processed material, either as planetary nebulae or in supernova explosions, which is diluted into the galactic medium to form new stars. Life is more interesting in binaries. When one star in a binary becomes large enough that it transfers mass to its companion, its evolutionary path completely changes. In binary red giants this is often through a common-envelope ejection. The star prematurely loses mass compared to its single-star equivalent, so there is less “not-hydrogen”. Asymptotic giant branch stars, for example, produce less carbon and barium in binaries because of stellar interaction. After mass transfer, the remaining -possibly compact- binary may then exhibit nucleosynthetic exotica such as thermonuclear novae, type-Ia supernovae and merging neutron stars. I will demonstrate how we are trying to quantitatively investigate nucleosynthesis in binary stars through stellar-population simulations, and outline some of the challenges we face in the future.

It is now clear, after many years of heated debate, that a significant fraction of planetary nebulae are the product of close-binary evolution. I will highlight the current state of our understanding, and the nascent relationships between binary and nebular parameters which have only begun to be unveiled in recent years. The importance of close-binary planetary nebulae for understanding the common envelope phase as well as post-common-envelope phenomena is discussed, in particular the implications of the apparently large fraction of double degenerate nuclei in understanding the origins of supernovae type Ia.

Most planetary nebulae (PNe) show beautiful, axisymmetric morphologies despite their progenitor stars being essentially spherical. Angular momentum provided by a close binary companion is widely invoked as the main agent that would help eject an axisymmetric nebula, after a brief common envelope (CE) stage. We examine the total mass (ionised+molecular) of a sample of post-CE planetary nebulae (PNe) and compare it to a larger sample of regular PNe. We find the median mass of post-CE PNe to be very similar to that of regular PNe. The results also show a bi-modal distribution, where PNe arising from double-degenerate central stars tend to be substantially more massive than their single-degenerate counterparts, constituting preliminary evidence of the higher efficiency of compact companions in common envelope ejection.

The abundances of heavy elements in ionised nebulae are subject to a systematic discrepancy in which recombination lines give higher values than collisionally excited lines. The discrepancy is typically a factor of 2-3 but in planetary nebulae (PNe), the distribution has a long tail that extends to more than two orders of magnitude. It has been established in recent years that the highest values of the abundance discrepancy occur in planetary nebulae with close binary central stars. I will discuss the nebular properties that set post-common-envelope PNe apart from the rest of the PN population, and outline the evidence for a nova-like ejection of hydrogen-deficient material associated with the common envelope phase as the cause of extreme abundance discrepancies.

The number of white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ~30 in 2003 to over 3000. However, in the majority of systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive A, F, G or K type companions have remained elusive due to the large difference in optical brightness between the two stars. In this talk I will present our work to uncover this hidden population of systems. I'll also present our follow up observations of these systems designed to identify the post common envelope systems and characterise the stars in order to determine their past and future evolution. Finally, I will discuss the potential of this sample for testing thermonuclear supernovae formation channels

Brown dwarfs with masses of less than 70 times that of Jupiter are the lowest mass objects known to survive common envelope evolution. There are currently 8 systems known where the brown dwarf has survived the death of its host star, and has been left in a close, white dwarf-brown dwarf binary system with a period of a few hours. There are a similar number of brown dwarfs also knowns in exist in cataclysmic variable systems, and systems with a hot subdwarf primary star. In this talk I will focus on the white dwarf-brown dwarf systems, discussing the mass range of the primary and secondary stars, the periods and the effect of the white dwarf on the brown dwarf atmosphere.

Unstable mass transfer from one star to another can lead to the formation of a shared gaseous shell where both stars orbit (the common envelope). The end of this phase is marked by the quick spiral-in of the secondary star towards its companion, leading to violent interactions between the components, when the whole, or part of the binary's common envelope gets ejected. This last phase has been serendipitously witnessed as astrophysical transients called luminous red novae (LRNe). In this talk, I will present the main observational properties for Galactic and extragalactic LRNe. Recent theoretical models have provided clues to interpret their progenitor populations, as well as the diverse observable characteristics for merger and common envelope ejection bursts. I will review some of these scenarios and introduce future challenges in this field to be addressed by large forthcoming transient surveys.

Galactic transients widely known as 'red novae' are thought to be stellar-merger eruptions. These systems go through the common envelope phase just before their outbursts and in one case, V1309 Sco, the common envelope evolution was observed photometrically in real time. I am going to present and discuss new interferometric observations of the Galactic red nova remnants which shed light on their physical properties before and after the merger. I am going to emphasize the efforts to understand the common-envelope physics from these observations.

Observational characteristics of luminous red novae can be explained within the context of binary mergers by a long-lasting mass loss from the second Lagrange point L2 and its subsequent interaction with a later, spherical dynamical ejection of material. The initial slow rise in luminosity is a consequence of internal shocks in the material escaping the L2 point and the collision between the fast spherical ejecta and material in the orbital plane produces two peaks in the light curve. This interaction also naturally leads to bipolar remnant morphology of common envelope events.

We study the kinematic properties of mass loss from the vicinity of the L2 point in ballistic approximation, investigating the possible final outcomes for particle trajectories as a function of a wider range of initial conditions. We find that L2 mass loss can lead to unbound outflow, formation of a decretion disk, collision with the binary surface or trajectory self-intersection near the binary implying a hydrodynamic shock. Additionally, we find that the distribution of final outcomes in the surroundings of the L2 point is non-trivial, namely that unbound outflows can be initiated near L2 even with velocities slower than corotation.

Stellar wind collisions are very energetic phenomena which leave observational signatures across the whole electromagnetic spectrum. In this study, we present a statistical study of the clump formation process as a result of unstable radiative wind collisions. Such clumps are though to be formed through hydrodynamic instabilities, mostly via the non-linear thin shell instability. To do so, we develop 3D high resolution hydrodynamical simulations of stellar wind collisions with the adaptive-mesh refinement grid-based code Ramses. We present a parameter study in order to characterise the physical properties of the clumps. Furthermore, we discuss applications of these computational tools and models in the Galactic Centre environment, as well as in future work on stellar mergers and classical novae.