Common-envelope events (CEEs), during which two stars temporarily orbit within a shared envelope, are believed to be vital for the formation of a wide range of close binaries. For decades, the only evidence that CEEs actually occur has been indirect, based on the existence of systems that could not be otherwise explained. Here we propose a direct observational signature of CEE arising from a physical model where emission from matter ejected in a CEE is controlled by a recombination front as the matter cools. The natural range of timescales and energies from this model, as well the expected colors, lightcurve shapes, ejection velocities and event rate, match those of a recentlyrecognized class of red transient outbursts.Many binary star systems, including X-ray binaries, cataclysmic variables, close doubleneutron stars, and the potential progenitors of Type Ia supernovae and short-duration γ-ray bursts, are thought to be formed by CEEs. Because most stellar-mass binary merger sources for gravitational waves have experienced a CEE in their past, improved knowledge of CEEs should decrease the large uncertainty in theoretically-predicted merger rates. However, the short timescale expected for CEEs suggested that we would never directly observe them, allowing us only to draw inferences from the systems produced.A CEE begins when a binary orbit becomes unstable and decays. This might, for example, be driven purely by tidal forces (i.e. the Darwin instability), although CEEs are more commonly imagined as following a period of rapid mass transfer from one star to the other (1). In some cases the rate of transfer is so high that the receiving star is unable to accrete all the matter without forming a shared common envelope (CE) around the binary. This CE causes drag on one or both stars and hence orbital decay, with orbital energy and angular momentum 1

One of the two outcomes of a common envelope event is a merger of the two stars. To date, the best known case of a binary merger is the V1309 Sco outburst, where the orbital period was known and observed to decay up to the outburst. Using the hydrodynamical code StarSmasher, we study in detail which characteristics of the progenitor binary affect the outburst and produce the best match with observations. We have developed a set of tools in order to quantify any common envelope event. We consider binaries consisting of a 1.52M giant and a 0.16M companion with P orb ∼ 1.4 days, varying the nature of the companion and its synchronization. We show that all considered progenitor binaries evolve towards the merger primarily due to the Darwin instability. The merger is accompanied by mass ejection that proceeds in several separate mass outbursts and takes away a few percent of the donor mass. This very small mass, nonetheless, is important as it is not only sufficient to explain the observed light-curve, but it also carries away up to 1/3 of the both initial total angular momentum and initial orbital energy. We find that all synchronized systems experience L 2 mass loss that operates during just a few days prior to merger and produces ring-shaped ejecta. The formed star is always a strongly heated radiative star that differentially rotates. We conclude that the case of a synchronized binary with a main-sequence companion gives the best match with observations of V1309 Sco.

Collisions of main-sequence stars occur frequently in dense star clusters. In open and globular clusters, these collisions produce merger remnants that may be observed as blue stragglers. Detailed theoretical models of this process require lengthy hydrodynamic computations in three dimensions. However, a less computationally expensive approach, which we present here, is to approximate the merger process (including shock heating, hydrodynamic mixing, mass ejection, and angular momentum transfer) with simple algorithms based on conservation laws and a basic qualitative understanding of the hydrodynamics. These algorithms have been fine-tuned through comparisons with the results of our previous hydrodynamic simulations. We find that the thermodynamic and chemical composition profiles of our simple models agree very well with those from recent SPH (smoothed particle hydrodynamics) calculations of stellar collisions, and the subsequent stellar evolution of our simple models also matches closely that of the more accurate hydrodynamic models. Our algorithms have been implemented in an easy-to-use software package, which we are making publicly available. 4 This software could be used in combination with realistic dynamical simulations of star clusters that must take into account stellar collisions.

We continue our exploration of collisionally merged stars in the blue straggler region of the colormagnitude diagram. We report the results of new smoothed particle hydrodynamics (SPH) calculations of parabolic collisions between two main-sequence stars, with the initial structure and composition proÐles of the parent stars having been determined from stellar evolution calculations. Parallelization of the SPH code has permitted much higher numerical resolution of the hydrodynamics. We also present evolutionary tracks for the resulting collision products, which emerge as rapidly rotating blue stragglers. The rotating collision products are brighter, bluer, and remain on the main sequence longer than their nonrotating counterparts. In addition, they retain their rapid rotation rates throughout their mainsequence lifetime. Rotationally induced mixing strongly a †ects the evolution of the collision products, although it is not sufficient to mix the entire star. We discuss the implications of these results for studies of blue straggler populations in clusters. This work shows that o †-axis collision products cannot become blue stragglers unless they lose a large fraction of their initial angular momentum. The mechanism for this loss is not apparent, although some possibilities are discussed.

In this Letter, we investigate the role of recombination energy during a common envelope event. We confirm that taking this energy into account helps to avoid the formation of the circumbinary envelope commonly found in previous studies. For the first time, we can model a complete common envelope event, with a clean compact double white dwarf binary system formed at the end. The resulting binary orbit is almost perfectly circular. In addition to considering recombination energy, we also show that between 1/4 and 1/2 of the released orbital energy is taken away by the ejected material. We apply this new method to the case of the double white dwarf system WD 1101+364, and we find that the progenitor system at the start of the common envelope event consisted of an ∼1.5 M⊙ red giant star in an ∼30 d orbit with a white dwarf companion.