Publications of the Astronomical Society of Australia (PASA) c Astronomical Society of Australia 2016; published by Cambridge University Press. doi: 10.1017/pas.2016.xxx. The impact of companions on stellar evolution Orsola De Marco1;2 & Robert G. Izzard3 1Department of Physics & Astronomy, Macquarie University, Sydney, NSW 2109, Australia 2Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia 3Institute of Astronomy, University of Cambridge, Cambridge, CB3 0HA, United Kingdom. Abstract Stellar astrophysicists are increasingly taking into account the effects of orbiting companions on stellar evolution. New discoveries, many thanks to systematic time-domain surveys, have underlined the role of binary star interactions in a range of astrophysical events, including some that were previously interpreted as due uniquely to single stellar evolution. Here, we review classical binary phenomena such as type Ia supernovae, and discuss new phenomena such as intermediate luminosity transients, gravitational wave- producing double black holes, or the interaction between stars and their planets. Finally, we examine the reassessment of well-known phenomena in light of interpretations that include both single and binary stars, for example supernovae of type Ib and Ic or luminous blue variables. At the same time we contextualise the new discoveries within the framework and nomenclature of the corpus of knowledge on binary stellar evolution. The last decade has heralded an era of revival in stellar astrophysics as the complexity of stellar observations is increasingly interpreted with an interplay of single and binary scenarios. The next decade, with the advent of massive projects such as the Large Synoptic Survey Telescope, the Square Kilometre Array, the James Webb Space Telescope and increasingly sophisticated computational methods, will see the birth of an expanded framework of stellar evolution that will have repercussions in many other areas of astrophysics such as galactic evolution and nucleosynthesis. Keywords: stars: evolution – stars: binaries: close – ISM: jets and outflows – methods: numerical – surveys 1 INTRODUCTION their companion(s) during their lifetime (e.g., Sana et al. 2012a, Kiminki & Kobulnicky 2012, Kobulnicky Many classes of stars are either known or presumed to et al. 2012, Kobulnicky et al. 2014). This discovery sug- be binaries and many astrophysical observations can be gests that many massive star phenomena are related to explained by interactions in binary or multiple star sys- the presence of a binary companion, for example, type tems. This said, until recently duplicity was not widely Ib and Ic supernovae (Podsiadlowski et al. 1992; Smith considered in stellar evolution, except to explain certain et al. 2011a) or phenomena such as luminous blue vari- types of phenomena. ables (Smith & Tombleson 2015). Additionally, the re- Today, the field of stellar astrophysics is fast evolv- cent detection of gravitational waves has confirmed the ing. Primarily, time-domain surveys have revealed a existence of binary black holes (Abbott et al. 2016d), plethora of astrophysical events many of which can which are a likely end-product of the most massive bi- be reasonably ascribed to binary interactions. These nary evolution. surveys reveal the complexity of binary interactions arXiv:1611.03542v1 [astro-ph.SR] 10 Nov 2016 Finally, we now know that planets exist frequently and also provide sufficient number of high quality ob- around main-sequence stars. Planets have also been dis- servations to sample such diversity. In this way, new covered orbiting evolved stars, close enough that an in- events can be classified and some can be related to well teraction must have taken place. These discoveries open known phenomena for which a satisfactory explanation the possibility that interactions between stars and plan- had never been found. Binary-star phenomena are thus ets may change not only the evolution of the planet or linked to outstanding questions in stellar evolution, such planetary system, but the evolution of the mother star. as the nature of type Ia, Ib and Ic supernovae, and hence The fast-growing corpus of binary interaction obser- are of great importance in several areas astrophysics. vations also allows us to conduct experiments with stel- Another recent and important discovery is that most lar structure, because companions perturb the star. Ex- main-sequence, massive stars are in multiple systems, amples are the "heartbeat stars", which are eccentric with ∼70 per cent of them predicted to interact with 1 2.1 The multiplicity fraction of main sequence stars binaries with intermediate orbital separation. At pe- riastron the star is "plucked" like a guitar string and the resulting oscillation spectrum, today studied thanks to high precision observations such as those of Kepler Space Telescope, can be used to study the stellar layers below the photosphere (Welsh et al. 2011). Alongside new observations, creative new methods exist to model binaries. One-dimensional stellar struc- ture and evolution codes such as the new Modules for Experiments in Stellar Astrophysics (mesa) are used to model not just stellar evolution, but binary processes such as accretion. Multi-dimensional simulations able to model accurately both the stars and the interaction are currently impossible because of the challenge in mod- elling simultaneously a large range of space and time Figure 1. Citations to the seminal paper on the common enve- scales. However, great progress over recent years has lope binary interactions of Webbink (1984). The relative increase starting in approximately 2006 cannot be explained by the over- been made in 3D hydrodynamics with 3D models of in- all increase in the number of astrophysics papers over the same dividual stars (e.g., Chiavassa et al. 2011, where impor- period, demonstrating an increase in interest in this interaction tant relevant physics is captured) and numerical tech- over the last 10 years. Figure sourced from the Astrophysics Data niques being refined and developed to be adopted for Service. binary modelling (e.g., Ohlmann et al. 2016a; Quataert et al. 2016). These studies are part of a revival in the field of stellar astrophysics, the start of which may be and lengthen the orbital separation, bringing two stars observed in the increase by over a factor of 10 in cita- within each other’s influence or allowing them to avoid tions to seminal binary interaction papers such as that an interaction altogether. Here, we define the binary of Webbink (1984, Fig. 1), well above the factor of 2.3 fraction to be the fraction of systems that are multi- increase in the overall volume of astrophysics papers ple rather than the companion frequency, which can be that has been witnessed between 1985 and 2015. larger than unity when there is more than one compan- This review is arranged as follows. We start with a ion per primary on average. summary of the properties of main sequence binary pop- We use the same naming conventions as Duchêne ulations (Section 2), including stars with planetary sys- & Kraus (2013). Massive stars are more massive than tems (Section 2.5). In Section 3 we briefly discuss bi- about 8 M , intermediate-mass stars have masses nary pathways and list binary classes. In Section 4 we from about 1.5 to 5 M (spectral types B5 to F2), discuss current and future observational platforms par- Solar-type stars have masses in the approximate range ticularly suited to the study of binary phenomena as 0.7 to 1.3 M (spectral types F through mid-K), low- well as modelling tools used to interpret observations mass stars between 0.1 and 0.5 M (spectral types and make predictions. We then, in Section 5, emphasise M0 to M6) and very-low-mass stars and brown dwarfs how certain classes of binaries allow us to carry out stel- are less massive than about 0.1 M (spectral types lar experiments, while in Section 6 we report a range of M8 and later). interesting phenomena, which have been explained by including the effects of interactions between stars or be- tween stars and planets. In Section ?? we discuss the 2.1 The multiplicity fraction of main exciting and expanding field of stellar transients, includ- sequence stars ing the newly detected gravitational waves. We conclude in Section 8. Our knowledge of the fraction of massive stars that have companions was considerably revised by the Galactic O- star survey of Sana et al. (2012a) and Kiminki & Kobul- 2 MAIN-SEQUENCE BINARY STARS nicky (2012). Sana et al. (2012a) show that the fraction In this section we summarise the frequencies of main of O stars that have companions that will interact dur- sequence binaries as well as their period and mass ratio ing the lifetime of the O star, i.e. with periods shorter distributions. For a recent review of stellar multiplic- than about 1500 days, is 71 per cent. Of these, one third ity in pre-main sequence and main sequence stars see merge. The overall binary fraction is established to be Duchêne & Kraus (2013). more than 60 per cent in early B stars and in excess of The fraction of stars that interact with their com- 80 per cent in O stars. There is evidence that the binary panions depends on these frequencies and on the ac- fraction in clusters and in the field are similar (Duchêne tion of tides and mass loss, which can both shorten & Kraus 2013). 2 2.3 The mass ratio distribution of main sequence binaries The fraction of intermediate-mass stars in binary with their companions compared to more massive stars systems is substantially lower than for the most mas- that are not only in binaries more often, but that have sive stars. Amongst the entire group of F2 to B5 type systematically closer companions. stars, the binary frequency is greater than about 50 per Additional and updated information on the period cent, as determined from the Sco-Cen OB association distribution can be found in Moe & Di Stefano (2016) (Kouwenhoven et al.