Publications of the Astronomical Society of Australia (PASA) © 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 importance of binary star interactions to 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 in- teraction between stars and their planets. Finally, we examine the reassessment of well-known phenomena in light of interpretations that include both single stars and binary interactions, 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 nomen- clature 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 bi- nary 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 nicky et al. 2014). This discovery suggests that many mas- sive star phenomena are related to the presence of a bi- Many classes of stars are either known or presumed to be nary companion, for example, type Ib and Ic supernovae binaries and many astrophysical observations can be ex- (Podsiadlowski et al. 1992; Smith et al. 2011a) or phenom- plained by interactions in binary or multiple star systems. ena such as luminous blue variables (Smith & Tombleson This said, until recently duplicity was not widely consid- 2015). Additionally, the recent detection of gravitational ered in stellar evolution, except to explain certain types of waves (Abbott et al. 2016d) has confirmed the existence of phenomena. binary black holes, which are a likely end-product of the Today, the field of stellar astrophysics is fast evolving. most massive binary evolution. Primarily, time-domain surveys have revealed a plethora Finally, we now know that planets exist frequently of astrophysical events many of which can be reasonably around main-sequence stars. Planets have also been dis- ascribed to binary interactions. These surveys reveal the covered orbiting evolved stars, close enough that an in- complexity of binary interactions and also provide suffi- teraction must have taken place. These discoveries open cient number of high quality observations. In this way, new the possibility that interactions between stars and planets events can be classified and some can be related to well may change not only the evolution of the planet or plane- known phenomena for which a satisfactory explanation tary system, but the evolution of the mother star. had never been found. Binary-star phenomena are thus The fast-growing corpus of binary interaction observa- linked to outstanding questions in stellar evolution, such tions also allows us to conduct experiments with stellar as the nature of type Ia, Ib and Ic supernovae, and hence structure, because companions perturb the star. Example are of great importance in several areas astrophysics. are the “heartbeat stars", which are eccentric binaries with Another recent and important discovery is that most intermediate orbital separation. At periastron the star is main-sequence, massive stars are in multiple systems, “plucked" like a guitar string and the resulting oscillation with 70 per cent of them predicted to interact with their » spectrum, today studied thanks to high precision observa- companion(s) during their lifetime (e.g., Sana et al. 2012a, tions such as those of Kepler Space Telescope, can be used Kiminki & Kobulnicky 2012, Kobulnicky et al. 2012, Kobul- 1 2.1 The multiplicity fraction of main sequence stars to study the stellar layers below the photosphere (Welsh et al. 2011). Alongside new observations, creative new methods ex- ist to model binaries. One-dimensional stellar structure and evolution codes such as the new Modules for Experi- ments in stellar Astrophysics (MESA) are used to model not just stellar evolution, but binary processes such as accre- tion. Multi-dimensional simulations able to model accu- rately both the stars and the interaction are currently im- possible because of the challenge in modelling simultane- ously a large range of space and time scales. However, great progress over recent years has been made in 3D hydrody- namics with 3D models of individual stars (e.g., Chiavassa et al. (2011), where important relevant physics is captured) and numerical techniques being refined and developed Figure 1. Citations to the seminal paper on the common envelope binary to be adopted for binary modelling (e.g., Ohlmann et al. interactions of Webbink (1984). The relative increase starting in approxi- mately 2006 cannot be explained by the overall increase in the number of 2016a; Quataert et al. 2016). These studies promise a re- astrophysics papers over the same period, demonstrating an increase in vival, in the field of stellar astrophysics, the start of which interest in this interaction over the last 10 years. Figure sourced from the may be observed in the increase by over a factor of 10 in Astrophysics Data Service. citations to seminal binary interaction papers such as that of Webbink (1984, Figure 1), well above the factor of 2.3 increase in the overall volume of astrophysics papers that We use the same naming conventions as Duchêne & has been witnessed between 1985 and 2015. Kraus (2013). Massive stars are more massive than about This review is arranged as follows. We start with a sum- 8 M , intermediate-mass stars have masses from about ¯ mary of the properties of main sequence binary popula- 1.5 to 5 M (spectral types B5 to F2), Solar-type stars have ¯ tions (Section 2), including stars with planetary systems masses in the approximate range 0.7 to 1.3 M (spectral ¯ (Section 2.5). In Section 3 we briefly discuss binary path- types F through mid-K), low-mass stars between 0.1 and ways and list binary classes. In Section 4 we discuss cur- 0.5 M (spectral types M0 to M6) and very-low-mass stars ¯ rent and future observational platforms particularly suited and brown dwarfs are less massive than about 0.1 M ¯ to the study of binary phenomena as well as modelling (spectral types M8 and later). tools used to interpret observations and make predictions. We then, in Section 5, emphasise how certain classes of 2.1 The multiplicity fraction of main sequence stars binaries allow us to carry out stellar experiments, while in Section 6 we report a range of interesting phenomena, Our knowledge of the fraction of massive stars that have which have been explained by including the effects of in- companions was considerably revised by the Galactic O- teractions between stars or between stars and planets. In star survey of Sana et al. (2012a). They show that the frac- Section 7 we discuss the exciting and expanding field of tion of O stars that have companions that will interact dur- stellar transients, including the newly detected gravita- ing the lifetime of the O star, i.e. with periods shorter than tional waves. We conclude in Section 8. about 1500 days, is 71%. Of these, one third merge. The overall binary fraction is established to be more than 60% in early B stars and in excess of 80% in O stars. There is ev- 2 MAIN-SEQUENCE BINARY STARS idence that the binary fraction in clusters and in the field In this section we summarise the frequencies of main se- are similar (Duchêne & Kraus 2013). quence binaries as well as their period and mass ratio dis- The fraction of intermediate-mass stars in binary sys- tributions. For a recent review of stellar multiplicity in pre- tems is substantially lower than for the most massive stars. main sequence and main sequence stars see Duchêne & Amongst the entire group of F2 to B5 type stars, the bi- Kraus (2013). nary frequency is greater than about 50%, as determined The fraction of stars that interact with their companions from the Sco-Cen OB association (Kouwenhoven et al. depends on these frequencies and on the action of tides 2005; Fuhrmann & Chini 2012, 2015). and mass loss, which can both shorten and lengthen the As shown by Duquennoy & Mayor (1991) and, later, by orbital separation, bringing two stars within each other’s Raghavan et al. (2010), about 44 2% of G- and K-type § influence or allowing them to avoid an interaction alto- main sequence stars in the Solar neighbourhood have gether. Here, we define the binary fraction to be the frac- companions. They point out a low-significance difference tion of systems that are multiple rather than the compan- of 9 percent points between the immediately sub-Solar ion frequency, which can be larger than unity when there 41 3% (G2-K3) and immediately super-Solar, 50 4% (F6- § § is more than one companion per primary on average.