Institute of Astronomy

Abstracts (Programme)

Updated 21 July 2016 1125hrs

Monday 25 July - Morning

Binary star formation:

Simon Goodwin, The Theory of Multiple Star Formation
Many (most?) stars seem to form in high order multiple systems. Simulations seem to show that in a rapid, turbulent model of star formation it is easy to form such systems. The problem is that such systems are messy and chaotic and similar initial conditions can produce very different final results. Then systems can evolve internally and externally changing quite significantly. Therefore we face a difficult observational, theoretical and statistical problem in working out if we basically understand how multiples form, or if we have got it very wrong.

Alejandro Vigna-Gomez, COMPAS Compact Object Mergers: Population Astrophysics and Statistics
Advanced LIGO detections have confirmed the existence of coalescing compact objects, ushering in a new era of gravitational-wave astronomy. One of the proposed origins of these detections is through isolated binary evolution, during which two massive stars interact through a wide variety of processes, such as stellar winds, tides, mass transfer and common envelope events.

We report on the development of a new rapid binary population synthesis and statistical analysis code COMPAS (Compact Object Mergers: Population Astrophysics and Statistics) at the University of Birmingham, with examples of compact-object binary formation.

Max Moe, Mind your Ps and Qs: the Interrelation between Binary Orbital Periods P and Mass Ratios Q
We have compiled 30+ separate surveys of binary stars, corrected for their respective selection effects, and measured their statistical parameters in a homogenous manner. The intrinsic distributions of primary mass M1, mass ratio q, orbital period P, eccentricity e, and multiplicity are all interrelated with each other. We model the joint probability distribution function f(M1,q,P,e), which is not equal to f(M1)f(q)f(P)f(e). Separately adjusting the individual distribution functions to the extremes will still not encompass the true nature of the binary population. I will discuss how the intrinsic interrelations of the binary star properties provide insight into their formation processes, and I will highlight the implications for binary evolution. In particular, the updated initial conditions have important consequences for the predicted rates and properties of Type Ia supernovae and low-mass X-ray binaries.

Erez Michaely, Efficient Formation of Exotic Binaries from Hierarchical Triples and Wide Binaries in the Field
Collisional dynamics in dense stellar cluster catalyse and enhance the formation of compact binaries and collisional products in these environments. Production of similar objects in the field is thought to explained by stellar evolution. Here we show that collisional dynamics in the field can play a major rule in producing exotic binaries when mediated by very wide binaries \left(>10^{3}au\right). In particular hierarchical triple and wide binaries in the field undergo multiple stellar encounters during the lifetime of the system. These encounters change the orbital elements of the wide (outer) binary. If the binary becomes sufficiently eccentric during the process its components strongly interact lead to tidal captures, collisions or mergers. In triple systems changes in the orbital properties of the outer binary may lead the system into a Kozai (secular) evolution. In this case the secular evolution can serve as an efficient mediator for the close collisional interaction in the inner binary. We find the production rate for X-ray binaries gravitational waves sources and merger products in the field can be dominated by these. 

Bo Reipurth, Properties of Young Binaries and Multiples
Observational evidence increasingly supports that most, if not all, stars are formed in small multiple systems, which through internal instabilities and external perturbations decay to produce the separation distribution function and multiplicity statistics of the field. I will summarize observations of newborn stars that document their enhanced multiplicity fraction and discuss them in terms of dynamical evolution during the earliest evolutionary stages.  Various poorly understood phenomena in early stellar evolution find an explanation as a result of the dynamical evolution of multiple systems.

Kyle Conroy, Stellar triples in PHOEBE
The number of known mutually-eclipsing stellar triple and multiple systems has increased greatly during the Kepler era. These systems provide significant opportunities to both determine fundamental stellar parameters of benchmark systems to unprecedented precision as well as to study the dynamical interaction and formation mechanisms of stellar and planetary systems. Modeling these systems to their full potential, however, has not been feasible until recently. Most existing available codes are restricted to the two-body binary case and those that do provide N-body support for more components make sacrifices in precision by assuming no stellar surface distortion. We have completely redesigned and rewritten the PHOEBE binary modeling code to incorporate support for triple and higher-order systems while also robustly modeling data with Kepler precision. Here we present our approach, demonstrate several test cases based on real data, and discuss the current status of PHOEBE's support for modeling these types of systems.

Eleftheria-Panagota Christopoulou, Investigating Multiplicity of Binary Stars and the Nature of Substellar Companions with Greek 2.3m Aristarchos Telescope
Eleftheria-Panagiota Christopoulou1 & Athanasios Papageorgiou2
I will discuss results of recent observational studies of interesting eclipsing binaries (EBs) with the 2.3 Aristarchos telescope at Helmos Observatory, Greece  as part of a program  that aims  to 1) photometric follow up observations of KIC objects from Kepler field with Eclipse Timing Variation Signals and 2)  observations of low mass eclipsing binaries in order to infer the existence of substellar companions or circumbinary planets. Here we will describe the photometric selection criteria and the modern analysis techniques (heuristic scanning with parameter perturbation, genetic algorithm PIKAIA) that enable to reveal, optimize and constrain the astrophysical parameters of selected EBs. Finally I will discuss prospects of spectroscopic observations  

Pablo Marchant, The Many Faces of Very Massive Binaries in Tight Orbits
Recently, the evolution of massive binaries in orbits tight enough to induce strong rotational mixing and chemichally homogeneous evolution in both components has been put forward as a promising source for the formation of merging double black holes.  However, if this channel of evolution ocurrs in nature, it does not only have implications for gravitational wave astronomy, but also for our understanding of the evolution of very massive stars, in particular regarding the ocurrence of pair-instability supernovae (with possible implications for galactic chemical evolution) and gamma-ray bursts. Several of these systems should also undergo phases of deep contact early on in their lifes, synchronyzing their evolutionary state and bringing their mass ratio to unity. In addition,  potential progenitors for ultra-luminous X-ray binaries can be formed in systems where only the more massive component evolves homogeneously. In this talk I will provide an overview on how these systems are expected to evolve into these various phases, together with the observational evidence that supports it.

Monday 25 July - Afternoon

Binary star populations in clusters:

Alison Sills, Binary Star Populations in Clusters
I will review our current understanding of binary stars in open and globular clusters in the Milky Way. I will discuss what we can learn about clusters from their binary populations, and what we can learn about binaries by virtue of their location in clusters. Cluster dynamics can modify binary properties, create stellar exotica, and in turn be driven by the presence and creation of binary stars. I will touch on binaries in the youngest clusters, and how the clustered environment may impact overall properties of binaries in the field.

Robert MathieuAlternative Pathways of Stellar Evolution in Open Clusters
Our understanding of the evolution of single stars is one of the great intellectual accomplishments of the past century. Even so, from their beginnings cluster color-magnitude diagrams have shown many stars that do not lie along the main sequences and giant branches. The most famous of these are the blue stragglers, but there are numerous others such as the sub-subgiants, the yellow giants and evolved blue straggler stars. In fact, as many as 25% of the evolved stars in older open clusters do not fall on single-star isochrones. Rather than being anomalies, these stars represent frequently traveled alternative pathways of stellar evolution.

The overarching goal of our research program is to understand these alternative stellar evolution stories in open clusters. These narratives will start from the large populations of hard binary stars that have been found in open clusters. Their accounts will include both evolutionary processes internal to these binaries and external stellar dynamical influences. I will present the observational landscape for describing these alternative stellar evolution paths, including new asteroseismic results from our K2 M67 Kepler data and recent modeling work.

Natalie Gosnell, COS Spectroscopy of White Dwarf Companions to Blue Stragglers
Complete membership studies of open stellar clusters reveal that 25% of the evolved stars follow alternative pathways in stellar evolution, meaning something in the history of these stars changed their composition or mass (or both). In order to draw a complete picture of stellar evolution we must include these canonically "strange" stars in our definition of standard stellar populations. The formation mechanism of blue straggler stars, traditionally defined to be brighter and bluer than the main sequence turnoff in a star cluster, has been an outstanding question for almost six decades. Recent Hubble Space Telescope (HST) far-ultraviolet (far-UV) observations directly reveal that the blue straggler stars in the old (7 Gyr) open cluster NGC 188 are predominantly formed through mass transfer. I will present HST far-UV COS spectroscopy of white dwarf companions to blue stragglers. These white dwarfs are the remnants of the mass transfer formation process. The effective temperatures and surface gravities of the white dwarfs delineate the timeline of blue straggler formation in this cluster. The existence of these binaries in a well-studied cluster environment provides an unprecedented opportunity to observationally constrain mass transfer models and inform our understanding of many other alternative pathway stellar products.

Sverre Aarseth, The Life of a Black-hole Binary
The question of how a black hole binary interacts with the members of a star cluster has been addressed by N-body simulations. We concentrate on studying a single stellar-mass primordial black hole binary in a system of 50,000 or 100,000 stars subject to synthetic stellar evolution treated by the Cambridge STARS code. As expected, such a binary shrinks by imparting kinetic energy to field stars by the sling-shot mechanism. Since only a modest shrinkage is possible in practice, the onset of any relativistic effects requires a rather high eccentricity. This type of evolution appears to be characteristic of all the present models, thereby achieving a predicted maximum eccentricity exceeding 0.999 with many large actual values triggering post-Newtonian treatment. These sporadic events are induced by the Kozai-Lidov mechanism which is activated when a long-lived hierarchy is formed with high inclination. Alternatively, strong interactions may lead to the ejection of the massive black hole binary by recoil, in some cases with sufficient eccentricity for GR coalescence on a galactic time-scale. An observational feature of the binary evolution is that high-velocity escapers (> 100 km/s) are produced.

Binary star populations in the field:

Selma de Mink, Binary Star Populations: on Double Black Holes, Core Collapse Supernovae and Ionizing Photons
These are exciting times for the field of binary star evolution. Over the next three years we will see constraints on the BH-BH demographics, by far the most intriguing outcome of massive binary evolution, the coalescence of two gravitational singularities. Further great opportunities to learn about the complex of binary physics will likely be provided by the large scale transient surveys and increasing number of well studied resolved and unresolved stellar populations. Learning from these new sources requires both detailed accurate simulations as well as rapid populations synthesis simulations. I will discuss some recent advances in this area.

Ylva Götberg, Naked He-burning Stars: the Physics of the Stripping Mechanism and their UV Properties
Y. Götberg, S.E. de Mink, J. Groh, E. Zapartas, M. Renzo
Naked helium burning stars, i.e., stars stripped from their hydrogen envelope by a companion, represent the first long-lived post-interaction phase that almost all binaries go through. This is the first step in most of the complex scenarios that eventually give rise to a variety of exotic phenomena, such as X-ray binaries, Ib/c supernovae and double compact objects as sources of gravitational waves. They have also been suggested to be important as sources of ionising photons, but tailored atmosphere models have not been available so far.

We conduct an systematic computational study of the nature of these stripped stars using MESA. With the non-LTE, radiative transfer code CMFGEN we create the first extensive grid of tailored atmosphere models. This allows us for the first time to make reliable estimates for the expected UV spectra which we compare with observed systems such as the quasi WR in HD45166. We further study the effects of metallicity, mass loss and initial binary parameters to understand the physics of the envelope stripping mechanism. 

We quantify the ionising flux and conclude that stripped stars are not only important sources of hydrogen ionising radiation, but potentially also important sources of helium ionising radiation. We discuss how these computations can guide future observing campaigns to systematically search for stripped stars. Our results will be made available for studies of the integrated spectra of stellar populations in the context of the Epoch of Reionization.

Nami Mowlavi, Studying Binary Populations with Eclipsing Binaries in the GAIA Era
Launched at the end of 2013, ESA’s Gaia mission is repeatedly observing more than one billion stars. Several millions of them are expected to be eclipsing binaries, for which light curves, red and blue spectro-photometry, astrometry, and, depending on their brightness, stellar parameters, and radial velocity curves will be provided. This unprecedented all-sky sample of eclipsing binary systems will provide a detailed census of stellar populations as a function of galactic latitude in the Milky Way, as well as in the Magellanic Clouds, and will offer a unique opportunity to study binary systems in different states of evolution.

Pilot studies using real Kepler data resampled according to Gaia scanning law predict a successful detection and determination of the ephemerides for about half of eclipsing systems. The procedure implemented in Gaia’s Data Processing and Analysis Consortium identifies binary systems, finds their orbital periods, and computes eclipse widths and depths, inter-eclipse duration, inter-eclipse variability properties based on their light curve morphology. Several classification schemes are investigated, either based directly on the eclipsing binary parameters, or specific algorithms such as t-SNE (t-distributed Stochastic Neighbor Embedding), DBSCAN (Density-Based Spatial Clustering of Applications with Noise) or FPCA (Functional Principal Component Analysis), paving the road for a statistical analysis of binary populations. A realistic Roche model will subsequently be used to fit all available data (light curves, radial velocity curves for some of the brighter objects, as well as any other Gaia-derived quantity that constrains the system), with the aim of obtaining as much information about the physical parameters of the binary as allowed by the data.

We illustrate the method on eclipsing binary data collected by OGLE, Hipparcos and Kepler surveys, and present expectations for Gaia.

Tuesday 26 July - Morning

Paula Jofre Pfeil, Spectral Analyses of Blue Stragglers in the Field
The low-density environment of the field makes field blue stragglers good candidates to study mass transfer in binary evolution. Finding blue stragglers in the field is however very challenging because of the large variety of ages, metallicities and distances of the field population. Although more distant, the thick disk and the halo are ideal places to hunt blue stragglers as no young stars are expected and therefore anything blue or massive is a good candidate. I will focus on the discussion of the recently discovered "young alpha-rich" stars in the thick disk and how spectral analyses of these stars give us evidence that they are not young, but evolved blue stragglers. 

Edita Stonkute, Binaries and Large Spectroscopic Surveys
Stellar multiplicity is a key parameter for many astrophysical questions. Several interesting astronomical phenomena, such as gravitational waves and gamma-ray bursts, arise from binary stars, and the knowledge of multiplicity could provide constraints on possible channels of star formation and evolution in the Galaxy. For ongoing and coming large spectroscopic surveys, such as RAVE, APOGEE, LAMOST, Gaia−ESO, GALAH and 4MOST, it is important to identify the binaries to clean the survey products from potentially faulty results. However, little is known about the binary frequency in Milky Way field stars, particularly outside the Solar neighbourhood. I will present our models of the effect of binaries on high-resolution spectroscopic surveys, in order to determine how many binaries will be observed, whether unresolved binaries will contaminate measurements of chemical abundances, and how we can use spectroscopic surveys to better constrain the population of binaries in the Galaxy. As an application we model binary stars that mimic APOGEE red giants in the Galactic disc.

Planets in binary systems:

Kaitlin Kratter, Theory of Planets in Binary Systems
In this talk, I will provide an overview of theoretical investigations of s-type and p-type binary planetary systems. I will discuss the interplay between star formation and planet formation, as well as the unique dynamical influence of binaries on planetary systems. I will focus on a handful of peculiar systems to illustrate the constraints that binaries can provide for planet formation models.

Armaury Triaud, Observations of Planets in Binary Systems
Searching for planets in binary star systems is one of the only means we have to test the robustness of planet formation models. It is therefore important to search for such planets, measure their physical and orbital parameters, and estimate their mass, period, eccentricity distributions.
I will review the methods that have identified the presence of planets located in binary star systems and outline a few new results and prospects.

Alex Mustill, Dynamical Effects of Wide Binaries on Kepler Planets
Circumprimary planetary systems can be dynamically affected by a wide binary companion, for example through the Kozai effect or other secular perturbations. These dynamical effects range from excitation of mutual inclinations in multi-planet systems to destabilisation and loss of planets. I discuss how these affect the observed multiplicities of close-in planets observed with Kepler, both directly (binaries’ effects on the Kepler planets themselves) and indirectly (binaries’ effects on undetected planets on orbits wider than the Kepler planets, which in turn can affect the observed inner systems). An N-body population synthesis shows that up to 25% of multiple Kepler systems may be destabilised by wide binaries to the point of losing a planet, although in stable systems, mutual inclinations are only weakly excited, causing only a small effect on observed multiplicities.

Rebecca Martin, Planet Formation in Binary Systems
About half of observed exoplanets are estimated to be in binary systems. Thus, understanding planet formation and evolution in binaries is essential for explaining observed mexoplanet properties.   I will show how planet formation in binaries may be significantly different to planet formation around a single star because of the additional torque from the binary companion. Observations of protoplanetary discs show that they can be highly misaligned to the binary orbital plane.   A highly misaligned disc undergoes global Kozai-Lidov oscillations where the disc exchanges eccentricity and inclination. In a disc that is self-gravitating these oscillations may lead to planet formation through fragmentation. Planet-disc interactions in a mildly inclined disc around one component of a binary can lead to the formation of highly eccentric and highly inclined planets.

Nikolaos Georgakarakos, Analytic Orbit Propagation for Transiting Circumbinary Planets
We present an analytical framework that fully describes the motion of coplanar systems consisting of a stellar binary and a planet orbiting both stars on orbital as well as secular timescales. Perturbations of the Runge-Lenz vector are used to derive the short period evolution of the system, while octupole secular theory is applied to describe its long term behaviour. A post Newtonian correction for the stellar orbit is also included.

Our analytical estimates are tested against results from numerical integrations of the full equations of motion. The analytical model is then applied to investigate the dynamical history of some of the circumbinary planetary systems discovered by NASA’s Kepler spacecraft.

Tuesday 26 July - Afternoon

Common envelope evolution:

Natalia Ivanova, Theory of Common Envelope Evolution
Common-envelope events capture the imagination and are visually impressive, energetically noteworthy, and dramatically fate-defining episodes in the lives of close binary systems. During a common envelope event, two stars temporarily orbit within a shared envelope, and the episode ends with an exciting outburst, leaving behind either a significantly shrunk binary, or a single merged star. These episodes are believed to be vital for the formation of a wide range of extremely important astrophysical objects, including X-ray binaries, cataclysmic variables, close double-neutron stars, and the potential progenitors of Type Ia supernovae and gamma-ray bursts. While the problem is almost 40 years old, its theoretical foundation started to progress from the first simplified consideration only very recently, with the advances in our understanding of stellar structure, improvements of the numerical techniques for hydrodynamical simulations as well as recent observations of the Transient Universe. In this talk I will review the basics of the common envelope physics - the stages and the processes that are involved, and the recent progress that was made by the inclusion of recombination physics for understanding of the ejection during the plunge-in and during the slow spiral-in.

Sebastian Ohlmann, Hydrodynamics of the Common Envelope Phase in High Resolution
The common envelope (CE) phase is important for the evolution of close binary systems with compact stars.  Here, I present high-resolution hydrodynamics simulations of the CE phase of a two solar mass red giant interacting with different companions. The simulations employ the moving-mesh code AREPO that uses an unstructured, adaptive grid that moves with the fluid.  I will show that shear flows in the envelope lead to large-scale instabilities indicating the onset of convection. A resolution study shows in combination with an analysis of the angular momentum and energy transport that high resolutions are necessary for converged results.  Moreover, I will present results from the first magnetohydrodynamic simulations of the CE phase. These show that a dynamical amplification of magnetic fields is possible, saturating at fields of tens of kilogauss in the envelope. The dynamical impact of the magnetic fields on the evolution, however, is small on the simulated timescales.

Jan Staff, 3D AMR Hydrodynamic Simulations of Common Envelope Interactions between Red Giants and Low-mass Companions
We present results of 3 dimensional adaptive mesh refined grid-based hydrodynamic simulations of an RGB star in a common envelope with a low-mass main sequence companion or a planet. All low mass white dwarfs and short period white dwarf binaries must have undergone at least one common envelope interaction phase. White dwarf binaries may even lead to type Ia supernovae.  Many low mass white dwarfs and white dwarf binary systems are observed, and in order to understand how these systems formed it is therefore necessary to understand the common envelope interaction. Despite this, we find, as in many other simulations of this kind, that not much mass is being unbound in the process, and unless  additional physics is relevant for the interaction, the interaction will therefore lead to a merger. We calculate the drag force acting on the  companion as it spirals in through the envelope, and compare this force with analytic expressions for the force, and find reasonable agreements.

Matthew Clayton, Pulsation-driven Ejections in Common-envelope Objects
Common-envelope (CE) evolution remains one of the most important and least understood phases in the lives of binary star systems. Despite being a vital formation mechanism for many of the late-stage binary systems that are of greatest interest to astronomers, from double black hole and neutron star binaries to the progenitors of GRBs and SNe Ia, we still lack a theory of common envelopes which is able to predict the outcomes of this phase – whether the envelope is ejected, and the final separation of the binary if it is – with any degree of precision. A large part of the reason for this is that the difficulties associated with numerically modelling this phenomenon are formidable; the wide range of timescales that are important in the different phases of the process prevents any single 3-d or 1-d code from capturing its full dynamics. Although there has been some success in modelling fast ejections of common envelopes using 3-d hydrodynamics codes, simulations of the slow, self-regulating spiral-in phase which are restricted to 1-d have struggled to reveal the mechanisms underlying the delayed dynamical ejection of the envelope at this later stage.

I will report on hydrodynamical simulations of giant stars undergoing synthetic CE events in the self-regulated spiral-in phase, performed in 1-d using the stellar evolution code MESA, and building on the hydrostatic simulations performed by Ivanova, Justham and Podsiadlowski (2015). These simulations allow us to study the response of a giant envelope to the injection of heat expected during a CE event in different heating regimes.

In particular, I will report on the appearance of large amplitude Mira-like pulsations emerging as the giant envelope becomes increasingly dynamically unstable. In some cases these pulsations become supersonic, and the resulting shocks can lead to mass shells being dynamically ejected from the surface of the giant. These ejections may repeat on century timescales and constitute a rate of mass-loss as high as 0.001 solar masses per year. This mechanism is a promising candidate for the delayed dynamical ejection of common envelopes during the self-regulating spiral-in phase.

Orsola de Marco, Common Envelope Interactions from Observation to Theory and Back
What are the observations that can provide useable constraints for common envelope (CE) models? There are several ways to use post-CE binaries to impose some constraints on the physics of the interaction, but even more telling could be observations of the outflows caused by CE interactions. Among these, particularly useful are observations of classes known to be reasonably homogeneous, such as post-CE planetary nebulae, i.e., PN with a post-CE binary at their core, as well as of novae, for which the emergence of an outflow can be monitored and measured in real time. Both can provide energetics of the interaction from mass and energy measurements of the outflows. I will review these as well as other observations such as white dwarf binaries and other post-CE, post-giant, intermediate mass binaries to investigate their usability as model (particularly hydrodynamics model) constraints. I will also integrate these observations in the landscape of simulations, highlighting those simulation aspects that remain the most elusive.

Noam Soker, The role of jets in the CE and the GEE
I will present the view that jets launched by the more compact star during the common envelope evolution (CEE) supply the extra energy that is required to unbind the envelope. I will then present the first 3D simulations of the grazing envelope evolution (GEE). Jets launched from a secondary main sequence star manage to eject the envelope without any extra energy source. Such an event can be observed as intermediate luminosity optical transient (ILOT). I call for a paradigm shift in our approach to the CEE, where in the new paradigm jets play important role in ejecting the envelope and shaping the outflow. 

Devika Kamath, The Newly Discovered Dusty Post-RGB Binaries and their Implications on Stellar Evolution
Dusty Post-RGB stars are a newly discovered population of low-luminosity, low-metallicity, likely binaries in the Magellanic Clouds. They are likely to have evolved off the RGB via binary interaction. To place these objects in an evolutionary context and establish an evolutionary connection between RGB binaries (such as the Sequence E variables) and our new sample of objects we compare the theoretically predicted birthrates of the progeny of RGB binaries to the observational birthrates of the new sample of objects. In this talk I will present the obtained results and its implications on stellar (binary) evolution. I will also present the Galactic analogues of these objects and use their orbital motion and subsequently the full orbital observables like periods and mass functions to constrain precursors and progeny of this new population of dusty post-RGB binaries.

Onno Pols, Bridging the Gap in Binary Evolution
Binary mass transfer occurs by Roche-lobe overflow in relatively close systems, and by wind accretion in systems that are too wide for either star to fill its Roche lobe. In the case of red giant primaries, RLOF is often (though not always) unstable and results in a highly non-conservative common envelope event that shrinks the orbit. On the other hand, wind mass loss tends to widen the orbit, and accretion from the wind of an AGB star can leave chemical signatures in the companion star. This divergent orbital evolution is expected to produce a gap in the period distribution of mass-transfer remnant systems. However, observed systems are plentiful in this period range (roughly 100 days to a few years). In this intermediate regime the interaction processes are not well understood; wind interaction, Roche-lobe overflow and even common-envelope evolution are not easily distinguishable here. Remnants of such interaction, which include post-AGB binaries, barium stars and (at low metallicity) carbon-enhanced metal-poor stars, have properties that are at odds with the predictions of standard binary evolution models. In this contribution I will explore some of the processes that may be at work here.

Hans Van Winckel, Post AGB Binaries and their Circumbinary Discs
In this talk I will focus on evolved binaries of low to intermediate mass. In these systems, with orbital periods of one to several years, the most luminous object is an evolved post-AGB star and the unseen companion is suspected to be un-evolved. It is by now well established that these systems are often surrounded by a stable dusty circumbinary disk. Thanks to dedicated interferometric observations, we are now able to directly probe the structure and kinematics of these disks. I will hence review the properties of these disks and critically compare them with disks observed around young stellar objects. Recent results show that the presence of this new actor in the binary interaction landscape may have a strong influence on the orbital parameters, in particular by its ability to generate (pump) eccentricity. There is hence convincing evidence that these disks must play a lead role in the evolution of these binaries.

Wednesday 27 July - Morning

Merged binaries:

Simon Jeffery, Observational Evidence for Merging Binary Star

Pawel Pietrukowicz, Searches for Future Mergers Among ~23,000, Eclipsing Binary Stars from OGLE
The OGLE project is a long-term large-scale variability sky survey that monitors about 1.3 billion stars of the Milky Way and Magellanic System. Our recent searches for eclipsing binary stars in the OGLE-III Galactic bulge fields have led to the detection of nearly 23,000 binaries with orbital periods shorter than 4 days. In this contribution, we present the results of an analysis of period changes in this sample of binaries based on two decades of the OGLE observations.

Christopher Tout, Thorne-Zytkow Objects
A Thorne-Zytkow object has the structure of a red giant but with the electron degenerate core replaced by a neutron star. TZOs are powered by accretion on to the neutron degenerate core and this drives a convective envelope, the base of which is hot enough for nuclear burning when the envelope is massive. It is unclear whether this leads to a long-lived stable structure but existing models suggest that nuclear energy generation can dominate the accretion luminosity and we should expect rapid proton capture processes, with distinctive nucleosynthetic signatures, to be important. Recently a very bright red star HV2112, was identified as a possible candidate. The most likely formation mechanism for a TZO is the merging of a neutron star with the core of a giant during common envelope evolution. We discuss the likelihood that HV2112 is actually a TZO rather than the most likely alternative of a super-AGB star which would not have required a binary star progenitor.

Fabian Schneider, Rejuvenation of Stellar Mergers and the Origin of Magnetic Fields in Massive Stars
Approximately 10% of massive OBA main-sequence (MS) and pre-MS stars harbour strong, large-scale magnetic fields. At the same time there is a dearth of magnetic stars in close binaries. A process generating strong magnetic fields only in some stars must be responsible with the merging of pre-MS and MS stars being suggested as one such channel. Stars emerging from the coalescence of two MS stars are rejuvenated, appearing younger than they are. They can therefore be identified by comparison with reference clocks. Here we predict the rejuvenation of MS merger products over a wide range of masses and binary configurations calibrated to smoothed-particle-hydrodynamical merger models. We find that the rejuvenation is of the order of the nuclear timescale and is strongest in the lowest-mass mergers and the most evolved binary progenitors with the largest mass ratios. These predictions allow us to put constraints on the binary progenitors of merger products. We show that the magnetic stars HR2949 and tau Sco are younger than the potential binary companion HR2948 and the Upper-Sco association, respectively, making them promising merger candidates. We find that the age discrepancies and the potential binary progenitors of both are consistent with them being rejuvenated merger products, implying that their magnetic fields may originate from this channel. Searching for age discrepancies in magnetic stars is therefore a powerful way to explore which fraction of magnetic stars may have obtained their strong magnetic field in MS mergers and to improve our understanding of magnetism in massive stars and their remnants.

Stephan Rosswog, Theory of Merged Binaries

Philip Hall, Hydrogen in Helium White Dwarf Mergers
An isolated hot subdwarf might be formed by the merging of two helium-core white dwarfs. Before merging, the helium-core white dwarfs have hydrogen-rich envelopes. The mass of hydrogen that survives the merger and subsequently diffuses to the surface dictates the effective temperature of the resulting hot subdwarf. We find the mass of hydrogen at the start and end of a double helium white dwarf merger and thus test the extent to which the observed isolated hydrogen-rich hot subdwarfs can be explained as the remnants of these mergers.

Hagai Perets, Formation of Tidal Captured Binaries, Micro-tidal Disruption Events and Gravitational Wave Sources Following Neutron Star Natal
At birth neutron star (NSs) receive large natal velocity kicks. When accompanied by a binary companion such kicks may unbind the system or significantly change the binary orbit. In some cases the random kicks lead to very close approach between the newly born NS and its companion, at which point dissipative tidal forces and general relativistic effects become important. Here we focus on such events; we use binary stellar evolution models in which we incorporate natal kicks and follow the trajectories of the NSs to study the outcomes of such events in detail. We find that a such events could lead to high rates of collisional outcomes in the field (typically discussed only in the context of dense stellar clusters), including tidal capture formation of compact binaries, the tidal disruption of a main sequence or a white-dwarf companion by the NS (a micro - tidal disruption event), a direct collision with a companion star, or the production of a gravitational wave progenitors when the companion star is a compact object. We conclude that these little studied post-natal kick close approaches could play a major role in the formation of compact binaries and collisional outcomes in the field.

Ghina Halabi, Two-Dimensional Stellar Evolution: 2DStars
Ghina M. Halabi, Robert Izzard, Christopher Tout, Robert Cannon & Adam Jermyn.
Many stars are known to be rapid rotators, both at the surface and in their interiors. This rapid rotation causes aspherical distortion and surface temperature variations. It also drastically alters their chemistry, magnetic fields and future evolution. Thus, they can only be modelled properly in multi-dimensions.  In this talk, I will introduce 2DStars which is a general-use 2D, adaptable to 3D, stellar evolution code that is currently being developed at IoA. 2DStars will incorporate essential physics like rotation, magnetic fields, mixing, tides and mass transfer. In addition to modeling rotating stars, I will discuss the possible applications of the code which encompass a variety of multi-dimensional phenomena in stellar evolution like accretion and mass transfer in close binary systems, star formation and X-ray binaries.

Thursday 28 July - Morning

Compact binary evolution in the field:

Vicky Kalogera, The Dawn of Gravitational-Wave Astrophysics
The very first observing run of the Advanced LIGO detectors led to the discovery of binary-black-hole inspiral and merger events, and overnight we got launched into the era of gravitational-wave astrophysics. In this talk I will review the key results from the LIGO data search, signal characterization, and measurements of source properties and I will discuss what these observations imply for black-hole astrophysics.

Serena Repetto, On the Velocities that BH- and NS-XRBs Receive at Formation
The similarity between the Galactic distribution of black hole (BH) and neutron star (NS) X-ray binaries (XRBs) led Jonker \& Nelemans 2004 to suggest that BHs could potentially receive high natal kick (NK) at birth, similarly to NSs. Motivated by this idea, we investigate whether different assumptions on compact object formation (such as a different NK distribution and/or a different amount of mass ejected in the supernova, SN) have an imprint on the Galactic distribution of BH- and NS-XRBs, and we quantify these effects. We build synthetic populations of BH- and NS-XRBs and we model their binary evolution and their kinematics in the Galaxy. We find that the root mean square of the height above the Galactic plane of these binaries is a powerful proxy to discriminate among different formation scenarios, in particular when it comes to quantify how similar are the conditions at formation between NSs and BHs. Furthermore, we conclude that binary evolution following the BH/NS formation does not significantly affect the Galactic distributions of the binaries. When comparing our population synthesis results with the observed BH-XRBs, we find that a population model in which at least some BHs receive a high (or relatively high) NK fits the observed BH-XRBs best. This is an agreement with the results of Repetto et al. 2012, while now making use of the most recent measurements of BH-XRB distances and of a detailed modeling of the binary evolution of the sources. In the second part of the talk, we study the {\emph{minimal}} velocities of BH-XRBs rather than their {\emph{expected}} velocities. We analyze the simple method we previously used to estimate the minimal peculiar velocity of an individual BH-XRB at birth (Repetto et al. 2015), which was recently challenged by Mandel 2016. We find that this method may be less reliable in the bulge of the Galaxy and for certain models for the Galactic potential, but that our estimate is excellent for most of the BH-XRBs.

Christopher Berry, Binary Observations from Advanced LIGO
Advanced LIGO has finished its first observing run and announced the first detection of gravitational waves. From the first observing run, there were three interesting events (GW150914, LVT151012 and GW151226) identified as binary black hole coalescences. I will explain how the parameters of these systems are inferred, and discuss the results from our first observing run.

Sylvain ChatyThe Impact of High Mass X-ray Binaries on their Environment
I will report on a survey we recently performed with the Herschel satellite on the environment of High Mass X-ray Binaries (HMXB) and microquasars. In addition to Herschel PACS and SPIRE proprietary data, we retrieved multi-wavelength archival data (from Spitzer, GreenBank, VLA, etc.). We aimed at searching for interactions created by the impact and feedback of outflows emanating from the central sources, and also looked for potential induced stellar formation. The main question is: how do jets and wind alter the close environment of these high energy sources?

Krzysztof Belczynski, Theory of Compact Binary Evolution in the Field

The merger of two massive (about 30 solar masses) black holes has been detected in gravitational waves. This discovery validates recent predictions that massive binary black holes would constitute the first detection. Previous calculations, however, have not sampled the relevant binary-black-hole progenitors—massive, low-metallicity binary stars—with sufficient accuracy nor included sufficiently realistic physics to enable robust predictions to better than several orders of magnitude. Here we report high-precision numerical simulations of the formation of binary black holes via the evolution of isolated binary stars, providing a framework within which to interpret the first gravitational-wave source, GW150914, and to predict the properties of subsequent binary-black-hole gravitational-wave events. Our models imply that these events form in an environment in which the metallicity is less than ten per cent of solar metallicity, and involve stars with initial masses of 40–100 solar masses that interact through mass transfer and a common-envelope phase. These progenitor stars probably formed either about 2 billion years or, with a smaller probability, 11 billion years after the Big Bang. Most binary black holes form without supernova explosions, and their spins are nearly unchanged since birth, but do not have to be parallel. The classical field formation of binary black holes we propose, with low natal kicks (the velocity of the black hole at birth) and restricted common-envelope evolution, produces approximately 40 times more binary-black-holes mergers than do dynamical formation channels involving globular clusters; our predicted detection rate of these mergers is comparable to that from homogeneous evolution channels. Our calculations predict detections of about 1,000 black-hole mergers per year with total masses of 20–80 solar masses once second-generation ground-based gravitational-wave observatories reach full sensitivity.

Ilya Mandel, Massive Binary Paleontology with Gravitational Waves

Gravitational-wave astrophysics provides a unique opportunity to probe the evolution of massive stellar binaries via observations of their end products: merging compact remnants. In this talk, I will describe the ongoing efforts of our group to set up a framework for interpreting the observed populations in order to solve the inverse problem and determine the physics governing their evolution. I will focus on the statistical aspects of COMPAS (Compact Object Mergers: Population Astrophysics and Statistics), such as accounting for selection effects and measurement uncertainties. I will also discuss prospects for model-independent inference in the face of systematic model uncertainty.

Shenghua Yu, The Gravitational Waves Radiation from Populations of Galactic Double Compact Binary
In this talk, I will present a systematic study of the gravitational waves radiation from populations of double compact binary in the Galaxy with emphasis on the double white dwarfs, double neutron stars and double stellar mass black holes. These gravitational waves radiation may provide us an alternative means to study the relation between double compact binaries and the Galaxy. I will also show some results on the observability of the gravitational waves. 

Simon Stevenson, Insights into Stellar and Binary Evolution from Gravitational-wave Observations of Merging Compact Objects
Advanced LIGO recently announced the first direct observations of gravitational-waves from the merger of stellar mass black holes. It is reasonable to expect more detections to follow in the next few years. Compact binaries containing neutron stars and black holes form as the end product of the evolution of massive stars in close binaries. There remain many poorly understood processes in the lives of massive stars and the evolution of close binary systems. Examples of these processes include the distribution of supernova kicks received by compact objects at birth, uncertainties in mass transfer and the common envelope event. One way of attempting to understand these processes is to attempt to constrain them observationally. Here we present a framework for using future gravitational-wave observations of compact binary mergers to constrain stellar and binary evolution.

Thursday 28 July - Afternoon

Transient sources:

Samaya Nissanke, Transient Sources (theory) - The Astrophysics of Compact Object Mergers
New Astrophysics of Compact Object Mergers from Gravitational Wave and Electromagnetic Observations From September 2015 to January 2016, the first observational run of the Advanced LIGO detectors saw the first detections of gravitational waves from binary black holes. Future observational runs by advanced gravitational wave detectors should measure not only binary black hole mergers but compact object mergers that comprise neutron stars. These cosmic laboratories present us now with both a challenge and an opportunity. The challenge is to explain the rich physics at play in high velocity, strongly-curved spacetime in Universe for the first time. The opportunity is to detect both the accompanying electromagnetic and gravitational radiation for the first time with a suite of new time-domain telescopes and gravitational wave detectors.  In this talk, I will first give an introduction to optical and radio counterparts of neutron star binary mergers and then discuss the challenges that lie ahead in pinpointing and fully characterising the events on the sky. I will outline these efforts within the context of the recent efforts in the broadband follow-up of the binary black hole gravitational-wave transients GW150914 and GW151226. 

Priscilla CanizaresTimely Characterisation for Electromagnetic Follow-up of Gravitational Wave Mergers
The first detections of Gravitational Waves (GWs) have started a new era for GW Astronomy and will reshape astronomy and astrophysics as we know them. Prompt and precise alerts of a GW event are necessary to alert high energy-optical-radio observers and maximize the science from a GW detection. However in the first LIGO science run where the first GWs were detected, the localisation and characterisation of the GW source took from days to weeks. In the summer of 2016 LIGO will start to run its second science and we must be prepared to discover the first GW-EM merger. Within several years, the LIGO and Virgo GW detectors will realize their full potential and could detect GWs daily. Hence, it is now essential to perform rapid characterisation of GW sources and to send out timely alerts to the broader astronomy community. 

In this talk I will introduce a new methodology to address the problem of characterising GW signals for triggering timely alerts to EM observers. In particular, I will focus on compact object mergers, which are the main sources expected for electromagnetic follow-up.

Melvyn Davies, Mass Transfer in Eccentric Compact Binaries
We perform hydrodynamic simulations of mass transfer in eccentric white dwarf neutron star binaries. By measuring non-conservative mass transfer parameters we link the results of our simulations to a long-term evolution model. We find that the binaries containing a low mass helium white dwarf evolve into ultracompact X-ray binaries, whereas the systems with higher mass white dwarfs experience unstable mass transfer leading ultimately to the shredding of the white dwarf donors potentially producing the so-called Ca-rich transients.

Ross ChurchThe Binary Progenitors of Ca-rich Transients
One of the more intriguing results from the Palomar Transit Factory has been the identification of a new class of transients, commonly known as calcium-rich transients or gap transients. Ca-rich transients fall into the luminosity gap between novae and supernovae, and their spectra evolve rapidly to a nebular phase dominated by strong calcium lines. Most interestingly, though, they are often found at large spatial offsets from their host galaxies. Our observational work has shown that, in at least some cases, their progenitors can not have formed in situ but must have been ejected from their host stellar populations. I will review possible binary progenitor models for Ca-rich transients, in particular the possibility that they are formed from the tidal disruption of white dwarfs as they merge with neutron stars in binaries that have been ejected from the host galaxies by supernova kicks.

Joshua Bloom, Transient & Variable Star Observations: Some Challenges
The ever-increasing depth, cadence, and spectral coverage of the sky brings the potential for new insights across the breadth of the time-domain universe. Nevertheless, as discovery and follow-up resources remain relatively scarce, a particular onus rests on improving our computational capabilities to meet the inferential challenges on a time-scale appropriate for the science at hand. Here, I describe some efforts at bringing machine learning to bear on time-domain observations, couched in the context of specific scientific initiatives such as the identification of EM counterparts of gravitational wave events, discovery and follow-up of supernovae, and variable star studies. These efforts and others will be discussed within the landscape of follow up resources (both online and upcoming).

Grzegorz Wiktorowicz, Nature of the Extreme Ultraluminous X-ray Sources
In this proof-of-concept study we demonstrate that in a binary system mass can be transferred toward an accreting compact object at extremely high rate. If the transferred mass is efficiently converted to X-ray luminosity (with disregard of the classical Eddington limit) or if the X-rays are focused into a narrow beam then binaries can form extreme ULX sources with the X-ray luminosity of Lx >= 10^42 erg/s. For example, Lasota & King argued that the brightest known ULX (HLX-1) is a regular binary system with a rather low-mass compact object (a stellar-origin black hole or a neutron star).

The predicted formation efficiencies and lifetimes of binaries with the very high mass transfer rates are large enough to explain all observed systems with extreme X-ray luminosities. These systems are not only limited to binaries with stellar-origin black hole accretors. Noteworthy, we have also identified such objects with neutron stars. Typically, a 10 Msun black hole is fed by a massive (10 Msun) Hertzsprung gap donor with Roche lobe overflow rate of ~10^-3 Msun/yr (~ 2600 Mdot_Edd). For neutron star systems the typical donors are evolved low-mass (~2 Msun) helium stars with Roche lobe overflow rate of ~ 10^-2 Msun/yr.

Our study does not prove that any particular extreme ULX is a regular binary system, but it demonstrates that any ULX, including the most luminous ones, may potentially be a short-lived phase in the life of a binary star.

Wojciech Gladysz, Binary Black Hole Spin Evolution in Various Accretion Models
The scarcity and uncertainty of electromagnetic black hole spin measurements, together with a highly debatable issue of angular momentum transfer to a newly formed black hole, set ground for our theoretical effort. Using population synthesis method, we generated populations of binary star systems with black holes in the Milky Way environment.  Adopting various accretion models, we show the corresponding BH spin evolution. We also compare our results with (highly uncertain) estimates of available BH spins. It appears that LIGO/Virgo near-future detections of BH-BH mergers may provide the most reliable constraints on BH spins.

Elisabeth Guggenberger, A Spectroscopic Hunt for RR Lyrae Pulsators in Binary Systems
RR Lyrae stars are evolved pulsating stars burning helium in their cores. Their high-amplitude radial pulsation can lead to radial velocity changes of tens of km/s and brightness changes of about 1 mag. Since its discovery more than a century ago this class of stars has increased our knowledge of, among others, stellar pulsation theory, stellar evolution, and Milky Way structure. As standard candles RR Lyrae stars also serve as an important part of the cosmic distance ladder. Thanks to large surveys, ten thousands of RR Lyraes are known today. In spite of this large number only a handful of them have been found to reside in binary systems. Having an independent constraint of the mass from an orbit solution would be a crucial test for pulsation theory. The only eclipsing RR Lyrae pulsator that was found so far turned out to be an exotic object with an unexpectedly low mass - too low to ignite helium. 

We present here a spectroscopic study of RR Lyrae stars that are suspected to be binaries.  Extracting orbital parameters for RR Lyraes is very challenging due to their large intrinsic variability which can be an order of magnitude larger than the signal we are aiming to detect. We have been awarded 28 nights at the 2.1m telescope at McDonald observatory for our project. We present the pulsational RV curves and the derived center of mass RVs which we compare to previous measurements from the literature in order to find long-term changes induced by orbital motions.

Friday 29 July - Morning

Nucleosynthesis in binary stars:

Alain Jorissen, Observations of Nucleosynthesis in Binary Stars
I will discuss situations where nucleosynthesis and binary properties are tightly related:
(i) the Tc - Ru chronometer and Zr - Nb chronometer in intrinsic/extrinsic S stars;
(ii) the period - eccentricity diagram for barium stars in open clusters and in the field; 
(iii) the sample of Carbon-Enriched Metal-Poor stars.

Anindita Mondal, A Spectroscopic Analysis and Modelling of the Recurrent Nova RS Ophicuchi 2006 Outburst
Novae are close binary systems consisting of a white dwarf as the primary star and a red-giant or main sequence star as the secondary. They are mainly of two types, classical and recurrent. Here, we present an analysis of elemental abundances of ejecta of recurrent nova RS Oph using optical and NIR spectrum during 2006 outburst (12 February, 2006) taken from 2m HCT and 1.2m Mt Abu observatory. We use CLOUDY photoionization code to generate synthetic spectra by varying several parameters. We used χ2 (chi-square) minimization technique to fit multiple epochs of emission line spectra and obtained the best fit model parameters. These parameters are compatible with a hot white dwarf source with log(Blackbody Temperature) of 4.45 – 6.4 K and roughly constant log(Luminosity) of 36.9 – 37.1 erg/s. From the analysis, we find the following abundances (by number) of elements with respect to solar: He/He⊙ = 1.5 – 2.5, N/N⊙ = 10 - 12, O/O⊙ = 0.7 – 5.0, Ne/Ne⊙ = 0.5 – 2.0, Si/Si⊙ = 0.3 – 0.5, Fe/Fe⊙ = 0.7 – 3.8, Ar/Ar⊙ = 4.0 – 5.5, and Al/Al⊙ = 0.9 – 1.1, Ni/Ni⊙ = 1.5 – 2.0. We also estimate an ejected mass in the range of 3.4 - 4.9 x 10^(−6) M⊙ which is consistent with observational results. 

Joanna Mikolajewska, Extragalactic Symbiotic Stars: an Important Test for Binary Evolution Models
Symbiotic stars (SySt) are amongst the longest orbital period interacting binaries. The components are an evolved cool giant and an accreting, hot, luminous (usually a WD) companion, embedded in rich and complex surroundings, including both ionized and neutral regions, accretion/excretion discs, interacting winds and jest. surrounded by a dense ionized nebula.  SySt are are important tracers of late evolutionary phases of low- and medium-mass stars, and excellent laboratories to explore interactions and evolution in binary stars. In addition, the composition of SySt makes some of them a promising öfactoryö of SN Ia, independently of the scenario leading to their eruption.   While about 300 Galactic symbiotics are known, and a few dozen are relatively well studied, their distances (and hence their component luminosities and other distance-related parameters) are poorly determined. This makes comparison with the theoretical models for their interaction and evolution very challenging.  Fortunately, SySt are detectable throughout the Milky Way as tracers of luminous binary stars.  Here we will present some preliminary results of our ongoing systematic search for SySt in M33 and M31. Our goal is to obtain large, complete, luminosity-limited samples of extragalactic SySt to derive their total numbers and their spatial distribution in different type galaxies. Large SySt samples, all at the same distances, also enable us to determine the values and distribution of their luminosity-related parameters.  These will provide strong constraints on binary stellar evolution and the progenitor masses.

Hongwei Ge, Adiabatic and Thermal Equilibrium Mass Loss Models and Applications
Whether the mass transfer is dynamically stable or not is one of the basic question in binary evolution. Recently, many different authors have made progress on it by different methods. We have found that the main sequence donor stars with a radiative envelope will suffer a delayed dynamically unstable mass transfer if the initial mass ratio is too large; the local thermal timescale and the global dynamical timescale of the donor stars on the first or asymptotic giant branch have nearly the same order of magnitude. So the response to both thermal and dynamical timescale mass transfer of these donor stars is important. We build the adiabatic and the thermal equilibrium mass loss model to study the response of the donors to dynamical and thermal timescale mass loss.

Richard Stancliffe, The Theory of Nucleosynthesis in Binary Stars
Duplicity is important to our understanding of stellar nucleosynthesis. Binary systems open up nucleosynthetic pathways to produce isotopes that are not easily produced in other ways. In addition, the less evolved companions of binary stars can preserve a chemical record of the nucleosynthesis of their long-dead partners. Using the specific case of carbon-enhanced metal-poor stars, I will discuss nucleosynthesis in asymptotic giant branch stars and how our inferences are affected by what happens to material accreted by the companion.

Katelyn Milliman, Barium Enhanced Blue Stragglers in NGC 6819
Possible formation pathways for blue straggler stars include mergers in hierarchical triple systems, stellar collisions during dynamical encounters, and mass transfer from a giant companion. Extensive work on the blue stragglers in the old open cluster NGC 188 (7 Gyr) indicate that mass transfer is the dominant mechanism for blue straggler formation in open clusters. Such mass transfer events should pollute the surface abundance of the blue straggler with nucleosynthesis products from the evolved donor. The other formation pathways, mergers and collisions, are predicted to produce no such enhancements. I will present the results of a surface abundance study of the blue stragglers in the intermediate-aged open cluster NGC 6819 (2.5 Gyr). This work identified five blue stragglers with significant barium excess that indicates formation via mass transfer from an AGB donor, but surprisingly four of these stars show no radial-velocity evidence for a companion and the last star has a luminous secondary. I will discuss the conclusions we can draw from these results and our attempts to explain the origin of these blue straggler systems.

Robert Izzard, Binary Stars in the Galactic Thick Disc
Recent studies of old stars in the Galactic thick disc suggest it contains a population of “young” giant stars. These are chemically and kinematically members of the thick disc, but their masses -- measured by asteroseismology -- are up to a factor of two greater than expected given the age of the thick disc. Galactic astronomers have suggested they come from a population of stars that have migrated from the bar of the Galaxy. We show that they are binary stars which have undergone mass transfer, a process similar to that which forms blue stragglers. We populate the [C/N] vs mass parameter space with binary-star models which match the observations far better than we could have expected. We also predict binary fractions and other properties of these “young” stars, and hence show that these properties can potentially be used to pin down uncertain physics in binary systems, such as the efficiency of common envelope ejection.
[Authors: Robert Izzard, Paula Jofré, Thomas Masseron, Holly Preece, all of the University of Cambridge]

Ana Escorza, To Ba or not to Ba: the Formation of Barium Stars
In this contribution we will focus on the formation of chemically peculiar stars known as Barium stars. These stars have an excess of barium on their surface as well as several other chemical elements which are products of slow-neutron-capture nucleosynthesis (s-process). The presence of s-process elements on the surface of a main-sequence or a giant star cannot be explained by single star evolutionary theory, and Barium stars are understood as coming from a binary evolution channel in which the former AGB star (now a dim white dwarf) polluted the current Ba star with enriched products via a mass transfer process. A long-standing problem is, however, that the observed orbital characteristics of Ba stars are not predicted by the current binary evolution models. Important problems remain concerning in particular the persistence of high eccentricity in systems with relatively short periods. We will discuss the results of our on-going radial velocity programme and compare the orbital distribution of Ba dwarfs and Ba giants. We aim at identifying the initial parameters and the evolutionary channel that will make a binary system to form a Ba star. We will present the result of our first evolutionary models, calculated with the state-of-the-art BINSTAR code. Our goal is to investigate different possible mass-transfer scenarios as well as different eccentricity pumping mechanisms, which are needed to reproduce the observed periods and eccentricities, for both Ba dwarfs and Ba giants.

Friday 29 July - Afternoon

Supernovae in binaries:

Philipp Podsiadlowski, Theory of Supernovae in Binary Systems
It is now widely accepted that the large diversity of supernova types and sub-types is almost certainly caused by the large variety of binary interactions. I will review how the various binary interactions (mass loss, mass accretion, binary mergers, tidal coupling) modify the pre-supernova structure of massive stars, in particular their envelopes, and how this accounts for most of the observed diversity. However, as only has been realized in the last 15 years, binary interactions do not only affect the envelope structure of massive supernova progenitors they also determine the final fate of the core, specifically whether the core collapses to a neutron star or black hole or produces a gamma-ray burst or other exotic events. I will summarize how the various binary interactions affect the final fate of stars and its potential implications for a variety of "normal" and exotic supernova events, including supernovae with a circumstellar medium ("LBV supernovae"), superluminous supernovae, gamma-ray burst sources, pair-instability supernovae and aLIGO gravitational-waves sources.

Xiangcun Meng, A Common-envelope Wind Model for SNe Ia
We propose a new version of the SD model in which a common envelope (CE) is assumed
to form when the mass-transfer rate between the CO WD and its companion exceeds a critical accretion rate, instead of the onset of an optically thick wind (OTW). In this case, the WD may
gradually increase its mass at the base of the CE similar to the degenerate core in an asymptotic giant branch star. Due to a large nuclear luminosity for stable hydrogen burning, the CE may
expand to giant dimensions and will lose mass from the surface of the CE by a CE wind (CEW), which leads to a low density in the CE and a correspondingly low frictional luminosity between the binary system and the CE. As a result, the binary system will, within a large parameter range, avoid a strong spiral-in phase and finally re-emerge from the CE phase rather than merge completely.

Emmanouil Zapartas, Late Core-collapse Supernovae from Intermediate Mass Binaries
Core-collapse supernovae (ccSNe) mark the end of the lives of massive stars. The majority of these massive stars experience interaction with a binary companion before they explode, drastically affecting the evolution of both stars. We use a population synthesis approach to study the impact of binarity on the delay-time distribution of ccSNe, i.e. the supernova rate versus time after a sudden starburst.

We find that 10-30% of all ccSNe are “late”, i.e. they occur 50-200 Myrs after birth, when all single stars have already died. These late ccSNe originate from intermediate mass stars that gained mass from a binary companion. We investigate the main binary evolutionary channels, which include also exotic mergers, and discuss possible observational signatures of these late events. We also find up to 40% more ccSNe in a realistic population with massive binary stars compared to a pure single star population of the same total mass.

We conclude that the large majority of ccSNe originate from stars that have experienced binary interaction in the past, severely altering their properties. Our results will be relevant for future comparison with the results from all-sky automated transient surveys and as improved input models for stellar feedback in cosmological and galaxy evolution simulations.

Peter Eggleton and Ludmilla Kisseleva, Binaries of Red Giants and Main-sequence Dwarfs
Many comparisons have been made of eclipsing double-main-sequence binaries, and the result is usually good agreement between observation and theory, though with some qualifications. But more evolved stars are harder to compare because the orbits are usually larger, eclipses rarer, and radial velocities amplitudes less. We compare 57 such observed binaries with theoretical models that include mass loss rotationally driven by the RS CVn mechanism, and rotation affected by tidal friction.  Provided a rather specific formulation of convective core overshooting is used, we usually get reasonable agreement; except that there appears to be something inherently chaotic about the mass loss proces so that some systems lose 10 or more times more, and others 10 or more times less, in rather similar binaries. We note 2 systems where, we believe, an initially triple system has undergone a merger in a sub-binary, leading to an anomalous wide binary. Two systems have a blue giant or supergiant; and  seven have two red giants.

Alicia Soderberg, Observations of Supernovae in Binary Systems

Mathieu Renzo, Final Structure and Fate of Massive Stars: the Impact of Mass Loss by Winds and Roche-lobe Overflow
Contributors: M. Renzo, C. D. Ott, S. N. Shore, S. E. de Mink
Massive stars experience mass loss by stellar winds and as result of interaction with a binary companion. Both modes of mass loss, steady and episodic, have major impact on the final stellar structure. This can change the final fate and leaves imprints on the supernova lightcurve. To quantify the impact, we carry out a systematic study of mass loss algorithms using the MESA stellar evolutionary code. We employ a very extensive nuclear reaction network (~200 isotopes) and adopt ultra high resolution (10-100 times more mesh points than previously published models). We find this is crucial to resolve the delayed effect of mass loss on the final core structure.

Our results show that main sequence winds have the largest effect on the final core structure, even though they only remove a small fraction of the total mass. The predicted final total mass for a given star is uncertain by a factor 2 due to uncertainties in the stellar wind algorithms. The compactness, which has been suggested to be indicative of whether a massive star leaves a neutron star or black hole, is affected at least 30%.  We further discuss the impact of impulsive removal of large fraction of the envelope on the light curve morphology, peak luminosity, and duration.  We conclude that our current understanding of stellar mass loss constitutes a limiting factor in the study of supernova explosions."

Page last updated: 26 July 2016 at 14:03