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  Papers • Papers of 2016
The impact of companions on stellar evolution
O. De Marco, R.G. Izzard
A Dawes Review for PASA

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.
Cannibals in the thick disk: the young α−rich stars as evolved blue stragglers
P. Jofre, A. Jorissen, S. Van Eck, R.G. Izzard, T. Masseron, K. Hawkins, G. Gilmore, C. Paladini, A. Escorza, S. Blanco-Cuaresma, R. Manick
Spectro-seismic measurements of red giants enabled the recent discovery of stars in the thick disk that are more massive than 1.4 M. While it has been claimed that most of these stars are younger than the rest of the typical thick disk stars, we show evidence that they might be products of mass transfer in binary evolution, notably evolved blue stragglers. We took new measurements of the radial velocities in a sample of 26 stars from APOKASC, including 13 "young" stars and 13 "old" stars with similar stellar parameters but with masses below 1.2 M and found that more of the "young" stars appear to be in binary systems with respect to the "old" stars. Furthermore, we show that the "young" stars do not follow the expected trend of [C/H] ratios versus mass for individual stars. However, with a population synthesis of low-mass stars including binary evolution and mass transfer, we can reproduce the observed [C/N] ratios versus mass. Our study shows how asteroseismology of solar-type red giants provides us with a unique opportunity to study the evolution of field blue stragglers after they have left the main-sequence.
Core-collapse supernova progenitor constraints using the spatial distributions of massive stars in local galaxies
T. Kangas, L. Portinari, S. Mattila, M. Fraser, E. Kankare, R. G. Izzard, P. James, C. González-Fernández, J. R. Maund, A. Thompson
We study the spatial correlations between the Hα emission and different types of massive stars in two local galaxies, the Large Magellanic Cloud (LMC) and Messier 33. We compare these to correlations derived for core-collapse supernovae (CCSNe) in the literature to connect CCSNe of different types with the initial masses of their progenitors and to test the validity of progenitor mass estimates which use the pixel statistics method. We obtain samples of evolved massive stars in both galaxies from catalogues with good spatial coverage and/or completeness, and combine them with coordinates of main-sequence stars in the LMC from the SIMBAD database. We calculate the spatial correlation of stars of different classes and spectral types with Hα emission. We also investigate the effects of distance, noise and positional errors on the pixel statistics method. A higher correlation with Hα emission is found to correspond to a shorter stellar lifespan, and we conclude that the method can be used as an indicator of the ages, and therefore initial masses, of SN progenitors. We find that the spatial distributions of type II-P SNe and red supergiants of appropriate initial mass (≳9 M) are consistent with each other. We also find the distributions of type Ic SNe and WN stars with initial masses ≳20 M consistent, while supergiants with initial masses around 15 M are a better match for type IIb and II-L SNe. The type Ib distribution corresponds to the same stellar types as type II-P, which suggests an origin in interacting binaries. On the other hand, we find that luminous blue variable stars show a much stronger correlation with Hα emission than do type IIn SNe.
The proper motion of HV2112: A TZO candidate in the SMC
C.C. Worley, M.J. Irwin, C.A. Tout, A.N. Zytkow, M.F., R.G. Izzard
The candidate Thorne-Zytkow object (TZO), HV2112, is becoming a well-studied if enigmatic object. A key point of its candidacy as a TZO is whether or not it resides in the Small Magellanic Cloud (SMC). HV2112 has detections in a series of photometric catalogues which have resulted in contradictory estimates of its proper motion and, therefore, its membership within the SMC. This letter seeks to resolve the issue of the SMC membership of HV2112 through a reanalysis of extant photometric data. We also demonstrate the difficulties and downfalls inherent in considering a range of catalogue proper motions. We conclude that the proper motion, and associated ancillary radial velocity, positional and photometric properties, are fully consistent with HV2112 being within the SMC and thus it remains a candidate TZO.
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