Astrophysicists are increasingly taking into account the effects of orbiting companions on stellar evolution. New discoveries have underlined the role of binary star interactions in a range of astrophysical events, including some that were previously interpreted as being due uniquely to single stellar evolution. 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, and the interaction between stars and their planets. Finally, we reassess well-known phenomena, such as luminous blue variables, in light of interpretations that include both single and binary stars. At the same time we contextualise the new discoveries within the framework of binary stellar evolution. The last decade has seen a 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 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.
To investigate the effect of a companion star on stellar evolution the
binary_c code was
developed. The code uses the BSE package at its core, with
nucleosynthesis in parallel, to explore large parameter spaces in
reasonable periods of time. Mass lost from the binary system
contributes to the stellar yield and since the BSE code is
based on the SSE code nucleosynthesis for populations of single stars
is also calculated. The two are then compared to see if there is any
difference.
Binaries are important:
The presence of a companion affects evolution by mass loss and
gain. Good examples are RLOF due to interaction between a giant
(GB/AGB) star and a MS star. Truncation of the GB/AGB phase may
prevent dredge-up events and hence reduce the amount of nuclear
processing material undergoes prior to explusion to the ISM. Common
envelopes generally result and while the detailed physics is unclear
it is likely that mass is ejected to the ISM from some of these stars.
The most extreme example is a stellar merger, where two stars become one. What happens? We do not really know.
A companion star can be spun up and down by tides. This may be a good way to make rapidly rotating stars which become gamma-ray bursts, the most violent explosions observed in the Universe.
Novae and SNeIa only occur in binaries. What are the progenitors
of SNeIa? We do not know - despite what you may have been told!
Binary evolution is plagued with uncertainy, even compared to our understanding of single-star evolution. Effects which must be considered include:
Duplicity : single star or binary star.
Metallicity and, less importantly, initial abundance mix. The
initial abundance mix depends on the galactic evolution history and
even the solar mix is somewhat uncertain.
Initial distributions: What is the IMF? What is the initial
distribution for primary mass, secondary mass, separation/period and
eccentricity for binary stars?
Abundance changes at dredge-ups. These changes can depend on the
input physics, especially in the case of third dredge-up. Calibration
to observations is necessary in this case and leads to the
introduction of free parameters to increase the amount of
dredge-up. There is also great uncertainty with regard to the
s-process isotopes, in particular the size of the C13 pocket during
third dredge-up.
Wind loss rates. Mass loss due to stellar winds is a very
dodgy affair - most prescriptions in current use are quite
phenomenological and have little regard for actual physics. With this
in mind it is important to test a range of prescriptions.
Common envelope parameters - the parameters αCE
and λCE parameterise our ignorance of this complex
process, mainly because the mechanism for driving off the stellar
envelope is unclear (magnetic fields? friction? ionization? who
knows!).
Eddington limit : should this be imposed during accretion
processes?
HBB temperature : somewhat uncertain is the amount of HBB, this
can be varied in the model
Black hole formation : what is the mass of a black hole forming
from a given mass progenitor?
Supernova kicks : is there a kick at SN formation? What is the
magnitude/distribution of this kick? Pulsar peculiar velocities give
us an idea but are not necessarily the answer to the question.
Binary induced wind loss - see Chris Tout's PhD. Does the presence
of a binary companion increase wind loss rates?
Time evolution of the yields. Even if the integrated yield up to
(say) 15Gyr from a population of stars is similar when comparing
binary and single stars, the time evolution probably is not. For
example, nitrogen peaks far more quickly in single than in binary
stars because massive (M>4Msun) TPAGB stars in binaries
overflow their Roche lobes prior to HBB so C12 cannot be converted
into N14.
