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  Papers • Papers of 2011
Binaries are the best single stars
de Mink, S. E.; Langer, N.; Izzard, R. G.
Stellar models of massive single stars are still plagued by major uncertainties. Testing and calibrating against observations is essential for their reliability. For this purpose one preferably uses observed stars that have never experienced strong binary interaction, i.e. &nbquot;true single stars&nbquot;. However, the binary fraction among massive stars is high and identifying &nbquot;true single stars&nbquot; is not straight forward. Binary interaction affects systems in such a way that the initially less massive star becomes, or appears to be, single. For example, mass transfer results in a widening of the orbit and a decrease of the luminosity of the donor star, which makes it very hard to detect. After a merger or disruption of the system by the supernova explosion, no companion will be present. The only unambiguous identification of &nbquot;true single stars&nbquot; is possible in detached binaries, which contain two main-sequence stars. For these systems we can exclude the occurrence of mass transfer since their birth. A further advantage is that binaries can often provide us with direct measurements of the fundamental stellar parameters. Therefore, we argue these binaries are worth the effort needed to observe and analyze them. They may provide the most stringent test cases for single stellar models.

Schematic depiction of the evolutionary stages of a 20+15 M binary as a function of the initial orbital period (x-axis) and time (y-axis).
Proceedings of the 39th Liège International Astrophysical Colloquium The multi-wavelength view of Hot, Massive Stars, see also arxiv.org/1010.2200
The search for progenitor models of type Ia supernovae
Claeys, J. S. W.; Pols, O. R.; Vink, J.; Izzard, R. G.
The Delay Time Distribution (DTD) of SNe Ia in SNuM of the three different progenitor scenarios in our model (Left Fig. 𝛼CE=3. Right Fig. 𝛼CE=1). The
We show the preliminary results of our search for the progenitor systems of type Ia supernovae (SNe Ia). We model binary populations our aim being to compare these models with the observations of detailed element abundances of the hot Intra-Cluster Medium.
See also arxiv.org/1101.5601
The VLT-FLAMES Tarantula Survey I: Introduction and observational overview
C.J.Evans et al.
The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our scientific motivations and give an overview of the survey targets, including optical and near-infrared photometry and comprehensive details of the data reduction. One of the principal objectives was to detect massive binary systems via variations in their radial velocities, thus shaping the multi-epoch observing strategy. Spectral classifications are given for the massive emission-line stars observed by the survey, including the discovery of a new Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To illustrate the diversity of objects encompassed by the survey, we investigate the spectral properties of sixteen targets identified by Gruendl & Chu from Spitzer photometry as candidate young stellar objects or stars with notable mid-infrared excesses. Detailed spectral classification and quantitative analysis of the O- and B-type stars in the VFTS sample, paying particular attention to the effects of rotational mixing and binarity, will be presented in a series of future articles to address fundamental questions in both stellar and cluster evolution.

The Tarantula Nebula and overlaid with target stars of the survey.
Accepted for publication in Astronomy and Astrophysics, see also arxiv.org/1010.2200.
Common Envelope Evolution: 221KB PDF
Robert G. Izzard, Philip D. Hall, Thomas M. Tauris and Christopher A. Tout

Many binary star systems are not wide enough to contain the progenitor stars from which they were made. One explanation for this is that when one star becomes a red giant a common envelope forms around both stars in the binary system. The core of the giant and its companion star continue to orbit one another inside the envelope. Frictional energy deposited into the common envelope may lead to its ejection and, if so, a close binary system is formed from the core of the former giant star and its relatively untouched companion. When the primary is an asymptotic giant branch star the core becomes a hot carbon-oxygen white dwarf which may ionise the ejected envelope and illuminate a planetary nebula. Many other types of binary systems form through common envelope evolution such as low-mass X-ray binaries and cataclysmic variables. In the case of a failed envelope ejection when the cores merge, rapidly-rotating solitary giants similar to FK Comae stars form. In this short review we focus on attempts to constrain parameters of common envelope evolution models and also describe the latest efforts to model this elusive phase of binary stellar evolution.
Invited review talk at IAU Symposium 283 - Planetary Nebulae an Eye to the Future
How did Carbon-Enhanced Metal-Poor Stars form?
Abate, C.; Pols, O. R.; Izzard, R. G.; Mohamed, S.; de Mink, S. E.
Among the very metal-poor stars observed in the Galactic halo there is a large fraction of stars which show a great enhancement in the abundance of carbon and other heavy elements. These stars are called CEMP (carbon-enhanced metal-poor) stars. One explanation for these observed enhancements is that in the past CEMP stars have accreted mass from an AGB binary companion. This scenario is supported by the evidence that CEMP stars are mostly found in spectroscopic binary systems. Different aspects still need to be clarified: (1) the efficiency of the mass-transfer process and its effects on the dynamics of the system; (2) the consequences of this mass-transfer for the overall population of metal poor stars and (3) its implications for the initial mass function at early epochs. Recent hydrodynamical simulations suggest a new model for mass-transfer, intermediate between wind accretion and RLOF and applicable in the case of slow and dense AGB winds, which under specific conditions can lead to an accretion efficiency much higher than the classical predictions. We present preliminary results of population simulations in which we explore this new model and its consequences for the CEMP stars. First results suggest a doubling of the CEMP formation rate.

Evolution of compact binaries. Proceedings of a workshop held at Hotel San Martin, Vina del Mar, Chile 6–11 May 2011. Astronomical Society of the Pacific, 2011., p.81
Formation and Evolution of Carbon-Enhanced Metal-Poor Stars
Abate, C.; Pols, O. R.; Izzard, R. G.
Very metal-poor stars observed in the Galactic halo constitute a window on the primordial conditions under which the Milky Way was formed. A large fraction of these stars show a great enhancement in the abundance of carbon and other heavy elements. One explanation of this observation is that these stars have undergone mass transfer from an AGB binary companion. This scenario is supported by the evidence that these carbon-enhanced metal-poor (CEMP) stars are mostly found in spectroscopic binary systems. We have started a project with the aim to understand several aspects which still need to be clarified: (1) how the abundances of low-mass metal-poor stars evolve after accretion of material from an evolved asymptotic giant branch star, (2) what the consequences of this mass transfer are for the overall population of CEMP stars, and (3) what its implications are for the initial mass function at early epochs.
Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants. Proceedings of a conference held at University Campus, Vienna, Austria, 16-20 August 2010. Edited by F. Kerschbaum, T. Lebzelter, and R.F. Wing. San Francisco: Astronomical Society of the Pacific, 2011., p.445
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