Robert Izzard's Pages of Astronomical Happiness

  Science • Papers of 2013
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A nova re-accretion model for J-type carbon stars
Sutirtha Sengupta; Robert G. Izzard and Herbert. H.B. Lau

The J-type carbon (J)-stars constitute 10-15% of the observed carbon stars in both our Galaxy and the Large Magellanic Cloud (LMC). They are characterized by strong 13C absorption bands with low 12C/13C ratios along with other chemical signatures peculiar for typical carbon stars, e.g. a lack of s-process enhancement. Most of the J-stars are dimmer than the N-type carbon stars some of which, by hot-bottom burning, make 13C only in a narrow range of masses. We investigate a binary-star formation channel for J-stars involving re-accretion of carbon-rich nova ejecta on main-sequence companions to low-mass carbon-oxygen white-dwarfs. The subsequent evolution of the companion stars in such systems is studied with a rapid binary evolutionary code to predict chemical signatures of nova pollution in systems which merge into giant single stars. A detailed population synthesis study is performed to estimate the number of these mergers and compare their properties with observed J-stars. Our results predict that such nova polluted mergers evolve with low luminosities as well as low 12C/13C ratios like the majority of observed J-stars (e.g. in the LMC) but cannot account for the observed fraction of J-stars in existing surveys of carbon stars.
Planetary nebulae after common-envelope phases initiated by low-mass red giants
Philip D. Hall, Christopher A. Tout, Robert G. Izzard and Denise Keller
It is likely that at least some planetary nebulae are composed of matter which was ejected from a binary star system during common-envelope (CE) evolution. For these planetary nebulae the ionizing component is the hot and luminous remnant of a giant which had its envelope ejected by a companion in the process of spiralling-in to its current short-period orbit. A large fraction of CE phases which end with ejection of the envelope are thought to be initiated by low-mass red giants, giants with inert, degenerate helium cores. We discuss the possible end-of-CE structures of such stars and their subsequent evolution to investigate for which structures planetary nebulae are formed. We assume that a planetary nebula forms if the remnant reaches an effective temperature greater than 30kK within 104 yr of ejecting its envelope. We assume that the composition profile is unchanged during the CE phase so that possible remnant structures are parametrized by the end-of-CE core mass, envelope mass and entropy profile. We find that planetary nebulae are expected in post-CE systems with core masses greater than about 0.3M if remnants end the CE phase in thermal equilibrium. We show that whether the remnant undergoes a pre-white dwarf plateau phase depends on the prescribed end-of-CE envelope mass. Thus, observing a young post-CE system would constrain the end-of-CE envelope mass and post-CE evolution.

Only certain combinations of envelope and core mass can make planetary nebulae.
Eccentricity-pumping in post-AGB stars with circumbinary discs
Dermine, T.; Izzard, R. G.; Jorissen, A.; Van Winckel, H.

Evolutionary tracks during the post-AGB phase (black lines) in the eccentricity-log orbital period plane including the interaction with a circumbinary disc. The diamond symbols are observed post-AGB systems.
Circumbinary discs are commonly observed around post-asymptotic giant branch (post-AGB) systems and are known to play an important role in their evolution. Several studies have pointed out that a circumbinary disc interacts through resonances with the central binary and leads to angular momentum transfer from the central binary orbit to the disc. This interaction may be responsible for a substantial increase in the binary eccentricity. We investigate whether this disc eccentricity-pumping mechanism can be responsible for the high eccentricities commonly found in post-AGB binary systems.
Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars
C. Abate, O.R. Pols, R.G. Izzard, S.S. Mohamed, S.E. de Mink

Schematic view of a binary showing the stellar radius R, Roche-lobe radius RL and dust radius Rd.
Carbon-enhanced metal-poor stars (CEMP) are observed as a substantial fraction of the very metal-poor stars in the Galactic halo. Most CEMP stars are also enriched in s-process elements and these are often found in binary systems. This suggests that the carbon enrichment is due to mass transfer in the past from an asymptotic giant branch (AGB) star on to a low-mass companion. Models of binary population synthesis are not able to reproduce the observed fraction of CEMP stars without invoking non-standard nucleosynthesis or a substantial change in the initial mass function. This is interpreted as evidence of missing physical ingredients in the models. Recent hydrodynamical simulations show that efficient wind mass transfer is possible in the case of the slow and dense winds typical of AGB stars through a mechanism called wind Roche-lobe overflow (WRLOF), which lies in between the canonical Bondi-Hoyle-Lyttleton (BHL) accretion and Roche-lobe overflow. WRLOF has an effect on the accretion efficiency of mass transfer and on the angular momentum lost by the binary system. The aim of this work is to understand the overall effect of WRLOF on the population of CEMP stars. To simulate populations of low-metallicity binaries we combined a synthetic nucleosynthesis model with a binary population synthesis code. In this code we implemented the WRLOF mechanism. We used the results of hydrodynamical simulations to model the effect of WRLOF on the accretion efficiency and we took the effect on the angular momentum loss into account by assuming a simple prescription. As a result the number of CEMP stars predicted by our model increases by a factor 1.2-1.8 compared to earlier results that consider the BHL prescription. Moreover, higher enrichments of carbon are produced and the final orbital period distribution is shifted towards shorter periods.
The Rotation Rates of Massive Stars: The Role of Binary Interaction through Tides, Mass Transfer, and Mergers
de Mink, S. E.; Langer, N.; Izzard, R. G.; Sana, H.; de Koter, A.
Rotation is thought to be a major factor in the evolution of massive stars—especially at low metallicity—with consequences for their chemical yields, ionizing flux, and final fate. Deriving the birth spin distribution is of high priority given its importance as a constraint on theories of massive star formation and as input for models of stellar populations in the local universe and at high redshift. Recently, it has become clear that the majority of massive stars interact with a binary companion before they die. We investigate how this affects the distribution of rotation rates, through stellar winds, expansion, tides, mass transfer, and mergers. For this purpose, we simulate a massive binary-star population typical for our Galaxy assuming continuous star formation. We find that, because of binary interaction, 20+5–10% of all massive main-sequence stars have projected rotational velocities in excess of 200 km s–1. We evaluate the effect of uncertain input distributions and physical processes and conclude that the main uncertainties are the mass transfer efficiency and the possible effect of magnetic braking, especially if magnetic fields are generated or amplified during mass accretion and stellar mergers. The fraction of rapid rotators we derive is similar to that observed. If indeed mass transfer and mergers are the main cause for rapid rotation in massive stars, little room remains for rapidly rotating stars that are born single. This implies that spin-down during star formation is even more efficient than previously thought. In addition, this raises questions about the interpretation of the surface abundances of rapidly rotating stars as evidence for rotational mixing. Furthermore, our results allow for the possibility that all early-type Be stars result from binary interactions and suggest that evidence for rotation in explosions, such as long gamma-ray bursts, points to a binary origin.

BINSTAR: a new binary stellar evolution code. Tidal interactions
Siess, L.; Izzard, R. G.; Davis, P. J.; Deschamps, R.

We provide a detailed description of a new stellar evolution code, BINSTAR, which has been developed to study interacting binaries. Based on the stellar evolution code STAREVOL, it is specifically designed to study low- and intermediate-mass binaries. We describe the state-of-the-art input physics, which includes treatments of tidal interactions, mass transfer and angular momentum exchange within the system. A generalised Henyey method is used to solve simultaneously the stellar structure equations of each component as well as the separation and eccentricity of the orbit. Test simulations for cases A and B mass transfer are presented and compared with available models. The results of the evolution of Algol systems are in remarkable agreement with the calculations of the Vrije Universiteit Brussel (VUB) group, thus validating our code. We also computed a large grid of models for various masses (2 ≤ M/M ≤ 20) and seven metallicities (Z = 0.0001, 0.001, 0.004, 0.008, 0.01, 0.02, 0.03) to provide a useful analytical parameterisation of the tidal torque constant E2, which allows the determination of the circularisation and synchronisation timescales for stars with a radiative envelope and convective core. The evolution of E2 during the main sequence shows noticeable differences compared to available models. In particular, our new calculations indicate that the circularisation timescale is constant during core hydrogen burning. We also show that E2 weakly depends on core overshooting but is substantially increased when the metallicity becomes lower.
Challenges for Understanding the Evolution of Massive Stars: Rotation, Binarity, and Mergers
de Mink, S. E.; Brott, I.; Cantiello, M.; Izzard, R. G.; Langer, N.; Sana, H.
The evolutionary models of massive stars that are widely used in various branches of astronomy are still plagued by major uncertainties. In particular, the uncertainties related to mixing and mass loss are a concern as they affect massive stars during their earliest evolutionary stages and have consequences for their entire remaining life. Two major challenges for understanding the evolution of massive stars are: (1) their strong preference for close binaries and (2) they are often found to be rapid rotators. Rotation is now a standard ingredient in evolutionary models, but consideration of the effects of interaction with a companion is often limited to cases of peculiar systems and exotic phenomena for which binarity is clear, such as X-ray binaries. Binary interaction may leave an apparently single star behind, so disentangling effects of rotation and binarity is not straightforward. The effects of rotation and binarity interplay and lead to surprising evolutionary channels. We discuss examples of how binary interaction leads to high stellar rotation rates as a result of tides, mass transfer and mergers.

Type Ia Supernovae and the Uncertainties in their Progenitor Evolution
Claeys, J. S. W.; Pols, O. R.; Izzard, R. G.
We use binary population synthesis to study the main proposed channels leading to Type Ia supernovae, the single degenerate channel (SD) and double degenerate channel (DD). For this purpose, we discuss the progenitor evolution and the influence of the common envelope efficiency, αce, on the rate of the different channels. Our study demonstrates the large αce-dependence of both channels, especially for the SD channel.
Massive Binary Stars and Self-Enrichment of Globular Clusters
Izzard, Robert G.; de Mink, Selma E.; Pols, Onno R.; Langer, Norbert; Sana, Hugues; de Koter, Alex

Globular clusters contain many stars with surface abundance patterns indicating contributions from hydrogen burning products, as seen in the anti-correlated elemental abundances of e.g. sodium and oxygen, and magnesium and aluminium. Multiple generations of stars can explain this phenomenon, with the second generation forming from a mixture of pristine gas and ejecta from the first generation. We show that massive binary stars may be a source of much of the material that makes this second generation of stars. Mass transfer in binaries is often non-conservative and the ejected matter moves slowly enough that it can remain inside a globular cluster and remain available for subsequent star formation. Recent studies show that there are more short-period massive binaries than previously thought, hence also more stars that interact and eject nuclear-processed material.

Contribution to the proceedings of "Reading the book of globular clusters with the lens of stellar evolution", Rome, 26-28 November 2012
Stellar Evolution Models of Classical Cepheids Require Enhanced Mass Loss
Measurements of rates of period change of Classical Cepheids probe stellar physics and evolution. Additionally, better understanding of Cepheid structure and evolution provides greater insight into their use as standard candles and tools for measuring the Hubble constant. In this work, we compare rates of period change measured for about 200 Galactic Cepheids to population synthesis models of Cepheids including convective core overshooting and enhanced mass loss. Rates of period change predicted from stellar evolution models without mass loss do not agree with observed rates whereas including enhanced mass loss yield predicted rates in better agreement with observations. The results suggest that enhanced mass loss must be a ubiquitous property of Classical Cepheids.
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