Abundance anomalies observed in globular cluster stars indicate pollution with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch (AGB) stars and ...massive stars rotating near the break-up limit (spin stars). We propose massive binaries as an alternative source of processed material. We compute the evolution of a 20 ${M}_{\odot}$ star in a close binary considering the effects of non conservative mass and angular momentum transfer and of rotation and tidal interaction to demonstrate the principle. We find that this system sheds about 10 ${M}_{\odot}$ of material, nearly the entire envelope of the primary star. The ejecta are enriched in He, N, Na, and Al and depleted in C and O, similar to the abundance patterns observed in gobular cluster stars. However, Mg is not significantly depleted in the ejecta of this model. In contrast to the fast, radiatively driven winds of massive stars, this material is typically ejected with low velocity. We expect that it remains inside the potential well of a globular cluster and becomes available for the formation or pollution of a second generation of stars. We estimate that the amount of processed low-velocity material ejected by massive binaries is greater than the contribution of AGB stars and spin stars combined, assuming that the majority of massive stars in a proto-globular cluster interact with a companion and return their envelope to the interstellar medium. If we take the possible contribution of intermediate mass stars in binaries into account and assume that the ejecta are diluted with an equal amount of unprocessed material, we find that this scenario can potentially provide enough material to form a second generation of low-mass stars, which is as numerous as the first generation of low-mass stars, without the need to make commonly adopted assumptions, such as preferential loss of the first generation of stars, external pollution of the cluster, or an anomalous initial mass function.
It is thought that Type Ia supernovae (SNe Ia) are explosions of carbon-oxygen white dwarfs (CO WDs). Two main evolutionary channels are proposed for the WD to reach the critical density required for ...a thermonuclear explosion: the single degenerate (SD) scenario, in which a CO WD accretes from a non-degenerate companion, and the double degenerate (DD) scenario, in which two CO WDs merge. However, it remains difficult to reproduce the observed SN Ia rate with these two scenarios. With a binary population synthesis code we study the main evolutionary channels that lead to SNe Ia and we calculate the SN Ia rates and the associated delay-time distributions. We find that the DD channel is the dominant formation channel for the longest delay times. The SD channel with helium-rich donors is the dominant channel at the shortest delay times. Our standard model rate is a factor of five lower than the observed rate in galaxy clusters. We investigate the influence of ill-constrained aspects of single- and binary-star evolution and uncertain initial binary distributions on the rate of Type Ia SNe. These distributions, as well as uncertainties in both helium star evolution and common envelope evolution, have the greatest influence on our calculated rates. Inefficient common envelope evolution increases the relative number of SD explosions such that for αce = 0.2 they dominate the SN Ia rate. Our highest rate is a factor of three less than the galaxy-cluster SN Ia rate, but compatible with the rate determined in a field-galaxy dominated sample. If we assume unlimited accretion onto WDs, to maximize the number of SD explosions, our rate is compatible with the observed galaxy-cluster rate.
Carbon-enhanced metal-poor (CEMP) stars 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. The combination of these two effects widens the range of systems that become CEMP stars towards longer initial orbital periods and lower mass secondary stars. As a consequence 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 carbon-enhanced metal-poor (CEMP) stars constitute approximately one fifth of the metal-poor (${\rm Fe}/{\rm H} \la -2$) population but their origin is not well understood. The most widely ...accepted formation scenario, at least for the majority of CEMP stars which are also enriched in s-process elements, invokes mass-transfer of carbon-rich material from a thermally-pulsing asymptotic giant branch (TPAGB) primary star to a less massive main-sequence companion which is seen today. Recent studies explore the possibility that an initial mass function biased toward intermediate-mass stars is required to reproduce the observed CEMP fraction in stars with metallicity ${\rm Fe}/{\rm H}<-2.5$. These models also implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars which is not seen. In this paper we investigate whether the observed CEMP and NEMP to extremely metal-poor (EMP) ratios can be explained without invoking a change in the initial mass function. We construct binary-star populations in an attempt to reproduce the observed number and chemical abundance patterns of CEMP stars at a metallicity ${\rm Fe}/{\rm H}\sim-2.3$. Our binary-population models include synthetic nucleosynthesis in TPAGB stars and account for mass transfer and other forms of binary interaction. This approach allows us to explore uncertainties in the CEMP-star formation scenario by parameterization of uncertain input physics. In particular, we consider the uncertainty in the physics of third dredge up in the TPAGB primary, binary mass transfer and mixing in the secondary star. We confirm earlier findings that with current detailed TPAGB models, in which third dredge up is limited to stars more massive than about $1.25~M_{\odot}$, the large observed CEMP fraction cannot be accounted for. We find that efficient third dredge up in low-mass (less than $1.25~M_{\odot}$), low-metallicity stars may offer at least a partial explanation for the large observed CEMP fraction while remaining consistent with the small observed NEMP fraction.
Hierarchical triple systems are common among field stars yet their long-term evolution is poorly understood theoretically. In such systems Kozai cycles can be induced in the inner binary system ...during which the inner orbit eccentricity and the inclination between both binary orbits vary periodically. These cycles, combined with tidal friction and gravitational wave emission, can significantly affect the inner binary evolution. To investigate these effects quantitatively we perform a population synthesis study of triple systems and focus on evolutionary paths that lead to mergers of carbon-oxygen (CO) white dwarfs (WDs), which constitute an important candidate progenitor channel for Type Ia supernovae (SNe Ia). We approach this problem by Monte Carlo sampling from observation-based distributions of systems within the primary mass range 1.0-6.5 M and inner orbit semimajor axes a
1 and eccentricities e
1 satisfying a
1(1 − e
2
1) > 12 au, i.e. non-interacting in the absence of a tertiary component. We evolve these systems by means of a newly developed algorithm that couples secular triple dynamics with an existing binary population synthesis code. We find that the tertiary significantly alters the inner binary evolution in about 24 per cent of all sampled systems. In particular, we find several channels leading to CO WD mergers. Amongst these is a novel channel in which a collision occurs in wide inner binary systems as a result of extremely high eccentricities induced by Kozai cycles. With assumptions on which CO WD mergers lead to a SN Ia explosion we estimate the SNe Ia delay time distribution resulting from triples and compare to a binary population synthesis study and to observations. Although we find that our triple rate is low, we have determined a lower limit of the triple-induced SNe Ia rate and further study is needed that includes triples with initially tighter inner orbits.
Carbon-enhanced metal-poor stars, CH stars, barium stars, and extrinsic S stars, among other classes of chemically peculiar stars, are thought to be the products of the interaction of low- and ...intermediate-mass binaries, which occurred when the most evolved star was in the asymptotic giant branch (AGB) phase. Binary evolution models predict that because of the large sizes of AGB stars, if the initial orbital periods of such systems are shorter than a few thousand days, their orbits should have circularised due to tidal effects. However, observations of the progeny of AGB binary stars show that many of these objects have substantial eccentricities, up to e ≈ 0.9. In this work we explore the impact of wind mass transfer on the orbital parameters of AGB binary stars by performing numerical simulations in which the AGB wind is modelled using a hydrodynamical code and the dynamics of the stars is evolved using an N-body code. We find that in most models the effect of wind mass transfer contributes to the circularisation of the orbit, but on longer timescales than tidal circularisation if e ≲ 0.4. For relatively low initial wind velocities and pseudo-synchronisation of the donor star, we find a structure resembling wind Roche-lobe overflow as the stars approach periastron. In this case, the interaction between the gas and the star is stronger than when the initial wind velocity is high and the orbit shrinks while the eccentricity decreases. In one of our models wind interaction is found to pump the eccentricity of the orbit on a similar timescale as tidal circularisation. However, since the orbit of this model is shrinking tidal effects will become stronger during the evolution of the system. Although our study is based on a small sample of models, it offers some insight into the orbital evolution of eccentric binary stars interacting via winds. A larger grid of numerical models for different binary parameters is needed to test if a regime exists where hydrodynamical eccentricity pumping can effectively counteract tidal circularisation, and if this can explain the puzzling eccentricities of the descendants of AGB binaries.
We present analytic formulae that approximate the evolution of stars for a wide range of mass M and metallicity Z. Stellar luminosity, radius and core mass are given as a function of age, M and Z, ...for all phases from the zero-age main sequence up to, and including, the remnant stages. For the most part we find continuous formulae accurate to within 5 per cent of detailed models. These formulae are useful for purposes such as population synthesis that require very rapid but accurate evaluation of stellar properties, and in particular for use in combination with N-body codes. We describe a mass-loss prescription that can be used with these formulae, and investigate the resulting stellar remnant distribution.
AGB stars are responsible for producing a variety of elements, including carbon, nitrogen, and the heavy elements produced in the slow neutron-capture process (selements). There are many ...uncertainties involved in modelling the evolution and nucleosynthesis of AGB stars, and this is especially the case at low metallicity, where most of the stars with high enough masses to enter the AGB have evolved to become white dwarfs and can no longer be observed. The stellar population in the Galactic halo is of low mass and only a few observed stars have evolved beyond the first giant branch. For most of the stars in our sample, we find that an episode of efficient wind mass transfer, combined with strong angular momentum loss, has occurred in the past. The discrepancies are probably caused by missing physical ingredients in our models of AGB nucleosynthesis and they provide indications of how to improve our knowledge of the process of nucleosynthesis in AGB stars.
In the cores of young dense star clusters, repeated stellar collisions involving the same object can occur. It has been suggested that this leads to the formation of an intermediate-mass black hole. ...To verify this scenario we compute the detailed evolution of the merger remnant of three sequences, then follow the evolution until the onset of carbon burning, and estimate the final remnant mass to determine the ultimate fate of a runaway merger sequence. We use a detailed stellar evolution code to follow the evolution of the collision product. At each collision we mix the two colliding stars, accounting for the mass loss during the collision. During the stellar evolution we apply mass-loss rates from the literature, as appropriate for the evolutionary stage of the merger remnant. We computed models for high ($Z = 0.02$) and low ($Z = 0.001$) metallicity to quantify metallicity effects. We find that the merger remnant becomes a Wolf-Rayet star before the end of core hydrogen burning. Mass loss from stellar winds dominates the mass increase due to repeated mergers for all three merger sequences that we consider. In none of our high-metallicity models an intermediate-mass black hole is formed, instead our models have a mass of 10–14 ${M}_\odot$ at the onset of carbon burning. For low metallicity the final remnant is more massive and may explode as a pair-creation supernova. We find that our metal-rich models become inflated as a result of developing an extended low-density envelope. This may increase the probability of further collisions, but self-consistent N-body calculations with detailed evolution of runaway mergers are required to verify this.
Massive stars that lose their hydrogen-rich envelope down to a few tenths of a solar mass explode as extended type IIb supernovae, an intriguing subtype that links the hydrogen-rich type II ...supernovae with the hydrogen-poor type Ib and Ic. The progenitors may be very massive single stars that lose their envelope due to their stellar wind, but mass stripping due to interaction with a companion star in a binary system is currently considered to be the dominant formation channel. Anticipating the upcoming automated transient surveys, we computed an extensive grid of binary models with the Eggleton binary evolution code. We identify the limited range of initial orbital periods and mass ratios required to produce type IIb binary progenitors. The rate we predict from our standard models, which assume conservative mass transfer, is about six times smaller than the current rate indicated by observations. It is larger but still comparable to the rate expected from massive single stars. We evaluate extensively the effect of various assumptions such as the adopted accretion efficiency, the binary fraction and distributions for the initial binary parameters. To recover the observed rate we must generously allow for uncertainties and consider low accretion efficiencies in combination with limited angular momentum loss from the system. Motivated by the claims of detection and non-detection of companions for a few IIb supernovae, we investigate the properties of the secondary star at the moment of explosion. We identify three cases: (1) the companion is predicted to appear as a hot O star in about 90% of the cases, as a result of mass accretion during its main sequence evolution, (2) the companion becomes an over-luminous B star in about 3% of the cases, if mass accretion occurred while crossing the Hertzsprung gap or (3) in systems with very similar initial masses the companion will appear as a K supergiant. The second case, which applies to the well-studied case of SN 1993J and possibly to SN 2001ig, is the least common case and requires that the companion very efficiently accretes the transferred material – in contrast to what is required to recover the overall IIb rate. We note that relative rates quoted above depend on the assumed efficiency of semi-convective mixing: for inefficient semi-convection the presence of blue supergiant companions is expected to be more common, occurring in up to about 40% of the cases. Our study demonstrates that type IIb supernovae have the potential to teach us about the physics of binary interaction and about stellar processes such as internal mixing and possibly stellar-wind mass loss. The fast increasing number of type IIb detections from automated surveys may lead to more solid constraints on these model uncertainties in the near future.
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