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 asymptotic giant branch star R Sculptoris is surrounded by a detached shell of dust and gas 1,2 . The shell originates from a thermal pulse during which the star undergoes a brief period of increased mass loss 3,4 . It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse − parameters that determine the lifetime on the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3". What was hitherto thought to be only a thin, spherical shell with a clumpy structure, is revealed to contain a spiral structure. Spiral structures associated with circumstellar envelopes have been seen previously, from which it was concluded that the systems must be binaries 5,6,7,8 . Using the data, combined with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse ≈1800 years ago, lasting ≈200 years. About 3×10 !3 M ! of mass was ejected at a velocity of 14.3 km s −1 and at a rate ≈30 times higher than the prepulse mass-loss rate. This shows that ≈3 times more mass is returned to the interstellar medium during and immediately after a pulse than previously thought. The detached shell around R Sculptoris was observed in CO(J = 3 − 2) emission at 345 GHz using the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 0 operations (Fig.1, and supplementary information). The data clearly show the well-centered detached shell with a radius of 18.5", and reveal a spiral structure extending from the central star outwards to the shell. Previous observations of R Sculptoris show structure in the form of clumps. However, this was interpreted as clumpy material within the shell itself, and not as a structure interior to the shell 2 . Until now no clear signs of binary companions have been observed in the detached shell sources (with a possible exception for the detached shell around TT Cyg 9 ). The observed structure around R Sculptoris, however, indicates the presence of a companion, shaping the stellar wind into a spiral shell structure 8 . Smoothed particle hydrodynamics (SPH) models show that a wide binary companion can have a significant effect in the shaping of the wind, leading to elliptical and spiral structures (e.g. as observed in the case of the envelope of AFGL 3068) 5,6 . The observed shapes of the circumstellar envelopes (CSEs) around binary AGB stars depend on the physical parameters of the binary system (e.g., separation and mass ratio 10 ), the density contrasts imprinted on the wind, the temperatures in the CSE, the viewing angle, and, in the case of the gas, the chemistry and excitation 11 . The temporal variations of the mass-loss-rate and the expansion velocity further affect the structure of the CSE. Hence, the observed spiral structure and detached shell allow us to measure these important properties, and to directly link them to th...

Abstract. Detached, symbiotic binaries are generally assumed to interact via Bondi-Hoyle-Littleton (BHL) wind accretion. However, the accretion rates and outflow geometries that result from this mass-transfer mechanism cannot adequately explain the observations of the nearest and best studied symbiotic binary, Mira, or the formation of some post-AGB binaries, e.g. barium stars. We propose a new mass-transfer mode for Mira-type binaries, which we call 'wind Roche-lobe overflow' (WRLOF), and which we demonstrate with 3D hydrodynamic simulations. Importantly, we show that the circumstellar outflows which result from WRLOF tend to be highly aspherical and strongly focused towards the binary orbital plane. Furthermore, the subsequent mass-transfer rates are at least an order of magnitude greater than the analogous BHL values. We discuss the implications of these results for the shaping of bipolar (proto)-planetary nebulae and other related systems.

Interstellar bubbles around O stars are driven by a combination of the star's wind and ionizing radiation output. The wind contribution is uncertain because the boundary between the wind and interstellar medium is difficult to observe. Mid-infrared observations (e.g., of the H ii region RCW 120) show arcs of dust emission around O stars, contained well within the H ii region bubble. These arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by density gradients and/or stellar motion. We present twodimensional, radiation-hydrodynamics simulations investigating the evolution of wind bubbles and H ii regions around massive stars moving through a dense (n H = 3000 cm −3 ), uniform medium with velocities ranging from 4 to 16 km s −1 . The H ii region morphology is strongly affected by stellar motion, as expected, but the wind bubble is also very aspherical from birth, even for the lowest space velocity considered. Wind bubbles do not fill their H ii regions (we find filling factors of 10-20 per cent), at least for a main sequence star with mass M ∼ 30 M . Furthermore, even for supersonic velocities the wind bow shock does not significantly trap the ionization front. X-ray emission from the wind bubble is soft, faint, and comes mainly from the turbulent mixing layer between the wind bubble and the H ii region. The wind bubble radiates <1 per cent of its energy in X-rays; it loses most of its energy by turbulent mixing with cooler photoionized gas. Comparison of the simulations with the H ii region RCW 120 shows that its dynamical age is 0.4 Myr and that stellar motion 4 km s −1 is allowed, implying that the ionizing source is unlikely to be a runaway star but more likely formed in situ. The region's youth, and apparent isolation from other O or B stars, makes it very interesting for studies of massive star formation and of initial mass functions.

We have mapped the 12 CO(3-2) line emission around the Mira AB system at 0. 5 resolution using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO map shows amazing complexity. The circumstellar gas has been shaped by different dynamical actors during the evolution of the system, and several morphological components can be identified. The companion is marginally resolved in continuum emission and is currently at 0. 487 ± 0. 006 separation. In the main line component, centered on the stellar velocity, spiral arcs around Mira A are found. The spiral appears to be relatively flat and oriented in the orbital plane. An accretion wake behind the companion is clearly visible, and the projected arc separation is about 5 . In the blue wing of the line emission, offset from the main line, several large (∼5-10 ) opposing arcs are found. We tentatively suggest that this structure is created by the wind of Mira B blowing a bubble in the expanding envelope of Mira A.

Betelgeuse, a nearby red supergiant, is a runaway star with a powerful stellar wind that drives a bow shock into its surroundings 1-4 . This picture has been challenged by the discovery of a dense and almost static shell 5 that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse's wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind 6 and forms an almost static, photoionization-confined shell. Other red supergiants should have significantly more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova lightcurve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction.Red supergiants are massive stars near the end of their lives, and are direct progenitors of core-collapse supernovae 7, 8 . They evolve from O-and B-type stars (hot, luminous sources of ionizing photons), and so these stars are often found together, within or near star clusters 9 . As a result, the cool stellar winds of red supergiants are often photoionized by external radiation fields [10][11][12][13] . To calculate the radiation hydrodynamics of a photoionized red supergiant wind, we simplify the problem by assuming spherical symmetry. We use an approximate two-temperature equation of state for the gas, for which both the neutral and photoionized gases are isothermal with 1 temperatures T = T n and T i ≫ T n , respectively. The ionized and neutral isothermal sound speeds similarly satisfy a i ≫ a n . The photoionized part of the red supergiant wind is accelerated as a result of ionization heating 14 , whereas the neutral part is decelerated 6 if the wind speed through the ionization front, v n , satisfies v n ≤ 2a i .The resulting flow is depicted in Fig. 1. The outermost layer is the interface where the wind meets the interstellar medium. For static stars this is a spherical, detached shell, and for stars moving supersonically it is a bow shock. A photoionization-confined shell -a dense, shocked layer separating the neutral inner wind from the ionized outer wind -forms closer to the star. We identify this with the recently-discovered shell in Betelgeuse's circumstellar medium 5 .The properties of the photoionization-confined shell are calculated analytically and verified with simulations in Methods. Its outer boundary, R IF , is calculated following previous work 10 (Extended Data Fig. 1), and the standing shock radius, R shell , is obtained by requiring pressur...

Recent interpretations of narrow, variable absorption lines detected in some Type Ia supernovae suggest that their progenitors are surrounded by dense, circumstellar material. Similar variations detected in the symbiotic recurrent nova system RS Oph, which undergoes thermonuclear outbursts every 20 years, making it an ideal candidate to investigate the origin of these lines. To this end, we present simulations of multiple mass transfer-nova cycles in RS Oph. We find that the quiescent mass transfer produces a dense, equatorial outflow, i.e., concentrated towards the binary orbital plane, and an accretion disc forms around the white dwarf. The interaction of a spherical nova outburst with these aspherical circumstellar structures produces a bipolar outflow, similar to that seen in HST imaging of the 2006 outburst. In order to produce an ionization structure that is consistent with observations, a mass-loss rate of 5 × 10 −7 M ⊙ yr −1 from the red giant is required. The simulations also produce a polar accretion flow, which may explain the broad wings of the quiescent H line and hard X-rays. By comparing simulated absorption line profiles to observations of the 2006 outburst, we are able to determine which components arise in the wind and which are due to the novae. We explore the possible behaviour of absorption line profiles as they may appear should a supernova occur in a system like RS Oph. Our models show similarities to supernovae like SN 2006X, but require a high mass-loss rate,Ṁ ∼ 10 −6 to 10 −5 M ⊙ yr −1 , to explain the variability in SN 2006X.

Betelgeuse, the bright, cool red supergiant in Orion, is moving supersonically relative to the local interstellar medium. The star emits a powerful stellar wind that collides with this medium, forming a cometary structure, a bow shock, pointing in the direction of motion. We present the first 3D hydrodynamic simulations of the formation and evolution of Betelgeuse's bow shock. The models include realistic low-temperature cooling and cover a range of plausible interstellar medium densities of 0.3-1.9 cm −3 and stellar velocities of 28-73 km s −1 . We show that the flow dynamics and morphology of the bow shock differ substantially because of the growth of Rayleigh-Taylor or Kelvin-Helmholtz instabilities. The former dominate the models with slow stellar velocities resulting in a clumpy bow shock substructure, whereas the latter produce a smoother, more layered substructure in the fast models. If the mass in the bow shock shell is low, as seems to be implied by the AKARI luminosities (∼3 × 10 −3 M ), then Betelgeuse's bow shock is very young and is unlikely to have reached a steady state. The circular nature of the bow shock shell is consistent with this conclusion. Thus, our results suggest that Betelgeuse only entered the red supergiant phase recently.