|The Ages of Young Star Clusters and the Upper Mass Limit of Stars: Analysing Age-dependent Stellar Mass Functions. |
|F.R.N. Schneider, R.G. Izzard, S.E. de Mink, N. Langer, A. Stolte, A. de Koter, V.V. Gvaramadze, B. Hussmann, A. Liermann, H. Sana.|
|Massive stars rapidly change their masses through strong stellar winds and mass transfer in binary systems. The latter aspect is important for populations of massive stars as more than 70% of all O-stars are expected to interact with a binary companion during their lifetime. We show that such mass changes leave characteristic signatures in stellar mass functions of young star clusters which can be used to infer their ages and to identify products of binary evolution. We model the observed present day mass functions of the young Galactic Arches and Quintuplet star clusters using our rapid binary evolution code. We find that shaping of the mass function by stellar wind mass loss allows us to determine the cluster ages to 3.5 ± 0.7 Myr and 4.8 ± 1.1 Myr, respectively. Exploiting the effects of binary mass exchange on the cluster mass function, we find that the most massive stars in both clusters are rejuvenated products of binary mass transfer, i.e. the massive counterpart of classical blue straggler stars. This resolves the problem of an apparent age spread among the most luminous stars exceeding the expected duration of star formation in these clusters. We perform Monte Carlo simulations to probe stochastic sampling, which support the idea of the most massive stars being rejuvenated binary products. We find that the most massive star is expected to be a binary product after 1.0 ± 0.7Myr in Arches and after 1.7 ± 1.0Myr in Quintuplet. Today, the most massive 9 ± 3 stars in Arches and 8 ± 3 in Quintuplet are expected to be such objects. Our findings have strong implications for the stellar upper mass limit and solve the discrepancy between the claimed 150M⨀ limit and observations of fours stars with initial masses of 165-320M⨀ in R136 and of SN 2007bi, which is thought to be a pair-instability supernova from an initial 250M⨀ star. Using the stellar population of R136, we revise the upper mass limit to values in the range 200-500M⨀. || |
Mass functions of stellar clusters including binary stars.
|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 envelopeejected 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.
|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|
|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. |
Schematic view of a binary showing the stellar radius R, Roche-lobe radius RL and dust radius Rd.
|Binary Interaction Dominates the Evolution of Massive Stars|
|Sana, H.; de Mink, S. E.; de Koter, A.; Langer, N.; Evans, C. J.; Gieles, M.; Gosset, E.; Izzard, R. G.; Le Bouquin, J.-B.; Schneider, F. R. N.|
|The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, x-ray binaries, and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. More than 70% of all massive stars will exchange mass with a companion, leading to a binary merger in one-third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae. |
Cumulative number distributions of logarithmic orbital periods (left) and mass ratios (right) for our sample of 71 O-type objects, of which 40 are identified binaries.
|The occurrence of nitrogen-enhanced metal-poor stars: implications for the initial mass function in the early Galactic halo|
|Pols, O. R.; Izzard, R. G.; Stancliffe, R. J.; Glebbeek, E.|
|Most carbon-enhanced metal-poor (CEMP) stars are thought to result from past mass transfer of He-burning material from an asymptotic giant branch (AGB) star to a low-mass companion star, which we now observe as a CEMP star. Because AGB stars of intermediate mass efficiently cycle carbon into nitrogen in their envelopes, the same evolution scenario predicts the existence of a population of nitrogen-enhanced metal-poor (NEMP) stars, with [N/Fe] > 1 and [N/C] > 0.5. Such NEMP stars are rare, although their occurrence depends on metallicity: they appear to be more common at [Fe/H] < - 2.8 by about a factor of 10 compared to less metal-poor stars. We analyse the observed sample of metal-poor stars with measurements of both carbon and nitrogen to derive firm constraints on the occurrence of NEMP stars as a function of metallicity. We compare these constraints to binary population synthesis calculations in which we vary the initial distributions of mass, mass ratio and binary orbital periods. We show that the observed paucity of NEMP stars at [Fe/H] > - 2.8 does not allow for large modifications in the initial mass function, as have been suggested in the literature to account for the high frequency of CEMP stars. The situation at lower metallicity is less clear, and we do not currently have stellar models to perform this comparison for [Fe/H] < -2.8. However, unless intermediate-mass AGB stars behave very differently at such low metallicity, the observed NEMP frequency at [Fe/H] < -2.8 appears incompatible with the top-heavy forms of the initial mass function suggested in the literature. || |
Dependence of the CEMP fraction (upper panel) and the fraction fNEMP of NEMP stars among all CEMP+NEMP stars (lower panel) for our default model A, as a function of the median mass and dispersion for a log-normal IMF.