Aims. We propose the Wind of Fast Rotating Massive Stars scenario to explain the origin of the abundance anomalies observed in globular clusters. Methods. We compute and present models of fast rotating stars with initial masses between 20 and 120 M for an initial metallicity Z = 0.0005 ([Fe/H] −1.5). We discuss the nucleosynthesis in the H-burning core of these objects and present the chemical composition of their ejecta. We consider the impact of uncertainties in the relevant nuclear reaction rates. Results. Fast rotating stars reach critical velocity at the beginning of their evolution and remain near the critical limit during the rest of the main sequence and part of the He-burning phase. As a consequence they lose large amounts of material through a mechanical wind which probably leads to the formation of a slow outflowing disk. The material in this slow wind is enriched in H-burning products and presents abundance patterns similar to the chemical anomalies observed in globular cluster stars. In particular, the C, N, O, Na and Li variations are well reproduced by our model. However the rate of the 24 Mg(p, γ) has to be increased by a factor 1000 around 50 × 10 6 K in order to reproduce the amplitude of the observed Mg-Al anticorrelation. We discuss how the long-lived low-mass stars currently observed in globular clusters could have formed out of the slow wind material ejected by massive stars.

A B S T R A C TThe chemical and spectrophotometric evolution of spiral galaxies is investigated with detailed models, making use of up-to-date ingredients (like metallicity-dependent stellar properties) and a prescription for the star formation rate (SFR) justified both empirically and theoretically. As a first application, the model is used to describe the evolution of the Milky Way. The role of the adopted scheme of disc formation (`inside-out') in shaping the various chemical and colour profiles is investigated, as well as the role of extinction. It is shown that the Solar neighbourhood does not evolve like the Milky Way as a whole and that one-zone models with a non-linear SFR prescription cannot be used to study the evolution of our Galaxy. Our model average SFR is shown to match well observations of external spirals.

We present a comprehensive study of the abundance evolution of the elements from H to U in the Milky Way halo and local disk. We use a consistent chemical evolution model, metallicity dependent isotopic yields from low and intermediate mass stars and yields from massive stars which include, for the first time, the combined effect of metallicity, mass loss and rotation for a large grid of stellar masses and for all stages of stellar evolution. The yields of massive stars are weighted by a metallicity dependent function of the rotational velocities, constrained by observations as to obtain a primary-like 14 N behavior at low metallicity and to avoid overproduction of s-elements at intermediate metallicities. We show that the solar system isotopic composition can be reproduced to better than a factor of two for isotopes up to the Fe-peak, and at the 10% level for most pure s-isotopes, both light ones (resulting from the weak s-process in rotating massive stars) and the heavy ones (resulting from the main s-process in low and intermediate mass stars). We conclude that the light element primary process (LEPP), invoked to explain the apparent abundance deficiency of the s-elements with A< 100, is not necessary. We also reproduce the evolution of the heavy to light s-elements abundance ratio ([hs/ls]) -recently observed in unevolved thin disk stars -as a result of the contribution of rotating massive stars at sub-solar metallicities. We find that those stars produce primary F and dominate its solar abundance and we confirm their role in the observed primary behavior of N. In contrast, we show that their action is insufficient to explain the small observed values of 12 C/ 13 C in halo red giants, which is rather due to internal processes in those stars.

No abstract

Aims. Galactic globular cluster (GC) stars exhibit abundance patterns that are not shared by their field counterparts, e.g. the welldocumented O-Na and Mg-Al anticorrelations. Recent spectroscopic observations of GC turnoff stars have provided compelling evidence that these abundance anomalies were already present in the gas from which the observed stars formed. A widely held hypothesis is that the gas was "polluted" by stars that were more massive (and evolving faster) than the presently observed low-mass stars. In the framework of this "self-enrichment" scenario for GCs, we present a new method of deriving the initial mass function (IMF) of the polluters, by using the O/Na abundance distribution. Methods. We focus on NGC 2808, a GC for which the largest sample of O and Na abundance determinations is presently available. We use the abundance distribution of [O/Na] to derive the amount of polluted material with respect to the original composition. We explore two scenarios in detail for the self-enrichment of the cluster, which differ by the assumptions made on the composition of the polluter ejecta. In each case we consider two classes of possible "culprits": massive asymptotic giant branch (AGB) stars (4−9 M ) and winds of massive stars (WMS) in the mass range 10−100 M . Results. We obtain upper limits for the slope of the IMF (assumed to be given by a power-law) of the stars initially more massive than the present turnoff mass. We also derive lower limits for the amount of stellar residues in NGC 2808. Conclusions. We find that the polluter IMF had to be much flatter than the presently observed IMFs in stellar clusters, which agrees with the results of two other GC IMF determination methods, which we also discuss. Likewise, we find that the present mass of the GC should be totally dominated by stellar remnants if the polluters were AGB stars, which is not the case if the culprits are WMS. We critically analyse the advantages and shortcomings of each potential polluter class and find the WMS scenario more attractive.

The first gamma-ray line originating from outside the solar system that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. Observations in the 1990's with OSSE/CGRO showed that the emission is strongly concentrated towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's INTEGRAL γ-ray observatory allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge/disk luminosity ratio is larger than observed in any other wavelength. This mapping prompted a number of novel explanations, including rather "exotic" ones (e.g. dark matter annihilation). However, conventional astrophysical sources, like type Ia supernovae, microquasars or X-ray binaries, are still plausible candidates for a large fraction of the observed total 511 keV emission of the bulge. A closer study of the subject reveals new layers of complexity, since positrons may propagate far away from their production sites, making it difficult to infer the underlying source distribution from the observed map of 511 keV emission. However, contrary to the rather well understood propagation of high energy (>GeV) particles of Galactic cosmic rays, understanding the propagation of low energy (∼MeV) positrons in the turbulent, magnetized interstellar medium, still remains a formidable challenge. We review the spectral and imaging properties of the observed 511 keV emission and we critically discuss candidate positron sources and models of positron propagation in the Galaxy.

We reassess the applicability of the Toomre criterion in galactic discs and we study the local star formation law in 16 disc galaxies for which abundance gradients are published. The data we use consist of stellar light profiles, atomic and molecular gas (deduced from CO with a metallicity‐dependent conversion factor), star formation rates (from Hα emissivities), metallicities, dispersion velocities and rotation curves. We show that the Toomre criterion applies successfully to the case of the Milky Way disc, but it has limited success with the data of our sample; depending on whether or not the stellar component is included in the stability analysis, we find average values for the threshold ratio of the gas surface density to the critical surface density in the range 0.5–0.7. We also test various star formation laws proposed in the literature, i.e. either the simple Schmidt law or modifications of it, that take into account dynamical factors. We find only small differences among them as far as the overall fit to our data is concerned; in particular, we find that all three star formation laws (with parameters derived from the fits to our data) match observations in the Milky Way disc particularly well. In all cases we find that the exponent n of our best‐fitting star formation rate has slightly higher values than in other recent works and we suggest several reasons that may cause that discrepancy.

No abstract