Search

Context. Carbon, nitrogen, and oxygen are the most abundant elements throughout the universe, after hydrogen and helium. Studying these elements in low-metallicity stars can provide crucial information on the chemical composition in the early Galaxy and possible internal mixing processes that can alter the surface composition of the stars. Aims. This work aims to investigate the chemical abundance patterns for CNO elements and Li in a homogeneously analyzed sample of 52 metal-poor halo giant stars. From these results, we have been able to determine whether internal mixing processes have taken place in these stars. Methods. We used high-resolution spectra with a high signal-to-noise ratio (S/N) to carry out a spectral synthesis to derive detailed C, N, O, and Li abundances for a sample of stars with metallicities in the range of −3.58 ≤ [Fe/H] ≤ −1.79 dex. Our study was based on the assumption of one-dimensional (1D) local thermodynamic equilibrium (LTE) atmospheres. Results. Based on carbon and nitrogen abundances, we investigated the deep mixing taking place within stars along the red giant branch (RGB). The individual abundances of carbon decrease towards the upper RGB while nitrogen shows an increasing trend, indicating that carbon has been converted into nitrogen. No signatures of ON-cycle processed material were found for the stars in our sample. We computed a set of galactic chemical evolution (GCE) models, implementing different sets of massive star yields, both with and without including the effects of stellar rotation on nucleosynthesis. We confirm that stellar rotation is necessary to explain the highest [N/Fe] and [N/O] ratios observed in unmixed halo stars. The predicted level of N enhancement varies sensibly in dependence of the specific set of yields that are adopted. For stars with stellar parameters similar to those of our sample, heavy elements such as Sr, Y, and Zr appear to have unchanged abundances despite the stellar evolution mixing processes. Conclusions. The unmixed RGB stars provide very useful constraints on chemical evolution models of the Galaxy. As they are more luminous than unevolved (main sequence and turnoff) stars, they also allow for stars to be probed at greater distances. The stellar CN-cycle clearly changes the atmospheric abundances of the lighter elements, but no changes were detected with respect to the heavy elements. [ABSTRACT FROM AUTHOR]

The variations of elemental abundance and their ratios along the Galactocentric radius result from the chemical evolution of the Milky Way discs. The |$\rm ^{12}C/^{13}C$| ratio in particular is often used as a proxy to determine other isotopic ratios, such as |$\rm ^{16}O/^{18}O$| and |$\rm ^{14}N/^{15}N$|⁠. Measurements of |$\rm ^{12}CN$| and |$\rm ^{13}CN$| (or |$\rm C^{15}N$|⁠) – with their optical depths corrected via their hyper-fine structure lines – have traditionally been exploited to constrain the Galactocentric gradients of the CNO isotopic ratios. Such methods typically make several simplifying assumptions (e.g. a filling factor of unity, the Rayleigh–Jeans approximation, and the neglect of the cosmic microwave background) while adopting a single average gas phase. However, these simplifications introduce significant biases to the measured |$\rm ^{12}C/^{13}C$| and |$\rm ^{14}N/^{15}N$|⁠. We demonstrate that exploiting the optically thin satellite lines of 12CN constitutes a more reliable new method to derive |$\rm ^{12}C/^{13}C$| and |$\rm ^{14}N/^{15}N$| from CN isotopologues. We apply this satellite-line method to new IRAM 30-m observations of 12CN, 13CN, and C15N N  = 1 → 0 towards 15 metal-poor molecular clouds in the Galactic outer disc (R gc > 12 kpc), supplemented by data from the literature. After updating their Galactocentric distances, we find that |$\rm ^{12}C/^{13}C$| and |$\rm ^{14}N/^{15}N$| gradients are in good agreement with those derived using independent optically thin molecular tracers, even in regions with the lowest metallicities. We therefore recommend using optically thin tracers for Galactic and extragalactic CNO isotopic measurements, which avoids the biases associated with the traditional method. [ABSTRACT FROM AUTHOR]

We investigate the origin of neutron-capture elements by analysing their abundance patterns and radial gradients in the Galactic thin disc. We adopt a detailed two-infall chemical evolution model for the Milky Way, including state-of-the-art nucleosynthesis prescriptions for neutron-capture elements. We consider r-process nucleosynthesis from merging neutron stars (MNS) and magneto-rotational supernovae (MR-SNe), and s-process synthesis from low- and intermediate-mass stars (LIMS) and rotating massive stars. The predictions of our model are compared with data from the sixth data release of the Gaia -ESO survey, from which we consider 62 open clusters with age ≳ 0.1 Gyr and ∼1300 Milky Way disc field stars. We conclude that: (i) the [Eu/Fe] versus [Fe/H] diagram is reproduced by both prompt and delayed sources, with the prompt source dominating Eu production; (ii) rotation in massive stars significantly contributes to the first peak s-process elements, but MNS and MR-SNe are necessary to match the observations; and (iii) our model slightly underpredicts Mo and Nd, while accurately reproducing the [Pr/Fe] versus [Fe/H] trend. Regarding the radial gradients, we find that: (i) our predicted [Fe/H] gradient slope agrees with observations from Gaia -ESO and other high-resolution spectroscopic surveys; (ii) the predicted [Eu/H] radial gradient slope is steeper than the observed one, regardless of how quick the production of Eu is, prompting discussion on different Galaxy-formation scenarios and stellar radial migration effects; and (iii) elements in the second s-process peak as well as Nd and Pr exhibit a plateau at low-Galactocentric distances, likely due to enhanced enrichment from LIMS in the inner regions. [ABSTRACT FROM AUTHOR]

The 18O/17O abundance ratio is, in principle, a powerful tool to estimate the relative contributions of massive stars and low- to intermediate-mass stars to the chemical enrichment of galaxies. We present 18O/17O ratios derived from simultaneous observations of C18O and C17O 1-0 towards 51 massive star-forming regions with the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. Simultaneous observations of HC18O+ 1-0 and HC17O+ 1-0 with the Yebes 40m telescope towards five sources from this sample were also done to test the consistency of 18O/17O ratios derived from different isotopic pairs. From our improved measurements, resulting in smaller errors than previous work in the literature, we obtain a clear trend of increasing 18O/17O ratio with increasing Galactocentric distance (D GC ), which provides a significant constraint on Galactic chemical evolution (GCE) models. Current GCE models have to be improved in order to explain the observed C18O/C17O 1-0 gradient. [ABSTRACT FROM AUTHOR]

We study the evolution of europium (Eu) and barium (Ba) abundances in Local Group dwarf spheroidal and ultrafaint dwarf galaxies by means of detailed chemical evolution models and compare our results with new sets of homogeneous abundances. The adopted models include gas infall and outflow and have been previously tested. We investigate several production scenarios for r-process elements: merging neutron stars and magnetorotational-driven supernovae. Production of Ba through the main s-process acting in low- and intermediate-mass stars is considered as well. We also test different sets of nucleosynthesis yields. For merging neutron stars we adopt either a constant and short delay time for merging or a delay time distribution function. Our simulations show that (i) if r-process elements are produced only by a quick source, it is possible to reproduce the [Eu/Fe] versus [Fe/H], but those models fail in reproducing the [Ba/Fe] versus [Fe/H]. (ii) If r-process elements are produced only with longer delays the opposite happens. (iii) If both a quick source and a delayed one are adopted, such as magnetorotational-driven supernovae and merging neutron stars with a delay time distribution, the [Eu/Fe] abundance pattern is successfully reproduced, but models still fail in reproducing the [Ba/Fe]. (iv) On the other hand, the characteristic abundances of Reticulum II can be reproduced only if both the Eu and the r-process fraction of Ba are produced on short and constant time delays during a single merging event. We discuss also other possible interpretations, including an inhomogeneous mixing of gas that might characterize this galaxy. [ABSTRACT FROM AUTHOR]

The cosmological 7Li problem consists in explaining why the primordial Li abundance, as predicted by the standard Big Bang nucleosynthesis theory with constraints from WMAP and Planck , is a factor of 3 larger than the Li abundance measured in the stars of the Spite plateau defined by old, warm dwarf stars of the Milky Way halo. Several explanations have been proposed to explain this difference, including various Li depletion processes as well as non-standard Big Bang nucleosynthesis, but the main question remains unanswered. In this paper, we present detailed chemical evolution models for dwarf spheroidal and ultra faint galaxies, compute the galactic evolution of 7Li abundance in these objects, and compare it with observations of similar objects. In our models, Li is mainly produced by novae and cosmic rays, and to a minor extent, by low and intermediate mass stars. We adopt the yield combination that best fits the Li abundances in the Milky Way stars. It is evident that the observations of dwarf objects define a Spite plateau, identical to that observed in the Milky Way, thus suggesting that the Spite plateau could be a universal feature and its meaning should be discussed. The predictions of our models for dwarf galaxies are obtained by assuming as Li primordial abundance either the one detected in the atmospheres of the oldest halo stars (Spite plateau; A(Li) ∼ 2.2 dex), or the one from cosmological observations (WMAP; A(Li) ∼ 2.66 dex). Finally, we discuss the implications of the universality of the Spite plateau results. [ABSTRACT FROM AUTHOR]

Chemical abundances and abundance ratios measured in galaxies provide precious information about the mechanisms, modes, and time-scales of the assembly of cosmic structures. Yet, the nucleogenesis and chemical evolution of elements heavier than helium are dictated mostly by the physics of the stars and the shape of the stellar mass spectrum. In particular, estimates of CNO isotopic abundances in the hot, dusty media of high-redshift starburst galaxies offer a unique glimpse into the shape of the stellar initial mass function (IMF) in extreme environments that cannot be accessed with direct observations (star counts). Underlying uncertainties in stellar evolution and nucleosynthesis theory, however, may hurt our chances of getting a firm grasp of the IMF in these galaxies. In this work, we adopt new yields for massive stars, covering different initial rotational velocities. First, we implement the new yield set in a well-tested chemical evolution model for the Milky Way. The calibrated model is then adapted to the specific case of a prototype submillimetre galaxy (SMG). We show that, if the formation of fast-rotating stars is favoured in the turbulent medium of violently star-forming galaxies irrespective of metallicity, the IMF needs to be skewed towards high-mass stars in order to explain the CNO isotopic ratios observed in SMGs. If, instead, stellar rotation becomes negligible beyond a given metallicity threshold, as is the case for our own Galaxy, there is no need to invoke a top-heavy IMF in starbursts. [ABSTRACT FROM AUTHOR]

We test the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the our Galaxy have been the building blocks of the Galactic halo by comparing their [O/Fe] and [Ba/Fe] vs. [Fe/H] patterns with the ones observed in Galactic halo stars. The [O/Fe] ratio deviates substantially from the observed abundance ratios in the Galactic halo stars for [Fe/H] > -2 dex, while they overlap for lower metallicities. On the other hand, for the neutron capture elements, the discrepancy is extended at all the metallicities, suggesting that the majority of stars in the halo are likely to have been formed in situ. We present the results for a model considering the effects of an enriched gas stripped from dwarf satellites on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the adopted infall time-scale, and the presence of a threshold in the gas for star formation. [ABSTRACT FROM AUTHOR]

We present different chemical evolution models for the ultrafaint dwarf galaxy Boötes I. We either assume that the galaxy accretes its mass through smooth infall of gas of primordial chemical composition (classical models) or adopt mass accretion histories derived from the combination of merger trees with semi-analytical modelling (cosmologically-motivated models). Furthermore, we consider models with and without taking into account inhomogeneous mixing in the interstellar medium within the galaxy, i.e. homogeneous versus inhomogeneous models. The theoretical predictions are then compared to each other and to the body of the available data. From this analysis, we confirm previous findings that Boötes I has formed stars with very low efficiency but, at variance with previous studies, we do not find a clear-cut indication that supernova explosions have sustained long-lasting galactic-scale outflows in this galaxy. Therefore, we suggest that external mechanisms such as ram pressure stripping and tidal stripping are needed to explain the absence of neutral gas in Boötes I today. [ABSTRACT FROM AUTHOR]

ABSTRACT Scaling relations of metallicity (O/H), star formation rate (SFR) and stellar mass ( Mstar) give important insight on galaxy evolution. They are obeyed by most galaxies in the Local Universe and also at high redshift. In a companion paper, we compiled a sample of ∼1100 galaxies from redshift 0 to ≳3, spanning almost two orders of magnitude in metal abundance, a factor of ∼106 in SFR and of ∼105 in stellar mass. We have characterized empirically the star formation 'main sequence' (SFMS) and the mass-metallicity relation (MZR) for this sample, and also identified a class of low-metallicity starbursts, rare locally but more common in the distant Universe. These galaxies deviate significantly from the main scaling relations, with high SFR and low metal content for a given Mstar. In this paper, we model the scaling relations and explain these deviations from them with a set of multi-phase chemical evolution models based on the idea that, independently of redshift, initial physical conditions in a galaxy's evolutionary history can dictate its location in the scaling relations. Our models are able to successfully reproduce the O/H, Mstar and SFR scaling relations up to z ≳ 3, and also successfully predict the molecular cloud fraction as a function of stellar mass. These results suggest that the scaling relations are defined by different modes of star formation: an 'active' starburst mode, more common at high redshift, and a quiescent 'passive' mode that is predominant locally and governs the main trends. [ABSTRACT FROM AUTHOR]

ABSTRACT Most galaxies follow well-defined scaling relations of metallicity (O/H), star formation rate (SFR) and stellar mass ( Mstar). However, low-metallicity starbursts, rare in the Local Universe but more common at high redshift, deviate significantly from these scaling relations. On the 'main sequence' of star formation, these galaxies have high SFR for a given Mstar; and on the mass-metallicity relation, they have excess Mstar for their low metallicity. In this paper, we characterize O/H, Mstar and SFR for these deviant 'low-metallicity starbursts', selected from a sample of ∼1100 galaxies, spanning almost two orders of magnitude in metal abundance, a factor of ∼106 in SFR, and of ∼105 in stellar mass. Our sample includes quiescent star-forming galaxies and blue compact dwarfs at redshift 0, luminous compact galaxies at redshift 0.3, and Lyman break galaxies at redshifts 1-3.4. Applying a principal component analysis (PCA) to the galaxies in our sample with Mstar ≤ 3 × 1010 M⊙ gives a Fundamental Plane (FP) of scaling relations; SFR and stellar mass define the plane itself, and O/H its thickness. The dispersion for our sample in the edge-on view of the plane is 0.17 dex, independently of redshift and including the metal-poor starbursts. The same FP is followed by 55 100 galaxies selected from the Sloan Digital Sky Survey, with a dispersion of 0.06 dex. In a companion paper, we develop multi-phase chemical evolution models that successfully predict the observed scaling relations and the FP; the deviations from the main scaling relations are caused by a different (starburst or 'active') mode of star formation. These scaling relations do not truly evolve, but rather are defined by the different galaxy populations dominant at different cosmological epochs. [ABSTRACT FROM AUTHOR]

ABSTRACT We model the evolution of manganese relative to iron in the progenitor system of the globular cluster Omega Centauri by means of a self-consistent chemical evolution model. We use stellar yields that already reproduce the measurements of [Mn/Fe] versus [Fe/H] in Galactic field disc and halo stars, in Galactic bulge stars and in the Sagittarius dwarf spheroidal galaxy. We compare our model predictions to the Mn abundances measured in a sample of 10 red giant members and six subgiant members of ω Cen. The low values of [Mn/Fe] observed in a few, metal-rich stars of the sample cannot be explained in the framework of our standard, homogeneous chemical evolution model. Introducing cooling flows that selectively bring to the cluster core only the ejecta from specific categories of stars does not help to heal the disagreement with the observations. The capture of field stars does not offer a viable explanation either. The observed spread in the data and the lowest [Mn/Fe] values could, in principle, be understood if the system experienced inhomogeneous chemical evolution. Such an eventuality is qualitatively discussed in this paper. However, more measurements of Mn in ω Cen stars are needed to settle the issue of Mn evolution in this cluster. [ABSTRACT FROM AUTHOR]

The uncertainties which still plague our understanding of the evolution of the light nuclides D, 3He and 4He in the Galaxy are described. Measurements of the local abundance of deuterium range over a factor of 3. The observed dispersion can be reconciled with the predictions on deuterium evolution from standard Galactic chemical evolution models, if the true local abundance of deuterium proves to be high, but not too high, and lower observed values are due to depletion onto dust grains. The nearly constancy of the 3He abundance with both time and position within the Galaxy implies a negligible production of this element in stars, at variance with predictions from standard stellar models which, however, do agree with the (few) measurements of 3He in planetary nebulae. Thermohaline mixing, inhibited by magnetic fields in a small fraction of low-mass stars, could in principle explain the complexity of the overall scenario. However, complete grids of stellar yields taking this mechanism into account are not available for use in chemical evolution models yet. Much effort has been devoted to unravel the origin of the extreme helium-rich stars which seem to inhabit the most massive Galactic globular clusters. Yet, the issue of 4He evolution is far from being fully settled even in the disc of the Milky Way. [ABSTRACT FROM PUBLISHER]

The deuterium abundances inferred from observations of the interstellar medium (ISM) within 1–2 kpc of the Sun range over a factor of 3 and the corresponding oxygen abundances show an even larger dispersion. While the lower D (and O) abundances likely result from depletion on to dust, the higher D abundances are consistent with the big bang nucleosynthesis-predicted primordial D abundance and chemical evolution models of the Galaxy with infall of primordial or nearly primordial material. The large ranges in deuterium and oxygen abundances suggest that the effects of depletion and/or infall have not been homogenized in the local ISM. [ABSTRACT FROM AUTHOR]

Chemical evolution models are presented for the anomalous globular cluster ω Centauri. After demonstrating that the chemical features of ω Cen cannot be reproduced in the framework of the closed-box self-enrichment scenario, we discuss a model in which this cluster is the remnant of a dwarf spheroidal galaxy evolved in isolation and then swallowed by the Milky Way. Both infall of primordial matter and metal-enriched gas outflows have to be considered in order to reproduce the stellar metallicity distribution function, the age–metallicity relation and several abundance ratios. Yet, as long as an ordinary stellar mass function and standard stellar yields are assumed, we fail by far to get the enormous helium enhancement required to explain the blue main sequence (and, perhaps, the extreme horizontal branch) stellar data. Rotating models of massive stars producing stellar winds with large helium excesses at low metallicities have been put forward as promising candidates to solve the ‘helium enigma’ of ω Cen. However, we show that for any reasonable choice of the initial mass function the helium-to-metal enrichment of the integrated stellar population is unavoidably much lower than 70 and conclude that the issue of the helium enhancement in ω Cen still waits for a satisfactory explanation. We briefly speculate upon possible solutions. [ABSTRACT FROM AUTHOR]

Estimates of the interstellar deuterium abundance span a wide range of values. Until recently, it was customary to adopt the abundance of deuterium measured in the Local Bubble as representative of the local one. Now, it is becoming unclear whether the true local deuterium abundance is significantly higher or lower than this value, depending on the interpretation given to current data. It is important to deal with the issue of the deuterium variation and see whether it challenges our current understanding of the Galaxy evolution. To this aim, we study the evolution of deuterium in the framework of successful models for the chemical evolution of the Milky Way able to reproduce the majority of the observational constraints for the solar neighbourhood and for the Galactic disc. We show that, in the framework of our models, the lowest D/H (deuterium-to-oxygen) values observed locally cannot be explained in terms of simple astration processes occurring during the Galaxy evolution. Indeed, the combination of a mild star formation and a continuous infall of unprocessed gas required to fit all the available observational data allows only a modest variation of the deuterium abundance from its primordial value. Therefore, we suggest that depletion of deuterium on to dust grains is the most likely physical mechanism proposed so far to explain the observed dispersion in the local data. [ABSTRACT FROM AUTHOR]

We present one-zone chemical evolution models for two dwarf starburst galaxies, NGC 1705 and NGC 1569. Though especially designed for the inner ∼1 kpc region, where numerous H ii regions and most of the stars are observed, the models also account for the presence of extended gaseous and dark matter haloes, and properly compute the binding energy of the gas heated by supernova explosions. Using information about the past star formation history and initial mass function of the systems previously obtained from Hubble Space Telescope optical and near-infrared colour–magnitude diagrams, we identify possible scenarios of chemical enrichment and development of galactic winds. We assume that the galactic winds are proportional to the Type II and Type Ia supernova rates. As a consequence, they do not necessarily go to zero when the star formation stops. In order not to overestimate the current metallicity of the interstellar gas inferred from H ii region spectroscopy, we suggest that the winds efficiently remove from the galaxies the metal-rich ejecta of dying stars. Conversely, requiring the final mass of neutral gas to match the value inferred from 21-cm observations implies a relatively low efficiency of interstellar medium entrainment in the outflow, thus confirming previous findings that the winds driving the evolution of typical starbursts are differential. These conclusions could be different only if the galaxies accrete huge fractions of unprocessed gas at late times. By assuming standard stellar yields we obtain a good fit to the observed nitrogen-to-oxygen (N/O) ratio of NGC 1569, while the mean N/O ratio in NGC 1705 is overestimated by the models. Reducing the extent of hot bottom burning in low-metallicity intermediate-mass stars does not suffice to solve the problem. Localized self-pollution from stars more massive than in NGC 1705 and/or funnelling of larger fractions of nitrogen through its winds are then left to explain the discrepancy between model predictions and observations. Inspection of the log(N/O) versus log(O/H)+12 diagram for a large sample of dwarf irregular and blue compact dwarf galaxies in the literature favours the latter hypothesis, but the physical mechanisms responsible for such a selective loss of metals remain unclear. [ABSTRACT FROM AUTHOR]

Despite the considerable progress in the theories of stellar evolution and nucleosynthesis, important questions related to the production sites and stellar yields of some of the CNO isotopes still remain unanswered. Single low-, intermediate- and high-mass stars as well as binary stars can produce CNO elements. These elements are then restored into the interstellar medium on different time scales, according to the different nature of the stellar progenitors. In this work, we use a detailed chemical evolution model for the Galactic halo and disc to shed light on the contributions to the abundances of the CNO group nuclei provided by different stellar factories during the whole Galactic lifetime. We suggest that the yields of the secondary isotopes require major revisions. We also show that, if novae actually contribute important amounts of the secondary isotopes 13C, 15N and 17O, as claimed by several authors, the behaviour of the 12C/13C, 14N/15N and 16O/17O ratios in the Milky Way is better explained. In particular, our chemical evolution model results are in agreement with recent claims that novae should be the major sources of 15N in a galaxy, while massive rotating stars should not produce 15N at a galactic level. However, in our opinion current nova yields should be revisited. In particular, it would be worthwhile to study the dependence of nova yields on metallicity. [Copyright &y& Elsevier]

We determined C, N and α-element relative abundances in the gas surrounding six quasi-stellar objects (QSOs) at an average redshift of , by studying six narrow associated absorption systems in Ultraviolet Visual Echelle Spectrograph (UVES) high-resolution spectra. We found five systems with a metallicity (measured by C/H) consistent or above the solar value. The ionization structure observed in the associated systems is clearly different from that of the intervening ones, indicating that the associated systems are influenced by the strong UV flux from the QSO. There is a possible correlation (anticorrelation) between [N/C] ([Si/C]) and [C/H] of the studied associated systems, and when . We have compared these observational results with the predictions of a model simulating the joint evolution of QSOs and their spheroidal hosts. The agreement turns out to be very good; in particular, the case envisaging massive haloes and high star formation rates recovers both the correlation between [N/C] and [C/H] and the anticorrelation for [Si/C] versus [C/H]. Narrow associated absorption systems prove to be powerful tracers of the chemical abundances in gas belonging to high-redshift spheroidal galaxies. The outflow of this same gas, triggered by the QSO feedback, is probably going to contribute to the early enrichment of the surrounding intergalactic medium. A larger sample, possibly increasing the number of ionization stages, chemical elements and the redshift range, would allow us to put firm constraints on detailed chemical evolution models of galaxies at high redshifts. [ABSTRACT FROM AUTHOR]

The recent determination of the baryon-to-photon ratio from Wilkinson Microwave Anisotropy Probe ( WMAP) data by Spergel et al. allows one to fix, with unprecedented precision, the primordial abundances of the light elements D, 3He, 4He and 7Li in the framework of the standard model of big bang nucleosynthesis. We adopt these primordial abundances and discuss the implications for Galactic chemical evolution, stellar evolution and nucleosynthesis of the light elements. The model predictions on D, 3He and 4He are in excellent agreement with the available data, while a significant depletion of 7Li in low-metallicity stars is required to reproduce the Spite plateau. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper we study the role played both by novae and single stars in enriching the interstellar medium of the Galaxy with CNO group nuclei, in the framework of a detailed successful model for the chemical evolution of both the Galactic halo and disc. First, we consider only the nucleosynthesis from single low-mass, intermediate-mass and massive stars. In particular, the nucleosynthesis prescriptions in the framework of the adopted model are such that (i) low- and intermediate-mass stars are responsible for the production of most of the Galactic [sup 12]C and [sup 14]N; (ii) massive stars produce the bulk of the Galactic [sup 16]O; (iii) [sup 13]C and [sup 17]O originate mostly in intermediate-mass stars, with only a minor contribution from low-mass and massive stars. In this context, we show that the behaviour of the [sup 12]C/[sup 13]C, [sup 14]N/[sup 15]N and [sup 16]O/[sup 17]O isotopic ratios, as inferred from observations, can be explained only allowing for a substantial revision of the available stellar yields. On the other hand, the introduction of nova nucleosynthesis allows us to better explain the temporal evolution of the CNO isotopic ratios in the solar neighbourhood as well as their trends across the Galactic disc. Once all the nucleosynthesis sources of CNO elements are taken into account, we conclude that [sup 13]C, [sup 15]N and [sup 17]O are likely to have both a primary and a secondary origin, in contrast to previous beliefs. We show that, when adopting the most recent [sup 17]O yields from intermediate-mass stars published in the literature so far, we still get a too large solar abundance for this element, a problem already encountered in the past by other authors using different yield sets. Therefore, we conclude that in computing the [sup 17]O yields from intermediate-mass stars some considerable sink of [sup 17]O is probably neglected. The situation for [sup 15]N is less clear than that for [sup 13]C and [sup 17]O, mainly due to contradictory observational findings. However, a stellar factory restoring [sup 15]N on quite long time-scales seems to be needed in order to reproduce the observed positive gradient of [sup 14]N/[sup 15]N across the disc, and novae are, at present, the best candidates for this factory. Given the uncertainties still present in the computation of theoretical stellar yields, our results can be used to put constraints on stellar evolution and nucleosynthesis models. [ABSTRACT FROM AUTHOR]

ABSTRACT We discuss the evolution of oxygen, carbon and nitrogen in galaxies of different morphological type by adopting detailed chemical evolution models with different star formation histories (continuous star formation or starbursts). In all the models detailed nucleosynthesis prescriptions from supernovae of all types and low- and intermediate-mass stars are taken into account. We start by computing chemical evolution models for the Milky Way with different stellar nucleosynthesis prescriptions. Then, a comparison between model results and ‘key’ observational constraints allows us to choose the best set of stellar yields. Once the best set of yields is identified for the Milky Way, we apply the same nucleosynthesis prescriptions to other spirals (in particular M101) and dwarf irregular galaxies. We compare our model predictions with the [C,N,O/Fe] versus [Fe/H], log(C/O) versus 12 + log(O/H), log(N/O) versus 12+ log(O/H) and [C/O] versus [Fe/H] relations observed in the solar vicinity and draw the following conclusions. (i) There is no need to invoke strong stellar winds in massive stars in order to explain the evolution of the C/O ratio, as often claimed in the literature. (ii) The predicted [O/Fe] ratio as a function of metallicity is in very good agreement with the most recent data available for the solar vicinity, especially for halo stars. This fact again suggests that the oxygen stellar yields in massive stars computed by either Woosley & Weaver or Thielemann, Nomoto & Hashimoto without taking into account mass loss, reproduce the observations well. (iii) We predict that the gap observed in the [Fe/O] versus [O/H] at [O/H]∼-0.3 dex should be observed also in C/O versus O/H. The existence of such a gap is predicted by our model for the Milky Way and is caused by a halt in the star formation between the end of the thick disc and the beginning of the thin disc phase. Such a halt is produced by the adopted threshold gas density for the star formation rate. (iv) This threshold is also responsible for the prediction of a very slow chemical enrichment between the time of formation of the solar system (4.5 Gyr ago) and the present time, in agreement with new abundance measurements. (v) The chemical evolution models for dwarf irregulars and spirals, adopting the same nucleosynthesis prescriptions of the best model for the solar neighbourhood, well reproduce the available constraints for these objects. (vi) By taking into account the results obtained for all the studied galaxies (Milky Way, M101, dwarf galaxies and DLAs) we conclude that there is no need for claiming a strong primary component of N produced in massive stars (M > 10 M[sub ⊙]) . (vii) Moreover, there is a strong indication that C and N are mainly produced in low- and intermediate-mass stars, at variance with recent suggestions that most of the C should come from massive stars. In particular, intermediate-mass stars with masses between 4 and 8 M[sub ⊙] contribute mostly to N (both primary and secondary) whereas those with masses between 1 and 3 M[sub ⊙] contribute mostly to C. At the same time, our results suggest that the C yields computed for massive stars, without taking into account stellar rotation, are underestimated and should be at least a factor of 3 larger than the current values. [ABSTRACT FROM AUTHOR]

In this contribution we discuss the origin of the extreme helium-rich stars which inhabit the blue main sequence (bMS) of the Galactic globular cluster Omega Centauri. In a scenario where the cluster is the surviving remnant of a dwarf galaxy ingested by the Milky Way many Gyr ago, the peculiar chemical composition of the bMS stars can be naturally explained by considering the effects of strong differential galactic winds, which develop owing to multiple supernova explosions in a shallow potential well. [ABSTRACT FROM PUBLISHER]

The globular cluster ω Cen is an outstanding object in terms of both its chemical and kinematic properties. Its large mass, spread in element abundances, chemical and kinematical segregations, as well as its peculiar orbit, all suggest that it is the surviving remnant of a larger system. In this contribution we deal with the chemical evolution of ω Cen in the framework of a model where it is the remnant of a dwarf spheroidal galaxy evolved in isolation and then swallowed and partially disrupted by the Milky Way. Both infall of primordial matter and metal-enriched gas outflows are necessary in order to reproduce the observed stellar metallicity distribution function, age-metallicity relation and several abundance ratios. Yet, as long as an ordinary stellar initial mass function and standard stellar yields are adopted, we fail by far to get the enormous helium enhancement required to explain the double main sequence of ω Cen. [ABSTRACT FROM PUBLISHER]

Despite the many studies on stellar nucleosynthesis published so far, the scenario for the production of copper in stars remains elusive. In particular, it is still debated whether copper originates mostly in massive stars or in Type Ia supernovae. To answer this question, we compute self-consistent chemical evolution models taking into account the results of updated stellar nucleosynthesis. By contrasting copper evolution in ω Cen and the Milky Way, we end up with a picture where massive stars are the major factor responsible for the production of copper in ω Cen as well as the Galactic disc. [ABSTRACT FROM AUTHOR]

The article reports that in the paper "Chemical Abundances in QSO Host Galaxies and Environments From Narrow Absorption Line Systems" that was published in "Monthly Notices of Royal Astronomical Society" 351, 976-88 (2004), the addresses of two of the authors were corrupted.