Low-mass AGB Stars Research Articles

Stellar modelling: the AGB zoo

The Asymptotic Giant Branch phase of low and intermediate mass stars (1.2 < M/M⊙ < 6.0) synthesizes nearly half of the heavy elements (A > 56) in the Universe. This evolutionary phase represents an ideal laboratory to test our knowledge of stellar microphysics and macrophysics. In fact, the nucleosynthesis occurring in AGB stars sensibly depends on the adopted physical inputs and on the assumptions in the treatment of physical phenomena like convection, mass loss and rotation. In this contribution we illustrate the characteristics of low mass (1.5 < M/M⊙ < 3.0) AGB models at different metallicities (− 2.2 < [Fe/H] < 0.2). In particular, we highlight the effects induced by the use of proper C-enhanced low temperature opacities and the importance of the treatment of the envelope radiative/convective interface on the formation of the 13C-pocket (the main neutron source). We briefly describe main characteristics of the FRUITY database on AGB stars nucleosynthesis, created by directly coupling a full nuclear network with evolutionary models. Finally, we discuss the effects that the inclusion of rotation has on the s-process pattern in low-mass AGB stars.

CANDELS: THE EVOLUTION OF GALAXY REST-FRAME ULTRAVIOLET COLORS FROMz= 8 TO 4

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 < z < 8. We use new wide-field near-infrared data in GOODS-S from the CANDELS, HUDF09 and ERS programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z > 3.5, including 113 at z = 7 to 8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. The median value of beta evolves significantly from -1.82 (+0.00,-0.04) at z = 4, to -2.37 (+0.26,-0.06) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68 (+0.39,-0.24) (~ -2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require "exotic" stellar populations to explain their colors. This evolution can be explained by an increase in dust extinction, with the timescale consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2-sigma) correlation between beta and M_UV when measuring M_UV at a consistent rest-frame wavelength of 1500 A. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z > 7. Thus the stellar-mass - metallicity relation, previously observed up to z ~ 3, may extend out to z = 7 - 8.

Principal component analysis on chemical abundances spaces

In preparation for the HERMES chemical tagging survey of about a million Galactic FGK stars, we estimate the number of independent dimensions of the space defined by the stellar chemical element abundances [X/Fe]. [...] We explore abundances in several environments, including solar neighbourhood thin/thick disk stars, halo metal-poor stars, globular clusters, open clusters, the Large Magellanic Cloud and the Fornax dwarf spheroidal galaxy. [...] We find that, especially at low metallicity, the production of r-process elements is likely to be associated with the production of alpha-elements. This may support the core-collapse supernovae as the r-process site. We also verify the over-abundances of light s-process elements at low metallicity, and find that the relative contribution decreases at higher metallicity, which suggests that this lighter elements primary process may be associated with massive stars. [...] Our analysis reveals two types of core-collapse supernovae: one produces mainly alpha-elements, the other produces both alpha-elements and Fe-peak elements with a large enhancement of heavy Fe-peak elements which may be the contribution from hypernovae. [...] The extra contribution from low mass AGB stars at high metallicity compensates the dimension loss due to the homogenization of the core-collapse supernovae ejecta. [...] the number of independent dimensions of the [X/Fe]+[Fe/H] chemical space in the solar neighbourhood for HERMES is about 8 to 9. Comparing fainter galaxies and the solar neighbourhood, we find that the chemical space for fainter galaxies such as Fornax and the Large Magellanic Cloud has a higher dimensionality. This is consistent with the slower star formation history of fainter galaxies. [...]

S-PROCESSING IN THE GALACTIC DISK. I. SUPER-SOLAR ABUNDANCES OF Y, Zr, La, AND Ce IN YOUNG OPEN CLUSTERS

In a recent study, based on homogeneous barium abundance measurements in open clusters, a trend of increasing [Ba/Fe] ratios for decreasing cluster age was reported. We present here further abundance determinations, relative to four other elements hav- ing important s-process contributions, with the aim of investigating whether the growth found for [Ba/Fe] is or not indicative of a general property, shared also by the other heavy elements formed by slow neutron captures. In particular, we derived abundances for yttrium, zirconium, lanthanum and cerium, using equivalent widths measurements and the MOOG code. Our sample includes 19 open clusters of different ages, for which the spectra were obtained at the ESO VLT telescope, using the UVES spectrometer. The growth previously suggested for Ba is confirmed for all the elements analyzed in our study. This fact implies significant changes in our views of the Galactic chemical evolution for elements beyond iron. Our results necessarily require that very low-mass AGB stars (M < 1.5M\odot) produce larger amounts of s-process elements (hence acti- vate the 13 C-neutron source more effectively) than previously expected. Their role in producing neutron-rich elements in the Galactic disk has been so far underestimated and their evolution and neutron-capture nucleosynthesis should now be reconsidered.

A detailed look at chemical abundances in the Magellanic Clouds

AbstractWe determine elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe) using direct methods and a large set of observed ions, minimizing the need for ionization correction factors. In contrast to prior studies, we find a clear separation between Type I and non-Type I PNe in these low-metallicity environments, and no evidence that the O-N nucleosynthesis cycle is active in low-mass progenitors. We find that the S/O abundance ratio is anomalously low compared to H ii regions, confirming the “sulfur anomaly” found for Galactic PNe. We also found that Ne/O is elevated in some cases, raising the possibility that Ne yields in low-mass AGB stars may be enhanced at low metallicity.

Thesprocess: Nuclear physics, stellar models, and observations

Nucleosynthesis in the s process takes place in the He burning layers of low mass AGB stars and during the He and C burning phases of massive stars. The s process contributes about half of the element abundances between Cu and Bi in solar system material. Depending on stellar mass and metallicity the resulting s-abundance patterns exhibit characteristic features, which provide comprehensive information for our understanding of the stellar life cycle and for the chemical evolution of galaxies. The rapidly growing body of detailed abundance observations, in particular for AGB and post-AGB stars, for objects in binary systems, and for the very faint metal-poor population represents exciting challenges and constraints for stellar model calculations. Based on updated and improved nuclear physics data for the s-process reaction network, current models are aiming at ab initio solution for the stellar physics related to convection and mixing processes. Progress in the intimately related areas of observations, nuclear and atomic physics, and stellar modeling is reviewed and the corresponding interplay is illustrated by the general abundance patterns of the elements beyond iron and by the effect of sensitive branching points along the s-process path. The strong variations of the s-process efficiency with metallicity bear also interesting consequences for Galactic chemical evolution.

A holistic approach to carbon-enhanced metal-poor stars

By considering the various CEMP subclasses separately, we try to derive, from the specific signatures imprinted on the abundances, parameters (such as metallicity, mass, temperature, and neutron source) characterizing AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion.To put CEMP stars in a broad context, we collect abundances for about 180 stars of various metallicities, luminosity classes, and abundance patterns, from our own sample and from literature. First, we show that there are CEMP stars which share the properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This strongly points at the operation of the 13C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s- and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2nd and 3rd s-process peaks bears instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs stars exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. Finally, we argue that most CEMP-no stars (with no overabundances for the neutron-capture elements) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2). This trend is not predicted by any of the current AGB models.

Stellar sources of the short-lived radionuclides in the early solar system

We discuss the possible stellar sources of short-lived radionuclides (SLRs) known to have been present in the early solar system ( 26Al, 36Cl, 41Ca, 53Mn, 60Fe, 107Pd, 129I, 182Hf, 244Pu). SLRs produced primarily by irradiation ( 7Be, 10Be) are not discussed in this paper. We evaluate the role of the galactic background in explaining the inventory of SLRs in the early solar system. We review the nucleosynthetic processes that produce the different SLRs and place the processes in the context of stellar evolution of stars from 1 to 120 M ⊙ . The ejection of newly synthesized SLRs from these stars is also discussed. We then examine the extent to which each stellar source can, by itself, explain the relative abundances of the different SLRs in the early solar system, and the probability that each source would have been in the right place at the right time to provide the SLRs. We conclude that intermediate-mass AGB stars and massive stars in the range from ∼20 to ∼60 M ⊙ are the most plausible sources. Low-mass AGB stars fail to produce enough 60Fe. Core-collapse Type II supernovae from stars with initial masses of <20 M ⊙ produce too much 60Fe and 53Mn. Sources such as novae, Type Ia supernovae, and core-collapse supernovae of O–Ne–Mg white dwarfs do not appear to provide the SLRs in the correct proportions. However, intermediate-mass AGB stars cannot provide 53Mn or the r-process elements, so if an AGB star provided the 41Ca, 36Cl, 26Al, 60Fe, and 107Pd, and if a late stellar source is required for 53Mn and the r-process elements, then two types of sources would be required. A separate discussion of the production of r-process elements highlights the difficulties in modeling their production. There appear to be two sources of r-process elements, one that produces the heavy r-process elements, including the actinides, and one that produces the elements from N to Ge and the elements ∼110 < A < ∼130. These can be assigned to SNII explosions of stars of ⩽11 M ⊙ and stars of 12–25 M ⊙ , respectively. More-massive stars, which leave black holes as supernova remnants, apparently do not produce r-process elements.

EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES

The envelope of thermally pulsing AGB stars undergoing periodic third dredge-up episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, alpha and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 Msun), with metallicity ranging between Z=0.0138 (the solar one) and Z=0.0001, are presented. Main features are: i) a full nuclear network (from H to Bi) coupled to the stellar evolution code, ii) a mass loss-period-luminosity relation, based on available data for long period variables, and ii) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the third dredge up. For each model a detailed description of the physical and chemical evolution is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin 13C pocket, which forms in the core-envelope transition region after each third dredge up episode. The formation of this 13C pockets is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good.

FLUORINE IN ASYMPTOTIC GIANT BRANCH CARBON STARS REVISITED

A reanalysis of the fluorine abundance in three Galactic AGB carbon stars (TX Psc, AQ Sgr and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 $\mu$m. High-resolution (R$\sim 50000$) and high signal to noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon rich stars. Using spectral synthesis in LTE we derive for these stars fluorine abundances that are systematically lower by $\sim 0.8$ dex in average with respect to the sole previous estimates by Jorissen, Smith & Lambert (1992). The possible reasons of this discrepancy are explored. We conclude that the difference may rely on the blending with C-bearing molecules (CN and C$_2$) that were not properly taken into account in the former study. The new F abundances are in better agreement with the prediction of full network stellar models of low mass AGB stars. These models also reproduce the $s$-process elements distribution in the sampled stars. This result, if confirmed in a larger sample of AGB stars, might alleviate the current difficulty to explain the largest [F/O] ratios found by Jorissen et al. In particular, it may not be necessary to search for alternative nuclear chains affecting the production of F in AGB stars.

Mass Fraction of 13C-Pocket in Metal-Poor AGB Stars and the Primary Nature of Neutron Source

Chemical abundances of very metal-poor s-rich stars contain excellent information to set new constraints on models of neutron-capture processes at low metallicity. Using the parametric approach based on the radiative s-process nucleosynthesis model, we obtain the mass fraction q of13 C-pocket, the overlap factor r, the neutron exposure per interpulse Δτ, and the component coefficients of the s-process and the r-process for 25 s-rich stars, respectively. We find that q deduced for the lead stars is comparable to the overlap factor r, which is larger than the standard case (hereafter ST case) of the AGB model (q ˜ 0.05) about 10 times, and Δτ are about 10 times smaller than the ST case (Δτ = 7.0 mbarn−1). Although the two parameters obtained for the lead stars are very different from the ST case of the AGB stellar model, it is worth noting that the total amounts of 13C in metal-poor condition are close to the ST case. The above relation is a significant evidence for the primary nature of the neutron source and the lead stars could be polluted by low-mass AGB stars. Because interpulse period declines with increasing stellar mass, for high-mass AGB star, the neutron irradiation may be terminated due to their shorter interpulse period. Thus the neutron exposure per interpulse of the larger AGB stars should be about 10 times smaller than the ST case. In this case, the primary nature of the neutron source also exists.

Exploring the nucleosynthesis region of metal-poor Stars

AbstractThe chemical abundances of the very metal poor double-enhanced stars are excellent information to set new constraints on models of neutron-capture processes at low metallicity. There have been many theoretical studies of s-process nucleosynthesis in low-mass AGB stars. Using the parametric approach based on the radiative s-process nucleosynthesis model, we calculate the following five parameters for a series of metal-poor stars. They are: the mass fraction of 13C pocket q, the overlap factor r, the neutron exposure per interpulse Δτ, and the component coefficients that correspond to relative contribution from the s-process and the r-process. We find that the mass fraction of 13C pocket q deduced for the Pb stars is comparable to the overlap factor r, which is about 10 times larger than normal AGB model; q ~ 0.05; and the neutron exposure per interpulse Δτ for all Pb stars are about 10 times smaller than the ST case (Δτ ~ 7.0mb−1). Although the two fundamental parameters Δτ and q obtained for the Pb stars are very different from the AGB stellar model, the results of the larger value of q and the smaller value of Δτ can also explain the abundance distribution of the Pb stars. This suggest that the q change to larger than that of normal AGB model. Then, this factor will result in the descent of the density of 13C in the nuclear synthesis region directly. So, the neutron exposure Δτ will also decrease to the same extent. Although the neutron number density in the larger initial mass AGB stars (m &gt; 3M⊙) is high, the neutron irradiation time is shorter, obviously the neutron exposure per interpulse in the AGB stars should be smaller. It is noteworthy that the total amount of 13C in metal poor condition is close to the ST case, which is consistent with the primary nature of the neutron source.

Chemical analysis of carbon stars in the Local Group

Aims. We present new results of our ongoing chemical study of carbon stars in Local Group galaxies to test the critical dependence of s-process nucleosynthesis on the stellar metallicity. Methods. We collected optical spectra with the VLT/UVES instrument of two carbon stars found in the Carina Dwarf Spheroidal (dSph) galaxy, namely ALW-C6 and ALW-C7. We performed a full chemical analysis using the new generation of hydrostatic, spherically symmetric carbon-rich model atmospheres and the spectral synthesis method in LTE. Results. The luminosities, atmosphere parameters and chemical composition of ALW-C6 and ALW-C7 are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature (external pollution in a binary stellar system) cannot be definitively excluded. Our chemical analysis shows that the metallicity of both stars agree with the average metallicity ([Fe/H] ∼− 1.8 dex) previously derived for this satellite galaxy from the analysis of both low resolution spectra of RGB stars and the observed colour magnitude diagrams. ALW-C6 and ALW-C7 present strong s-element enhancements, [s/Fe] =+ 1.6, +1.5, respectively. These enhancements and the derived s-process indexes [ls/Fe], [hs/Fe] and [hs/ls] are compatible with theoretical s-process nucleosynthesis predictions in low mass AGB stars (∼1.5 M� ) on the basis that the 13 C(α,n) 16 O is the main source of neutrons. Furthermore, the analysis of C2 and CN bands reveals a large carbon enhancement (C/O ∼ 7 and 5, respectively), much larger than the values typically found in galactic AGB carbon stars (C/O ∼ 1−2). This is also in agreement with the theoretical prediction that AGB carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. However, theoretical low-mass AGB models apparently fail to simultaneously fit the observed s-element and carbon enhancements. On the other hand, Zr is found to be less enhanced in ALW-C7 compared to the other elements belonging to the same s-peak. Although the abundance errors are large, the fact that in this star the abundance of Ti (which has a similar condensation temperature to Zr) seems also to be lower than those of others metals, may indicate the existence of some depletion into dust-grains in its photosphere.

The s process in massive stars

In the past decades considerable progress has been made in the understanding of the various processes responsible for the nucleosynthesis of the elements in stars. This holds especially for the main s process in low-mass AGB stars, which is largely responsible for the production of about half of the elemental abundances in the mass range 90 ≤ A ≤ 209 . The weak s process, which produces elements with A ≤ 90 , however, is much less understood. A better characterization of the weak s component would help to disentangle the various processes contributing to element production in this region. In this respect, more accurate neutron capture cross sections in the mass range 56 ≤ A ≤ 90 are indispensable. Also, neutron captures on abundant light elements with A < 56 play an important role, since they act as neutron poisons and affect the stellar neutron balance. In this context, the impact of new results for neutron capture cross sections on light and medium mass nuclei is discussed.

A Mechanism for the Equilibrium Growth of Mineral Crystals in AGB Stars and Red Giants on 105yr Timescales

We show that for particle sizes ranging from a few hundred angstroms up to several tens of microns in diameter the force exerted by radiation pressure in some red giant and AGB stars exceeds the force of gravity and thus offers the potential for graphite, SiC, corundum, and spinel grains to grow to the size range observed in primitive meteorites (e.g., up to ~25 μm). In the highest mass AGB stars radiation pressure on growing grains greatly exceeds the force of gravity and thus ejects a grain from the star before it can grow larger than a few tens of nanometers. Only in very low mass AGB stars (less than 3 M☉) does the radiative force balance the gravitational force to such a fine degree that the net acceleration on individual particles ranging from a few nanometers up to about 25 μm produces particle velocities that are comparable to atmospheric turbulence. Our analysis shows that the large graphite, SiC, corundum, and spinel crystals found in primitive meteorites can only have formed in the atmospheres of the lowest mass red giant and AGB stars, where particle growth is able to occur on timescales of a hundred thousand years under near-equilibrium conditions. We note that this suggestion is contrary to the standard assumption that grains can only form in stellar winds and implies that there may be a class of grains that can form in chemical equilibrium deep within the stellar atmosphere, just above the photosphere.

Stellar nucleosynthetic contribution of extinct short-lived nuclei in the early solar system and the associated isotopic effects

Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short-lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short-lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf-Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low-mass AGB stars imply large isotopic anomalies in Ca-Al-rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short-lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high-injection mass cut is a possible scenario. There is a possibility of the contribution of short-lived nuclides by a 1.5–3 M⊙ AGB star as it implies the smallest shift in stable isotopes. A low-mass AGB star of relatively low metallicity would be even a better source of short-lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf-Rayet stars that evolved to supernovae Ib/c were involved.

The origin of the lead-rich stars in the Galactic halo: investigation of model parameters for the s-process

Several stars at the low-metallicity extreme of the Galactic halo show large spreads of lead and associated 'heavy' s-process elements ([Pb/hs]). Theoretically, an s-process pattern should be obtained from an AGB star with a fixed metallicity and initial mass. For the third dredge-up and the s-process model, several important properties depend primarily on the core mass of AGB stars. Zijlstra reported that the initial-to-final mass relation steepens at low metallicity, due to low mass-loss efficiency. This might affect the model parameters of the AGB stars, e.g. the overlap factor and the neutron irradiation time, in particular at low metallicity. The calculated results do indeed show that the overlap factor and the neutron irradiation time are significantly small at low metallicities, especially for 3.0 M ⊙ AGB stars. The scatter of [Pb/hs] found in low metallicities can therefore be explained naturally when varying the initial mass of the low-mass AGB stars.

The chemical evolution of barium and europium in the Milky Way

We compute the evolution of the abundances of barium and europium in the Milky Way and we compare our results with the observed abundances from the recent UVES Large Program "First Stars". We use a chemical evolution model which already reproduces the majority of observational constraints. We confirm that barium is a neutron capture element mainly produced in the low mass AGB stars during the thermal-pulsing phase by the 13C neutron source, in a slow neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H] as well as the Ba solar abundance, we suggest that Ba should be also produced as an r-process element by massive stars in the range 10-30 solar masses. On the other hand, europium should be only an r-process element produced in the same range of masses (10-30 solar masses), at variance with previous suggestions indicating a smaller mass range for the Eu producers. As it is well known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low metallicities, although smaller in the newest data. With our model we estimate for both elements (Ba and Eu) the ranges for the r-process yields from massive stars which better reproduce the trend of the data. We find that with the same yields which are able to explain the observed trends, the large spread in the [Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an inhomogeneous models for the chemical evolution of our Galaxy. We therefore derive the amount by which the yields should be modified to fully account for the observed spread. We then discuss several possibilities to explain the size of the spread. We finally suggest that the production ratio of [Ba/Eu] could be almost constant in the massive stars.

The evolution of barium and europium in local dwarf spheroidal galaxies

By means of a detailed chemical evolution model, we follow the evolution of barium and europium in four Local Group Dwarf Spheroidal Galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present day total mass and gas mass content of these galaxies, adopts up to date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that barium is a neutron capture element produced in low mass AGB stars by s-process but also in massive stars (in the mass range 10 - 30 M⊙) by r-process, during the explosive event of supernovae of type II, and that europium is a pure r-process element synthesized in massive stars also in the range of masses 10 - 30 M⊙, we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation efficiencies ( = 0.005 - 0.5 Gyr −1 ) and by the intense galactic winds (6-13 times the star formation rate) in the evolution of these galaxies. These low star formation efficiencies (compared to the one for the Milky Way disc) adopted for the Dwarf Spheroidal Galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available.

S-process nucleosynthesis in low mass AGB Stars: do we really need an improved determination of the 13C( α, n) 16O reaction rate?

Thermally pulsing Asymptotic Giant Branch stars are responsible for the nucleosynthesis of the main component of the cosmic s-elements. The most important neutron source is the 13 C( α , n ) 16 O reaction. Owing to the presence of a subthreshold resonance, the low energy extrapolation is a rather complex task. The rate quoted in the literature differ up to a factor of 4 at typical stellar energies. The latest improvements in computer power allows us to calculate the evolution of TP-AGB stars coupled with a full nuclear network, extending from hydrogen to lead. Here we discuss the effects of the variation of the 13 C( α , n ) 16 O rate on the predicted neutron capture nucleosynthesis.