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The main neutron source for the slow neutron capture process in low mass Asymptotic Giant Branch stars is the 13C(α,n)16O reaction. This reaction is responsible for the production of half of the natural heavy elements in the Universe. Up to now, no direct measurements have reached the energy region of interest for astrophysics, the so called Gamow window, which lies between 140 and 230 keV in the center of mass. In this paper we describe the experiment carried out at the LUNA experiment at the Laboratori Nazionali del Gran Sasso and present first preliminary results.

In the past, the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the so-called Asymptotic Giant Branch (or AGB) could not be explained by self-consistent models, forcing researchers to extensive parameterisations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding to the formation of 13C and then the activation of the 13C(α,n)16O reaction. In the last decade, some physically-based mixing mechanisms have been considered to solve this problem. Nowadays, a big step forward in s-process studies would be to understand what is among the suggested ones the physical model better accounts for the observational constrains. In this paper we analyse a model where the 13C forms as a feedback of MHD processes in the stellar plasma. We compare results of nucleosynthesis models for low mass AGB stars (M13C reservoir. We find that n-captures driven by magnetically-induced mixing can well account for the SiC data and that this is due to the fact that our 13C distribution fullfills the above constraints rather accurately. We show comparisons between model predictions and measurements for isotopes of Sr, Zr, Ba, Mo and Ru as representative examples of light and heavy s-elements.

Barium stars are now primaries in a binary system with a former asymptotic giant branch (AGB) star. Here we compare some available AGB nucleosynthesis models and the observed s-process abundances of individual Ba star measurements to constrain the nature of the s-process in low mass AGB stars. After correcting the models with a dilution factor calculated for [Ce/Fe], we found that some of the sample stars show higher abundances for light s-process elements than the model predictions. This might be attributed to diffusive mixing in the stars.

By means of a population synthesis code and by constructing a simple toy model of dust produced by asymptotic giant branch (AGB) stars, common envelope (CE) ejecta and binary merger ejecta, we estimate the dust product rates (DPRs) of these processes in the Milky Way. The total DPR from AGB stars is ~ 6.7 × 10 −4 M ⊙ yr −1 , in which about 73% of dust grains are carbon, 24% are silicates and 3% are iron. The total DPR from CE ejecta is ~ 4.2 × 10 −4 M ⊙ yr −1 , in which about 83% of dust grains are silicates, about 12% are carbon and 5% are iron. The DPR from binary merger ejecta is less than 1/3 that from AGB stars or CE ejecta, and it could even be negligible under certain circumstances. Therefore, compared with AGB stars and CE ejecta, the contribution of dust produced by binary merger ejecta to total dust grains in the Milky Way is smaller or can be negligible.

Two properties of Messier 13 are pertinent to the study of mass loss among metal-poor stars and the chemical evolution of globular clusters: (1) an extended blue horizontal branch, which seems to demand mass loss from red giant progenitor stars and possibly an enhanced helium abundance, and (2) the presence of internal abundance inhomogeneities of elements in the mass range from C to Al. A popular explanation for this second phenomenon is that M13 was self-enriched by intermediate-mass asymptotic giant branch (IM-AGB) stars of a type that may also have been able to instigate helium enrichment. Spectra of the λ10830 absorption feature produced by He I have been obtained by using the NIRSPEC spectrometer on the Keck 2 telescope for seven red giants in M13 chosen to have a range in λ3883 CN band strengths, oxygen, and sodium abundances. Whereas these spectra do reveal the presence of fast winds among some M13 red giants, they provide little support for helium abundance differences of the type that might have been generated by a burst of IM-AGB star activity within the M13 protocluster.

Using a non-local and time-dependent theory of convection, we have calculated the linear non-adiabatic oscillations of the radial and low-degree F-p39 modes for evolutionary models from the main sequence to the asymptotic giant branch for stars with solar abundance (X = 0. 7 0, Z = 0. 0 2) in the mass range of 0.6-3.0 M circle dot. The results show that low luminosity cool stars tend to be solar-like oscillators, whose low-order modes are stable, but intermediate and high order p-modes are pulsationally unstable; their unstable modes have a wide range in frequency and small values for amplitude growth rates. For stars with increasing luminosity and therefore lower temperature, the unstable modes shift towards lower orders, the corresponding range of frequency decreases, and the amplitude growth rate increases. High luminosity red giant stars behave like typical Mira-like oscillators. The effects of the coupling between convection and oscillations on pulsational instability have been carefully analyzed in this work. Our research shows that convection does not simply act as a damping mechanism for oscillations, and the complex nature of the coupling between convection and oscillations makes turbulent convection sometimes behave as damping, and sometimes as excitation. Such a picture can not only naturally account for the red edge of the instability strip, but also the solar-like oscillations in low luminosity red stars and Mira-like ones in high luminosity red giants.

In the WISE all-sky source catalogue there are 76 million mid-infrared (MIR) point sources that were detected at the first three WISE bands and have association with only one 2MASS near-IR source within 3 arcsec. We search for their identifications in the SIMBAD database and find 3.2 million identified sources. Based on these known sources, we establish three criteria for selecting candidate AGB stars in the Galaxy, which are three defined occupation zones in a color-color diagram, Galactic latitude |gb|< 20 deg, and "corrected" WISE third-band W3c < 11. Applying these criteria to the WISE+2MASS sources, 1.37 million of them are selected. We analyze the WISE third-band W3 distribution of the selected sources, and further establish that W3 < 8 is required in order to exclude a large fraction of normal stars in them. We therefore find 0.47 million candidate AGB stars in our Galaxy from the WISE source catalogue. Using W3c, we estimate their distances and derive their Galactic distributions. The candidates are generally located around the Galactic center uniformly, with 68% (1-sigma) of them within approximately 8 kpc. We discuss that optical spectroscopy can be used to verify the C-rich AGB stars in our candidates, and they will be good targets for the LAMOST survey that is planned to start from fall of 2012., 11 pages, 6 figures, accepted for publication in Research in Astron. and Astrophys

Asymptotic giant branch (AGB) stars are a main site of production of nuclei heavier than iron via the s process. In massive (>4 M⊙) AGB stars the operation of the 22 Ne neutron source appears to be confirmed by observations of high Rb enhancements, while the lack of Tc in these stars rules out 13 C as a main source of neutrons. The problem is that the Rb enhancements are not accompanied by Zr enhancements, as expected by s-process models. This discrepancy may be solved via a better understanding of the complex atmospheres of AGB stars. Second- generation stars in globular clusters (GCs), on the other hand, do not show enhancements in any s-process elements, not even Rb. If massive AGB stars are responsible for the composition of these GC stars, they may have evolved differently in GCs than in the field. In AGB stars of lower masses, 13 C is the main source of neutrons and we can potentially constrain the effects of rotation and proton-ingestion episodes using the observed composition of post-AGB stars and of stardust SiC grains. Furthermore, independent asteroseismology observations of the rotational velocities of the cores of red giants and of white dwarves will play a fundamental role in helping us to better constrain the effect of rotation. Observations of carbon-enhanced metal-poor stars enriched in both Ba and Eu may require a neutron flux in-between the s and the r process, while the puzzling increase of Ba as function of the age in open clusters, not accompanied by increase in any other element heavier than iron, require further observational efforts. Finally, stardust SiC provides us high-precision constraints to test nuclear inputs such as neutron-capture cross sections of stable and unstable isotopes and the impact of excited nuclear states in stellar environments.

In this article, a magnetohydrodynamic (MHD) model is presented that is aimed at understanding the anisotropic mass loss by an AGB star and subsequent bipolar planetary nebula formation. The main challenge in developing such a model was to simultaneously treat the dynamo engine in the core region of the AGB progenitor, magnetic buoyancy, meridional circulation, and gas ejection in a self-consistent manner. A previous article did not incorporate the dynamo and the feedback of the produced magnetic field on differential rotation.

The presence of abundance inhomogeneities in the CNO elements in globular clusters indicates that these objects sustained extended eras of star formation combined with chemical enrichment. Nonetheless, most globular clusters in the Milky Way are homogeneous in iron-peak elements and the heavier alpha elements. If globular clusters were sites of chemical evolution, then it seems to have occurred at constant iron-peak abundance. This might imply that only nonsupernova stars contributed to the chemical evolution. In this article another scenario is explored, one in which supernovae did participate, but in which the chemical evolution occurred in such a way as to maintain a constant iron-peak abundance, while at the same time establishing CNO and related element inhomogeneities. In order to keep the [Fe/H] abundance homogeneous, a scenario is studied that requires the net protocluster mass (gas plus stars) to be specifically related to the number of supernovae that occur. By comparing models from the literature for element production in metal-poor supernovae and intermediate-mass asymptotic giant branch (IM-AGB) stars, it is found that iron homogeneity plus a pattern of CNO-Na abundance inhomogeneities that reasonably match Milky Way globular clusters can be obtained by combining very low mass supernovae (~10 M⊙) with relatively high mass IM-AGB stars. Nucleosynthesis within metal-poor stars in the mass range of 7-9 M⊙ could prove useful to understanding the abundances of globular cluster stars.

The identification of individual stars in crowded environments using photometric information alone is confounded by source confusion. However, with the addition of spectroscopic information it is possible to distinguish between blends and areas where the light is dominated by a single star, using the widths of absorption features. We describe a procedure for identifying locations in kinematically hot environments where the light is dominated by a single star, and apply this method to spectra with 0.1'' angular resolution covering the 2.1-2.3 μm interval in the central regions of M32. Targets for detailed investigation are selected as areas of localized brightness enhancement. Three locations where at least 60% of the K-band light comes from a single bright star, and another with light that is dominated by two stars with very different velocities, are identified. The dominant stars are evolving near the tip of the asymptotic giant branch (AGB), and have M5 III spectral type. The lack of a dispersion in spectral type suggests that the upper AGB within the central arcsecond of M32 has a dispersion in J - K of only a few hundredths of a magnitude, in agreement with what is seen at larger radii. One star has weaker atomic absorption lines than the others, such that [M/H] is 0.2 dex lower. Such a difference in metallicity is consistent with the metallicity dispersion inferred from the photometric width of the AGB in M32. The use of line width to distinguish between blends involving many relatively faint stars, none of which dominate the light output, and areas that are dominated by a single intrinsically bright star could be extended to crowded environments in other nearby galaxies.

The thermonuclear 19F(p, )16O reaction rate in the temperature region 0.007–10 GK has been derived by re-evaluating the available experimental data, together with the low-energy theoretical R-matrix extrapolations. Our new rate deviates by up to about 30% compared to the previous results, although all rates are consistent within the uncertainties. At very low temperature (e.g. 0.01 GK) our reaction rate is about 20% lower than the most recently published rate, because of a difference in the low energy extrapolated S-factor and a more accurate estimate of the reduced mass used in the calculation of the reaction rate. At temperatures above ~1 GK, our rate is lower, for instance, by about 20% around 1.75 GK, because we have re-evaluated the previous data (Isoya et al., Nucl. Phys. 7, 116 (1958)) in a meticulous way. The present interpretation is supported by the direct experimental data. The uncertainties of the present evaluated rate are estimated to be about 20% in the temperature region below 0.2 GK, and are mainly caused by the lack of low-energy experimental data and the large uncertainties in the existing data. Asymptotic giant branch (AGB) stars evolve at temperatures below 0.2 GK, where the 19F(p, )16O reaction may play a very important role. However, the current accuracy of the reaction rate is insufficient to help to describe, in a careful way, the fluorine over-abundances observed in AGB stars. Precise cross section (or S factor) data in the low energy region are therefore needed for astrophysical nucleosynthesis studies.

Asymptotic giant branch (AGB) stars of intermediate mass have been implicated in the origin of abundance inhomogeneities within globular clusters because of their ability to synthesize elements such as nitrogen and sodium. Such stars are also thought to be sites for the synthesis of s-process elements. Abundance data from the literature are used to study the behavior of sodium relative to the s-process element barium and the r-process element europium in a sample of 14 globular clusters. These all show large intracluster scatter in [Na/Fe], however, most have minimal internal dispersion in barium and europium abundances beyond what can be attributed to observational measurement error. Upon subdividing each cluster into samples of Na-enhanced and Na-normal stars, it is found that differences in their mean [Ba/Fe] and [Eu/Fe] abundances rarely exceed 0.1 dex. It is concluded that whatever stars were responsible for enriching the high-Na red giants in globular clusters were not, by and large, substantial producers of s or r-process elements. Thermally pulsating AGB stars of mass greater than 5 M⊙ may satisfy this constraint. There are a few clusters in which the available data do provide evidence of either barium or europium inhomogeneities, such as M15, NGC 6752, M4, and NGC 3201. However, there is no consistency in regard to whether Ba or Eu enhancements correlate with sodium abundance. At least some of the stars that contributed to the element inhomogeneities within these clusters may have formed in an external environment, such that there was decoupling between the masses of the enriching stars and the time at which their ejecta became incorporated into a cluster.

In order to explain the lack of carbon stars in the Galactic bulge, we have made a detailed study of thermal pulse-asymptotic giant branch (TP-AGB) stars by using a population synthesis code. The effects of the oxygen overabundance and the mass loss rate on the ratio of the number of carbon stars to that of oxygen stars in the Galactic bulge are discussed. We find that the oxygen overabundance which is about twice as large as that in the solar neighbourhood (close to the present observations) is insufficient to explain the rareness of carbon stars in the bulge. We suggest that the large mass loss rate may serve as a controlling factor in the ratio of the number of carbon stars to that of oxygen stars.

Based on the s-process nucleosynthesis model with the 13C(α,n)16O reaction occurring under radiative conditions in the interpulse phases, we investigate the characteristics of the distribution of neutron exposure in low-mass Asymptotic Giant Branch (AGB) stars. We introduce a new concept, the distribution of neutron exposures of the Galaxy (NEG), to study the chemical evolution characteristics of the Galaxy for s-process elements. Using a chemical evolution model of the Galaxy, we develop a model for the NEG and obtain the evolution results of the NEG in different epochs. The present results appear to reasonably reproduce the distribution of neutron exposures of the solar system (hereafter NES). The main component and the strong component in the NES are built up in different epochs. The strong component of the s-process is mainly synthesised in the low-mass and metal-poor AGB stars, and the main component is produced by the s-process in the low-mass AGB stars with higher metallicities.

Correlations between the presence of the 6707 A line of lithium and the resonance lines of technetium (4238 and 4262 A) in a large sample of Galactic S stars are analyzed. Half of the sample stars are intrinsic S stars (those exhibiting technetium in their spectra), and 1/3 of these stars also have strong lithium lines in their spectra. Stars having both lithium and technetium in their spectra are interpreted as intermediate-mass thermally pulsating asymptotic giant branch (TP-AGB) stars in which lithium is produced by the Cameron-Fowler mechanism. The production of lithium is predicted to occur in high-luminosity (Mbol ≤ −6) TP-AGB stars by the hot-bottom burning (HBB) mechanism. Data on the carbon isotope ratios of stars in our sample agree with the predictions of HBB; however, oxygen isotope ratios in these stars do not agree with the predictions of HBB. Furthermore, the available luminosities for our sample stars are below the minimum value necessary for HBB to occur in available models. Cool-bottom processing (CBP) is one possible explanation for the presence of lithium in the spectra of these stars. Intrinsic S stars having technetium but no lithium in their spectra are interpreted as lower mass (1.5-3 M⊙) thermally pulsating AGB stars that have not undergone CBP. Extrinsic S stars constitute the remaining half of the sample. Carbon and oxygen isotope ratios, as well as the lack of technetium and lithium in the spectra of these stars, are consistent with these being low-mass red giant branch stars (1-2 M⊙), with mass transfer from a now extinct thermally pulsating AGB star being responsible for the enhanced abundance of s-process elements.

Keck Observatory LRIS-B (Low Resolution Imaging Spectrometer) spectra are reported for six red giant stars in the Draco dwarf spheroidal galaxy and several comparison giants in the globular cluster M13. Indexes that quantify the strengths of the Ca ii H and K lines, the l3883 and l4215 CN bands, and the l4300 G band have been measured. These data confirm evidence of metallicity inhomogeneity within Draco obtained by previous authors. The four brightest giants in the sample have absolute magnitudes in the range . They have a range in color, metallicities less than that of M13, and exhibit a dispersion 2.6 ! M ! 2.3 B V V in both calcium and CH indices. Our data indicate that a spread in (C/H) likely exists among the Draco tip giants and that some giants have higher (C/Fe) ratios than is typical of giants in the globular clusters M13 and M92. Several suggestions are made as to why some Draco stars may have higher (C/Fe) ratios than globular cluster red giants: deep mixing might be inhibited in these Draco stars, they may formerly have been mass-transfer binaries that acquired carbon from a more massive companion, or the Draco dwarf galaxy may have experienced relatively slow chemical evolution over a period of several billion years, allowing carbon-enhanced ejecta from intermediate-mass asymptotic giant branch stars to enrich the interstellar medium while star formation was still occurring.

A carbon-oxygen white dwarf may explode in a Type Ia supernova by accreting matter from its companion via either Roche lobe overflow or from winds, but there exists a critical accretion rate of the progenitor system for the explosion. We study the relation between the critical accretion rate and the metallicity via an AGB star approach. The result indicates that the critical accretion rate depends not only on the hydrogen mass fraction and the white dwarf mass, but also on the metallicity. The effect of the metallicity is smaller than that of the white dwarf mass. We show that it is reasonable to use the model with stellar mass 1.6 M-. for real white dwarfs.

V725 Sgr is a unique pulsating variable star, first recognized by H. Swope, which increased its period from 12 to 21 days between 1926 and 1935. It was then neglected for three decades. We have used visual and photoelectric photometry of V725 Sgr from 1985 to 2004 to study its recent variability, using light‐curve, Fourier, least‐squares, and self‐correlation analyses. Whereas the period was 12 days in 1926, it is now about 90 days. The range is typically a magnitude, in V. Its spectrum and variability, which earlier were those of a Cepheid, are now those of a red semiregular variable. The star's behavior is consistent with a thermal flash and blue loop in the H‐R diagram, from the asymptotic giant branch to the Cepheid instability strip and back again.

We have obtained deep HST imaging of two candidate dwarf systems in the nearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surface brightness systems, but with likely very different star formation histories based on their mean colors and color fluctuations. The color-magnitude diagrams of these galaxies have been used to estimate their distances using the tip of the red giant branch (RGB), and their mean metallicities from the colors of the RGB. For DDO 44 we find a distance of D = 3.01 ± 0.18 Mpc—confirming it to be a member of the M81 Group—and a mean metallicity of [Fe/H] = -1.54 ± 0.14. We also find from the properties and numbers of a population of luminous asymptotic giant branch (AGB) stars that about 20% of the luminous population of DDO 44 consists of "intermediate-age" stars (with ages between about 2 and 8 Gyr) that give rise to the observed AGB. There is no difference in the spatial distribution of the RGB and AGB stars in this galaxy. For UGC 4998 we derive a metallicity of [Fe/H] = -1.58 ± 0.21 and a distance of D = 8.24 ± 0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group, confirming results from previous measurements. We find this galaxy to contain a population of young (< 50 Myr old) stars, and there is evidence of older RGB stars. The young stars are considerably more centrally concentrated than the older stars. The current star formation rate in UGC 4998 is ~9 × 10-4 M⊙ yr–1 over the entire galaxy, or ~5 × 10-4 M⊙ yr–1 kpc–2. Given its H i mass (1 × 107 M⊙), this galaxy can continue to form stars at this rate for another ~11 Gyr.

An increasing amount of literature reports the detection of magnetic fields in asymptotic giant branch (AGB) stars and in central stars of planetary nebulae (PNs). These detections lead to claims that the magnetic fields are the main agent shaping the PNs. In this paper, I examine the energy and angular momentum carried by magnetic fields expelled from AGB stars, as well as other physical phenomena that accompany the presence of large scale fields, such as those claimed in the literature. I show that a single star cannot supply the energy and angular momentum if the magnetic fields have the large coherent structure required to shape the circumstellar wind. Therefore, the structure of non-spherical planetary nebulae cannot be attributed to dynamically important large scale magnetic fields. I conclude that the observed magnetic fields around evolved stars can be understood by locally enhanced magnetic loops which can have a secondary role in the shaping of the PN. The primary role, I argue, rests with the presence of a companion., PASP, 2006, in press. (This paper was rejected by MNRAS and ApJ; my criticism of the referee reports are in: Soker, N. astro-ph/0508525.)

The globular cluster Messier 13 (NGC 6205) has been shown by a number of different studies to be chemically inhomogeneous with regard to elements ranging from C through Al. A database of λ3883 CN-band indices, carbon, oxygen, and sodium abundances for red giants in M13 has been compiled and homogenized from a variety of literature sources. The data are used to document the distribution of CN band strength among both red giant branch (RGB) and asymptotic giant branch (AGB) stars brighter than MV = +0.8, as well as the relationships between the CN inhomogeneities and the dispersions in other elements. The CN distribution among the M13 RGB stars is bimodal, although a few intermediate-CN red giants are present in the cluster. The AGB stars show uniformly weak CN bands. The spread in CN band strength, once empirically corrected for effective temperature and gravity differences among the red giants, is very well correlated with the sodium abundance and is anticorrelated with oxygen. Above MV = +0.8, the carbon abundance of red giants decreases with increasing luminosity, as first found by N. Suntzeff. There is a rather modest anticorrelation between CN band strength and [C/Fe] among RGB stars of comparable magnitude. The abundance patterns within M13 can be interpreted as a primordial (or pre-RGB phase) abundance spread, coupled with the later effects of deep mixing within the red giants. In the CN-weak giants and those CN-strong stars with [O/Fe] ≈ -0.2 ± 0.2 dex on the upper RGB, deep mixing seems to have been limited largely to the dredge-up of C → N-processed material. By contrast, there are some M13 stars, most notably a group of very oxygen-deficient giants near the RGB tip, but perhaps also a fraction of CN-strong giants with lesser oxygen depletions, in which some measure of O → N-processed material also appears to have been brought to the surface.

The chemical classification of IRAS 17515-2407 has been debated for a long time. Up to now there are two contenders, oxygen-rich or carbon-rich. We believe that IRAS 17515-2407 is an oxygen-rich source: because (i) it shows the silicate self-absorbed emission; (ii) in the near infrared-IRAS diagram it is located in the oxygen-rich object region and (iii) particularly, it has detected SiO maser emission.

The 13C(α, n)16O reaction is believed to be the main neutron source reaction for the s-process in asymptotic giant branch (AGB) stars. The astrophysical S-factors of this reaction have been determined based on an evaluation of the α spectroscopic factor of the 1/2+ subthreshold state in 17O (Ex = 6.356 MeV) by using the 13C(11B, 7Li)17O α transfer reaction. Our result confirms that the 1/2+ subthreshold resonance is dominant for the 13C(α, n)16O reaction at low energies of astrophysical interest.

The heavy elements formed by neutron capture processes have an interesting history from which we can extract useful clues to and constraints upon both the characteristics of the processes themselves and the star formation and nucleosynthesis history of Galactic matter. Of particular interest in this regard are the heavy element compositions of extremely metal-deficient stars. At metallicities [Fe/H] = 130-140 have been found (in non carbon-rich stars) to be pure r-process products. The identification of an environment provided by massive stars and associated Type II supernovae as an r-process site seems compelling. Increasing levels of heavy s-process (e.g., barium) enrichment with increasing metallicity, evident in the abundances of more metal-rich halo stars and disk stars, reflect the delayed contributions from the low- and intermediate-mass (M \~ 1-3 Msol) stars that provide the site for the main s-process nucleosynthesis component during the AGB phase of their evolution. New abundance data in the mass region 60, Comment: 47 pages, 2 tables, 11 figures; To appear in PASP

Measurements of the l3883 CN band absorption strength in spectra of red giant members of the globular cluster M3 have been compiled from several sources in the literature, including the pioneering survey of N. B. Suntzeff, and transformed onto a common system. The homogenized indices give a picture of the behavior of the CN band strength over a range in stellar magnitude of ∼3.6 mag, from the tip of the red giant branch (RGB) to . As with other well-studied globular clusters, the red giant branch stars of M3 exhibit M ∼ 1.0 V a bimodal distribution of CN band strengths that extends to the limit of the available survey. The asymptotic giant branch stars also show evidence of an intrinsic spread in CN content. Anticorrelations exist between the CN band strength and both carbon and oxygen abundances, suggesting that the high nitrogen abundances of the CN-strong giants have derived from a conversion of C and/or O into nitrogen, either within the cluster stars themselves or in some primordial site. Among both CN-strong and CN-weak stars, the (C/Fe) abundance decreases with increasing luminosity on the RGB. Stellar atmosphere models indicate that in the magnitude range , the C # N abundance product is roughly constant among the CN-weak giants, while it decreases 1.5 ! M ! 0 V toward the upper RGB among CN-strong stars. This decrease in the product of C # N may indicate that carbon- to-nitrogen conversion plus deep mixing is occurring within the CN-strong giants to a greater degree than in the CN-weak giants. These results, when interpreted with additional constraints from the giant branch luminosity function of M3, suggest that the C and N abundance pattern requires a hybrid scenario involving interior mixing within red giants as well as primordial abundance inhomogeneities that precede the RGB phase of evolution.

We construct a speculative scenario for the evolution of Wolf-Rayet central stars of planetary nebula. It is clear from the latest infra-red observations that a new perspective has to be adopted: the simultaneous presence of carbon- and oxygen-rich dust (double dust chemistry), while being a rare phenomenon for H-rich central stars, is found around the vast majority of cool Wolf-Rayet central stars. This correlation between Wolf-Rayet characteristics and double dust chemistry points to a common mechanism. Within the binary evolution framework established by Soker, two scenarios are proposed, responsible for the majority (80-85%) and minority (15-20%) of Wolf-Rayet central stars. In the first scenario, proposed here for the first time, a low mass main sequence star, brown dwarf or planet spirals into the Asymptotic Giant Branch (AGB) star inducing extra mixing, hence a chemistry change, and terminating the AGB evolution. In the second scenario, previously proposed, a close binary companion is responsible for the formation of a disk around either the binary or the companion. This long-lived disk harbors the O-rich dust., Comment: PASP, in press; 21 pages

Low mass stars contribute to the chemical evolution of the Galaxy as well as more massive supernova progenitors. Indeed the limited amount of processed matter released into the interstellar medium by small objects is compensated by the large number of them. At the late stages of their evolution, stars with mass smaller than 3M⊙ undergo the Asymptotic Giant Branch phase, which has been found to be a unique site for synthesis of some nuclei heavier than Fe through slow neutron capture reactions. AGB nucleosynthesis is also characterized by H-burning coupled with mixing phenomena, which has been proved to account for anomalies in light element isotopic abundances from Li to Al observed in stellar spectra and meteorites. We present here the case of a large number of meteorite grains, whose isotopic composition offers a puzzles that only Nuclear and Stellar Physics coupled together might solve.

NGC 147 and NGC 185 are two of the most massive satellites of the Andromeda galaxy (M 31). With similar mass and morphological type dE, they possess different amounts of interstellar gas and tidal distortion. The question therefore is, how do their histories compare? We present the first reconstruction of the star formation histories of NGC 147 and NGC 185 using long-period variable stars (LPVs). LPVs are low- to intermediate-mass stars at the asymptotic giant branch, which their luminosity is related to their birth mass. Combining near-infrared photometry with stellar evolution models, we construct the mass function and hence the star formation history. For NGC 185 we found that the main epoch of star formation occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate. In the case of NGC 147, the star formation rate peaked only 7 Gyr ago, staying intense until ~ 3 Gyr ago, but no star formation has occurred for at least 300 Myr. Despite their similar masses, NGC 147 has evolved more slowly than NGC 185 initially, but more dramatically in more recent times.

We use the Modules for Experiments in Stellar Astrophysics (MESA) code to calculate the yields of in the ejecta of classical novae, occurring on the surface of carbon-oxygen (CO) and oxygen-neon-magnesium (ONeMg) white dwarfs (WDs). Taking the masses and the mass accretion rates of WDs as input parameters, this article presents a grid showing the yield of from classical novae, including a total of 79 models. Under similar input parameters, the yields in our models are consistent with previous ones in the literature. yields recently inferred spectroscopically from classical nova V1369 Cen are then compared with the theoretical values obtained in this work. Using the grid as its basis and population synthesis as its method, this paper estimates the amount of produced by classical novae compared to the total in the Galaxy. Our simulations suggest that novae can eject about of every year. Hence, one may estimate that approximately 10% of in the Galaxy originates from classical novae, which is significantly higher than the produced by asymptotic giant branch stars.

It was a year in which some topics selected themselves as important through the sheer numbers of papers published. These include the connection(s) between galaxies with active central engines and galaxies with starbursts, the transition from asymptotic giant branch stars to white dwarfs, gamma-ray bursters, solar data from three major satellite missions, and the cosmological parameters, including dark matter and very large scale structure. Several sections are oriented around processes - accretion, collimation, mergers, and disruptions - shared by a number of kinds of stars and galaxies. And, of course, there are the usual frivolities of errors, omissions, exceptions, and inventories.

The abundance of Li in stars formed within the past 5 Gyr is log N(Li) = 3.2(± 0.2), while the corresponding value for the oldest stars in the Galaxy is log N(Li) = 2.2(± 0.2). The global evidence suggests that the latter represents the full, or the major, part of the primordial abundance, so that the difference of an order of magnitude is due to Li produced in the Galaxy. It is well known that spallation of interstellar CNO by 4He and protons in Galactic cosmic rays (GCRs) can produce Li, but models yield a shortfall of almost an order of magnitude compared with the current observed abundance range. Another GCR reaction, α+α fusion, has been invoked to explain some Li production in the early Galaxy, but application of this to the disk yielded too much early Li or too little current Li. These failures led to a search for alternative mechanisms, essentially stellar, at particular phases of evolution: the helium flash phase in asymptotic giant branch stars, in novae, and in supernovae (SNe).Here we stress the importance of the observed upper envelope in the plot of Li versus Fe in stars as a constraint on any mechanism in any model aiming to account for disk Li. We show that a good match can be found assuming that low-energy GCRs produce the Li, with the α+α reaction as the key mechanism, although production in supernovae cannot at this stage be excluded. There is an apparent time delay in the Li production, relative to O and Fe, which if confirmed could be explained by the origin of a low-energy α-particle component in processes associated with stars of intermediate and low mass. The α-flux at a given epoch would then be proportional to the amount of gas expelled by low- and intermediate-mass stars in the Galaxy, though the acceleration of these α-particles could still be linked to more energetic events as supernova explosions. The present scenario appears to account coherently for the closely related observations of the temporal evolution in the Galaxy (halo+disk) of abundances of 12C, 13C, 14N, 16O, 26Fe, the two main peaks (one in the halo and one in the disk) in the G-dwarf stellar frequency distribution, and the evolution of 9Be and 10B+11B via GCR spallation reactions without requiring the very high local cosmic-ray fluxes implied by the spallation close to SN. Adding a natural mechanism of differential depletion in red supergiant envelopes, we can explain the observed time evolution of the abundance of D and that of the isotopic ratios 7Li/6Li and 11B/10B starting from a standard big bang nucleosynthesis model with baryon density ~0.05. Our model also predicts the second Li "plateau" found for [Fe/H] between –0.2 and +0.2, due to the "loop back" implied for Li (also for 9Be and B) because of the required infall of low-metallicity gas to the disk. Without ruling out other mechanisms for the main production of Li in the Galactic disk, the low-energy α+α fusion reaction in the interstellar medium offers a promising contribution.

Chemical compositions are derived from high-resolution spectra for three stars classed as SRd variables in the General Catalogue of Variable Stars. These stars are shown to be metal-poor supergiants: WY And with [Fe/H] = -1.0, VW Eri with [Fe/H] = -1.8, and UW Lib with [Fe/H] = -1.2. Their compositions are identical to within the measurement errors with the compositions of subdwarfs, subgiants, and less evolved giants of the same FeH. The stars are at the tip of the first giant branch or in the early stages of evolution along the asymptotic giant branch (AGB). There is no convincing evidence that these SRd variables are experiencing thermal pulsing and the third dredge-up on the AGB. The SRds appear to be the cool limit of the sequence of RV Tauri variables., 14 pages, 1 figure, 4 tables

A number of relatively bright metal-poor red giants from the HD and BD catalogs are found to have been detected by the IRAS satellite. Data for these stars have been retrieved from the IRAS Point Source Catalog (PSC) and/or the Faint Source Catalog (FSC). The majority of metal-poor giants in these samples fall along relatively well-defined sequences in plots of V-[12] versus B - V and V - I; for these stars, the 12 μm flux detected is presumed to arise from the photosphere. Only a subset of stars detected at 12 μm were detected at 25 μm; these are displayed in a plot of [12]-[25] versus V-[12]. There are a small number of giants that exhibit notable 12 and/or 25 μm excesses relative to the mean sequences defined by the bulk of the sample. Those stars with the most unambiguous evidence for infrared excesses are variable stars, either long-period or semiregular variables or RV Tauri stars. As such, those stars exhibiting infrared excesses in the metal-poor giant sample are likely in the asymptotic giant branch (AGB) or post-AGB phase of evolution. There is no clear evidence for nonvariable first-ascent red giants having been detected among the infrared-excess stars. In fact, some metal-poor red giants known to exhibit outflows in their chromospheres do not show infrared excesses. A Population II star ascending the red giant branch for the first time appears to have too low a mass-loss rate to be recognizable as an infrared-excess star in the IRAS PSC or FSC.

A survey of red giants in the globular cluster M3 with the Hydra multi-object spectrograph on the WIYN 3.5 m telescope indicated a prominent Li i 6707 A feature in the red giant vZ 1050. Followup spectroscopy with the ARC 3.5 m telescope confirmed this observation and yielded a derived abundance of A(Li)NLTE = 1.6 ± 0.05. In addition, the high oxygen and low sodium abundances measured from the same spectrum suggest that vZ 1050 is a first generation cluster star. The location of vZ 1050 above the horizontal branch and blueward of the red giant branch in the cluster's color–magnitude diagram places vZ 1050 on M3's asymptotic giant branch. The likely source for the enhanced lithium abundance is the Cameron–Fowler mechanism operating in vZ 1050 itself.

Asymptotic Giant Branch (AGB) stars and post-AGB stars are thought to be major dust sources in galaxies. Investigation of dust in planetary nebulae (PNe), their successors, reveals properties of dust grains injected into the interstellar medium. The near-infrared (2.5-5.0 μm) emission of PNe consists of hot dust continuum, emission lines of ionized gases, and emission bands of carbonaceous dust grains. This wave range, however, has not been well investigated due to the lack of instruments with a high sensitivity. We conducted spectroscopic observations of PNe with the AKARI, obtaining 2.5-5.0 μm spectra of 72 Galactic PNe with a sensitivity of a few mJy. The spectra and the intensities of emission features are compiled as a catalog. The characteristics of the catalog are described.

Post-AGB (PAGB) stars are short-lived, low-intermediate mass objects transitioning from the asymptotic giant branch (AGB) to the white dwarf (WD) phase. These objects are characterised by a constant, core-mass dependent luminosity and a large infrared excess from the dusty envelope ejected at the top of the AGB. PAGB stars provide insights into the evolution of their direct descendants, planetary nebulae (PNe). Calculation of physical characteristics of PAGB are dependent on accurately determined distances scarcely available in the literature. Using the Torun catalogue for PAGB objects, supplemented with archival data, we have determined distances to the known population of Galactic PAGB stars. This is by modelling their spectral energy distributions (SED) with black bodies and numerically integrating over the entire wavelength range to determine the total integrated object flux. For most PAGB stars we assumed their luminosities are based on their positional characteristics and stellar evolution models. RV Tauri stars however are known to follow a period-luminosity relation (PLR) reminiscent of type-2 Cepheids. For these variable PAGB stars we determined their luminosities via the PLR and hence their distances. This allows us to overcome the biggest obstacle to characterising these poorly understood objects that play a vital part in Galactic chemical enrichment.

ALMA cycles 1 and 3 observations of CO isotopologues and 1.3mm continuum are used in a study of the circumstellar environment of the binary HD 101584, a post-AGB star and a low-mass companion that is most likely a post-common-envelope-evolution system. These data are supplemented with new information from OH maser emission. It is inferred that the large- scale circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic, ≥ 150 km s-1, jet. Significant amount of material still resides in the central region. It is proposed that the circumstellar morphology is related to an event which took place ≤ 500 yr ago, possibly a capture event where the companion spiralled in towards the AGB star. Several observed features remain to be explained, and may hint to a more complicated scenario.

We report new abundances of Rb and Zr in a sample of massive Galactic asymptotic giant branch (AGB) stars that were previously studied with hydrostatic models by using more realistic dynamical model atmospheres. We use a modified version of the spectral synthesis code Turbospectrum, and consider the presence of a circumstellar envelope and a radial wind in the modelling of these Galactic AGB stars. The Rb and Zr are determined from the 7800 A Rb I resonant line and the 6474 A ZrO bandhead, respectively, and they are compared with the AGB nucleosynthesis theoretical predictions. The derived Rb abundances are much lower (~1-2 dex) with the new dynamical models, while the Zr abundances, however, are closer to the hydrostatic values. The new model atmospheres can help to resolve the problem of the mismatch between the observations and the nucleosynthesis theoretical predictions of massive AGB stars.

The chemical evolution of asymptotic giant branch (AGB) stars models depends greatly on the input physics (e.g. convective model, mass loss recipe). Variations of hot bottom burning (HBB) strength, or third dredge-up (TDU) efficiency are among the main consequences of adopting different input physics in the AGB models. The ATON evolutionary code stands apart from others in that it uses the Full Spectrum of Turbulence convective model. Here we present the first results of a newly developed s-process nucleosynthesis module for ATON AGB models. Our results are compared also with observations and theoretical predictions of present AGB nucleosynthesis models using different input physics.

The stellar evolution from the asymptotic giant branch (AGB) to planetary nebulae (PN) contains some of the most interesting physical and chemical processes in the Universe. Within a time period of one million years starting from the nucleosynthesis of carbon in the core, we witness the chemical synthesis of molecules in the atmosphere, followed by the condensation of minerals and organics in the stellar outflow. Different phases of supersonic stellar winds, both spherical symmetric and highly collimated, and their interactions lead to a series of dynamical processes and morphological transformation of the stellar ejecta. Most interestingly, PN are now known to be major sources of complex organics in the Galaxy. Organic compounds of mixed aromatic and aliphatic structures have been observed to form in the post-AGB evolution over time scales as short as hundreds of years. There is likely that these stellar organics journeyed through the Galaxy and were embedded in early Solar System.

I present our ALMA observations of the CO emission around the carbon AGB star R Sculptoris. The data reveal the known detached shell and a previously unknown, binary induced, spiral shape. The observations confirm a formation of the shell during a thermal pulse about 2300 years ago. The full analysis of the ALMA data shows that the shell around R Scl in fact is entirely filled with molecular gas, and hence not as detached as previously thought. This has implications for the mass-loss rate evolution immediately after the pulse, indicating a much higher mass-loss rate than previously assumed. Comparing the ALMA images to our optical observations of polarised, dust scattered light, we further show that the distributions of the dust and gas coincide almost perfectly, implying a common evolution of the dust and gas, and constraining the wind-driving mechanism. The mass-loss process and amount of mass lost during the thermal pulse cycle affect the chemical evolution of the star, its lifetime on the AGB, and the return of heavy elements to the ISM. New high-resolution ALMA observations constrain the parameters of the binary system and the inner spiral, and will allow for a detailed hydrodynamical modelling of the gas and dust during and after the last thermal pulse. Our results present the only direct measurements of the thermal pulse evolution currently available. They greatly increase our understanding of this fundamental period of stellar evolution, and the implications it has for the chemical evolution of evolved stars, the ISM, and galaxie.

Spitzer/IRS spectra of eleven heavily obscured C-rich sources rapidly evolving from asymptotic giant branch (AGB) stars to Planetary Nebulae are presented. IRAM 30m observations for three of these post-AGBs are also reported. A few (3) of these sources are known to exhibit strongly variable maser emission of O-bearing molecules such as OH and H2 O, suggesting a transitory O-rich chemistry because of the quickly changing physical and chemical conditions in this short evolutionary phase. Interestingly, the Spitzer/IRS spectra show a rich circumstellar carbon chemistry, as revealed by the detection of small hydrocarbon molecules such as C2H2, C4H2, C6H2, C6H6, and HCN. Benzene is detected towards two sources, bringing up to three the total number of Galactic post-AGBs where this molecule has been detected. In addition, we report evidence for the possible detection of other hydrocarbon molecules like HC3N, CH3C2H, and CH3 in several of these sources. The available IRAM 30m data confirm that the central stars are C-rich - in despite of the presence of O-rich masers - and the presence of high velocity molecular outflows together with extreme AGB mass-loss rates (~10-4 Mʘ /yr). Our observations confirm the polymerization model of Cernicharo [1] that predicts a rich photochemistry in the neutral regions of these objects on timescales shorter than the dynamical evolution of the central HII region, leading to the formation of small C-rich molecules and a transitory O-rich chemistry.

Post-Asymptotic Giant Branch (post-AGB) stars bear signatures of the entire chemical and morphological changes that occur prior to and during the AGB phase of evolution. These objects also provide vital clues on the ultimate fate of the star. Detailed chemical abundance studies of some of these objects have shown that they are chemically much more diverse than anticipated. As expected, some are the most s-process enriched objects known to date while others are not s-process enriched. Our recent study has revealed a star in the Small Magellanic Cloud, J005252.87-722842.9, which displays a peculiar chemical signature that does not correspond to the expected chemical diversity observed in these objects. This unique object reveals the possibility of a new stellar evolutionary channel where the star evolves without any third dredge-up episodes or during its evolution becomes devoid of its nucleosynthetic history.

In the present paper we investigate the possible connection between s-process nucleosynthesis occurring during the asymptotic giant branch (AGB) phase of low-mass stars (LMS) and the isotopic anomalies of the "Fe-group" elements observed in several macroscopic samples of meteorites or in grains formed as circumstellar condensates (hereafter CIRCONs). The available measurements of chromium, iron, and nickel are well reproduced by stellar models, which account for the largest shifts in the heaviest isotopes of each element: in particular ^(54)Cr, ^(58)Fe, and ^(64)Ni. Moreover, many circumstellar condensates reflect ^(50)Ti excesses and some production of ^(46, 47, 49)Ti, as predicted by slow-neutron captures in AGB stars. Nevertheless, some difficulties are found in comparing theoretical calculations of s-process nucleosynthesis with calcium, silicon, and zinc isotopic anomalies.