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Notes: Abstract We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with improved opacities, which recently succeeded in accounting for the third dredge-up. This opens the possibility of understanding low luminosity C stars (enriched in s-elements) as the normal outcome of AGB evolution, characterized by production of 12C and neutron-rich nuclei in the He intershell and by mass loss from strong stellar winds. Neutron captures in AGB stars are driven by two reactions: 13C(alpha,n)16O, which provides the bulk of the neutron flux at low neutron densities (Nn〈= 107 n/cm3), and 22Ne(alpha,n)25Mg, which is mildly activated at higher temperatures and mainly affects the production of s-nuclei depending on reaction branchings. The first reaction is now known to occur in the radiative interpulse phase, immediately below the region previously homogenized by third dredge-up. The second reaction occurs during the convective thermal pulses. The resulting nucleosynthesis phenomena are rather complex and rule out any analytical approximation (exponential distribution of neutron fluences). Nucleosynthesis in AGB stars, modeled at different metallicities, account for several observational constraints, coming from a wide spectrum of sources: evolved red giants rich in s-elements, unevolved stars at different metallicities, presolar grains recovered from meteorites, and the abundances of s-process isotopes in the solar system. In particular, a good reproduction of the solar system main component is obtained as a result of Galactic chemical evolution that mixes the outputs of AGB stars of different stellar generations, born with different metallicities and producing different patterns of s-process nuclei. The main solar s-process pattern is thus not considered to be the result of a standard archetypal s-process occurring in all stars. Concerning the 13C neutron source, its synthesis requires penetration of small amounts of protons below the convective envelope, where they are captured by the abundant 12C forming a 13C-rich pocket. This penetration cannot be modeled in current evolutionary codes, but is treated as a free parameter. Future hydrodynamical studies of time dependent mixing will be required to attack this problem. Evidence of other insufficiencies in the current mixing algorithms is common throughout the evolution of low and intermediate mass stars, as is shown by the inadequacy of stellar models in reproducing the observations of CNO isotopes in red giants and in circumstellar dust grains. These observations require some circulation of matter between the bottom of convective envelopes and regions close to the H-burning shell (cool bottom processing). AGB stars are also discussed in the light of their possible contribution to the inventory of short-lived radioactivities that were found to be alive in the early solar system. We show that the pollution of the protosolar nebula by a close-by AGB star may account for concordant abundances of 26Al, 41Ca, 60Fe, and 107Pd. The AGB star must have undergone a very small neutron exposure, and be of small initial mass ( ). There is a shortage of 26Al in such models, that however remains within the large uncertainties of crucial reaction rates. The net 26Al production problem requires further investigation.

Notes: Abstract UBV photometry of the peculiar star FK Comae, obtained during the period January–June 1983, is presented. The amplitude of the photometric variation has raised with respect to the previous observations performed in 1981 and 1982, attaining about 0.18 mag in theV filter. Colour curves indicate that the star is redder at minimum light; this fact is consistent with both the models presented to explain the properties of FK Comae. A Fourier analysis of the light curves available in the literature has been performed; the phases of minimum and maximum light turn out to be noticeably stable in time. This fact is difficult to be reconciled with the hypothesis of dark spots that are observed to migrate in other active stars. Rather, it could be explained by the model of Walter and Basri (1982a, b), implying an extremely evolved Algol-type system accreting mass on its primary component.

Notes: Abstract We present new mid-IR images of AGB and post-AGB circumstellar envelopes (CSEs) obtained with the 10–20 µm imaging camera CAMIRAS mounted at the TIRGO infrared telescope (Gornergrat, Switzerland). Diffraction limited images of the sources are obtained, and extended emission for the brightest sources is detected for several arcseconds. Simple radiative transfer modelling is necessary to fit the observed data and derive the physical and chemical state of the envelope. This kind of analysis is an important test for dust formation theories, because it gives indications of dust properties along their condensation path through the envelope.

Notes: Abstract The contribution to the galactic abundance of He and heavy elements by stellar nucleosynthesis is calculated as a function of time, keeping account of present knowledge about stellar and galactic evolution. A model is used which distinguishes the phase of the contracting halo from the subsequent history of the disc. Various uncertainties involved both in stellar and in galactic evolutionary theory are discussed. The amount of4He produced by stars of different masses and ejected in interstellar medium is fairly well known from stellar theory, while we have assumed its primordial abundance as a free parameter, ranging from 0 up to 0.4. We find that stellar activity provides a significant contribution to the cosmic4He, though not sufficient to explain the observed abundance. The best agreement with observational data (Y ⊙≃0.26 andY now≃0.28) is obtained starting with a primordial abundanceY ρ=(0.20–0.23), which is consisten with the ‘Big-Bang theory’ predictions and with recent observational estimates. The contribution to the abundance of heavy elements depends on the last stellar stages and on the final explosion mechanism, which are only now beginning to be understood. Nevertheless, in the framework of present theories, we individuate a stellar evolutionary scheme reproducing the observedZ abundances for Populationi and Populationii stars, with the correctly estimated ΔY/ΔZ value. In this scheme, only stars belonging to two narrow mass ranges (10≤m/m ⊙≤15 andm/m ⊙≥80) are allowed to eject metal-enriched matter, possibly with the solar (C+O)/(Si+Fe) ratio.

Notes: Abstract The chemical enrichment in the galactic halo is studied, on the basis of the numerical model developed in Paper I, with paricular attention to the overabundances of O and light elements with respect to Fe shown by metal poor stars. Some representative nucleosynthesis pictures for stars of both Population I and Population II are considered and their yields are compared with observations of relative abundances in the Sun and in the halo, to identify the possible reasons of the observed compositional differences. It is found that solar elemental ratios can be reproduced if intermediate mass stars are allowed to give some contribution to the production of Fe by type-I supernovae, while the ratios of abundances observed in the halo are more similar to the relative yields produced by massive stars. These features are shared by all the nucleosynthesis schemes which have been considered. Using the best model of Paper I, we show that the steep star formation induced by the collapse has a decisive effect in maintaining the overabundances of light elements during the whole evolution of the halo. The relevance of this conclusion is discussed also in the light of a possible interpretation of the differences between the two abundance scales for globular clusters.

Notes: Abstract Models for the chemical evolution of the galactic halo are proposed, to reproduce the distribution of metals in globular clusters and in the stars of the bulge component of our Galaxy, with particular attention to the observedZ(N) relation. The present picture of the galactic halo, as it results from the new abundance scale for globular clusters and from recent data about the mass distribution, is outlined and some observational suggestions for the bulge component of the Galaxy are reviewed. These features are considered as constraints to the numerical models. Since it has been demonstrated (Hartwick, 1976) that chemical distributions of the type observed for population II stars can be theoretically reproduced if mass is temporarily removed from the star-forming regions, two possible mechanisms of mass segregation are proposed to account for this phenomenon. The first one comes from the different dynamical behaviour of the gas and of the stars, since stars are likely to decouple from the gas when they are formed, without participating to the subsequent collapse. The second one results from considering a star formation law conditioned by the ionization by early-type stars. On one side, the advanced stages of evolution of Hii regions can enhance or even generate star formation; on the other hand, the ionizing radiation segregates a fraction of the mass in Strömgren spheres, thus inhibiting new stellar birth in these regions for a certain time. Models based on simple schematizations of these phenomena and on an initial mass function allowing for highM/L ratios are found to reproduce observational data for both metal abundances and mass distribution.

Notes: Abstract We present the main characteristics of a new mid-IR camera, TIRCAM, operating at the 1.5 m Italian Infrared Telescope.

Notes: Abstract A mid-infrared (8 – 22 µm) camera, equipped with a Hughes hybrid bulk Si:As array detector of 10 × 64 pixels was developed for the 1.5 m Italian Infrared Telescope (TIRGO). The instrument was used for narrow-band 8–14 µm high-background astronomical observations. Here we present a short description of the camera, including its fast readout electronics and its optical design. When used at TIRGO, the camera has a pixel scale of 1.23 arcsec/pix. From the observations of infrared standard stars we derived a noise equivalent flux density (NEFD) that, for all the filters, is in the range 0.5–0.7 Jy × arcsec−2 xmin−1/2 and a point spread function (PSF) of the order of 3 arcsec(FWHM). Mosaics of the extended sources Mars at 8.8, 9.8, and 12.5 µm, and of the star forming region Orion BN-KL Nebula at 12.5 µm, are shown. The camera performances are also illustrated on the basis of the image of the post-AGB source CRL618.

Notes: [Auszug] Meteoritic silicon carbide grains, formed in the winds from carbon stars, contain noble gases and other species whose elemental and isotopic abundances are a probe of stellar nucleosynthesis. Theoretical models of carbon stars can explain a variety of measured abundances, in particular the range of ...

Notes: Abstract The polymer of ethylenesulfonic acid (U-9843) is a potent inhibitor of HIV-1 RT (reverse transcriptase) and the drug possesses excellent antiviral activity at nontoxic doses in HIV-infected lymphocytes grown in tissue culture. The drug also inhibits RTs isolated from other species such as AMV and MLV retroviruses. Enzymatic kinetic studies of the HIV-1 RT catalyzed RNA-directed DNA polymerase function, using synthetic template: primers, indicate that the drug acts generally noncompetitively with respect to the template: primer binding site but the specific inhibition patterns change somewhat depending on the drug concentration. The inhibitor acts noncompetitively with respect to the dNTP binding sites. Hence, the drug inhibits this RT polymerase function by interacting with a site distinct from the template: primer and dNTP binding sites. In addition, the inhibitor also impairs the DNA-dependent DNA polymerase activity of HIV-1 RT and the RNase H function. This indicates that the drug interacts with a target site essential for all three HIV RT functions addressed (RNA- and DNA-directed DNA polymerases, RNase H).