Why are massive O-rich AGB stars in our Galaxy not S-stars?

We present the main results derived from a chemical analysis carried out on a large sample of galactic O-rich AGB stars using high resolution optical spectroscopy (R~40,000-50,000) with the intention of studying their lithium abundances and/or possible s-process element enrichment. Our chemical analysis shows that some stars are lithium overabundant while others are not. The observed lithium overabundances are interpreted as a clear signature of the activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive galactic O-rich AGB stars, as predicted by the models. However, these stars do not show the zirconium enhancement (taken as a representative for the s-process element enrichment) associated to the third dredge-up phase following thermal pulses. Our results suggest that the more massive O-rich AGB stars in our Galaxy behave differently from those in the Magellanic Clouds, which are both Li- and s-process-rich (S-type stars). Reasons for this unexpected result are discussed. We conclude that metallicity is probably the main responsible for the differences observed and suggest that it may play a more important role than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference "Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul 02, 200

CORS Baade-Wesselink method in the Walraven photometric system: the period-radius and the period-luminosity relation of classical Cepheids

We present a new derivation of the CORS Baade-Wesselink method in the Walraven photometric system. We solved the complete Baade-Wesselink equation by calibrating the surface brightness function with a recent grid of atmosphere models. The new approach was adopted to estimate the mean radii of a sample of Galactic Cepheids for which are available precise light curves in the Walraven bands. Current radii agree, within the errors, quite well with Cepheid radii based on recent optical and near-infrared interferometric measurements. We also tested the impact of the projection factor on the Period-Radius relation using two different values (p=1.36, p=1.27) that bracket the estimates available in the literature. We found that the agreement of our Period-Radius relation with similar empirical and theoretical Period-Radius relations in the recent literature, improves by changing the projection factor from p=1.36 to p=1.27. Our Period-Radius relation is log(R)=(0.75\pm 0.03)log(P)+(1.10 \pm 0.03), with a rms=0.03 dex. Thanks to accurate estimates of the effective temperature of the selected Cepheids, we also derived the Period-Luminosity relation in the V band and we found Mv=(-2.78 \pm 0.11)log(P)+(-1.42 \pm 0.11) with rms=0.13 mag, for p=1.27. It agrees quite well with recent results in the literature, while the relation for p=1.36 deviates by more than 2sigma. We conclude that, even taking into account the intrinsic dispersion of the obtained Period-Luminosity relations, that is roughly of the same order of magnitude as the effect of the projection factor, the results of this paper seem to favour the value p = 1.27.Comment: 17 pages, 14 figures, accepted for publication in MNRA

Pdms-based ligands for quantum dots in silicones

The invention provides a process for the production of a light converter comprising a siloxane polymer matrix with light converter nano particles embedded therein, the process comprising (a) mixing (i) light converter nano particles having an outer surface grafted with grafting ligands and (ii) curable siloxane polymers, and (b) curing the curable siloxane polymers, thereby producing the light converter; wherein the grafting ligands comprise siloxane grafting ligands having x1 Si backbone elements, wherein at least one Si backbone element of each siloxane grafting ligand comprises a side group having a grafting functionality; wherein the curable siloxane polymers have y1 Si backbone elements; and wherein x1 is at least 20, wherein y1 is at least 2, and wherein x1/y1≥0.8.</p

Lithium and zirconium abundances in massive Galactic O-rich AGB stars

Lithium and zirconium abundances (the latter taken as representative for s-process enrichment) are determined for a large sample of massive Galactic O-rich AGB stars, for which high resolution optical spectroscopy has been obtained (R=40,000-50,000). This is done by computing synthetic spectra based on classical hydrostatic model atmospheres for cool stars using extensive line lists. The results obtained are discussed in the framework of hot bottom burning (HBB) and nucleosynthesis models. The complete sample is studied attending to various observational properties such as the position of the stars in the IRAS two-colour diagram ([12]-[25] vs [25]-[60]), Galactic distribution, expansion velocity (derived from the OH maser emission) and period of variability (when available). We conclude that a considerable fraction of the sources observed are actually massive AGB stars (M > 3-4 solar masses) experiencing HBB, as deduced from the strong Li overabundances found. A comparison of our results with similar studies carried out in the past for the Magellanic Clouds (MCs) reveals that, in contrast to MC AGB stars, our Galactic sample does not show any indication of s-process element enrichment. The differences observed are explained as a consequence of metallicity effects. Finally, we discuss the results obtained in the framework of stellar evolution by comparing our results with the data available in the literature for Galactic post-AGB stars and PNe

Structure and Evolution of Nearby OB Associations

We present the first results of a comprehensive census of the stellar content of the nearby OB associations based on Hipparcos positions, proper motions and parallaxes for 12842 candidate member stars distributed over 21 fields on the sky. We use a new method to identify moving groups in these fields (see de Bruijne et al., these proceedings). Previously, astrometric membership in nearly all the nearby OB associations was known only for stars with spectral types earlier than B5. The Hipparcos measurements now allow us to identify members down to late F. This census provides a firm basis for studies of galactic and extragalactic star forming regions.Comment: 6 pages, 4 Postscript figures, LaTeX using venice97.sty, twocolumn.sty; to appear in Proceedings of the Hipparcos Venice '97 Symposium, ESA-SP 40

A multi-color and Fourier study of RR Lyrae variables in the globular cluster NGC 5272 (M3)

We have performed a detailed study of the pulsational and evolutionary characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3) using highly accurate BVI data taken on 5 separate epochs. M3 seems to contain no less than ~32% of Blazhko stars, and the occurrence and characteristics of the Blazhko effect have been analyzed in detail. We have identified a good number (~ 14%) of overluminous RR Lyrae stars that are likely in a more advanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physical parameters (i.e. temperature, luminosity, mass) have been derived from (B--V) colors and accurate color-temperature calibration, and compared with Horizontal Branch evolutionary models and with the requirements of stellar pulsation theory. Additional analysis by means of Fourier decomposition of the V light curves confirms, as expected, that no metallicity spread is present in M3. Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko stars at low amplitude phase do affect [Fe/H] distributions as they appear more metal-rich. Absolute magnitudes derived from Fourier coefficients might provide useful average estimates for groups of stars, if applicable, but do not give reliable {\em individual} values. Intrinsic colors derived from Fourier coefficients show significant discrepancies with the observed ones, hence the resulting temperatures and temperature-related parameters are unreliable.Comment: 86 pages, 19 figures, 13 tables, in press A