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ABSTRACT: 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, 2005
09/2005;
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C. Alard,
J. Blommaert,
C. Cesarsky,
N. Epchtein,
M. Felli,
P. Fouque,
S. Ganesh,
R. Genzel,
G. Gilmore,
I. Glass, [......],
D. Minniti,
C Nelson,
B. Peterson,
P. Popowski,
M. Pratt,
P. Quinn,
W. Sutherland,
A. Tomaney,
T. Vandehei,
D. Welch
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ABSTRACT: By cross-correlating the results of two recent large-scale surveys, the general properties of a well defined sample of semi-regular variable stars have been determined. ISOGAL mid-infrared photometry and MACHO lightcurves are assembled for approximately 300 stars in the Baade's Windows of low extinction towards the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log~period -- bolometric magnitude relation or set of such relations. Approximate mass-loss rates in the range of 1e-8 to 5e-7 M_sun per year are derived from ISOGAL mid-infrared photometry and models of stellar spectra adjusted for the presence of optically-thin circumstellar silicate dust. Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass as rapidly as short-period Miras but do not show Mira-like amplitudes. A period of 70 days or longer is a necessary but not a sufficient condition for mass loss to occur. For AGB stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity is found to be in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGB stars in the Large Magellanic Cloud, we get the general result for AGB stars that mass-loss rate is proportional to luminosity^{2.7}, valid for AGB stars with 10^{-8} to 10^{-4} M_sun per year (Abridged). Comment: to appear in The Astrophysical Journal, 51 pages, 9 figures, 3 tables; table 1 will be available in machine-readable format at the electronic ApJ
The Astrophysical Journal 01/2001; 552(1):289. · 6.02 Impact Factor
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ABSTRACT: DENIS observations in the J (1.2 micron) and K_S (2.15 micron) bands together with isochrones calculated for the RGB and AGB phase are used to draw an extinction map of the inner Galactic Bulge. The uncertainty in this method is mainly limited by the optical depth of the Bulge itself. A comparison with fields of known extinction shows a very good agreement. We present an extinction map for the inner Galactic Bulge (approx. 20 sq. deg.)
10/1999;
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A. Omont,
G Simon,
D. Ojha,
J. Blommaert,
M. Felli,
G. Gilmore, H. Habing,
S. Price,
A. Robin,
B. de Batz,
C. Cesarsky,
D. Elbaz,
N. Epchtein,
S. Guest,
D. Levine,
A. Pollock,
T. Prusti,
R. Siebenmorgen,
L. Testi
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ABSTRACT: . ISOGAL is a 15 ¯m ISOCAM survey of ¸ 12 deg 2 in the Galactic Plane interior to j`j = 45 ffi . In combination with IJK data from the near-infrared southern sky survey DENIS, the ISO images allow the first detailed study of stellar populations throughout the inner Galaxy. We present preliminary results from a test observation at ` = Gamma45 ffi with 6" pixels and completeness limit 8 mJy. Of the ¸ 3000 sources deg Gamma2 detected, about half are KM giants, seen through extinction of up to A v ¸ 30, while most of the remainder are probably dusty young stars. Although away from bright IRAS regions, the field displays spectacular emission features, and, unexpectedly, a number of regions which are optically thick at 15 ¯m. The dark regions are presumably dense filaments with A v ? 25. Key words: The Galaxy: stellar content -- Interstellar medium: general -- Interstellar medium: dust, extinction -- Infrared: galaxies -- Infrared: stars Send offprint requests to: M. P'erault, m...
04/1997;
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M. Perault,
A. Omont,
G Simon,
P Seguin,
D. Ojha,
JADL Blommaert,
M. Felli,
G. Gilmore,
F. Guglielmo, H. Habing, [......],
D. Elbaz,
N. Epchtein,
P Fougue,
S. Guest,
D. Levine,
AMT Pollock,
T. Prusti,
R. Siebenmorgen,
L. Testi,
D. Tiphene
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ABSTRACT: Lithium and zirconium abundances (the latter taken as representative of 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 ($R\sim 40\,000{-}50\,000$). This was done by computing synthetic spectra based on classical hydrostatic model atmospheres for cool stars and using extensive line lists. The results are discussed in the framework of “hot bottom burning” (HBB) and nucleosynthesis models. The complete sample is studied for various observational properties such as the position of the stars in the IRAS two-colour diagram ([ 12$] - [$25] vs. [ 25$] - [$60] ), Galactic distribution, expansion velocity (derived from the OH maser emission), and period of variability (when available). We conclude that a considerable fraction of these sources are actually massive AGB stars ($M>3{-}4$ $M_{\odot}$) experiencing HBB, as deduced from the strong Li overabundances we 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.
http://dx.doi.org/10.1051/0004-6361:20065785.
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Proceedings of the International Astronomical Union 169:347.
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ABSTRACT: Using high resolution optical spectroscopy (R˜40,000-50,000) we
have derived the Li and Zr abundances of a large sample of galactic
O-rich AGB stars. Our chemical analysis shows that some stars are Li
overabundant while others are not. The observed Li overabundances are
attributed to the activation of the so-called ``hot bottom burning''
(HBB) process, confirming that they are actually massive AGB stars.
However, these stars do not show the zirconium enhancement (taken as a
representative for the s-process element enrichment) usually associated
to the third dredge-up. Our study reveals that the s-process element
abundances of the more massive O-rich AGB stars in our Galaxy are
dramatically different from those found in the equivalent population of
AGB stars in the Magellanic Clouds. We conclude that probably the
different metallicity environment can explain the differences observed.
Memorie della Societa Astronomica Italiana. 77:967.
