Katia Cunha

Fluorine Abundances in Galactic Asymptotic Giant Branch Stars

An analysis of the fluorine abundance in Galactic asymptotic giant branch (AGB) carbon stars (24 N-type, 5 SC-type, and 5 J-type) is presented. This study uses the state-of-the-art carbon-rich atmosphere models and improved atomic and molecular line lists in the 2.3 μm region. Significantly lower F abundances are obtained in comparison to previous studies in the literature. This difference is mainly due to molecular blends. In the case of carbon stars of SC-type, differences in the model atmospheres are also relevant. The new F enhancements are now in agreement with the most recent theoretical nucleosynthesis models in low-mass AGB stars, solving the long-standing problem of F in Galactic AGB stars. Nevertheless, some SC-type carbon stars still show larger F abundances than predicted by stellar models. The possibility that these stars are of larger mass is briefly discussed.

Chemical Evolution of the Orion Association. IV. The Oxygen and Iron Abundances of F and G Stars

Oxygen and iron abundances are derived for a sample of pre-main-sequence F and G stars of the Orion association. Results for association members are compared with results published previously for main-sequence B stars in the association. The abundances reveal that the F and G stars exhibit the pattern of abundances shown by the main-sequence B stars: the stars have a single iron abundance, but there is a star-to-star variation of oxygen abundance. Oxygen-rich and oxygen-poor stars are roughly segregated on the sky.

BERYLLIUM AND IRON ABUNDANCES OF THE SOLAR TWINS 16 CYGNI A AND B

Red (signal-to-noise ratio of S/N D 1000 pixel~1) and ultraviolet pixel~1) Keck High (S/N Z 100 Resolution Echelle Spectrograph (HIRES) spectra (R D 45,000 3 pixels) are used to derive the iron (Fe) and beryllium (Be) abundances in each of the solar twins 16 Cygni A and B. Self-consistent spectroscopic solutions yield, for 16 Cyg A and B, respectively, and 5760 ^ 20 K, log g 4.30 ^ 0.06 T eff 5795 ^ 20 and 4.40 ^ 0.06, m 1.25 ^ 0.05 and 1.12 ^ 0.05 km s~1, and (Fe/H) 0.04 ^ 0.02 and 0.06 ^ 0.02. If Fe is used as a surrogate for metallicity, this represents an average metallicity of 11% ^ 5% above solar. These are in excellent agreement with other recent studies of this (wide) binary. Whereas it can be argued that no single study is conclusive, the consistent —ndings of these various studies oUer compelling evidence that these stars have just barely supersolar metallicity, that 16 Cyg A is just hotter than the Sun, and that 16 Cyg B is just cooler. We have previously reported (based on Keck HIRES data) a diUerence in the lithium (Li) abundances of these stars of at least a factor of 4.5; for 16 Cyg A we detected a Li abundance of a factor of D2 above solar, and for 16 Cyg B we placed a conservative upper limit of a factor of D3 below solar. We detect Be in both stars and —nd that, if there is any diUerence between them, it must be much smallerconservatively no more than 0.2 dex. Evidence suggests that solar-type stars deplete their surface Li abundance during the main sequence, a feat that the standard stellar evolution theory has, thus far, been unable to accomplish. Whatever physical mechanism depletes the surface Li abundance must create far less of a spread in the Be abundances than it does in the Li abundances. We —nd that our Li and Be results are consistent with the predictions of Yale models that include rotationally induced mixing driven by angular momentum loss. Our results provide no evidence for a small (D0.05 dex) enhancement in the 9Be abundance of the A component relative to the B com- ponent expected if the starsLi abundance diUerence was due to accretion of planetary material by the A component. Given the errors, however, neither are we able to —rmly preclude such a signature.

Boron Abundances of B Stars of the Orion Association

The boron abundance of four B-type stars from the Orion association has been determined from Goddard High Resolution Spectrograph (GHRS)/Hubble Space Telescope (HST) spectra covering the B II 1362 A resonance line. After correction for large non-LTE effects, the mean boron abundance is similar to the meteoritic abundance, but a factor of 4 (0.6 dex) greater than published abundances of boron in interstellar gas. This difference in boron abundance between young stars and gas is unexplained. The Orion stars were selected by their oxygen abundances: two are enriched in oxygen by about 60% relative to the others. Analysis of the GHRS spectra shows that the O-rich stars are deficient in boron by about 250% (0.4 dex) relative to the O-poor stars. Then, if the differences in oxygen abundance reflect differing levels of contamination of the natal clouds with (O-rich) ejecta of local Type II supernovae, the ejecta were not enriched in boron synthesized by the ν-process.

The First Fluorine Abundance Determinations in Extragalactic Asymptotic Giant Branch Carbon Stars

Fluorine (19F) abundances (or upper limits) are derived in six extragalactic asymptotic giant branch (AGB) carbon stars from the HF(1-0) R9 line at 2.3358 ?m in high-resolution spectra. The stars belong to the Local Group galaxies, Large Magellanic Cloud, Small Magellanic Cloud, and Carina dwarf spheroidal, spanning more than a factor of 50 in metallicity. This is the first study to probe the behavior of F with metallicity in intrinsic extragalactic C-rich AGB stars. Fluorine could be measured only in four of the target stars, showing a wide range in F enhancements. Our F abundance measurements together with those recently derived in Galactic AGB carbon stars show a correlation with the observed carbon and s-element enhancements. The observed correlations, however, display a different dependence on the stellar metallicity with respect to theoretical predictions in low-mass, low-metallicity AGB models. We briefly discuss the possible reasons for this discrepancy. If our findings are confirmed in a larger number of metal-poor AGBs, the issue of F production in AGB stars will need to be revisited.

The origin of fluorine: abundances in AGB carbon stars revisited

Revised spectroscopic parameters for the HF molecule and a new CN line list in the 2.3 mu region have been recently available, allowing a revision of the F content in AGB stars. AGB carbon stars are the only observationally confirmed sources of fluorine. Nowadays there is not a consensus on the relevance of AGB stars in its Galactic chemical evolution. The aim of this article is to better constrain the contribution of these stars with a more accurate estimate of their fluorine abundances. Using new spectroscopic tools and LTE spectral synthesis, we redetermine fluorine abundances from several HF lines in the K-band in a sample of Galactic and extragalactic AGB carbon stars of spectral types N, J and SC spanning a wide range of metallicities. On average, the new derived fluorine abundances are systematically lower by 0.33 dex with respect to previous determinations. This may derive from a combination of the lower excitation energies of the HF lines and the larger macroturbulence parameters used here as well as from the new adopted CN line list. Yet, theoretical nucleosynthesis models in AGB stars agree with the new fluorine determinations at solar metallicities. At low metallicities, an agreement between theory and observations can be found by handling in a different way the radiative/convective interface at the base of the convective envelope. New fluorine spectroscopic measurements agree with theoretical models at low and at solar metallicity. Despite this, complementary sources are needed to explain its observed abundance in the solar neighbourhood.

Contributions to the Galactic halo from in-situ, kicked-out, and accreted stars

Where did stars in the Milky Ways halo form? The LCDM model predicts that the Milky Ways halo was built in a bottom-up fashion, and this view is now generally accepted due to overwhelming evidence of the relics of past mergers. It is still uncertain, however, what fraction of the halo is made up of such accreted debris. Close to the time of accretion, a group of stars formed in a particular satellite of the Milky Way will show coherence spatially, kinematically, and chemically. In the inner halo where dynamical timescales are short, spatial coherence will become blurred quickly, although kinematical and chemical coherence remain. Kinematics alone may still lead to ambiguity, as a merger event can cause stars formed in the Milky Way to redistribute into rings in the halo (kicked out disk stars) and these rings can be difficult to distinguish from accreted satellite stars. Thus, to get a more complete profile of a stars formation history, both kinematical and chemical information are needed.

Stellar Parameters for a Sample of Stars with Planets

The study of chemical abundances in stars with planets is an important ingredient for the models of formation and evolution of planetary systems. In order to determine accurate abundances, it is crucial to have a reliable set of atmospheric parameters. In this work, we describe the homogeneous determination of effective temperatures, surface gravities and iron abundances for a large sample of stars with planets as well as a control sample of stars without giant planets. Our results indicate that the metallicity distribution of the stars with planets is more metal rich by ~0.13 dex than the control sample stars.