arXiv subjects

Amanda I. Karakas

Publications and source records attributed to Amanda I. Karakas.

95 records · Page 6Linked to original sources

arXiv2008

Chemical abundances in giants stars of the tidally disrupted globular cluster NGC 6712 from high-resolution infrared spectroscopy

We present abundances of C, N, O, F, Na, and Fe in six giant stars of the tidally disrupted globular cluster NGC 6712. The abundances were derived by comparing synthetic spectra with high resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. We find large star-to-star abundance variations of the elements C, N, O, F, and Na. NGC 6712 and M4 are the only globular clusters in which F has been measured in more than two stars, and both clusters reveal F abundance variations whose amplitude is comparable to, or exceeds, that of O, a pattern which may be produced in M > 5M_sun AGB stars. Within the limited samples, the F abundance in globular clusters is lower than in field and bulge stars at the same metallicity. NGC 6712 and Pal 5 are tidally disrupted globular clusters whose red giant members exhibit O and Na abundance variations not seen in comparable metallicity field stars. Therefore, globular clusters like NGC 6712 and Pal 5 cannot contribute many field stars and/or field stars do not form in environments with chemical enrichment histories like that of NGC 6712 and Pal 5. Although our sample size is small, from the amplitude of the O and Na abundance variations, we infer a large initial cluster mass and tentatively confirm that NGC 6712 was once one of the most massive globular clusters in our Galaxy.

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arXiv2007

Stellar Models and Yields of Asymptotic Giant Branch Stars

We present stellar yields calculated from detailed models of low and intermediate-mass asymptotic giant branch (AGB) stars. We evolve models with a range of mass from 1 to 6Msun, and initial metallicities from solar to 1/200th of the solar metallicity. Each model was evolved from the zero age main sequence to near the end of the thermally-pulsing AGB phase, and through all intermediate phases including the core He-flash for stars initially less massive than 2.5Msun. For each mass and metallicity, we provide tables containing structural details of the stellar models during the TP-AGB phase, and tables of the stellar yields for 74 species from hydrogen through to sulphur, and for a small number of iron-group nuclei. All tables are available for download. Our results have many applications including use in population synthesis studies and the chemical evolution of galaxies and stellar systems, and for comparison to the composition of AGB and post-AGB stars and planetary nebulae.

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arXiv2007

Yields from single AGB stars

In order to understand the composition of planetary nebulae we first need to study the nucleosynthesis occurring in the progenitor star during the thermally-pulsing Asymptotic Giant Branch (AGB) phase. I present an overview of single AGB evolution, with an emphasis on the mixing processes that alter the envelope composition, followed by a discussion of the stellar yields available from single AGB stellar models.

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arXiv2004

HR4049: signature of nova nucleosynthesis ?

The post-Asymptotic Giant Branch (AGB) star HR4049 is in an eccentric binary system with a relatively short period probably surrounded by a dusty circumbinary disk. Extremely anomalous oxygen isotopic ratios, O16/O17 ~ O16/O18 ~ 7, have been measured from CO_2 molecules likely residing in the disk. Such a composition cannot be explained in the framework of AGB and post-AGB evolution while it can be qualitatively associated with the nucleosynthesis occurring during nova outbursts. We discuss nova models, the presence of a white dwarf companion to HR4049 and possible scenarios for the dynamical evolution of this binary system. Circumbinary disks in which mixing occurs between red-giant and nova material may also be invoked as the site of formation of some rare types of meteoritic presolar grains.

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arXiv2004

Reaction Rates Uncertainties and the Production of F19 in AGB Stars

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge up and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(alpha,gamma)18O and the 19F(alpha,p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. The importance of the partial mixing zone is reduced when using our estimate for the 14C(alpha,gamma)18O rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, also in relation to reducing the uncertainties of the 14C(alpha,gamma)18O reaction rate. At the same time the possible effect of Cool Bottom Processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could in principle help matching the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

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