Uncertainties in AGB Evolution and Nucleosynthesis

We summarise the evolution and nucleosynthesis in AGB and Super-AGB stars. We then examine the major sources of uncertainty, especially mass-loss.Comment: 8 pages, no figures. Invited review presented at The 11th Pacific Rim Conference on Stellar Astrophysics "Physics and Chemistry of the Late Stages of Stellar Evolution

Updated stellar yields from Asymptotic Giant Branch models

An updated grid of stellar yields for low to intermediate-mass thermally-pulsing Asymptotic Giant Branch (AGB) stars are presented. The models cover a range in metallicity Z = 0.02, 0.008, 0.004, and 0.0001, and masses between 1Msun to 6Msun. New intermediate-mass Z = 0.0001 AGB models are also presented, along with a finer mass grid than used in previous studies. The yields are computed using an updated reaction rate network that includes the latest NeNa and MgAl proton capture rates, with the main result that between ~6 to 30 times less Na is produced by intermediate-mass models with hot bottom burning. In low-mass AGB models we investigate the effect on the production of light elements of including some partial mixing of protons into the intershell region during the deepest extent of each third dredge-up episode. The protons are captured by the abundant 12C to form a 13C pocket. The 13C pocket increases the yields of 19F, 23Na, the neutron-rich Mg and Si isotopes, 60Fe, and 31P. The increase in 31P is by factors of ~4 to 20, depending on the metallicity. Any structural changes caused by the addition of the 13C pocket into the He-intershell are ignored. However, the models considered are of low mass and any such feedback is likely to be small. Further study is required to test the accuracy of the yields from the partial-mixing models. For each mass and metallicity, the yields are presented in a tabular form suitable for use in galactic chemical evolution studies or for comparison to the composition of planetary nebulae.Comment: Accepted for publication in MNRAS; 15 page

The Effects of Fuel Type Above Adsorbtive Properties of the Nickel Ferrite Nanoparticles synthesized with Microwave Method

In this study, we were able to develop a new and practical method for the synthesis of the NiFe2O4 nanoparticles. The synthesis of nickel ferrite nanoparticles was used various fuel substances such as glycine, urea and citric acid. The synthesis mixture prepared in stoichiometric rates was put in to the kitchen type microwave oven. In the end of reaction time was obtained a brown-black solid. The obtained solid was characterized with X-Ray Powder Diffraction and Scanning Electron Microscopy. The results of this analysis showed that all of the obtained particles has got nano-size particle size distribution. Later, the nanoparticles were analyzed by using a surface area analyzer and their adsorptive properties were investigated such as surface area and average pore size. We observed that the nanoparticles prepared with urea has the highest surface area. However, fuel type used in synthesis is quite effective on the surface properties of NiFe2O4 nanoparticles. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3527

The Effects of Fuel Type Above Magnetic Properties of the Nickel Ferrite Nanoparticles Synthesized with Microwave Method

The synthesis of nickel ferrite nanoparticles was used various fuel substances such as glycine, urea and citric acid. The mixture prepared in stoichiometric rates was put in to the kitchen type microwave oven. In the end of reaction time was obtained a brown-black solid. The obtained solid was characterized with X-Ray Powder Diffraction and Scanning Electron Microscopy. The results of this analysis showed that all of the obtained particles have got nano-size particle size distribution. To determine the magnetic properties of the nanoparticles were analyzed by using a vibrating sample magnetometer. Fuel type used in synthesis is quite effective on the magnetic properties of NiFe2O4 nanoparticles. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3527

Reaction Rate Uncertainties: NeNa and MgAl in AGB Stars

We study the effect of uncertainties in the proton-capture reaction rates of the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB nucleosynthesis is associated with the production of sodium, radioactive Al26 and the heavy magnesium isotopes, and it is possibly responsible for the O, Na, Mg and Al abundance anomalies observed in globular cluster stars. We model HBB with an analytic code based on full stellar evolution models so we can quickly cover a large parameter space. The reaction rates are varied first individually, then all together. This creates a knock-on effect, where an increase of one reaction rate affects production of an isotope further down the reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most susceptible to current nuclear reaction rate uncertainties.Comment: Presented at NIC-IX, International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200

Heavy Element Nucleosynthesis in the Brightest Galactic Asymptotic Giant Branch stars

We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5Msun to 9Msun, with an initial metallicity of Z =0.02 ([Fe/H] = 0.14). In our previous studies we used the Vassiliadis & Wood mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted that for stars over 2.5Msun the superwind should be delayed until P ~ 750 days at 5Msun. We calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P ~ 700-800 days in models of M = 5, 6, and 7Msun. Post-processing nucleosynthesis calculations show that the 6 and 7Msun models produce the most Rb, with [Rb/Fe] ~ 1 dex, close to the average of most of the Galactic Rb-rich stars ([Rb/Fe] ~ 1.4 plus or minus 0.8 dex). Changing the rate of the 22Ne + alpha reactions results in variations of [Rb/Fe] as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the 22Ne(a,n)25Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final [Rb/Fe] of ~ 1.4 dex although with C/O ratios > 1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.Comment: 37 pages, accepted for publication in the Astrophysical Journal, minor modifications to text and Tables 2 and 3, reference adde