Search

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). 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(a, g)18O and 19F(a, p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(a, g)18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass [?]3 M and of about a factor of 7 in stellar models of mass [?]5 M. This larger effect at high masses is due to the high uncertainties of the 19F(a, p)22Ne reaction 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, as well as in relation to reducing the uncertainties of the 14C(a, g)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 to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

A method is proposed for deriving the form of a turbulence-distorted wave front from Shack–Hartmann data by modeling the polychromatic imaging process in detail. In this way a higher order of reconstruction is possible than by methods that use only the barycenters (centers of gravity) of the individual spots. Simulations are presented showing the possible gains for a range of conditions of turbulence and photon flux. When photon noise is negligible the residual phase errors are approximately three times smaller than what is achievable by use of only the barycenters. The choice of functions for representing wave-front distortions is also considered with regard to the quality of the reconstruction obtained and to numerical efficiency.

For many applications in adaptive optics the eigenfunctions of the power spectrum of turbulence-induced distortions on the actual telescope aperture constitute the ideal basis for modeling wave fronts. The mathematical derivation of these functions is reviewed, and a procedure for their calculation, allowing for a circular central obscuration and leaving considerable freedom in the form of the structure function, is described. Examples of wave-front simulations are presented for a range of power-law structure functions and for Kolmogorov structure functions with an outer scale smaller than the diameter of the region being modeled.

We present a simple analytic model of a composite polytropic star, which exhibits a limiting Scho¨nberg–Chandrasekhar core mass fraction strongly analogous to the classic numerical result for an isothermal core, a radiative envelope and a μ‐jump (i.e. a molecular weight jump) at the interface. Our model consists of an nc= 5 core, an ne= 1 envelope and a μ‐jump by a factor ≥ 3; the core mass fraction cannot exceed 2/π. We use the classic UVplane to show that composite models will exhibit a Scho¨nberg–Chandrasekhar limit only if the core is ‘soft’, i.e. has nc≥ 5, and the envelope is ‘hard’, i.e. has ne< 5; in the critical case (nc= 5), the limit only exists if the μ‐jump is sufficiently large, ≥ 6/(ne+ 1).

Shipley-Hartford (SH) and Wechsler-Bellevue (WB) scores were compared for a random group of psychiatric patients. It was found that the SH serves as an extremely accurate screening test for selection decisions (rather than classification decisions) since all subjectsobtaining at least an Average SH IQ achieved the same or a higher level on the WB. The importance of the SH as a rapid and accurate intellectual screening instrument is discussed with reference to military, industrial, trade school, and hospital settings.