Oxygen isotopic ratios in first dredge-up red giant stars and nuclear reaction rate uncertainties revisited

We describe a general yet simple method to analyse the propagation of nuclear reaction rate uncertainties in a stellar nucleosynthesis and mixing context. The method combines post-processing nucleosynthesis and mixing calculations with a Monte Carlo scheme. With this approach we reanalyze the dependence of theoretical oxygen isotopic ratio predictions in first dredge-up red giant branch stars in a systematic way. Such predictions are important to the interpretation of pre-solar Al_2 O_3 grains from meteorites. The reaction rates with uncertainties were taken from the NACRE compilation (Angulo etal., 1999). We include seven reaction rates in our systematic analysis of stellar models with initial masses from 1 to 3 M_sun. We find that the uncertainty of reaction rate for reaction O18(p,alpha)N15 typically causes an error in the theoretical O16/O18 ratio of about +20/-5 per cent. The error of the O16/O17 prediction is +-10 to 40 per cent depending on the stellar mass, and is persistently dominated by the comparatively small uncertainty of the O16(p,gamma)F17 reaction. With the new estimates on reaction rate uncertainties by the NACRE compilation, the p-capture reactions O17(p,alpha)N14 and O17(p,gamma)F18 have virtually no impact on theoretical predictions for stellar mass 1.5 M_sun, where core mixing and subsequent envelope mixing interact. In these cases where core mixing complicates post-dredge-up surface abundances, uncertainty in other reactions have a secondary but noticeable effect on surface abundances

Meso-Nh simulations of the atmospheric flow above the Internal Antarctic Plateau

Mesoscale model such as Meso-Nh have proven to be highly reliable in reproducing 3D maps of optical turbulence (see Refs. 1, 2, 3, 4) above mid-latitude astronomical sites. These last years ground-based astronomy has been looking towards Antarctica. Especially its summits and the Internal Continental Plateau where the optical turbulence appears to be confined in a shallow layer close to the icy surface. Preliminary measurements have so far indicated pretty good value for the seeing above 30-35 m: 0.36" (see Ref. 5) and 0.27" (see Refs. 6, 7) at Dome C. Site testing campaigns are however extremely expensive, instruments provide only local measurements and atmospheric modelling might represent a step ahead towards the search and selection of astronomical sites thanks to the possibility to reconstruct 3D Cn2 maps over a surface of several kilometers. The Antarctic Plateau represents therefore an important benchmark test to evaluate the possibility to discriminate sites on the same plateau. Our group8 has proven that the analyses from the ECMWF global model do not describe with the required accuracy the antarctic boundary and surface layer in the plateau. A better description could be obtained with a mesoscale meteorological model. In this contribution we present the progress status report of numerical simulations (including the optical turbulence - Cn2) obtained with Meso-Nh above the internal Antarctic Plateau. Among the topic attacked: the influence of different configurations of the model (low and high horizontal resolution), use of the grid-nesting interactive technique, forecasting of the optical turbulence during some winter nights.Comment: 12 pages, 4 figures, SPIE 2008 conferenc

Wide field adaptive optics upper limit performances

Wide Field Adaptive Optics (WFAO) is a new proposed astronomical adaptive optics mode allowing a significant improvement of the seeing limited point spread function characteristics over large fields -- several arc minutes in diameter, using only one deformable mirror optically conjugated to an optimal altitude. In this paper, we present the WFAO upper limit performances, based on the assumption that the refractive index fluctuation field above the telescope is perfectly known. Our results are based on analytical developments for the residual phase power spectrum after WFAO correction, implemented in PAOLA, an analytical AO simulation tool, developed at the Herzberg Institute of Astrophysics. Results are presented for several sites: Mauna Kea, Cerro Tololo, Cerro Paranal. For each of these locations, we give the WFAO-PSF properties as a function of the field angle, the conjugation altitude of the deformable mirror, the imaging infrared wavelength, and the cone aperture angle over which the tomographic information is averaged to drive the deformable mirror actuators

An analytic model for natural guide star wide-field adaptive optics

NRC publication: Ye

Evaluation of the on-sky performance of Altair

NRC publication: Ye