We present the results of our monitoring study of the IR photometric and spectroscopic variability of the T Tau multiple system. We also present data on the apparent position of T Tau S with respect to T Tau N, and two new spatially resolved observations of the T Tau Sa-Sb binary. T Tau N has not varied by more than 0.2 magnitudes in K and L' flux during the 8 years of our observations, though its Br gamma and Br alpha emission line fluxes have varied by nearly a factor of four during this time. For the unresolved T Tau S system, we have derived a 20 year light curve based on our images and on measurements available in the literature. T Tau S varies by 2-3 magnitudes in K and L'-band brightness in a ``redder when faint'' manner, consistent with changes along the line of sight in the amount of material that follows an ISM extinction law. Absorption in the 3.05um water-ice feature is seen only in the spectra of T Tau S and it displays variations in depth and profile. H_2 (2.12 um) emission is also detected only at the position of T Tau S; the H_2, Br gamma and Br alpha emission line fluxes also vary. We have found that the apparent positions of T Tau S with respect to T Tau N and T Tau Sb with respect to Sa are consistent with gravitationally bound orbital motion. The possible orbits of the T Tau S binary imply that Sa is likely the most massive component in this young triple. A reanalysis of the motion of the radio source associated with T Tau S provides no evidence for an ejection event in the T Tau system.Comment: 49 pages, including 17 figures. Accepted for publication in the Astrophysical Journa

The atmospheric properties above three sites (Dome C, Dome A and the South Pole) on the Internal Antarctic Plateau are investigated for astronomical applications using the monthly median of the analyses from ECMWF (the European Centre for Medium‐Range Weather Forecasts). Radiosoundings extended on a yearly time‐scale at the South Pole and Dome C are used to quantify the reliability of the ECMWF analyses in the free atmosphere as well as in the boundary and surface layers, and to characterize the median wind speed in the first 100 m above the two sites. Thermodynamic instability properties in the free atmosphere above the three sites are quantified with monthly median values of the Richardson number. We find that the probability to trigger thermodynamic instabilities above 100 m is smaller on the Internal Antarctic Plateau than on mid‐latitude sites. In spite of the generally more stable atmospheric conditions of the Antarctic sites compared to mid‐latitude sites, Dome C shows worse thermodynamic instability conditions than those predicted above the South Pole and Dome A above 100 m. A rank of the Antarctic sites done with respect to the strength of the wind speed in the free atmosphere (ECMWF analyses) as well as the wind shear in the surface layer (radiosoundings) is presented.

In recent 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 rather good values for the seeing above 30-35 m: around 0.3 arcsec at Dome C. Site-testing campaigns are however extremely expensive; instruments provide only local measurements and atmospheric modelling might represent a step ahead in the search and selection of astronomical sites, thanks to the possibility of reconstructing three-dimensional (3D) C 2 N maps over a surface of several km. The Antarctic Plateau therefore represents an important benchmark test to evaluate the possibility of discriminating between sites on the same plateau. Our group has proven that the analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) global model do not describe the Antarctic boundary and surface layers in the plateau with the required accuracy. A better description could be obtained with a mesoscale meteorological model. The mesoscale model Meso-NH has proven to be reliable in reproducing 3D maps of optical turbulence above mid-latitude astronomical sites. In this paper we study the ability of the Meso-NH model to reconstruct the meteorological parameters as well as the optical turbulence above Dome C with different model configurations (monomodel and grid-nesting). We concentrate our attention on the abilities of the model in reproducing the optical turbulence surface-layer thickness (h sl ) and the integral of C 2 N in the free atmosphere and in the surface layer. It is worth highlighting that these are the first estimates ever made with a mesoscale model of the optical turbulence above the internal Antarctic Plateau.

Using five independent analytic and Monte Carlo simulation codes, we have studied the performance of wide field ground layer adaptive optics (GLAO), which can use a single, relatively low order deformable mirror to correct the wavefront errors from the lowest altitude turbulence. GLAO concentrates more light from a point source in a smaller area on the science detector, but unlike traditional adaptive optics, images do not become diffraction-limited. Rather the GLAO point spread function (PSF) has the same functional form as a seeing-limited PSF, and can be characterized by familiar performance metrics such as Full-Width Half-Max (FWHM). The FWHM of a GLAO PSF is reduced by 0.1 ′′ or more for optical and near-infrared wavelengths over different atmospheric conditions. For the Cerro Pachón atmospheric model this correction is even greater when the image quality is worst, which effectively eliminates "bad-seeing" nights; the best seeing-limited image quality, available only 20% of the time, can be achieved 60 to 80% of the time with GLAO. This concentration of energy in the PSF will reduce required exposure times and improve the efficiency of an observatory up to 30 to 40%. These performance gains are relatively insensitive to a number of trades including the exact field of view of a wide field GLAO system, the conjugate altitude and actuator density of the deformable mirror, and the number and configuration of the guide stars.

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 reanalyse 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 Al2O3 grains from meteorites. The reaction rates with uncertainties were taken from the NACRE compilation of Angulo et al. We include seven reaction rates in our systematic analysis of stellar models with initial masses from 1 to 3 M⊙. We find that the uncertainty of the 18O(p, α)15N reaction rate typically causes an error in the theoretical 16O/18O ratio of ≃+20/− 5 per cent. The error of the 16O/17O prediction is 10–40 per cent depending on the stellar mass, and is persistently dominated by the comparatively small uncertainty of the 16O(p, γ)17F reaction. With the new estimates on reaction rate uncertainties by the NACRE compilation, the p‐capture reactions 17O(p, α)14N and 17O(p, γ)18F have virtually no impact on theoretical predictions for stellar mass ≤1.5 M⊙. However, the uncertainty in 17O(p, α)14N has an effect comparable to or greater than that of 16O(p, γ)17F for masses >1.5 M⊙, 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.