Total and positronium formation cross sections for positron scattering from H2O and HCOOH

Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV (full-width at half-maximum (FWHM)). The energy range covered is 0.5–60 eV, including an investigation of the behavior of the onset of the positronium formation channel using measurements with a 50 meV energy step, the result of which shows no evidence of any channel coupling effects or scattering resonances for either molecule

The CHARA Array Angular Diameter of HR 8799 Favors Planetary Masses for Its Imaged Companions

HR 8799 is an hF0 mA5 gamma Doradus, lambda Bootis, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799's limb-darkened angular diameter to be 0.342 +/- 0.008 mas; this is the smallest interferometrically measured stellar diameter to date, with an error of only 2%. By combining our measurement with the star's parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 +/- 0.06 R_Sun), effective temperature (7193 +/- 87 K, consistent with F0), luminosity (5.05 +/- 0.29 L_Sun), and the extent of the habitable zone (1.62 AU to 3.32 AU). These improved stellar properties permit much more precise comparisons with stellar evolutionary models, from which a mass and age can be determined, once the metallicity of the star is known. Considering the observational properties of other lambda Bootis stars and the indirect evidence for youth of HR 8799, we argue that the internal abundance, and what we refer to as the effective abundance, is most likely near-solar. Finally, using the Yonsei-Yale evolutionary models with uniformly scaled solar-like abundances, we estimate HR 8799's mass and age considering two possibilities: 1.516 +0.038/-0.024 M_Sun and 33 +7/-13 Myr if the star is contracting toward the zero age main-sequence or 1.513 +0.023/-0.024 M_Sun and 90 +381/-50 Myr if it is expanding from it. This improved estimate of HR 8799's age with realistic uncertainties provides the best constraints to date on the masses of its orbiting companions, and strongly suggests they are indeed planets. They nevertheless all appear to orbit well outside the habitable zone of this young star.Comment: Accepted for publication in ApJ; 37 pages, 6 tables, 13 figure

Low-energy positron interactions with xenon

Low-energy interactions of positrons with xenon have been studied both experimentally and theoretically. The experimental measurements were carried out using a trap-based positron beam with an energy resolution of ̃80 meV, while the theoretical calculat

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Low energy positron interactions with the rare gases

This thesis presents measurements of low-energy positron scattering from the noble gases. Sets of total cross section measurements are presented for Ne-Xe, and are compared with prior experimental and theoretical results. The present results represent a substantial improvement on previous experiments, due to improved statistical and systematic uncertainty, as well as an energy resolution that is better than that achieved in most other experiments. Methods for calculating cross sections in a strong magnetic field are discussed, along with improvements made to the techniques for analysis, and the handling and correction, or minimisation, of systematic errors. Preliminary tests of the application of effective range theory have been conducted using theoretical total cross sections, calculated in the polarized orbital approach. These tests provide estimates of the range of applicability of effective range theory. Using this knowledge, analysis of the low-energy cross section, for energies below about 5 eV, has been carried out, using effective range theory to extract the s-wave scattering length. This work is in preparation for future measurements of low-energy total cross sections from metal vapours, where knowledge of the scattering length will provide a means of inferring the presence of theoretically predicted positron-atom bound states. Measurements of the elastic total cross section for positron scattering from He-Xe, in the region of the Ps formation threshold, are presented. These results indicate the presence of pronounced Wigner cusps in each cross section, resulting from channel coupling between the elastic and Ps formation scattering channels. These measurements are compared and contrasted with other recent experimental measurements. The trend of the channel coupling behaviour is discussed with respect to theoretical predictions

Low energy positron scattering from helium

A new experiment has been developed for high resolution studies of positron scattering from atoms and molecules. Based on the Surko trap technology, a pulsed positron beam has been used to obtain preliminary measurements of low energy, differential elastic scattering cross sections from helium. The operation of the beamline is described and preliminary absolute cross section values for scattering energies of 5, 10 and 15 eV are presented and compared with contemporary theoretical calculations

Low energy positron scattering from helium A

Positron scattering from helium has been historically limited to experiments with energy resolution of 0.5 eV or worse. A new apparatus has been developed that allows the investigation of scattering processes with an energy resolution as good as 30 meV, allowing new experimental insight into positron helium interactions. High resolution measurements of low energy scattering from helium will be presented, with a particular emphasis on the development of benchmark cross sections, the investigation of threshold behaviour and the search for positron scattering resonances

A search for Wigner cusps and resonances in positron scattering by atoms and molecules

Detailed elastic cross-section measurements in the vicinity of the positronium (Ps) formation threshold for He, Ne, Ar, Kr and Xe show a cusp-like feature, made visible by a small and localised depression of between (2 to 15%) of the cross-section value