Positronium ionisation in collision with He atoms

Positronium is the lightest known atom, consisting of an electron and its antiparticle the positron. Because of its light mass (comparable with that of the electron and positron, rather than conventional atoms), recoil effects are expected to play an important role in its scattering from atomic and molecular targets. In Positronium the centres of charge and mass coincide, leading to a zero-static interaction and enhancing the relative importance of electron-exchange effects. Up until now, Positronium beam experiments have been restricted to total cross-section measurements from simple target atoms and molecules i.e. H2, He and Ar. Significant discrepancies exist among various (theoretical and experimental) determinations of the Positronium-He total cross-section. In addition to their intrinsic interest. Positronium-atom partial-cross-sections are expected to provide a more sensitive test of our understanding of this collision system. In this work, the ionisation cross-section of Positronium has been measured for the first time. A monoenergetic Positronium beam has been created through charge exchange of positrons in a gaseous target and positrons, originating from the break-up of positronium in collision with He atoms, have been detected with a time-of-flight system. Measurements are presented in the energy range 10 - 40eV and absolute break-up cross-section values have been achieved by measuring explicitly both the positron and Positronium detection efficiencies. From the measured times-of-flight, longitudinal energy spreads of the residual positrons have also been obtained. The distributions have been found to be peaked at around 50% of the residual energy, suggesting a strong correlation between the residual particles. The present work is expected to stimulate further theoretical and experimental activity in the study of Positronium-atom interactions. Possible future new directions are discussed

Evidence for universality in the initial planetesimal mass function

Planetesimals may form from the gravitational collapse of dense particle clumps initiated by the streaming instability. We use simulations of aerodynamically coupled gas-particle mixtures to investigate whether the properties of planetesimals formed in this way depend upon the sizes of the particles that participate in the instability. Based on three high resolution simulations that span a range of dimensionless stopping time $6 \times 10^{-3} \leq \tau \leq 2$ no statistically significant differences in the initial planetesimal mass function are found. The mass functions are fit by a power-law, ${\rm d}N / {\rm d}M_p \propto M_p^{-p}$, with $p=1.5-1.7$ and errors of $\Delta p \approx 0.1$. Comparing the particle density fields prior to collapse, we find that the high wavenumber power spectra are similarly indistinguishable, though the large-scale geometry of structures induced via the streaming instability is significantly different between all three cases. We interpret the results as evidence for a near-universal slope to the mass function, arising from the small-scale structure of streaming-induced turbulence.Comment: 7 pages, 4 figures, accepted to ApJ Letters after minor modifications, including two new figures and some new text that better clarify our result

Magnetically driven accretion in protoplanetary discs

We characterize magnetically driven accretion at radii between 1 au and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision