A Bayesian 3-D linear gravity inversion for complex density distributions: application to the Puysegur subduction system

SUMMARY We have developed a linear 3-D gravity inversion method capable of modelling complex geological regions such as subduction margins. Our procedure inverts satellite gravity to determine the best-fitting differential densities of spatially discretized subsurface prisms in a least-squares sense. We use a Bayesian approach to incorporate both data error and prior constraints based on seismic reflection and refraction data. Based on these data, Gaussian priors are applied to the appropriate model parameters as absolute equality constraints. To stabilize the inversion and provide relative equality constraints on the parameters, we utilize a combination of first and second order Tikhonov regularization, which enforces smoothness in the horizontal direction between seismically constrained regions, while allowing for sharper contacts in the vertical. We apply this method to the nascent Puysegur Trench, south of New Zealand, where oceanic lithosphere of the Australian Plate has underthrust Puysegur Ridge and Solander Basin on the Pacific Plate since the Miocene. These models provide insight into the density contrasts, Moho depth, and crustal thickness in the region. The final model has a mean standard deviation on the model parameters of about 17 kg m–3, and a mean absolute error on the predicted gravity of about 3.9 mGal, demonstrating the success of this method for even complex density distributions like those present at subduction zones. The posterior density distribution versus seismic velocity is diagnostic of compositional and structural changes and shows a thin sliver of oceanic crust emplaced between the nascent thrust and the strike slip Puysegur Fault. However, the northern end of the Puysegur Ridge, at the Snares Zone, is predominantly buoyant continental crust, despite its subsidence with respect to the rest of the ridge. These features highlight the mechanical changes unfolding during subduction initiation.

Mathematical Model of Mechanical Virion-Cell Interaction During Early Engulfment in HIV

While HIV entry into host cells has been extensively studied from a biological and biochemical perspective, the influence of mechanical parameters of virions and cells on engulfment and invagination is not well understood. The present work aimed at developing a mathematical model to quantify effects of mechanical and morphological parameters on engulfment forces and energies of HIV particles. Invagination force and engulfment energy were described as analytical functions of radius and elastic modulus of virion and cell, ligand-receptor energy density, receptor complex density, and engulfment depth for early stage engulfment. The models were employed to study the effects of (a) virion-membrane contact geometry on required invagination force for global cell geometries and ultrastructural cell membrane features, and (b) virion radius and number of gp120 proteins on engulfment energy. The invagination force was equal for cells of various sizes (i.e. macrophages and lymphocytes), but lower when considering ultrastructural membrane. The magnitude of the normalised engulfment energy was higher for a mature than for an immature, larger virion with the same number of 72 gp120 spikes, but it decreased for a mature virion with a reduced number of gp120 spikes. The results suggest that for early stage engulfment (1) localised cell membrane features promote invagination and may play a role in entry ability, and (2) shedding of gp120 proteins during maturation reduces engulfment energy which is expected to reduce entry ability.

Influence of the elastic deformations on the form factor of polyhedral nanocrystals: the illustrative example of the pseudomorphic inclusion

Using an analytically tractable example, the pseudomorphic inclusion, this article examines the influence of elastic deformations on the form factor of polyhedral nanocrystals. A control parameter, the total amplitude of the variation of the complex density phase, is identified and it is shown that for low enough deformations the characteristic asymptotic behaviours as a function of the scattering vector associated with the polyhedral crystal shape are preserved, leading to a strong contrast in the dependence of the form factor on orientation. Using the sections method, it is explained why these results can be generalized to more realistic elastic situations.

Magnetic fields in Bok globules: multi-wavelength polarimetry as tracer across large spatial scales

Context. The role of magnetic fields in the process of star formation is a matter of continuous debate. Clear observational proof of the general influence of magnetic fields on the early phase of cloud collapse is still pending. In an earlier study on Bok globules with simple structures, we find strong indications of dominant magnetic fields across large spatial scales. Aims. The aim of this study is to test the magnetic field influence across Bok globules with more complex density structures. Methods. We apply near-infrared polarimetry to trace the magnetic field structure on scales of 104–105 au (~0.05–0.5pc) in selected Bok globules. The combination of these measurements with archival data in the optical and sub-mm wavelength range allows us to characterize the magnetic field on scales of 103–106 au (~0.005–5pc). Results. We present polarimetric data in the near-infrared wavelength range for the three Bok globules CB34, CB56, and [OMK2002]18, combined with archival polarimetric data in the optical wavelength range for CB34 and CB56, and in the submillimeter wavelength range for CB34 and [OMK2002]18. We find a strong polarization signal (P ≥ 2%) in the near-infrared for all three globules. For CB34, we detect a connection between the structure on scales of 104–105 au (~0.05–0.5pc) to 105–106 au (~0.5–5pc). For CB56, we trace aligned polarization segments in both the near-infrared and optical data, suggesting a connection of the magnetic field structure across the whole globule. In the case of [OMK2002]18, we find ordered polarization structures on scales of 104–105 au (~0.05–0.5pc). Conclusions. We find strongly aligned polarization segments on large scales which indicate dominant magnetic fields across Bok globules with complex density structures. To reconcile our findings in globules, the lowest mass clouds known, and the results on intermediate (e.g. Taurus) and more massive (e.g. Orion) clouds, we postulate a mass-dependent role of magnetic fields, whereby magnetic fields appear to be dominant on low and high mass but rather subdominant on intermediate mass clouds.

Satisfying positivity requirement in the Beyond Complex Langevin approach

The problem of finding a positive distribution, which corresponds to a given complex density, is studied. By the requirement that the moments of the positive distribution and of the complex density are equal, one can reduce the problem to solving the matching conditions. These conditions are a set of quadratic equations, thus Groebner basis method was used to find its solutions when it is restricted to a few lowest-order moments. For a Gaussian complex density, these approximate solutions are compared with the exact solution, that is known in this special case.