η Carinae: high angular resolution continuum, H30α and He30α ALMA images

ABSTRACT We present images of η Carinae in the recombination lines H30α and He30α and the underlying continuum with 50 mas resolution (110 au), obtained with ALMA. For the first time, the 230 GHz continuum image is resolved into a compact core, coincident with the binary system position, and a weaker extended structure to the NW of the compact source. Iso-velocity images of the H30α recombination line show at least 16 unresolved sources with velocities between −30 and −65 km s−1 distributed within the continuum source. A NLTE model, with density and temperature of the order of 107 cm−3 and 104 K, reproduce both the observed H30α line profiles and their underlying continuum flux densities. Three of these sources are identified with Weigelt blobs D, C, and B; estimating their proper motions, we derive ejection times (in years) of 1952.6, 1957.1, and 1967.6, respectively, all of which are close to periastron passage. Weaker H30α line emission is detected at higher positive and negative velocities, extending in the direction of the Homunculus axis. The He30α recombination line is also detected with the same velocity of the narrow H30α line. Finally, the close resemblance of the H30α image with that of an emission line that was reported in the literature as HCO+(4–3) led us to identify this line as H40δ instead, an identification that is further supported by modelling results. Future observations will enable to determine the proper motions of all the compact sources discovered in the new high angular resolution data of η Carinae.

Towards an analytical description of active microswimmers in clean and in surfactant-covered drops

Geometric confinements are frequently encountered in the biological world and strongly affect the stability, topology, and transport properties of active suspensions in viscous flow. Based on a far-field analytical model, the low-Reynolds-number locomotion of a self-propelled microswimmer moving inside a clean viscous drop or a drop covered with a homogeneously distributed surfactant, is theoretically examined. The interfacial viscous stresses induced by the surfactant are described by the well-established Boussinesq-Scriven constitutive rheological model. Moreover, the active agent is represented by a force dipole and the resulting fluid-mediated hydrodynamic couplings between the swimmer and the confining drop are investigated. We find that the presence of the surfactant significantly alters the dynamics of the encapsulated swimmer by enhancing its reorientation. Exact solutions for the velocity images for the Stokeslet and dipolar flow singularities inside the drop are introduced and expressed in terms of infinite series of harmonic components. Our results offer useful insights into guiding principles for the control of confined active matter systems and support the objective of utilizing synthetic microswimmers to drive drops for targeted drug delivery applications. Graphical abstract

High Resolution Imaging of the South Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

<p>Large quantities of fluids are predicted to be expelled from compacting sediments on subduction margins. Fluid expulsion is thought to be focussed, but its exact locations are usually constrained on very small scales and rarely can be resolved using velocity images obtained from traditional velocity analysis and ray-based tomography because of their resolution and accuracy limitation. However, with recent advancement in computing power, the full waveform inversion (FWI) is a powerful alternative to those traditional approaches as it uses phase and amplitude information contained in seismic data to yield a high-resolution velocity model of the subsurface.</p><p>Here, we applied elastic FWI along an 85 Km long 2D multichannel seismic profile on the southern Hikurangi margin, New Zealand. Our processing sequence includes: (1) downward continuation, (2) 2D traveltime tomography, and (3) full waveform inversion of wide-angle seismic data. We will present the final high-resolution velocity model and our interpretation of the fluid flow regimes associated with both the deforming overriding plate and the subducting plate.</p>

Elastic least-squares reverse time migration with density variations

Elastic least-squares reverse time migration (ELSRTM) is a powerful tool to retrieve high-resolution subsurface images of the earth’s interior. By minimizing the differences between synthetic and observed data, ELSRTM can improve spatial resolution and reduce migration artifacts. However, conventional ELSRTM methods usually assume constant density models, which cause inaccurate amplitude performance in resulting images. To partially remedy this problem, we have developed a new ELSRTM method that considers P- and S-wave velocity and density variations. Our method can simultaneously obtain P- and S-wave velocity and density images with enhanced amplitude fidelity and suppressed parameter crosstalk. In addition, it can provide subsurface elastic impedance images by summing the inverted velocity images with the density image. Compared with the conventional ELSRTM method, our method can improve the quality of final images and provide more accurate reflectivity estimates. Numerical experiments on a horizontal reflector model and a Marmousi-II model demonstrate the effectiveness of this method.