Helical magnetorotational instability in a Taylor-Couette flow with strongly reduced Ekman pumping

The magnetorotational instability (MRI) is thought to play a key role in the formation of stars and black holes by sustaining the turbulence in hydrodynamically stable Keplerian accretion discs. In previous experiments the MRI was observed in a liquid metal Taylor-Couette flow at moderate Reynolds numbers by applying a helical magnetic field. The observation of this helical MRI (HMRI) was interfered with a significant Ekman pumping driven by solid end-caps that confined the instability only to a part of the Taylor-Couette cell. This paper describes the observation of the HMRI in an improved Taylor-Couette setup with the Ekman pumping significantly reduced by using split end-caps. The HMRI, which now spreads over the whole height of the cell, appears much sharper and in better agreement with numerical predictions. By analyzing various parameter dependencies we conclude that the observed HMRI represents a self-sustained global instability rather than a noise-sustained convective one.Comment: 30 pages, 22 figures, submitted to PR

Three-dimensional evolution of magnetic fields in a differentially rotating stellar radiative zone

The question of the origin and evolution of magnetic fields in stars possessing a radiative envelope, like the A-type stars, is still regarded as a challenge for stellar physics. Those zones are likely to be differentially rotating, which suggests that strong interactions between differential rotation and magnetic fields could be at play. We numerically compute the joint evolution of the magnetic and velocity fields in a 3D spherical shell starting from an initial profile for the poloidal magnetic field and differential rotation. The poloidal magnetic field is initially wound-up by the differential rotation to produce a toroidal field which becomes unstable. In the particular setup studied here where the differential rotation is dominant, the magneto-rotational instability is triggered. The growth rate of the instability depends mainly on the initial rotation rate, while the background state typically oscillates over a poloidal Alfv\'en time. We thus find that the axisymmetric magnetic configuration is strongly modified by the instability only if the ratio between the poloidal Alfv\'en frequency and the rotation rate is sufficiently small. An enhanced transport of angular momentum is found in the most unstable cases: the typical time to flatten the rotation profile is then much faster than the diffusion time scale. We conclude that the magneto-rotational instability is always favored (over the Tayler instability) in unstratified spherical shells when an initial poloidal field is sheared by a sufficiently strong cylindrical differential rotation. A possible application to the magnetic desert observed among A stars is given. We argue that the dichotomy between stars exhibiting strong axisymmetric fields (Ap stars) and those harboring a sub-Gauss magnetism could be linked to the threshold for the instability.Comment: Accepted for publication in A&A, 21 pages, 13 figure

Synthesis facilitates an understanding of the structural basis for translation inhibition by the lissoclimides

International audienceThe lissoclimides are unusual succinimide-containing labdane diterpenoids that were reported to be potent cytotoxins. Our short semisynthesis and analogue-oriented synthesis approaches provide a series of lissoclimide natural products and analogues that expand the structure-activity relationships (SARs) in this family. The semisynthesis approach yielded significant quantities of chlorolissoclimide (CL) to permit an evaluation against the National Cancer Institute's 60-cell line panel and allowed us to obtain an X-ray co-crystal structure of the synthetic secondary metabolite with the eukaryotic 80S ribosome. Although it shares a binding site with other imide-based natural product translation inhibitors, CL engages in a particularly interesting and novel face-on halogen-π interaction between the ligand's alkyl chloride and a guanine residue. Our analogue-oriented synthesis provides many more lissoclimide compounds, which were tested against aggressive human cancer cell lines and for protein synthesis inhibitory activity. Finally, computational modelling was used to explain the SARs of certain key compounds and set the stage for the structure-guided design of better translation inhibitors