Turbulent transport and dynamo in sheared MHD turbulence with a non-uniform magnetic field

We investigate three-dimensional magnetohydrodynamics turbulence in the presence of velocity and magnetic shear (i.e., with both a large-scale shear flow and a nonuniform magnetic field). By assuming a turbulence driven by an external forcing with both helical and nonhelical spectra, we investigate the combined effect of these two shears on turbulence intensity and turbulent transport represented by turbulent diffusivities (turbulent viscosity, α and β effect) in Reynolds-averaged equations. We show that turbulent transport (turbulent viscosity and diffusivity) is quenched by a strong flow shear and a strong magnetic field. For a weak flow shear, we further show that the magnetic shear increases the turbulence intensity while decreasing the turbulent transport. In the presence of a strong flow shear, the effect of the magnetic shear is found to oppose the effect of flow shear (which reduces turbulence due to shear stabilization) by enhancing turbulence and transport, thereby weakening the strong quenching by flow shear stabilization. In the case of a strong magnetic field (compared to flow shear), magnetic shear increases turbulence intensity and quenches turbulent transport

Transverse spin physics at STAR

The transverse spin physics program at STAR is pursuing two complementary goals. The first is to extract quark transversity, and the second is to understand the origin of large transverse single spin asymmetry (AN) observed in the forward region. The mid-rapidity Collins and Interference Fragmentation Function (IFF) measurements at √s = 200 GeV are aimed at the extraction of transversity. In the forward region, the measurements of inclusive π0 and η meson AN, as functions of xF and pT at √s = 200 GeV and 500 GeV, allow for comparisons against theoretical models that incorporate the effects of either higher-twist, or partonic transverse momenta. The planned forward upgrade will enable the measurements of AN for prompt photons, jets, and the Drell-Yan process

Generation of coherent magnetic fields in sheared inhomogeneous turbulence: No need for rotation?

Coherent magnetic fields are often believed to be generated by the combination of stretching by differential rotation and turbulent amplification of magnetic field, via the so-called alpha effect. The latter is known to exist in helical turbulence, which is envisioned to arise due to both rotation and convection in solar-type stars. In this contribution, a turbulent flow driven by a nonhelical inhomogeneous forcing and its kinematic dynamo action are studied for a uniform magnetic field in the background of a linear shear flow. By using a quasilinear analysis and a nonperturbative method utilizing a time-dependent wave number, turbulence property and electromotive force are computed for arbitrary shear strength. Due to the large-scale shear flow, the turbulence is highly anisotropic, as a consequence, so is the electromotive force. The latter is found to exist even without rotation due to the combined effect of shear flow and inhomogeneous forcing, containing not only the alpha effect but also magnetic pumping (the gamma effect representing a transport of magnetic flux by turbulence). Specifically, without shear, only the magnetic pumping exists, aligned with the direction of inhomogeneity. For a weak but nonzero shear, the combined effects of shear and inhomogeneous forcing modify the structure of the magnetic pumping when the inhomogeneity is in the plane of the shear flow, the magnetic pumping becoming bidimensional in that plane. It also induces an alpha tensor which has nondiagonal components. When the inhomogeneity is perpendicular to the plane of the shear flow, the alpha effect has three nonzero diagonal components and one off-diagonal component. However, for a sufficiently strong shear, the gamma and alpha effects are suppressed due to shear stabilization which damps turbulence. A simplified dynamo model is then proposed where a large-scale dynamo arises due to the combined effect of shear flow and inhomogeneous forcing. In particular, the growth of a large-scale axisymmetric magnetic field is demonstrated in case of an inhomogeneity which is perpendicular to the plane of the shear flow. Interesting implications of these results for the structure of magnetic fields in star with slow rotation are discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3551700

The organic chemistry in the innermost, infalling envelope of the Class 0 protostar L483

Context: The protostellar envelopes, outflow and large-scale chemistry of Class~0 and Class~I objects have been well-studied, but while previous works have hinted at or found a few Keplerian disks at the Class~0 stage, it remains to be seen if their presence in this early stage is the norm. Likewise, while complex organics have been detected toward some Class~0 objects, their distribution is unknown as they could reside in the hottest parts of the envelope, in the emerging disk itself or in other components of the protostellar system, such as shocked regions related to outflows. Aims: In this work, we aim to address two related issues regarding protostars: when rotationally supported disks form around deeply embedded protostars and where complex organic molecules reside in such objects. Methods: We observed the deeply embedded protostar, L483, using Atacama Large Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a high angular resolution down to $\sim$~0.1$^{\prime\prime}$ (20~au) scales. Results: We find that the kinematics of CS~$J=7$--$6$ and H$^{13}$CN~$J=4$--$3$ are best fitted by the velocity profile from infall under conservation of angular momentum and not by a Keplerian profile. The spatial extents of the observed complex organics are consistent with an estimated ice sublimation radius of the envelope at $\sim$~50~au, suggesting that the complex organics exist in the hot corino of L483. Conclusions: We find that L483 does not harbor a Keplerian disk down to at least $15$~au in radius. Instead, the innermost regions of L483 are undergoing a rotating collapse. This result highlights that some Class~0 objects contain only very small disks, or none at all, with the complex organic chemistry taking place on scales inside the hot corino of the envelope, in a region larger than the emerging disk.Comment: 19 pages, 11 figure

Dynamo quenching due to shear flow

We provide a theory of dynamo (α effect) and momentum transport in three-dimensional magnetohydrodynamics. For the first time, we show that the α effect is reduced by the shear even in the absence of magnetic field. The α effect is further suppressed by magnetic fields well below equipartition (with the large-scale flow) with different scalings depending on the relative strength of shear and magnetic field. The turbulent viscosity is also found to be significantly reduced by shear and magnetic fields, with positive value. These results suggest a crucial effect of shear and magnetic field on dynamo quenching and momentum transport reduction, with important implications for laboratory and astrophysical plasmas, in particular, for the dynamics of the Sun

Surface Modification of Melt Extruded Poly(ε-caprolactone) Nanofibers: Toward a New Scalable Biomaterial Scaffold.

A photochemical modification of melt-extruded polymeric nanofibers is described. A bioorthogonal functional group is used to decorate fibers made exclusively from commodity polymers, covalently attach fluorophores and peptides, and direct cell growth. Our process begins by using a layered coextrusion method, where poly(ε-caprolactone) (PCL) nanofibers are incorporated within a macroscopic poly(ethylene oxide) (PEO) tape through a series of die multipliers within the extrusion line. The PEO layer is then removed with a water wash to yield rectangular PCL nanofibers with controlled cross-sectional dimensions. The fibers can be subsequently modified using photochemistry to yield a "clickable" handle for performing the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction on their surface. We have attached fluorophores, which exhibit dense surface coverage when using ligand-accelerated CuAAC reaction conditions. In addition, an RGD peptide motif was coupled to the surface of the fibers. Subsequent cell-based studies have shown that the RGD peptide is biologically accessible at the surface, leading to increased cellular adhesion and spreading versus PCL control surfaces. This functionalized coextruded fiber has the advantages of modularity and scalability, opening a potentially new avenue for biomaterials fabrication

A microfabricated steel and glass radiation detector with inherent wireless signaling

This paper describes an investigation of the performance compromises imposed by a manufacturing approach that utilizes lithographic micromachining processes to fabricate a wireless beta/gamma radiation detector. The device uses in-package assembly of stainless steel electrodes and glass spacers. These elements are micromachined using photochemical etching and powder blasting, respectively. The detector utilizes a commercial, TO-5 package that is hermetically sealed at 760 Torr with an Ar fill-gas. Gas microdischarges between the electrodes, which are initiated by the radiation, transmit wideband wireless signals. The detector diameter and height are 9 and 9.6 mm, respectively, and it weighs 0.97 g. The device performance has been characterized using various sealed, radioisotope sources, e.g., 30–99 µCi from 137 Cs (which is a beta and gamma emitter) and 0.1 µCi from 90 Sr (which is a pure beta emitter). It has a measured output of >15.5 counts s _1 when in close proximity to 99 µCi from 137 Cs. The wireless signaling spans 1.25 GHz at receiving antenna-to-detector distances >89 cm, when in close proximity to a 0.1 µCi 90 Sr source. The estimated intrinsic detection efficiency (i.e. with the background rate subtracted) is 3.34% as measured with the biasing arrangement described in the paper.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90792/1/0960-1317_21_1_015003.pd