Dynamics of magnetic flux tubes in close binary stars I. Equilibrium and stability properties

Surface reconstructions of active close binary stars based on photometric and spectroscopic observations reveal non-uniform starspot distributions, which indicate the existence of preferred spot longitudes (with respect to the companion star). We consider the equilibrium and linear stability of toroidal magnetic flux tubes in close binaries to examine whether tidal effects are capable to initiate the formation of rising flux loops at preferred longitudes near the bottom of the stellar convection zone. The tidal force and the deviation of the stellar structure from spherical symmetry are treated in lowest-order perturbation theory assuming synchronised close binaries with orbital periods of a few days. The frequency, growth time, and spatial structure of linear eigenmodes are determined by a stability analysis. We find that, despite their small magnitude, tidal effects can lead to a considerable longitudinal asymmetry in the formation probability of flux loops, since the breaking of the axial symmetry due to the presence of the companion star is reinforced by the sensitive dependence of the stability properties on the stellar stratification and by resonance effects. The orientation of preferred longitudes of loop formation depends on the equilibrium configuration and the wave number of the dominating eigenmode. The change of the growth times of unstable modes with respect to the case of a single star is very small.Comment: 11 pages, 11 figures, accepted for publication in A&

Kinematically complete study on Positron impact ionization of Helium

Within this work presented here, single ionisation of helium by impact of 80eV positrons was studied in a kinematically complete experiment. Therefore, a dedicated reaction microscope was connected to the beamline of the NEPOMUC positronmsource located at the research reactor FRMII in Garching. Using a reaction microscope the momenta of all particles in the final state can be detected. In order to guide the positrons from the high magnetic field of the beamline into the lower field of the reaction microscope with a minimal loss on intensity, solenoidal coils producing an adiabatic field change have been installed at the transition region. A sophisticated method for the data analysis was developed to reconstruct the time origin of the ionisation process also for the case of a continuous beam. Finally, we were able to identify triple coincident events originating from the fragmentation. The three-dimensional momentum vectors of the three particles in the final state could be obtained for these events. The gained cross sections show that the projectile charge which is inverse compared to electron collisions causes a high mnission of the ionised electron with the projectile in forward direction

Enhancing the Neuronal Differentiation of Mouse Embryonic Stem Cells Using Biomaterials.

Nervous system injuries remain significant clinical issues that affect hundreds of thousands of individuals each year. Spinal cord injuries are especially difficult since the wound healing process results in a glial scar, inhibiting regeneration. Current strategies for dealing with spinal cord injuries focus on stabilization and rehabilitation with minimal likelihood of any significant functional regeneration. The outlook for peripheral injuries is not quite as dire as peripheral nerve has the capacity to regenerate. However, if the defect is beyond a critical size of around 1cm, that ability is compromised. The gold standard for treating large defects is an autologous nerve graft, but there are significant drawbacks. Clearly there is room for improvement. In this study, conducting composite electrospun nanofibrous substrates were fabricated to investigate if topography and electrical stimulation could control embryonic stem cell (ESC) differentiation. First, it was determined that poly (l-lactic acid) (PLLA) nanofibers of at least 900nm promoted neuronal differentiation and neurite outgrowth. ESCs interacted with these fibers through integrin α6β1 and induced differentiation via early ERK activation. Fiber size did not have a significant effect on p38 activity. Next, PLLA was doped with polypyrrole (PPy) to improve its electrical properties. Nerve conduits fabricated with PPy/PDLLA were successful at promoting the regeneration of a rat sciatic nerve defect on par with an autologous graft. The sciatic function indices (SFI), nerve conduction velocities, triceps surae weights, and nerve fiber morphologies were all comparable between the PPy/PDLLA conduits and autografts, and both significantly better than the PDLLA conduits. Finally, PPY/PLLA was electrospun into random and aligned nanofibers. ESCs seeded on these aligned nanofibers were stimulated with 100mV for 2hrs, which induced a higher percentage of neurite-bearing cells, from around 13% to around 23%, and longer neurites, from around 80µm to around 130µm. Thus, it is clear that conducting aligned fibers combined with electrical stimulation improves ESC neurite growth. Determining the links between nanofiber-stimulated differentiation and neurogenesis allow for a better understanding of the application of ESCs in neural tissue engineering. These results provide guidance to neural tissue engineering scaffold design and insight into how ESCs interact with different topographies.PhDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111621/1/jmholz_1.pd

Field dynamics and kink-antikink production in rapidly expanding systems

Field dynamics in a rapidly expanding system is investigated by transforming from space-time to the rapidity - proper-time frame. The proper-time dependence of different contributions to the total energy is established. For systems characterized by a finite momentum cut-off, a freeze-out time can be defined after which the field propagation in rapidity space ends and the system decays into decoupled solitons, antisolitons and local vacuum fluctuations. Numerical simulations of field evolutions on a lattice for the (1+1)-dimensional $\Phi^4$ model illustrate the general results and show that the freeze-out time and average multiplicities of kinks (plus antikinks) produced in this 'phase transition' can be obtained from simple averages over the initial ensemble of field configurations. An extension to explicitly include additional dissipation is discussed. The validity of an adiabatic approximation for the case of an overdamped system is investigated. The (3+1)-dimensional generalization may serve as model for baryon-antibaryon production after heavy-ion collisions.Comment: 18 pages, 7 figures. Two references added. New subsection III.E added. Final version accepted for publication in PR

Towards a practical approach for self-consistent large amplitude collective motion

We investigate the use of an operatorial basis in a self-consistent theory of large amplitude collective motion. For the example of the pairing-plus-quadrupole model, which has been studied previously at equilibrium, we show that a small set of carefully chosen state-dependent basis operators is sufficient to approximate the exact solution of the problem accuratly. This approximation is used to study the interplay of quadrupole and pairing degrees of freedom along the collective path for realistic examples of nuclei. We show how this leads to a viable calculational scheme for studying nuclear structure, and discuss the surprising role of pairing collapse.Comment: 19 pages, 8 figures Revised version To be published in Phys. Rev.

Theoretical mass loss rates of cool main-sequence stars

We develop a model for the wind properties of cool main-sequence stars, which comprises their wind ram pressures, mass fluxes, and terminal wind velocities. The wind properties are determined through a polytropic magnetised wind model, assuming power laws for the dependence of the thermal and magnetic wind parameters on the stellar rotation rate. We use empirical data to constrain theoretical wind scenarios, which are characterised by different rates of increase of the wind temperature, wind density, and magnetic field strength. Scenarios based on moderate rates of increase yield wind ram pressures in agreement with most empirical constraints, but cannot account for some moderately rotating targets, whose high apparent mass loss rates are inconsistent with observed coronal X-ray and magnetic properties. For fast magnetic rotators, the magneto-centrifugal driving of the outflow can produce terminal wind velocities far in excess of the surface escape velocity. Disregarding this aspect in the analyses of wind ram pressures leads to overestimations of stellar mass loss rates. The predicted mass loss rates of cool main-sequence stars do not exceed about ten times the solar value. Our results are in contrast with previous investigations, which found a strong increase of the stellar mass loss rates with the coronal X-ray flux. Owing to the weaker dependence, we expect the impact of stellar winds on planetary atmospheres to be less severe and the detectability of magnetospheric radio emission to be lower then previously suggested. Considering the rotational evolution of a one solar-mass star, the mass loss rates and the wind ram pressures are highest during the pre-main sequence phase

Octave Spanning Frequency Comb on a Chip

Optical frequency combs have revolutionized the field of frequency metrology within the last decade and have become enabling tools for atomic clocks, gas sensing and astrophysical spectrometer calibration. The rapidly increasing number of applications has heightened interest in more compact comb generators. Optical microresonator based comb generators bear promise in this regard. Critical to their future use as 'frequency markers', is however the absolute frequency stabilization of the optical comb spectrum. A powerful technique for this stabilization is self-referencing, which requires a spectrum that spans a full octave, i.e. a factor of two in frequency. In the case of mode locked lasers, overcoming the limited bandwidth has become possible only with the advent of photonic crystal fibres for supercontinuum generation. Here, we report for the first time the generation of an octave-spanning frequency comb directly from a toroidal microresonator on a silicon chip. The comb spectrum covers the wavelength range from 990 nm to 2170 nm and is retrieved from a continuous wave laser interacting with the modes of an ultra high Q microresonator, without relying on external broadening. Full tunability of the generated frequency comb over a bandwidth exceeding an entire free spectral range is demonstrated. This allows positioning of a frequency comb mode to any desired frequency within the comb bandwidth. The ability to derive octave spanning spectra from microresonator comb generators represents a key step towards achieving a radio-frequency to optical link on a chip, which could unify the fields of metrology with micro- and nano-photonics and enable entirely new devices that bring frequency metrology into a chip scale setting for compact applications such as space based optical clocks

Characterization of a 450-km Baseline GPS Carrier-Phase Link using an Optical Fiber Link

A GPS carrier-phase frequency transfer link along a baseline of 450 km has been established and is characterized by comparing it to a phase-stabilized optical fiber link of 920 km length, established between the two endpoints, the Max-Planck-Institut f\"ur Quantenoptik in Garching and the Physikalisch-Technische Bundesanstalt in Braunschweig. The characterization is accomplished by comparing two active hydrogen masers operated at both institutes. The masers serve as local oscillators and cancel out when the double differences are calculated, such that they do not constitute a limitation for the GPS link characterization. We achieve a frequency instability of 3 x 10^(-13) in 30 s and 5 x 10^(-16) for long averaging times. Frequency comparison results obtained via both links show no deviation larger than the statistical uncertainty of 6 x 10^(-16). These results can be interpreted as a successful cross-check of the measurement uncertainty of a truly remote end fiber link.Comment: 14 pages, 6 figure