Effects of mixed rare earth occupancy on the low temperature properties of (R, R',R''...)Ni2Ge2 single crystals

Temperature and applied magnetic field dependent magnetization measurements on 34 single crystalline samples of (R, R',R''...)Ni2Ge2 compounds (R, R', R'', etc. being primarily Gd-Lu, Y), were made. These measurements reveal that, despite extremes in local moment anisotropy, the average de Gennes parameter is a remarkably good predictor of the paramagnetic to antiferromagnetic ordering temperature. In addition, the pronounced metamagnetic phase transitions seen in the low temperature phase of TbNi2Ge2 are found to be remarkably robust to high substitution levels of Gd and 25% substitutions of other heavy rare earths

Two Wide Planetary-mass Companions to Solar-type Stars in Upper Scorpius

At wide separations, planetary-mass and brown dwarf companions to solar-type stars occupy a curious region of parameter space not obviously linked to binary star formation or solar system scale planet formation. These companions provide insight into the extreme case of companion formation (either binary or planetary), and due to their relative ease of observation when compared to close companions, they offer a useful template for our expectations of more typical planets. We present the results from an adaptive optics imaging survey for wide (~50–500 AU) companions to solar-type stars in Upper Scorpius. We report one new discovery of a ~14 M_J companion around GSC 06214−00210and confirm that the candidate planetary-mass companion 1RXS J160929.1−210524 detected by Lafrenière et al. is in fact comoving with its primary star. In our survey, these two detections correspond to ~4% of solar-type stars having companions in the 6–20 M_J mass and ~200–500 AU separation range. This figure is higher than would be expected if brown dwarfs and planetary-mass companions were drawn from an extrapolation of the binary mass function. Finally, we discuss implications for the formation of these objects

Broken Symmetry as a Stabilizing Remnant

The Goldberger-Wise mechanism enables one to stabilize the length of the warped extra dimension employed in Randall-Sundrum models. In this work we generalize this mechanism to models with multiple warped throats sharing a common ultraviolet brane. For independent throats this generalization is straight forward. If the throats possess a discrete interchange symmetry like Z_n the stabilizing dynamics may respect the symmetry, resulting in equal throat lengths, or they may break it. In the latter case the ground state of an initially symmetric configuration is a stabilized asymmetric configuration in which the throat lengths differ. We focus on two- (three-) throat setups with a Z_2 (Z_3) interchange symmetry and present stabilization dynamics suitable for either breaking or maintaining the symmetry. Though admitting more general application, our results are relevant for existing models in the literature, including the two throat model with Kaluza-Klein parity and the three throat model of flavor based on a broken Z_3 symmetry.Comment: 23 pages; v2 minor cosmetic chang

The role of annexin 2 in RPE phagocytosis of photoreceptor outer segments

The Retinal Pigmented Epithelium (RPE) has many functions, one of which is the phagocytosis of shed photoreceptor outer segments (POS). This process is vital to the maintenance of both the RPE and photoreceptors. Outer segment shedding and internalisation are under circadian regulation, such that shedding is followed by a burst of phagocytosis at the onset of light. Annexin 2 is well placed to have a role in this process. Its direct involvement in actin dynamics and association to vesicle membranes during endocytosis may be significant in RPE outer segment phagocytosis, as this process requires extensive re-organisation of actin and re-distribution of membrane on the apical processes of the RPE. This thesis examines cell differentiation in two RPE cell lines and in primary porcine RPE cells, in order to evaluate the best system for conducting phagocytosis experiments. In vitro experiments provided evidence that annexin 2 localises to the phagocytic cup during POS internalisation but dissociates once internalisation is complete. Following knock down of annexin 2, phagocytosis was shown to decrease. Furthermore annexin 2 was shown to be phosphorylated during phagocytosis. We also found that c-Src is phosphorylated alongside annexin 2 and therefore may phosphorylate annexin 2, which contains a c-Src phosphorylation site. To investigate the circadian aspects of POS phagocytosis by the RPE, apical and basal phagosomes were quantified in the RPE from annexin 2 knock out and wild type eyes, harvested before and after light onset. Phagosomes from eyes harvested one hour after light onset were also mapped relative to Bruch’s membrane. The annexin 2 knock out animals lack the characteristic burst of phagocytosis one hour after light onset exhibited in wild type animals. Phagosomes were also retarded in the apical processes one hour after light onset, at the peak of phagocytosis, when they are normally internalised into the cell body for processing and degradation. Lysates from wild type eyes showed that annexin 2 is phosphorylated before light onset along with c-Src and FAK, key molecules in the RPE phagocytic machinery. Importantly, the absence of annexin 2 in knock out eyes delayed phosphorylation of c-Src and FAK. This delay in phosphorylation of two key RPE phagocytosis molecules may account for the delay in ingestion of outer segments into the cell body and the accumulation of phagosomes in the apical processes observed in the knock out animals. In conclusion, work in this thesis has demonstrated that annexin 2 is required for efficient RPE internalisation of rod outer segments both in vitro and in vivo. Annexin 2 is required for the timely phosphorylation of FAK and c-Src, which may account for the delay in POS internalisation observed in the annexin 2 knock out mice

Statistical mechanics of ecosystem assembly

We introduce a toy model of ecosystem assembly for which we are able to map out all assembly pathways generated by external invasions. The model allows to display the whole phase space in the form of an assembly graph whose nodes are communities of species and whose directed links are transitions between them induced by invasions. We characterize the process as a finite Markov chain and prove that it exhibits a unique set of recurrent states (the endstate of the process), which is therefore resistant to invasions. This also shows that the endstate is independent on the assembly history. The model shares all features with standard assembly models reported in the literature, with the advantage that all observables can be computed in an exact manner.Comment: Accepted for publication in Physical Review Letter

Wave attenuation and dispersion due to floating ice covers

Experiments investigating the attenuation and dispersion of surface waves in a variety of ice covers are performed using a refrigerated wave flume. The ice conditions tested in the experiments cover naturally occurring combinations of continuous, fragmented, pancake and grease ice. Attenuation rates are shown to be a function of ice thickness, wave frequency, and the general rigidity of the ice cover. Dispersion changes were minor except for large wavelength increases when continuous covers were tested. Results are verified and compared with existing literature to show the extended range of investigation in terms of incident wave frequency and ice conditions

Quantum fluctuations in coupled dark solitons in trapped Bose-Einstein condensates

We show that the quantum fluctuations associated with the Bogoliubov quasiparticle vacuum can be strongly concentrated inside dark solitons in a trapped Bose Einstein condensate. We identify a finite number of anomalous modes that are responsible for such quantum phenomena. The fluctuations in these anomalous modes correspond to the `zero-point' oscillations in coupled dark solitons.Comment: 4 pages, 3 figure

Robo-AO Discovery and Basic Characterization of Wide Multiple Star Systems in the Pleiades, Praesepe, and NGC 2264 Clusters

We identify and roughly characterize 66 candidate binary star systems in the Pleiades, Praesepe, and NGC 2264 star clusters based on robotic adaptive optics imaging data obtained using Robo-AO at the Palomar 60" telescope. Only $\sim$10% of our imaged pairs were previously known. We detect companions at red optical wavelengths having physical separations ranging from a few tens to a few thousand AU. A 3-sigma contrast curve generated for each final image provides upper limits to the brightness ratios for any undetected putative companions. The observations are sensitive to companions with maximum contrast $\sim$6$^m$ at larger separations. At smaller separations, the mean (best) raw contrast at 2 arcsec is 3.8$^m$ (6$^m$), at 1 arcsec is 3.0$^m$ (4.5$^m$), and at 0.5 arcsec is 1.9$^m$ (3$^m$). PSF subtraction can recover close to the full contrast in to the closer separations. For detected candidate binary pairs, we report separations, position angles, and relative magnitudes. Theoretical isochrones appropriate to the Pleiades and Praesepe clusters are then used to determine the corresponding binary mass ratios, which range from 0.2-0.9 in $q=m_2/m_1$. For our sample of roughly solar-mass (FGK type) stars in NGC 2264 and sub-solar-mass (K and early M-type) primaries in the Pleiades and Praesepe, the overall binary frequency is measured at $\sim$15.5% $\pm$ 2%. However, this value should be considered a lower limit to the true binary fraction within the specified separation and mass ratio ranges in these clusters, given that complex and uncertain corrections for sensitivity and completeness have not been applied.Comment: Accepted to A

Magnetic Resonance Imaging of Optic Nerve Traction During Adduction in Primary Open-Angle Glaucoma With Normal Intraocular Pressure.

PurposeWe used magnetic resonance imaging (MRI) to ascertain effects of optic nerve (ON) traction in adduction, a phenomenon proposed as neuropathic in primary open-angle glaucoma (POAG).MethodsSeventeen patients with POAG and maximal IOP ≤ 20 mm Hg, and 31 controls underwent MRI in central gaze and 20° to 30° abduction and adduction. Optic nerve and sheath area centroids permitted computation of midorbital lengths versus minimum paths.ResultsAverage mean deviation (±SEM) was -8.2 ± 1.2 dB in the 15 patients with POAG having interpretable perimetry. In central gaze, ON path length in POAG was significantly more redundant (104.5 ± 0.4% of geometric minimum) than in controls (102.9 ± 0.4%, P = 2.96 × 10-4). In both groups the ON became significantly straighter in adduction (28.6 ± 0.8° in POAG, 26.8 ± 1.1° in controls) than central gaze and abduction. In adduction, the ON in POAG straightened to 102.0% ± 0.2% of minimum path length versus 104.5% ± 0.4% in central gaze (P = 5.7 × 10-7), compared with controls who straightened to 101.6% ± 0.1% from 102.9% ± 0.3% in central gaze (P = 8.7 × 10-6); and globes retracted 0.73 ± 0.09 mm in POAG, but only 0.07 ± 0.08 mm in controls (P = 8.8 × 10-7). Both effects were confirmed in age-matched controls, and remained significant after correction for significant effects of age and axial globe length (P = 0.005).ConclusionsAlthough tethering and elongation of ON and sheath are normal in adduction, adduction is associated with abnormally great globe retraction in POAG without elevated IOP. Traction in adduction may cause mechanical overloading of the ON head and peripapillary sclera, thus contributing to or resulting from the optic neuropathy of glaucoma independent of IOP