Can low metallicity binaries avoid merging?

Rapid mass transfer in a binary system can drive the accreting star out of thermal equilibrium, causing it to expand. This can lead to a contact system, strong mass loss from the system and possibly merging of the two stars. In low metallicity stars the timescale for heat transport is shorter due to the lower opacity. The accreting star can therefore restore thermal equilibrium more quickly and possibly avoid contact. We investigate the effect of accretion onto main sequence stars with radiative envelopes with different metallicities. We find that a low metallicity (Z<0.001), 4 solar mass star can endure a 10 to 30 times higher accretion rate before it reaches a certain radius than a star at solar metallicity. This could imply that up to two times fewer systems come into contact during rapid mass transfer when we compare low metallicity. This factor is uncertain due to the unknown distribution of binary parameters and the dependence of the mass transfer timescale on metallicity. In a forthcoming paper we will present analytic fits to models of accreting stars at various metallicities intended for the use in population synthesis models.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New Mexico, July 16-20, 2007, 3 pages, 2 figure

X-discontinuity and transition zone structure beneath Hawaii suggests a heterogeneous plume

The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. The thermal and compositional nature of the plume alters the mantle phase transitions, which can be seen in the depth and amplitude of seismic discontinuities. This study utilises $>$ 5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases to image seismic discontinuities between 200 to 800 km depth. Common-conversion point stacks of the data are used to map out lateral variations in converted phase observations, while slowness stacks allow differentiation between true conversions from discontinuities and multiples. We find that the 410 discontinuity is depressed by 20 km throughout our study region, while the main 660 is around average depth throughout most of the area. To the southwest of the Big Island we observe splitting of the 660, with a major peak at 630 km, and a minor peak appearing at 675 km depth. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition. In the upper mantle, a discontinuity is found across the region at depths varying between 290 to 350 km. Identifying multiples from this depth confirms the presence of a so-called X-discontinuity. To the east of the Big Island the X-discontinuity lies around 336 km and the associated multiple is particularly coherent and strong in amplitude. Strikingly, the discontinuity around 410 km disappears in this area. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite, consistent with widespread ponding of silica-saturated material at these depths around the plume. This material could represent eclogite enriched material, which is relatively silica-rich compared to pyrolite, spreading out from the plume to the east as a deep eclogite pool, a hypothesis which is consistent with dynamical models of thermochemical plumes. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume

X-discontinuity and transition zone structure beneath Hawaii suggests a heterogeneous plume

The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. The thermal and compositional nature of the plume alters the mantle phase transitions, which can be seen in the depth and amplitude of seismic discontinuities. This study utilises >5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases to image seismic discontinuities between 200 to 800 km depth. Common-conversion point stacks of the data are used to map out lateral variations in converted phase observations, while slowness stacks allow differentiation between true conversions from discontinuities and multiples. We find that the 410 discontinuity is depressed by 20 km throughout our study region, while the main 660 is around average depth throughout most of the area. To the southwest of the Big Island we observe splitting of the 660, with a major peak at 630 km, and a minor peak appearing at 675 km depth. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition. In the upper mantle, a discontinuity is found across the region at depths varying between 290 to 350 km. Identifying multiples from this depth confirms the presence of a so-called X-discontinuity. To the east of the Big Island the X-discontinuity lies around 336 km and the associated multiple is particularly coherent and strong in amplitude. Strikingly, the discontinuity around 410 km disappears in this area. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite, consistent with widespread ponding of silica-saturated material at these depths around the plume. This material could represent eclogite enriched material, which is relatively silica-rich compared to pyrolite, spreading out from the plume to the east as a deep eclogite pool, a hypothesis which is consistent with dynamical models of thermochemical plumes. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume

Characterizing a cluster's dynamic state using a single epoch of radial velocities

Radial velocity measurements can be used to constrain the dynamical state of a stellar cluster. However, for clusters with velocity dispersions smaller than a few km/s the observed radial velocity distribution tends to be dominated by the orbital motions of binaries rather than the stellar motions through the potential well of the cluster. Our goal is to characterize the intrinsic velocity distribution of a cluster from a single epoch of radial velocity data, even for a cluster with a velocity dispersion of a fraction of a km/s, using a maximum likelihood procedure. Assuming a period, mass ratio, and eccentricity distribution for the binaries in the observed cluster this procedure fits a dynamical model describing the velocity distribution for the single stars and center of masses of the binaries, simultaneously with the radial velocities caused by binary orbital motions, using all the information available in the observed velocity distribution. We find that the fits to the intrinsic velocity distribution depend only weakly on the binary properties assumed, so the uncertainty in the fitted parameters tends to be dominated by statistical uncertainties. Based on Monte Carlo simulations we provide an estimate of how these statistical uncertainties vary with the velocity dispersion, binary fraction, and the number of observed stars, which can be used to estimate the sample size needed to reach a specific accuracy. Finally we test the method on the well-studied open cluster NGC 188, showing that it can reproduce a velocity dispersion of only 0.5 km/s using a single epoch of the multi-epoch radial velocity data. If the binary period, mass ratio, and eccentricity distribution of the observed stars are roughly known, this procedure can be used to correct for the effect of binary orbital motions on an observed velocity distribution. [Abridged]Comment: 11 pages, 6 figures, accepted by A&

A Refined Approach to Model Anisotropy in the Lowermost Mantle

© 2018 Institute of Physics Publishing. All rights reserved. Seismic anisotropy in the lowermost mantle has been attributed to texture development during mantle convection. This study models texture evolution in a subducting slab impinging on the core-mantle boundary. Using a 3-dimensional geodynamic model with tracers recording the deformation history, a visco-plastic self-consistent (VPSC) model for polycrystal deformation, and relying on experimentally determined slip systems, aggregate grains with volume fractions of 60% orthorhombic silicate perovskite (MgSiO3), 20% cubic calcium perovskite (CaSiO3), and 20% cubic ferropericlase ((Mg,Fe)O) were deformed plastically and developed crystal preferred orientation. Forward and reverse perovskite (Pv)-postperovskite (PPv) phase transitions were included by allowing for likely orientation variant selections. Grain orientations, P (compression) and S (shear) wave velocity pole figures were calculated for each phase as well as the aggregate. The results show that dominant (001) slip on PPv can produce strong texture and shear wave anisotropy of 0.01-3.07% with VSH >VSV which agrees with seismological observations in selected areas of the D" layer

Towards non-parametric fiber-specific T1T_1 relaxometry in the human brain

Purpose: To estimate fiber-specific $T_1$ values, i.e. proxies for myelin content, in heterogeneous brain tissue. Methods: A diffusion-$T_1$ correlation experiment was carried out on an in vivo human brain using tensor-valued diffusion encoding and multiple repetition times. The acquired data was inverted using a Monte-Carlo inversion algorithm that retrieves non-parametric distributions $\mathcal{P}(\mathbf{D},R_1)$ of diffusion tensors and longitudinal relaxation rates $R_1 = 1/T_1$. Orientation distribution functions (ODFs) of the highly anisotropic components of $\mathcal{P}(\mathbf{D},R_1)$ were defined to visualize orientation-specific diffusion-relaxation properties. Finally, Monte-Carlo density-peak clustering (MC-DPC) was performed to quantify fiber-specific features and investigate microstructural differences between white-matter fiber bundles. Results: Parameter maps corresponding to $\mathcal{P}(\mathbf{D},R_1)$'s statistical descriptors were obtained, exhibiting the expected $R_1$ contrast between brain-tissue types. Our ODFs recovered local orientations consistent with the known anatomy and indicated possible differences in $T_1$ relaxation between major fiber bundles. These differences, confirmed by MC-DPC, were in qualitative agreement with previous model-based works but seem biased by the limitations of our current experimental setup. Conclusions: Our Monte-Carlo framework enables the non-parametric estimation of fiber-specific diffusion-$T_1$ features, thereby showing potential for characterizing developmental or pathological changes in $T_1$ within a given fiber bundle, and for investigating inter-bundle $T_1$ differences.Comment: 11 pages, 6 figures, submitted to Magnetic Resonance in Medicine (MRM) on the 14th of June 202

Disambiguating brain functional connectivity

Functional connectivity (FC) analyses of correlations of neural activity are used extensively in neuroimaging and electrophysiology to gain insights into neural interactions. However, analyses assessing changes in correlation fail to distinguish effects produced by sources as different as changes in neural signal amplitudes or noise levels. This ambiguity substantially diminishes the value of FC for inferring system properties and clinical states. Network modelling approaches may avoid ambiguities, but require specific assumptions. We present an enhancement to FC analysis with improved specificity of inferences, minimal assumptions and no reduction in flexibility. The Additive Signal Change (ASC) approach characterizes FC changes into certain prevalent classes of signal change that involve the input of additional signal to existing activity. With FMRI data, the approach reveals a rich diversity of signal changes underlying measured changes in FC, suggesting that it could clarify our current understanding of FC changes in many contexts. The ASC method can also be used to disambiguate other measures of dependency, such as regression and coherence, providing a flexible tool for the analysis of neural data

Geochemical Constraints on the Structure of the Earth's Deep Mantle and the Origin of the LLSVPs

Funder: Royal Commission for the Exhibition of 1851; Id: http://dx.doi.org/10.13039/501100000700Funder: University of Cambridge; Id: http://dx.doi.org/10.13039/501100000735Funder: Geological Society of London; Id: http://dx.doi.org/10.13039/100008066Abstract: Geophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations