An equivariant discrete model for complexified arrangement complements

We define a partial ordering on the set $\mathcal {Q}=\mathcal {Q}(\mathsf {M})$ of pairs of topes of an oriented matroid $\mathsf {M}$, and show the geometric realization $\vert\mathcal {Q}\vert$ of the order complex of $\mathcal {Q}$ has the same homotopy type as the Salvetti complex of $\mathsf {M}$. For any element $e$ of the ground set, the complex $\vert\mathcal {Q}_e\vert$ associated to the rank-one oriented matroid on $\{e\}$ has the homotopy type of the circle. There is a natural free simplicial action of $\mathbb{Z}_4$ on $\vert\mathcal {Q}\vert$, with orbit space isomorphic to the order complex of the poset $\mathcal {Q}(\mathsf {M},e)$ associated to the pointed (or affine) oriented matroid $(\mathsf {M},e)$. If $\mathsf {M}$ is the oriented matroid of an arrangement $\mathscr {A}$ of linear hyperplanes in $\mathbb{R}^n$, the $\mathbb{Z}_4$ action corresponds to the diagonal action of $\mathbb{C}^*$ on the complement $M$ of the complexification of $\mathscr {A}$: $\vert\mathcal {Q}\vert$ is equivariantly homotopy-equivalent to $M$ under the identification of $\mathbb{Z}_4$ with the multiplicative subgroup $\{\pm 1, \pm i\}\subset \mathbb{C}^*$, and $\vert\mathcal {Q}(\mathsf {M},e)\vert$ is homotopy- equivalent to the complement of the decone of $\mathscr {A}$ relative to the hyperplane corresponding to $e$. All constructions and arguments are carried out at the level of the underlying posets.We also show that the class of fundamental groups of such complexes is strictly larger than the class of fundamental groups of complements of complex hyperplane arrangements. Specifically, the group of the non- Pappus arrangement is not isomorphic to any realizable arrangement group. The argument uses new structural results concerning the degree-one resonance varieties of small matroids

Final spins from the merger of precessing binary black holes

The inspiral of binary black holes is governed by gravitational radiation reaction at binary separations r < 1000 M, yet it is too computationally expensive to begin numerical-relativity simulations with initial separations r > 10 M. Fortunately, binary evolution between these separations is well described by post-Newtonian equations of motion. We examine how this post-Newtonian evolution affects the distribution of spin orientations at separations r ~ 10 M where numerical-relativity simulations typically begin. Although isotropic spin distributions at r ~ 1000 M remain isotropic at r ~ 10 M, distributions that are initially partially aligned with the orbital angular momentum can be significantly distorted during the post-Newtonian inspiral. Spin precession tends to align (anti-align) the binary black hole spins with each other if the spin of the more massive black hole is initially partially aligned (anti-aligned) with the orbital angular momentum, thus increasing (decreasing) the average final spin. Spin precession is stronger for comparable-mass binaries, and could produce significant spin alignment before merger for both supermassive and stellar-mass black hole binaries. We also point out that precession induces an intrinsic accuracy limitation (< 0.03 in the dimensionless spin magnitude, < 20 degrees in the direction) in predicting the final spin resulting from the merger of widely separated binaries.Comment: 20 pages, 16 figures, new PN terms, submitted to PR

Total cross section measurements for positron scattering from acetone

We report results from total cross section measurements for positron-acetone scattering. The energy range of these experiments was 0.2-23 eV, while the energy resolution of our positron beam was ~260 meV. The present data clearly highlight the important role played by the strong permanent dipole moment and significant dipole polarisability of the acetone molecule on the low-energy scattering dynamics of this system. For positron energies above about 6 eV the present data is found to be in quite good agreement with the only other total cross section results available in the literature from the Yamaguchi group, however, at lower energies the level of agreement is rather poor. To the best of our knowledge, no theoretical calculations are currently available for positron-acetone scattering

Low-frequency variability in the Gulf of Alaska from coarse and eddy-permitting ocean models

[1] An eddy-permitting ocean model of the northeast Pacific is used to examine the ocean adjustment to changing wind forcing in the Gulf of Alaska (GOA) at interannual-to-decadal timescales. It is found that the adjustment of the ocean model in the presence of mesoscale eddies is similar to that obtained with coarse-resolution models. Local Ekman pumping plays a key role in forcing pycnocline depth variability and, to a lesser degree, sea surface height (SSH) variability in the center of the Alaska gyre and in some areas of the eastern and northern GOA. Westward Rossby wave propagation is evident in the SSH field along some latitudes but is less noticeable in the pycnocline depth field. Differences between SSH and pycnocline depth are also found when considering their relationship with the local forcing and leading modes of climate variability in the northeast Pacific. In the central GOA pycnocline depth variations are more clearly related to changes in the local Ekman pumping than SSH. While SSH is marginally correlated with both Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) indices, the pycnocline depth evolution is primarily related to NPGO variability. The intensity of the mesoscale eddy field increases with increasing circulation strength. The eddy field is generally more energetic after the 1976–1977 climate regime shift, when the gyre circulation intensified. In the western basin, where eddies primarily originate from intrinsic instabilities of the flow, variations in eddy kinetic energy are statistically significant correlated with the PDO index, indicating that eddy statistics may be inferred, to some degree, from the characteristics of the large-scale flow

Facilitating knowledge transfer: decision support tools in environment and health

The HENVINET Health and Environment Network aimed to enhance the use of scientific knowledge in environmental health for policy making. One of the goals was to identify and evaluate Decision Support Tools (DST) in current use. Special attention was paid to four “priority” health issues: asthma and allergies, cancer, neurodevelopment disorders, and endocrine disruptors

A metal-poor damped Ly-alpha system at redshift 6.4

We identify a strong Ly-alpha damping wing profile in the spectrum of the quasar P183+05 at z=6.4386. Given the detection of several narrow metal absorption lines at z=6.40392, the most likely explanation for the absorption profile is that it is due to a damped Ly-alpha system. However, in order to match the data a contribution of an intergalactic medium 5-38% neutral or additional weaker absorbers near the quasar is also required. The absorption system presented here is the most distant damped Ly-alpha system currently known. We estimate an HI column density ($10^{20.68\pm0.25}\,$cm$^{-2}$), metallicity ([O/H]$=-2.92\pm 0.32$), and relative chemical abundances of a system consistent with a low-mass galaxy during the first Gyr of the universe. This object is among the most metal-poor damped Ly-alpha systems known and, even though it is observed only ~850 Myr after the big bang, its relative abundances do not show signatures of chemical enrichment by Population III stars.Comment: Updated to match published versio

Aurora B kinase and protein phosphatase 1 have opposing roles in modulating kinetochore assembly

The outer kinetochore binds microtubules to control chromosome movement. Outer kinetochore assembly is restricted to mitosis, whereas the inner kinetochore remains tethered to centromeres throughout the cell cycle. The cues that regulate this transient assembly are unknown. We find that inhibition of Aurora B kinase significantly reduces outer kinetochore assembly in Xenopus laevis and human tissue culture cells, frog egg extracts, and budding yeast. In X. leavis M phase extracts, preassembled kinetochores disassemble after inhibiting Aurora B activity with either drugs or antibodies. Kinetochore disassembly, induced by Aurora B inhibition, is rescued by restraining protein phosphatase 1 (PP1) activity. PP1 is necessary for kinetochores to disassemble at the exit from M phase, and purified enzyme is sufficient to cause disassembly on isolated mitotic nuclei. These data demonstrate that Aurora B activity is required for kinetochore maintenance and that PP1 is necessary and sufficient to disassemble kinetochores. We suggest that Aurora B and PP1 coordinate cell cycle–dependent changes in kinetochore assembly though phosphorylation of kinetochore substrates