PHASES High Precision Differential Astrometry of delta Equulei

delta Equulei is among the most well-studied nearby binary star systems. Results of its observation have been applied to a wide range of fundamental studies of binary systems and stellar astrophysics. It is widely used to calibrate and constrain theoretical models of the physics of stars. We report 27 high precision differential astrometry measurements of delta Equulei from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES). The median size of the minor axes of the uncertainty ellipses for these measurements is 26 micro-arcseconds. These data are combined with previously published radial velocity data and other previously published differential astrometry measurements using other techniques to produce a combined model for the system orbit. The distance to the system is determined to within a twentieth of a parsec and the component masses are determined at the level of a percent. The constraints on masses and distance are limited by the precisions of the radial velocity data; we outline plans improve this deficiency and discuss the outlook for further study of this binary.Comment: Accepted by AJ. Complete versions of tables 2-7 now available at http://stuff.mit.edu/~matthew1/deltaEquTables/ (removed from astroph server

Enabling Star Formation Via Spontaneous Molecular Dipole Orientation In Icy Solids

It is shown here how new experimental data, for the electrical properties of solid CO, can be used to fill important gaps in our understanding of the evolution of prestellar cores. Dust grains with a mantle of CO lead to a reduction in the degree of ionization in these cores by a factor of between 5 and 6. The lifetimes for expulsion of magnetic fields from cores, a process generally necessary for gravitational collapse, are reduced from current estimates of several megayears, by a similar factor. This removes a major inconsistency, since lifetimes now tally with typical ages of prestellar cores of a few hundred thousand to 106 yr, derived from observations. With the reduced timescales, cores also escape disruption by Galactic supernova remnants. Our results provide a natural mechanism for the generation of so-called magnetically supercritical cores, in which the magnetic field alone cannot prevent gravitational collapse. In addition, we find a minimum value for the density of prestellar cores of ≥(1.1 ± 0.1) × 104 H2 cm−3, in agreement with observations

Structure and nature of ice XIX

Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken. Instead, hydrochloric-acid-doped ice VI undergoes an alternative type of phase transition upon cooling at high pressure as the orientationally disordered ice remains disordered but undergoes structural distortions. As seen with in-situ neutron diffraction, the resulting phase of ice, ice XIX, forms through a Pbcn-type distortion which includes the tilting and squishing of hexameric clusters. This type of phase transition may provide an explanation for previously observed ferroelectric signatures in dielectric spectroscopy of ice VI and could be relevant for other icy materials

Detailed crystallographic analysis of the ice V to ice XIII hydrogen-ordering phase transition

Ice V is a structurally highly complex material with 28 water molecules in its monoclinic unit cell. It is classified as a hydrogen-disordered phase of ice. Yet, some of its hydrogen-bonded water molecules display significant orientational order. Upon cooling pure ice V, additional orientational ordering cannot be achieved on the experimental time scale. Doping with hydrochloric acid has been shown to be most effective in enabling the phase transition of ice V to its hydrogen-ordered counterpart ice XIII. Here, we present a detailed crystallographic study of this phase transition investigating the effects of hydrochloric and hydrofluoric acid as well as lithium and potassium hydroxide doping. The magnitudes of the stepwise changes in the lattice constants during the phase transition are found to be more sensitive indicators for the extent of hydrogen order in ice XIII than the appearance of new Bragg peaks. Hydrofluoric acid and lithium hydroxide doping enable similar ordering processes as hydrochloric acid but with slower kinetics. The various possible space groups and ordered configurations of ice XIII are examined systematically, and the previously determined P21/a structure is confirmed. Interestingly, the partial hydrogen order already present in ice V is found to perpetuate into ice XIII, and these ordering processes are found to be independent of pressure. Overall, the hydrogen ordering goes along with a small increase in volume, which appears to be the origin of the slower hydrogen-ordering kinetics under pressure. Heating pressure-quenched samples at ambient pressure revealed low-temperature "transient ordering" features in both diffraction and calorimetry

Preparation and Structure of the Ion-Conducting Mixed Molecular Glass Ga2I3.17

Modern functional glasses have been prepared from a wide range of precursors, combining the benefits of their isotropic disordered structures with the innate functional behavior of their atomic or molecular building blocks. The enhanced ionic conductivity of glasses compared to their crystalline counterparts has attracted considerable interest for their use in solid-state batteries. In this study, we have prepared the mixed molecular glass Ga2I3.17 and investigated the correlations between the local structure, thermal properties, and ionic conductivity. The novel glass displays a glass transition at 60 °C, and its molecular make-up consists of GaI4– tetrahedra, Ga2I62– heteroethane ions, and Ga+ cations. Neutron diffraction was employed to characterize the local structure and coordination geometries within the glass. Raman spectroscopy revealed a strongly localized nonmolecular mode in glassy Ga2I3.17, coinciding with the observation of two relaxation mechanisms below Tg in the AC admittance spectra

Origin of the low-temperature endotherm of acid-doped ice VI: new hydrogen-ordered phase of ice or deep glassy states?

On the basis of a low-temperature endotherm, it has recently been argued that cooling acid-doped ice VI at high pressures leads to a new hydrogen-ordered phase. We show that the endotherms are in fact caused by the glass transitions of deep glassy states related to ice VI. As expected for such endothermic overshoot effects, they display a characteristic dependence on pressure and cooling rate, they can be produced by sub-T_{g} annealing at ambient pressure, and they can be made to appear or disappear depending on the heating rate and the initial extent of relaxation. It is stressed that the existence of a new crystalline phase, as it has been suggested, cannot depend on the heating rate at which it is heated. X-ray diffraction shows that samples for which the low-temperature endotherm is present, weak or absent, as observed at a heating rate of 5 K min⁻¹, are structurally very similar. Furthermore, we show that the reported shifts of the (102) Bragg peak upon heating are fully consistent with our scenario and also with our earlier neutron diffraction study. Deuterated acid-doped ice VI cooled at high pressure also displays a low-temperature endotherm and its neutron diffraction pattern is consistent with deep glassy ice VI. Accessing deep glassy states of ice with the help of acid doping opens up a fascinating new chapter in ice research. Compared to pure ice VI, the glass transition temperature is lowered by more than 30 K by the acid dopant. Future work should focus on the deep glassy states related to all the other hydrogen-disordered ices including the ‘ordinary’ ice Ih

Impact of oxygen chemistry on model interstellar grain surfaces

Temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O2) beams on amorphous silica (aSiO2) and water (H2O) surfaces (porous-amorphous solid water; p-ASW, compact amorphous solid water; c-ASW, and crystalline solid water; CSW). Altering the deposition method of O2 is shown to result in different desorption energies of O2 due to differences in O2 film morphology when deposited on the aSiO2 surface. O2 enthalpy of formation is dissipated into the aSiO2 substrate without changes in the silica network. However, on the H2O surfaces, O2 formation enthalpy release is dissipated into the H-bonded matrix leading to morphological changes, possibly compacting p-ASW into c-ASW while CSW appears to undergo amorphisation. The enthalpy release from O2 formation is, however, not enough to result in reactive desorption of O2 or H2O under the current experimental circumstances. Further to this, O2 formation on sub-monolayer quantities of H2O leads to enhanced de-wetting and a greater degree of H-bond reconnection in H2O agglomerates. Lastly, O3 is observed from the O + O2 reaction on all surfaces studied

Amorphous Mixtures of Ice and C₆₀ Fullerene

Carbon and ice make up a substantial proportion of our universe. Recent space exploration has shown that these two chemical species often coexist such as on comets and asteroids and in the interstellar medium. Here, we prepare mixtures of C60 fullerene and H2O by vapor codeposition at 90 K with molar C60/H2O ratios ranging from 1:1254 to 1:5. The C60 percolation threshold is found between the 1:132 and 1:48 samples, corresponding to a transition from matrix-isolated C60 molecules to percolating C60 domains that confine H2O. Below this threshold, the crystallization and thermal desorption properties of H2O are not significantly affected by C60, whereas the crystallization temperature of H2O is shifted toward higher temperatures for the C60-rich samples. These C60-rich samples also display exotherms corresponding to the crystallization of C60 as the two components undergo phase separation. More than 60 vol % C60 is required to significantly affect the desorption properties of H2O. A thick blanket of C60 on top of pure amorphous ice is found to display large cracks due to water desorption. These findings may help us to understand the recently observed unusual surface features and the H2O weather cycle on the 67P/Churyumov–Gerasimenko comet

Interleukin-1beta and tumor necrosis factor-alpha are expressed by different subsets of microglia and macrophages after ischemic stroke in mice

<p>Abstract</p> <p>Background</p> <p>Interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) are expressed by microglia and infiltrating macrophages following ischemic stroke. Whereas IL-1β is primarily neurotoxic in ischemic stroke, TNF-α may have neurotoxic and/or neuroprotective effects. We investigated whether IL-1β and TNF-α are synthesized by overlapping or segregated populations of cells after ischemic stroke in mice.</p> <p>Methods</p> <p>We used flow cytometry and immunohistochemistry to examine cellular co-expression of IL-1β and TNF-α at 6, 12 and 24 hours after permanent middle cerebral artery occlusion in mice, validating the results by the use of bone marrow chimeric mice.</p> <p>Results</p> <p>We found that IL-1β and TNF-α were expressed in largely segregated populations of CD11b⁺CD45^dimmicroglia and CD11b⁺CD45^highmacrophages, with cells expressing both cytokines only rarely. The number of Gr1⁺granulocytes producing IL-1β or TNF-α was very low, and we observed no IL-1β- or TNF-α-expressing T cells or astrocytes.</p> <p>Conclusion</p> <p>Taken together, the results show that IL-1β and TNF-α are produced by largely segregated populations of microglia and macrophages after ischemic stroke in mice. Our findings provide evidence of a functional diversity among different subsets of microglia and macrophages that is potentially relevant to future design of anti-inflammatory therapies in stroke.</p

Surface heterogeneity and inhomogeneous broadening of vibrational line profiles

The surface heterogeneity of amorphous silica (aSiO2) has been probed using coverage dependent temperature programmed desorption (TPD) of a simple probe molecule, carbon monoxide (CO). The resulting distribution of interaction energies is the foundation from which an environmentally broadened vibrational line profile synthesis has been undertaken. These simulations are compared with measured line profiles recorded at 0.1 cm−1 resolution using reflection–absorption infrared spectroscopy (RAIRS). A comparison of such line profile synthesis for CO on amorphous silica and on porous amorphous solid water (p-ASW) is also reported and conclusions are drawn as to the vibrational relaxation and surface dynamics of the CO molecule on the two surfaces