Porous mandrels provide uniform deformation in hydrostatic powder metallurgy

Porous copper mandrels prevent uneven deformation of beryllium machining blanks. The beryllium powder is arranged around these mandrels and hot isostatically pressed to form the blanks. The mandrels are then removed by leaching

Cooperative DNA-binding by Bicoid provides a mechanism for threshold-dependent gene activation in the Drosophila embryo

The Bicoid morphogen directs pattern formation along the anterior-posterior (A-P) axis of the Drosophila embryo. Bicoid is distributed in a concentration gradient that decreases exponentially from the anterior pole, however, it transcribes target genes such as hunchback in a step-function-like pattern; the expression domain is uniform and has a sharply defined posterior boundary. A 'gradient-affinity' model proposed to explain Bicoid action states that (i) cooperative gene activation by Bicoid generates the sharp on/off switch for target gene transcription and (ii) target genes with different affinities for Bicoid are expressed at different positions along the A-P axis. Using an in vivo yeast assay and in vitro methods, we show that Bicoid binds DNA with pairwise cooperativity; Bicoid bound to a strong site helps Bicoid bind to a weak site. These results support the first aspect of the model, providing a mechanism by which Bicoid generates sharp boundaries of gene expression. However, contrary to the second aspect of the model, we find no significant difference between the affinity of Bicoid for the anterior gene hunchback and the posterior gene knirps, We propose, instead, that the arrangement of Bicoids bound to the target gene presents a unique signature to the transcription machinery that, in combination with overall affinity, regulates the extent of gene transcription along the A-P axis

A New Evolutionary Path to Type Ia Supernovae: Helium-Rich Super-Soft X-Ray Source Channel

We have found a new evolutionary path to Type Ia supernovae (SNe Ia) which has been overlooked in previous work. In this scenario, a carbon-oxygen white dwarf (C+O WD) is originated, not from an asymptotic giant branch star with a C+O core, but from a red-giant star with a helium core of $\sim 0.8-2.0 M_\odot$. The helium star, which is formed after the first common envelope evolution, evolves to form a C+O WD of $\sim 0.8-1.1 M_\odot$ with transferring a part of the helium envelope onto the secondary main-sequence star. This new evolutionary path, together with the optically thick wind from mass-accreting white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A part of the progenitor systems are identified as the luminous supersoft X-ray sources or the recurrent novae like U Sco, which are characterized by the accretion of helium-rich matter. The white dwarf accretes hydrogen-rich, helium-enhanced matter from a lobe-filling, slightly evolved companion at a critical rate and blows excess matter in the wind. The white dwarf grows in mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical estimate indicates that this channel contributes a considerable part of the inferred rate of SNe Ia in our Galaxy, i.e., the rate is about ten times larger than the previous theoretical estimates for white dwarfs with slightly evolved companions.Comment: 19 pages including 12 figures, to be published in ApJ, 519, No.

A Wide Symbiotic Channel to Type Ia Supernovae

As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a symbiotic binary system consisting of a white dwarf (WD) and a low mass red-giant (RG), where strong winds from the accreting WD play a key role to increase the WD mass to the Chandrasekhar mass limit. Here we propose two new evolutionary processes which make the symbiotic channel to SNe Ia much wider. (1) We first show that the WD + RG close binary can form from a wide binary even with such a large initial separation as $a_i \lesssim 40000 R_\odot$. Such a binary consists of an AGB star and a low mass main-sequence (MS) star, where the AGB star is undergoing superwind before becoming a WD. If the superwind at the end of AGB evolution is as fast as or slower than the orbital velocity, the wind outflowing from the system takes away the orbital angular momentum effectively. As a result the wide binary shrinks greatly to become a close binary. Therefore, the WD + RG binary can form from much wider binaries than our earlier estimate. (2) When the RG fills its inner critical Roche lobe, the WD undergoes rapid mass accretion and blows a strong optically thick wind. Our earlier analysis has shown that the mass transfer is stabilized by this wind only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that the WD wind can strip mass from the RG envelope, which could be efficient enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds 1.15. With the above two new effects (1) and (2), the symbiotic channel can account for the inferred rate of SNe Ia in our Galaxy.Comment: 29 pages including 14 firgures, to be published in ApJ, 521, No.

Synchronized Formation of Sub-Galactic Systems at Cosmological Reionization: Origin of Halo Globular Clusters

Gas rich sub-galactic halos with mass Mt <= 10^7.5 Msun, while incapable of forming stars due to lack of adequate coolants, contain a large fraction of baryonic mass at cosmological reionization. We show that the reionization of the universe at z=10-20 has an interesting physical effect on these halos. The external radiation field causes a synchronous inward propagation of an ionization front towards each halo, resulting in an inward, convergent shock. The resident gas of mass Mb~10^4-10^7 Msun in low spin (initial dimensionless spin parameter lambda <= 0.01) halos with a velocity dispersion sigmav <= 11km/s would be compressed by a factor of ~100 in radius and form self -gravitating baryonic systems. Under the assumption that such compressed gaseous systems fragment to form stars, the final stellar systems will have a size 2-40pc, velocity dispersion 1-10km/s and a total stellar mass of M* 10^3-10^6 Msun. The characteristics of these proposed systems seem to match the observed properties of halo globular clusters. The expected number density is consistent with the observed number density of halo globular clusters. The observed mass function of slope ~-2 at the high mass end is predicted by the model. Strong correlation between velocity dispersion and luminosity (or surface brightness) and lack of correlation between velocity dispersion and size, in agreement with observations, are expected. Metallicity is, on average, expected to be low and should not correlate with any other quantities of globular clusters, except that a larger dispersion of metallicity among globular clusters is expected for larger galaxies. The observed trend of specific frequency with galaxy type may be produced in the model. We suggest that these stellar systems are seen as halo globular clusters today.Comment: accepted to ApJ, 7 ApJ page

Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers

Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous imaging. A full study has been done for spherical particles with a +-20% size distribution. Stick-slip motion due to repetitive fluidization of the layer occurs for low driving velocities. Between major slip events, slight creep occurs that is variable from one event to the next. The effects of changing the stiffness k and velocity V of the driving system are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, but becomes irregular when k is large and V is relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period T and amplitude A of the relative motion are presented as a function of V. At a critical value Vc, a transition to continuous sliding motion occurs that is discontinuous for k not too large. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The force is a multi-valued function of the instantaneous velocity, with pronounced hysteresis and a sudden drop prior to resticking. Measurements of vertical displacement reveal a small dilation of the material (about one tenth of the mean particle size in a layer 20 particles deep) associated with each slip event. Finally, optical imaging reveals that localized microscopic rearrangements precede (and follow) each slip event. The behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.

Density waves and 1/f1/f density fluctuations in granular flow

We simulate the granular flow in a narrow pipe with a lattice-gas automaton model. We find that the density in the system is characterized by two features. One is that spontaneous density waves propagate through the system with well-defined shapes and velocities. The other is that density waves are so distributed to make the power spectra of density fluctuations as $1/f^{\alpha}$ noise. Three important parameters make these features observable and they are energy dissipation, average density and the rougness of the pipe walls.Comment: Latex (with ps files appended

Systematic detection of magnetic fields in massive, late-type supergiants

We report the systematic detection of magnetic fields in massive (M > 5 M$_\odot$) late-type supergiants, using spectropolarimetric observations obtained with ESPaDOnS at the Canada-France-Hawaii Telescope. Our observations reveal detectable Stokes V Zeeman signatures in Least-Squares Deconvolved mean line profiles in one-third of the observed sample of more than 30 stars. The signatures are sometimes complex, revealing multiple reversals across the line. The corresponding longitudinal magnetic field is seldom detected, although our longitudinal field error bars are typically 0.3 G ($1\sigma$). These characteristics suggest topologically complex magnetic fields, presumably generated by dynamo action. The Stokes V signatures of some targets show clear time variability, indicating either rotational modulation or intrinsic evolution of the magnetic field. We also observe a weak correlation between the unsigned longitudinal magnetic field and the CaII K core emission equivalent width of the active G2Iab supergiant $\beta$~Dra and the G8Ib supergiant $\epsilon$~Gem.Comment: 8 pages, 1 table, 6 figures, accepted for publication in MNRA