322 research outputs found
Sher 25: pulsating but apparently alone
The blue supergiant Sher25 is surrounded by an asymmetric, hourglass-shaped
circumstellar nebula, which shows similarities to the triple-ring structure
seen around SN1987A. From optical spectroscopy over six consecutive nights, we
detect periodic radial velocity variations in the stellar spectrum of Sher25
with a peak-to-peak amplitude of ~12 km/s on a timescale of about 6 days,
confirming the tentative detec-tion of similar variations by Hendry et al. From
consideration of the amplitude and timescale of the signal, coupled with
observed line profile variations, we propose that the physical origin of these
variations is related to pulsations in the stellar atmosphere, rejecting the
previous hypothesis of a massive, short-period binary companion. The radial
velocities of two other blue supergiants with similar bipolar nebulae, SBW1 and
HD 168625, were also monitored over the course of six nights, but these did not
display any significant radial velocity variations.Comment: 9 pages, 7 figures. Accepted for publication in MNRA
A randomized library approach to identifying functional lox site domains for the Cre recombinase
The bacteriophage P1 Cre/loxP site-specific recombination system is a useful tool in a number of genetic engineering processes. The Cre recombinase has been shown to act on DNA sequences that vary considerably from that of its bacteriophage recognition sequence, loxP. However, little is known about the sequence requirements for functional lox-like sequences. In this study, we have implemented a randomized library approach to identify the sequence characteristics of functional lox site domains. We created a randomized spacer library and a randomized arm library, and then tested them for recombination in vivo and in vitro. Results from the spacer library show that, while there is great plasticity, identity between spacer pairs is the most important factor influencing function, especially in in vitro reactions. The presence of one completely randomized arm in a functional loxP recombination reaction revealed that only three wild-type loxP arms are necessary for successful recombination in Cre-expressing bacteria, and that there are nucleotide preferences at the first three and last three positions of the randomized arm for the most efficiently recombined sequences. Finally, we found that in vitro Cre recombination reactions are much more stringent for evaluating which sequences can support efficient recombination compared to the 294-CRE system
Effective Temperatures of a Driven System Near Jamming
Fluctuations in a model of a sheared, zero-temperature foam are studied
numerically. Five different quantities that reduce to the true temperature in
an equilibrium thermal system are calculated. All five have the same shear-rate
dependence, and three have the same value. Near the onset of jamming, the
relaxation time is the same function of these three temperatures in the sheared
system as of the true temperature in an unsheared system. These results imply
that statistical mechanics is useful for the system and provide strong support
for the concept of jamming.Comment: 4 pages, 4 postscript figure
Statistics of Shear-Induced Rearrangements in a Two-Dimensional Model Foam
Under steady shear, a foam relaxes stress through intermittent rearrangements of bubbles accompanied by sudden drops in the stored elastic energy. We use a simple model of foam that incorporates both elasticity and dissipation to study the statistics of bubble rearrangements in terms of energy drops, the number of nearest neighbor changes, and the rate of neighbor-switching (T1) events. We do this for a two-dimensional system as a function of system size, shear rate, dissipation mechanism, and gas area fraction. We find that for dry foams, there is a well-defined quasistatic limit at low shear rates where localized rearrangements occur at a constant rate per unit strain, independent of both system size and dissipation mechanism. These results are in good qualitative agreement with experiments on two-dimensional and three-dimensional foams. In contrast, we find for progessively wetter foams that the event size distribution broadens into a power law that is cut off only by system size. This is consistent with criticality at the melting transition
The Melting of Aqueous Foams
Diffusing-wave spectroscopy measurements show that ordinarily solid aqueous foams flow by a series of stick-slip avalanche-like rearrangements of neighboring bubbles from one tight packing configuration to another. Contrary to a recent prediction, the distribution of avalanche sizes do not obey a power-law distribution characteristic of self-organized criticality. This can be understood from a simple model of foam mechanics based on bubble-bubble interactions
Quantum Nucleation of Vortex String Loops
We investigate quantum nucleation of vortex string loops in the relativistic
quantum field theory of a complex scalar field by using the Euclidean path
integral. Our initial metastable homogeneous field dominated by the
symmetric bounce solution. The nucleation rate and the critical vortex loop
size are obtained approximately. Gradually the initial current will be reduced
to zero as the induced current inside vortex loops is opposite to the initial
current. We also discuss a similar process in Maxwell-Higgs systems and
possible physical implications.Comment: phyzzx.tex, 13 pages: A correction to the final state of the
nucleation of local vortex string
SUMOylation inhibits FOXM1 activity and delays mitotic transition
The forkhead box transcription factor FOXM1 is an essential effector of G2/M-phase transition, mitosis and the DNA damage response. As such, it is frequently deregulated during tumorigenesis. Here we report that FOXM1 is dynamically modified by SUMO1 but not by SUMO2/3 at multiple sites. We show that FOXM1 SUMOylation is enhanced in MCF-7 breast cancer cells in response to treatment with epirubicin and mitotic inhibitors. Mutation of five consensus conjugation motifs yielded a SUMOylation-deficient mutant FOXM1. Conversely, fusion of the E2 ligase Ubc9 to FOXM1 generated an auto-SUMOylating mutant (FOXM1-Ubc9). Analysis of wild-type FOXM1 and mutants revealed that SUMOylation inhibits FOXM1 activity, promotes translocation to the cytoplasm and enhances APC/Cdh1-mediated ubiquitination and degradation. Further, expression of the SUMOylation-deficient mutant enhanced cell proliferation compared with wild-type FOXM1, whereas the FOXM1-Ubc9 fusion protein resulted in persistent cyclin B1 expression and slowed the time from mitotic entry to exit. In summary, our findings suggest that SUMOylation attenuates FOXM1 activity and causes mitotic delay in cytotoxic drug response
Hubble Space Telescope WFPC-2 Imaging of Cassiopeia A
The young SNR Cassiopeia A was imaged with WFPC-2 through four filters
selected to capture the complete velocity range of the remnant's main shell in
several important emission lines. Primary lines detected were [O III]
4959,5007, [N II] 6583, [S II] 6716,6731 + [O II] 7319,7330 + [O I] 6300,6364,
and [S III] 9069,9532. About 3/4th of the remnant's main shell was imaged in
all four filters. Considerable detail is observed in the reverse-shocked ejecta
with typical knot scale lengths of 0.2"-0.4" (1 - 2 x 10^16 cm). Both bright
and faint emission features appear highly clumped. Large differences in [S III]
and [O III] line intensities indicating chemical abundance differences are also
seen, particularly in knots located along the bright northern limb and near the
base of the northeast jet. A line of curved overlapping filament in the
remnant's northwestern rim appears to mark the location of the remnant's
reverse shock front in this region. Finger-like ejecta structures elsewhere
suggest cases where the reverse shock front is encountering the remnant's
clumped ejecta. Narrow-band [N II] images of the remnant's circumstellar knots
("QSFs") reveal them to be 0.1"-0.6" thick knots and filaments, often with
diffuse edges facing away from the center of expansion. Three color composite
images of the whole remnant and certain sections along with individual filter
enlargements of selected regions of the bright optical shell are presented and
discussed.Comment: 26 pages, 12 figures Accepted to the Astronomical Journa
Force distributions near the jamming and glass transitions
We calculate the distribution of interparticle normal forces near the
glass and jamming transitions in model supercooled liquids and foams,
respectively. develops a peak that appears near the glass or jamming
transitions, whose height increases with decreasing temperature, decreasing
shear stress and increasing packing density. A similar shape of was
observed in experiments on static granular packings. We propose that the
appearance of this peak signals the development of a yield stress. The
sensitivity of the peak to temperature, shear stress and density lends credence
to the recently proposed generalized jamming phase diagram.Comment: 4 pages, 3 postscript figures;Version 3 replaces figure 1 and removes
figure 2 from version 1. Significant rewording of version 1 to emphasize the
formation of peak in P(F) when these systems jam along five different routes
of the recently proposed jamming phase diagram. Version 2 displayed the
incorrect abstrac
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