926 research outputs found
Seeding of the nematic-isotropic phase transition by an electric field
In this paper, we use a relatively simple continuum model to investigate the effects of dielectric inhomogeneity within confined liquid crystal cells. Specifically, we consider, in planar, cylindrical and spherical geometries, the stability of a nematic-isotropic interface subject to an applied voltage. Depending on the magnitude of this voltage, the temperature and the geometry of the cell, the nematic region may shrink until the material is completely isotropic within the cell, grow until the nematic phase cells the cell or, in certain geometries, coexist with the isotropic phase. For planar geometry, no coexistence is found, but we are able to give analytical expressions for the critical voltage for an electric-field-induced phase transition as well as the critical wetting layer thickness for arbitrary applied voltage. In cells with cylindrical and spherical geometries, however, stable nematic-isotropic coexistence is predicted, the thickness of the nematic region being controllable by alteration of the applied voltage.</p
Multistable alignment states in nematic liquid crystal filled wells
Two distinct, stable alignment states have been observed for a nematic liquid crystal confined in a layer with thickness of 12 μm and in square wells with sides of length between 20 and 80 μm. The director lies in the plane of the layer and line defects occur in two corners of the squares. The positions of the defects determine whether the director orientation is across the diagonal or is parallel to two opposite edges of the square. The device is multistable because both the diagonal and parallel states are stable when rotated by multiples of 90° in plane
ALMA CO J=6-5 observations of IRAS16293-2422: Shocks and entrainment
Observations of higher-excited transitions of abundant molecules such as CO
are important for determining where energy in the form of shocks is fed back
into the parental envelope of forming stars. The nearby prototypical and
protobinary low-mass hot core, IRAS16293-2422 (I16293) is ideal for such a
study. The source was targeted with ALMA for science verification purposes in
band 9, which includes CO J=6-5 (E_up/k_B ~ 116 K), at an unprecedented spatial
resolution (~0.2", 25 AU). I16293 itself is composed of two sources, A and B,
with a projected distance of 5". CO J=6-5 emission is detected throughout the
region, particularly in small, arcsecond-sized hotspots, where the outflow
interacts with the envelope. The observations only recover a fraction of the
emission in the line wings when compared to data from single-dish telescopes,
with a higher fraction of emission recovered at higher velocities. The very
high angular resolution of these new data reveal that a bow shock from source A
coincides, in the plane of the sky, with the position of source B. Source B, on
the other hand, does not show current outflow activity. In this region, outflow
entrainment takes place over large spatial scales, >~ 100 AU, and in small
discrete knots. This unique dataset shows that the combination of a
high-temperature tracer (e.g., CO J=6-5) and very high angular resolution
observations is crucial for interpreting the structure of the warm inner
environment of low-mass protostars.Comment: Accepted for publication in A&A Letter
The RMS Survey: Resolving kinematic distance ambiguities towards a sample of compact HII regions using HI absorption
We present high-resolution HI data obtained using the Australia Telescope
Compact Array to resolve the near/far distance ambiguities towards a sample of
compact HII regions from the Red MSX Source (RMS) survey. The high resolution
data are complemented with lower resolution archival HI data extracted from the
Southern and VLA Galactic Plane surveys. We resolve the distance ambiguity for
nearly all of the 105 sources where the continuum was strong enough to allow
analysis of the HI absorption line structure. This represents another step in
the determination of distances to the total RMS sample, which with over 1,000
massive young stellar objects and compact HII regions, is the largest and most
complete sample of its kind. The full sample will allow the distribution of
massive star formation in the Galaxy to be examined.Comment: Accepted by MNRAS. This paper consists of 15 pages and contains 10
figures and 5 table
Classifying the embedded young stellar population in Perseus and Taurus & the LOMASS database
Context. The classification of young stellar objects (YSOs) is typically done
using the infrared spectral slope or bolometric temperature, but either can
result in contamination of samples. More accurate methods to determine the
evolutionary stage of YSOs will improve the reliability of statistics for the
embedded YSO population and provide more robust stage lifetimes. Aims. We aim
to separate the truly embedded YSOs from more evolved sources. Methods. Maps of
HCO+ J=4-3 and C18O J=3-2 were observed with HARP on the James Clerk Maxwell
Telescope (JCMT) for a sample of 56 candidate YSOs in Perseus and Taurus in
order to characterize emission from high (column) density gas. These are
supplemented with archival dust continuum maps observed with SCUBA on the JCMT
and Herschel PACS to compare the morphology of the gas and dust in the
protostellar envelopes. The spatial concentration of HCO+ J=4-3 and 850 micron
dust emission are used to classify the embedded nature of YSOs. Results.
Approximately 30% of Class 0+I sources in Perseus and Taurus are not Stage I,
but are likely to be more evolved Stage II pre-main sequence (PMS) stars with
disks. An additional 16% are confused sources with an uncertain evolutionary
stage. Conclusions. Separating classifications by cloud reveals that a high
percentage of the Class 0+I sources in the Perseus star forming region are
truly embedded Stage I sources (71%), while the Taurus cloud hosts a majority
of evolved PMS stars with disks (68%). The concentration factor method is
useful to correct misidentified embedded YSOs, yielding higher accuracy for YSO
population statistics and Stage timescales. Current estimates (0.54 Myr) may
overpredict the Stage I lifetime on the order of 30%, resulting in timescales
of 0.38 Myr for the embedded phase.Comment: 33 pages, 21 figures, 6 tables, Accepted to be published in A&
Bloodstream form trypanosoma brucei depend upon multiple metacaspases associated with RAB11-positive endosomes
Trypanosoma brucei possesses five metacaspase genes. Of these, MCA2 and MCA3 are expressed only in the mammalian bloodstream form of the parasite, whereas MCA5 is expressed also in the insect procyclic form. Triple RNAi analysis showed MCA2, MCA3 and MCA5 to be essential in the bloodstream form, with parasites accumulating pre-cytokinesis. Nevertheless, triple null mutants (Δmca2/3Δmca5) could be isolated after sequential gene deletion. Thereafter, Δmca2/3Δmca5 mutants were found to grow well both in vitro in culture and in vivo in mice. We hypothesise that metacaspases are essential for bloodstream form parasites, but they have overlapping functions and their progressive loss can be compensated for by activation of alternative biochemical pathways. Analysis of Δmca2/3Δmca5 revealed no greater or lesser susceptibility to stresses reported to initiate programmed cell death, such as treatment with prostaglandin D2. The metacaspases were found to colocalise with RAB11, a marker for recycling endosomes. However, variant surface glycoprotein (VSG) recycling processes and the degradation of internalised anti-VSG antibody were found to occur similarly in wild type, Δmca2/3Δmca5 and triple RNAi induced parasites. Thus, the data provide no support for the direct involvement of T. brucei metacaspases in programmed cell death and suggest that the proteins have a function associated with RAB11 vesicles that is independent of known recycling processes of RAB11-positive endosomes
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