3,041 research outputs found
Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope
We have used the Hubble Space Telescope to directly measure the angular size
of the large Kuiper belt object 2003 UB313. By carefully calibrating the point
spread function of a nearby field star, we measure the size of 2003 UB313 to be
34.31.4 milliarcseconds, corresponding to a diameter of 2400100 km or
a size % larger than Pluto. The V band geometric albedo of 2003 UB313 is
%. The extremely high albedo is consistent with the frosty methane
spectrum, the lack of red coloring, and the lack of observed photometric
variation on the surface of 2003 UB313. Methane photolysis should quickly
darken the surface of 2003 UB313, but continuous evaporation and redeposition
of surface ices appears capable of maintaining the extreme alebdo of this body
Origin of spatial variations of scattering polarization in the wings of the Ca {\sc i} 4227 \AA line
Polarization that is produced by coherent scattering can be modified by
magnetic fields via the Hanle effect. According to standard theory the Hanle
effect should only be operating in the Doppler core of spectral lines but not
in the wings. In contrast, our observations of the scattering polarization in
the Ca {\sc i} 4227 \AA line reveals the existence of spatial variations of the
scattering polarization throughout the far line wings. This raises the question
whether the observed spatial variations in wing polarization have a magnetic or
non-magnetic origin. A magnetic origin may be possible if elastic collisions
are able to cause sufficient frequency redistribution to make the Hanle effect
effective in the wings without causing excessive collisional depolarization, as
suggested by recent theories for partial frequency redistribution with coherent
scattering in magnetic fields. To model the wing polarization we apply an
extended version of the technique based on the "last scattering approximation".
This model is highly successful in reproducing the observed Stokes
polarization (linear polarization parallel to the nearest solar limb),
including the location of the wing polarization maxima and the minima around
the Doppler core, but it fails to reproduce the observed spatial variations of
the wing polarization in terms of magnetic field effects with frequency
redistribution. This null result points in the direction of a non-magnetic
origin in terms of local inhomogeneities (varying collisional depolarization,
radiation-field anisotropies, and deviations from a plane-parallel atmospheric
stratification).Comment: Accepted in May 2009 for publication in The Astrophysical Journa
High-throughput Quantum Chemistry: Empowering the Search for Molecular Candidates behind Unknown Spectral Signatures in Exoplanetary Atmospheres
The identification of molecules in exoplanetary atmospheres is only possible
thanks to the availability of high-resolution molecular spectroscopic data.
However, due to its intensive and time-consuming generation process, at
present, only on order 100 molecules have high-resolution spectroscopic data
available, limiting new molecular detections.
Using routine quantum chemistry calculations (i.e., scaled harmonic frequency
calculations using the B97-1/def2-TZVPD model chemistry with median errors of
10cm-1), here we present a complementary high-throughput approach to rapidly
generate approximate vibrational spectral data for 2743 molecules made from the
biologically most important elements C, H, N, O, P and S. Though these data are
not accurate enough to enable definitive molecular detections and does not seek
to replace the need for high-resolution data, it has powerful applications in
identifying potential molecular candidates responsible for unknown spectral
features. We explore this application for the 4.1 micron (2439cm-1) feature in
the atmospheric spectrum of WASP-39b, listing potential alternative molecular
species responsible for this spectral line, together with SO2. Further
applications of this big data compilation also include identifying molecules
with strong absorption features that are likely detectable at quite low
abundances, and training set for machine learning predictions of vibrational
frequencies.
Characterising exoplanetary atmospheres through molecular spectroscopy is
essential to understand the planet's physico-chemical processes and likelihood
of hosting life. Our rapidly generated quantum chemistry big data set will play
a crucial role in supporting this understanding by giving directions into
possible initial identifications of the more unusual molecules to emerge
The Processing Pathway of Prelamin A
The conversion of mammalian prelamin A to mature lamin A proceeds through the removal of 18 amino acids from the carboxyl terminus. The initial step in this processing is the isoprenylation of a CAAX box cysteine. This proteolytic event is distinctive for prelamin A among the known prenylated mammalian proteins. Since the carboxyl terminus of prelamin A is removed during maturation, it is not obvious that this protein would undergo the two reactions subsequent to prenylation observed in other CAAX box proteins-the endoproteolytic removal of the carboxyl-terminal 3 amino acids and the subsequent methylation of the now carboxyl-terminal cysteine. To characterize the maturation of prelamin A further, we have developed a CHO-K1 cell line that possesses a dexamethasone-inducible human prelamin A against a genetic background of high mevalonate uptake. Utilizing this cell line in association with antibodies specific to the transgenic prelamin A, we have been able to demonstrate directly in vivo that prelamin A undergoes farnesylation and carboxymethylation prior to conversion to lamin A, as is the case for other prenylated proteins. We have demonstrated previously that in the absence of isoprenylation, conversion of prelamin A to lamin A is blocked, but that unprocessed prelamin A is transported to the nucleus where it can still undergo maturation. Consistent with the implications of these prior studies, we now demonstrate the presence of both subunits of farnesyl-protein transferase in the nucleus
Segregation by thermal diffusion in granular shear flows
Segregation by thermal diffusion of an intruder immersed in a sheared
granular gas is analyzed from the (inelastic) Boltzmann equation. Segregation
is induced by the presence of a temperature gradient orthogonal to the shear
flow plane and parallel to gravity. We show that, like in analogous systems
without shear, the segregation criterion yields a transition between upwards
segregation and downwards segregation. The form of the phase diagrams is
illustrated in detail showing that they depend sensitively on the value of
gravity relative to the thermal gradient. Two specific situations are
considered: i) absence of gravity, and ii) homogeneous temperature. We find
that both mechanisms (upwards and downwards segregation) are stronger and more
clearly separated when compared with segregation criteria in systems without
shear.Comment: 8 figures. To appear in J. Stat. Mec
Thermal diffusion segregation in granular binary mixtures described by the Enskog equation
Diffusion induced by a thermal gradient in a granular binary mixture is
analyzed in the context of the (inelastic) Enskog equation. Although the Enskog
equation neglects velocity correlations among particles which are about to
collide, it retains spatial correlations arising from volume exclusion effects
and thus it is expected to apply to moderate densities. In the steady state
with gradients only along a given direction, a segregation criterion is
obtained from the thermal diffusion factor measuring the amount of
segregation parallel to the thermal gradient. As expected, the sign of the
factor provides a criterion for the transition between the Brazil-nut
effect (BNE) and the reverse Brazil-nut effect (RBNE) by varying the parameters
of the mixture (masses, sizes, concentration, solid volume fraction, and
coefficients of restitution). The form of the phase diagrams for the BNE/RBNE
transition is illustrated in detail for several systems, with special emphasis
on the significant role played by the inelasticity of collisions. In
particular, an effect already found in dilute gases (segregation in a binary
mixture of identical masses and sizes {\em but} different coefficients of
restitution) is extended to dense systems. A comparison with recent computer
simulation results shows a good qualitative agreement at the level of the
thermal diffusion factor. The present analysis generalizes to arbitrary
concentration previous theoretical results derived in the tracer limit case.Comment: 7 figures, 1 table. To appear in New J. Phys., special issue on
"Granular Segregation
Human Amniocytes Are Receptive to Chemically Induced Reprogramming to Pluripotency
Restoring pluripotency using chemical compounds alone would be a major step forward in developing clinical-grade pluripotent stem cells, but this has not yet been reported in human cells. We previously demonstrated that VPA_ AFS cells, human amniocytes cultivated with valproic acid (VPA) acquired functional pluripotency while remaining distinct from human embryonic stem cells (hESCs), questioning the relationship between the modulation of cell fate and molecular regulation of the pluripotency network. Here, we used single-cell analysis and functional assays to reveal that VPA treatment resulted in a homogeneous population of self-renewing non-transformed cells that fulfill the hallmarks of pluripotency, i.e., a short G1 phase, a dependence on glycolytic metabolism, expression of epigenetic modifications on histones 3 and 4, and reactivation of endogenous OCT4 and downstream targets at a lower level than that observed in hESCs. Mechanistic insights into the process of VPA-induced reprogramming revealed that it was dependent on OCT4 promoter activation, which was achieved independently of the PI3K (phosphatidylinositol 3-kinase)/ AKT/ mTOR (mammalian target of rapamycin) pathway or GSK3 beta inhibition but was concomitant with the presence of acetylated histones H3K9 and H3K56, which promote pluripotency. Our data identify, for the first time, the pluripotent transcriptional and molecular signature and metabolic status of human chemically induced pluripotent stem cells
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