1,006 research outputs found
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
Investigating the pre-main sequence magnetic chemically peculiar system HD 72106
The origin of the strong magnetic fields observed in chemically peculiar Ap
and Bp stars stars has long been debated. The recent discovery of magnetic
fields in the intermediate mass pre-main sequence Herbig Ae and Be stars links
them to Ap and Bp stars, providing vital clues about Ap and Bp stars and the
origin and evolution of magnetic fields in intermediate and high mass stars. A
detailed study of one young magnetic B star, HD 72106A, is presented. This star
appears to be in a binary system with an apparently normal Herbig Ae star. A
maximum longitudinal magnetic field strength of +391 +/- 65 G is found in HD
72106A, as are strong chemical peculiarities, with photospheric abundances of
some elements ranging up to 100x above solar.Comment: 8 pages, 6 figures. Proceeding of the 2006 conference of the Special
Astrophysical Observatory of the Russian Academy of Science
Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations
Recent spectropolarimetric studies of 7 SPB and Cep stars have
suggested that photospheric magnetic fields are more common in B-type pulsators
than in the general population of B stars, suggesting a significant connection
between magnetic and pulsational phenomena. We present an analysis of new and
previously published spectropolarimetric observations of these stars. New
Stokes observations obtained with the high-resolution ESPaDOnS and Narval
instruments confirm the presence of a magnetic field in one of the stars
( Lup), but find no evidence of magnetism in 5 others. A re-analysis
of the published longitudinal field measurements obtained with the
low-resolution FORS1/2 spectropolarimeters finds that the measurements of all
stars show more scatter from zero than can be attributed to Gaussian noise,
suggesting the presence of a signal and/or systematic under-estimation of error
bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO
archive demonstrates that small changes in reduction procedure lead to
substantial changes in the inferred longitudinal field, and substantially
reduces the number of field detections at the 3 level. Furthermore, we
find that the published periods are not unique solutions to the time series of
either the original or the revised FORS1/2 data. We conclude that the reported
field detections, proposed periods and field geometry models for Pyx,
15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction
procedures, and that magnetic fields at the reported strength are no more
common in SPB/ Cep stars than in the general population of B stars.Comment: 10 pages, 5 figures, accepted for publication in ApJ, 2012, typo
correcte
The Globular Cluster Systems of NGC 1400 and NGC 1407
The two brightest elliptical galaxies in the Eridanus A group, NGC 1400 and
NGC 1407, have been observed in both the Washington T_1 and Kron-Cousins I
filters to obtain photometry of their globular cluster systems (GCSs). This
group of galaxies is of particular interest due to its exceptionally high M/L
value, previously estimated at ~3000h, making this cluster highly
dark-matter-dominated. NGC 1400's radial velocity (549 km/s) is extremely low
compared to that of the central galaxy of Eridanus A (NGC 1407 with =
1766 km/s) and the other members of the system, suggesting that it is a
foreground galaxy projected by chance onto the cluster. Using the shapes of the
globular cluster luminosity functions, however, we derive distances of 17.6 +/-
3.1 Mpc to NGC 1407 and 25.4 +/- 7.0 Mpc to NGC 1400. These results support
earlier conclusions that NGC 1400 is at the distance of Eridanus A and
therefore has a large peculiar velocity. Specific frequencies are also derived
for these galaxies, yielding values of S_N = 4.0 +/- 1.3 for NGC 1407 and S_N =
5.2 +/- 2.0 for NGC 1400. In this and other respects, these two galaxies have
GCSs which are consistent with those observed in other galaxies.Comment: 14 pages (AASTeX), 3 postscript figures, submitted to the
Astronomical Journa
A biophysical model of decision making in an antisaccade task through variable climbing activity
We present a biophysical model of saccade initiation based on
competitive integration of planned and reactive cortical saccade decision signals
in the intermediate layer of the superior colliculus. In the model, the variable
slopes of the climbing activities of the input cortical decision signals are
produced from variability in the conductances of Na+, K+, Ca2+ activated K+,
NMDA and GABA currents. These cortical decision signals are integrated in
the activities of buildup neurons in the intermediate layer of the superior
colliculus, whose activities grow nonlinearly towards a preset criterion level.
When the level is crossed, a movement is initiated. The resultant model
reproduces the unimodal distributions of saccade reaction times (SRTs) for
correct antisaccades and erroneous prosaccades as well as the variability of
SRTs (ranging from 80ms to 600ms) and the overall 25% of erroneous
prosaccade responses in a large sample of 2006 young men performing an
antisaccade task
Colloids in light fields: particle dynamics in random and periodic energy landscapes
The dynamics of colloidal particles in potential energy landscapes have
mainly been investigated theoretically. In contrast, here we discuss the
experimental realization of potential energy landscapes with the help of light
fields and the observation of the particle dynamics by video microscopy. The
experimentally observed dynamics in periodic and random potentials are compared
to simulation and theoretical results in terms of, e.g. the mean-squared
displacement, the time-dependent diffusion coefficient or the non-Gaussian
parameter. The dynamics are initially diffusive followed by intermediate
subdiffusive behaviour which again becomes diffusive at long times. How
pronounced and extended the different regimes are, depends on the specific
conditions, in particular the shape of the potential as well as its roughness
or amplitude but also the particle concentration. Here we focus on dilute
systems, but the dynamics of interacting systems in external potentials, and
thus the interplay between particle-particle and particle-potential
interactions, is also mentioned briefly. Furthermore, the observed dynamics of
dilute systems resemble the dynamics of concentrated systems close to their
glass transition, with which it is compared. The effect of certain potential
energy landscapes on the dynamics of individual particles appears similar to
the effect of interparticle interactions in the absence of an external
potential
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 . The helium star, which is formed after the first common envelope
evolution, evolves to form a C+O WD of 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.
Resolving the Mystery of X-ray Faint Elliptical Galaxies: Chandra X-ray Observations of NGC 4697
Chandra observations of the X-ray faint elliptical galaxy NGC 4697 resolve
much of the X-ray emission (61% within one effective radius) into ~80 point
sources, of which most are low mass X-ray binaries (LMXBs). These LMXBs provide
the bulk of the hard emission and much of the soft emission as well. Of the
remaining unresolved emission, it is likely that about half is from fainter
LMXBs, while the other half (~23% of the total emission) is from interstellar
gas. Three of the resolved sources are supersoft sources. In the outer regions
of NGC 4697, eight of the LMXBs (about 25%) are coincident with candidate
globular clusters, which indicates that globulars have a high probability of
containing X-ray binaries compared to the normal stellar population. The X-ray
luminosities (0.3-10 keV) of the resolved LMXBs range from ~5e37 to ~2.5e39
ergs/s. The luminosity function of the LMXBs has a "knee" at 3.2e38 ergs/s,
which is roughly the Eddington luminosity of a 1.4 M_sun neutron star (NS);
this knee might be useful as a distance indicator. The highest luminosity
source has the Eddington luminosity of a ~20 M_sun black hole (BH). The
presence of this large population of NS and massive BH stellar remnants in this
elliptical galaxy shows that it (or its progenitors) once contained a large
population of massive main sequence stars.Comment: Astrophysical Journal Letters, in press. 4 pages in emulateapj5 style
with 4 embedded Postscript figures. Higher quality version of paper and
figures 1 and 2 are available at
http://www.astro.virginia.edu/~cls7i/papers/NGC4697-Chandra.ps.gz,
http://www.astro.virginia.edu/~cls7i/papers/NGC4697-Chandra_fig1.ps.gz, and
http://www.astro.virginia.edu/~cls7i/papers/NGC4697-Chandra_fig2.ps.g
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