4,757 research outputs found
Putative intermediates in the nerve cell differentiation pathway in hydra have properties of multipotent stem cells
We have investigated the properties of nerve cell precursors in hydra by analyzing the differentiation and proliferation capacity of interstitial cells in the peduncle of Hydra oligactis, which is a region of active nerve cell differentiation. Our results indicate that about 50% of the interstitial cells in the peduncle can grow rapidly and also give rise to nematocyte precursors when transplanted into a gastric environment. If these cells were committed nerve cell precursors, one would not expect them to differentiate into nematocytes nor to proliferate apparently without limit. Therefore we conclude that cycling interstitial cells in peduncles are not intermediates in the nerve cell differentiation pathway but are stem cells. The remaining interstitial cells in the peduncle are in G1 and have the properties of committed nerve cell precursors (Holstein and David, 1986). Thus, the interstitial cell population in the peduncle contains both stem cells and noncycling nerve precursors. The presence of stem cells in this region makes it likely that these cells are the immediate targets of signals which give rise to nerve cells
Gravitational-wave detectability of equal-mass black-hole binaries with aligned spins
Binary black-hole systems with spins aligned or anti-aligned to the orbital
angular momentum provide the natural ground to start detailed studies of the
influence of strong-field spin effects on gravitational wave observations of
coalescing binaries. Furthermore, such systems may be the preferred end-state
of the inspiral of generic supermassive binary black-hole systems. In view of
this, we have computed the inspiral and merger of a large set of binary systems
of equal-mass black holes with spins parallel to the orbital angular momentum
but otherwise arbitrary. Our attention is particularly focused on the
gravitational-wave emission so as to quantify how much spin effects contribute
to the signal-to-noise ratio, to the horizon distances, and to the relative
event rates for the representative ranges in masses and detectors. As expected,
the signal-to-noise ratio increases with the projection of the total black hole
spin in the direction of the orbital momentum. We find that equal-spin binaries
with maximum spin aligned with the orbital angular momentum are more than
"three times as loud" as the corresponding binaries with anti-aligned spins,
thus corresponding to event rates up to 30 times larger. We also consider the
waveform mismatch between the different spinning configurations and find that,
within our numerical accuracy, binaries with opposite spins S_1=-S_2 cannot be
distinguished whereas binaries with spin S_1=S_2 have clearly distinct
gravitational-wave emissions. Finally, we derive a simple expression for the
energy radiated in gravitational waves and find that the binaries always have
efficiencies E_rad/M > 3.6%, which can become as large as E_rad/M = 10% for
maximally spinning binaries with spins aligned with the orbital angular
momentum.Comment: 18 pages, 11 figures, matches published versio
Dynamical overlap simulations using HMC
We apply the Hybrid Monte Carlo method to the simulation of overlap fermions.
We give the fermionic force for the molecular dynamics update. We present early
results on a small dynamical chiral ensemble.Comment: Lattice2004(machines), 3 pages; references updated, minor changes to
tex
Cell cycle length, cell size, and proliferation rate in hydra stem cells
We have analyzed the cell cycle parameters of interstitial cells in Hydra oligactis. Three subpopulations of cells with short, medium, and long cell cycles were identified. Short-cycle cells are stem cells; medium-cycle cells are precursors to nematocyte differentiation; long-cycle cells are precursors to gamete differentiation. We have also determined the effect of different cell densities on the population doubling time, cell cycle length, and cell size of interstitial cells. Our results indicate that decreasing the interstitial cell density from 0.35 to 0.1 interstitial cells/epithelial cell (1) shortens the population doubling time from 4 to 1.8 days, (2) increases the [3H]thymidine labeling index from 0.5 to 0.75 and shifts the nuclear DNA distribution from G2 to S phase cells, and (3) decreases the length of G2 in stem cells from 6 to 3 hr. The shortened cell cycle is correlated with a significant decrease in the size of interstitial stem cells. Coincident with the shortened cell cycle and increased growth rate there is an increase in stem cell self-renewal and a decrease in stem cell differentiation
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: III. X-ray spectral modelling
Following the Swift X-ray observations of the 2006 outburst of the recurrent
nova RS Ophiuchi, we developed hydrodynamical models of mass ejection from
which the forward shock velocities were used to estimate the ejecta mass and
velocity. In order to further constrain our model parameters, here we present
synthetic X-ray spectra from our hydrodynamical calculations which we compare
to the Swift data. An extensive set of simulations was carried out to find a
model which best fits the spectra up to 100 days after outburst. We find a good
fit at high energies but require additional absorption to match the low energy
emission. We estimate the ejecta mass to be in the range (2-5) x 10^{-7} solar
masses and the ejection velocity to be greater than 6000 km/s (and probably
closer to 10,000 km/s). We also find that estimates of shock velocity derived
from gas temperatures via standard model fits to the X-ray spectra are much
lower than the true shock velocities.Comment: 13 pages, 5 figures, Accepted for publication in Ap
What clients want: a conjoint analysis of precursors to coach selection
This study investigated individuals’ preference structures for workplace coaching providers. Guided by questions about relative weightings of seven important coach(ing) characteristics (i.e., coach work experience/background/gender; coaching training; personal recommendations; client feedback; coaching cost), we carried out a conjoint analysis, using a mixed occupational sample (N = 383). In addition, we conducted linear regression analyses to determine the extent to which coaches’ perceived competence, likeability and trustworthiness might impact on individuals’ decision-making processes. Potential coachees favoured professionally trained coaches with four to ten years’ experience and a similar background to themselves, were female, and charged below average fees. Personal recommendations and existing client feedback further influenced potential consumers’ decision-making. Moreover, perceived competence was highly predictive of potential coach selection
Accurate Realizations of the Ionized Gas in Galaxy Clusters: Calibrating Feedback
Using the full, three-dimensional potential of galaxy cluster halos (drawn
from an N-body simulation of the current, most favored cosmology), the
distribution of the X-ray emitting gas is found by assuming a polytropic
equation of state and hydrostatic equilibrium, with constraints from
conservation of energy and pressure balance at the cluster boundary. The
resulting properties of the gas for these simulated redshift zero clusters (the
temperature distribution, mass-temperature and luminosity-temperature
relations, and the gas fraction) are compared with observations in the X-ray of
nearby clusters. The observed properties are reproduced only under the
assumption that substantial energy injection from non-gravitational sources has
occurred. Our model does not specify the source, but star formation and AGN may
be capable of providing this energy, which amounts to 3 to 5 x10^{-5} of the
rest mass in stars (assuming ten percent of the gas initially in the cluster
forms stars). With the method described here it is possible to generate
realistic X-ray and Sunyaev-Zel'dovich cluster maps and catalogs from N-body
simulations, with the distributions of internal halo properties (and their
trends with mass, location, and time) taken into account.Comment: Matches ApJ published version; 30 pages, 7 figure
Reducing the weak lensing noise for the gravitational wave Hubble diagram using the non-Gaussianity of the magnification distribution
Gravitational wave sources are a promising cosmological standard candle
because their intrinsic luminosities are determined by fundamental physics (and
are insensitive to dust extinction). They are, however, affected by weak
lensing magnification due to the gravitational lensing from structures along
the line of sight. This lensing is a source of uncertainty in the distance
determination, even in the limit of perfect standard candle measurements. It is
commonly believed that the uncertainty in the distance to an ensemble of
gravitational wave sources is limited by the standard deviation of the lensing
magnification distribution divided by the square root of the number of sources.
Here we show that by exploiting the non-Gaussian nature of the lensing
magnification distribution, we can improve this distance determination,
typically by a factor of 2--3; we provide a fitting formula for the effective
distance accuracy as a function of redshift for sources where the lensing noise
dominates.Comment: matches PRD accepted version (expanded description of the
cosmological parameter space + minor changes
A multiwavelength study of the remnant of nova GK Persei
We present new observations of the nebular remnant of the old nova GK Persei
1901, in the optical using the 2m HCT and at low radio frequencies using the
GMRT. The evolution of the nova remnant indicates shock interaction with the
ambient medium, especially in the southwest quadrant. Application of a simple
model for the shock and its evolution to determine the time dependence of the
radius of the shell in the southwest quadrant indicates that the shell is now
expanding into an ambient medium that has a lower density compared to the
density of the ambient medium ahead of the shock in 1987.There are indications
of a recent interaction of the nova remnant with the ambient medium in the
northeast quadrant also. The nova remnant of GK Per is detected at all the
observed radio frequencies and is of similar extent as the optical remnant.
Putting together our radio observations with VLA archival data on GK Per from
1997, we obtain three interesting results: 1. The spectrum above 1.4 GHz
follows a power law with an index -0.7 and below 1.4 GHz follows a power law
with an index ~ -0.85. This could be due to the presence of at least two
populations of electrons dominating the global emission at different
frequencies. 2. We record an annual secular decrease of 2.1% in the flux
density of the nova remnant at 1.4 and 4.9 GHz between 1984 and 1997 which has
left the spectral index unchanged at -0.7. No such decrease is observed in the
flux densities below 1 GHz. 3. We record an increase in the flux density at
0.33 GHz compared to the previous estimate in 1987. We conclude that the
remnant of nova GK Per is similar to supernova remnants and in particular, to
the young supernova remnant Cas A.Comment: 10 pages; uses A&A style; figures 1, 2 & 6 are in JPEG format.
Accepted for publication in Astronomy & Astrophysics. Full paper including
Figures 1, 2 & 6 may be downloaded from
http://www.iiap.res.in/personnel/gca/gca.htm
New analytic running coupling in QCD: higher loop levels
The properties of the new analytic running coupling are investigated at the
higher loop levels. The expression for this invariant charge, independent of
the normalization point, is obtained by invoking the asymptotic freedom
condition. It is shown that at any loop level the relevant function has
the universal behaviors at small and large values of the invariant charge. Due
to this feature the new analytic running coupling possesses the universal
asymptotics both in the ultraviolet and infrared regions irrespective of the
loop level. The consistency of the model considered with the general definition
of the QCD invariant charge is shown.Comment: LaTeX 2.09, 12 pages with 5 EPS figures, uses mpla1.sty; enlarged
version is accepted for publication in Mod. Phys. Lett.
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