310 research outputs found
Effects of gravity on contractile proteins
A method was established for the isolation and purification of nuclei in high yield from the microplasmodia of Physarum flavicomum. Purified nuclei were resistant to breakage by methods commonly employed for isolated plant and animal nuclei. Several methods for the extraction of nuclear protein were compared. Incubation of nuclear lysates with either 2 M NaCl, with or without 5 M urea, or 1 M CaCl2 resulted in the extraction of nuclear action together with histones. The histones were chemically fractionated into the 5 basic groups common to other eucaryotic tissue. Amino acid analyses of the total histone were also performed. Nuclear actin was found to have a molecular weight of 41,000 ? 4,000 daltons as determined by SDS polyacrylamide gel electrophoresis. The amino acid composition of the nuclear action was established
The Nature and Frequency of Outflows from Stars in the Central Orion Nebula Cluster
Recent Hubble Space Telescope images have allowed the determination with
unprecedented accuracy of motions and changes of shocks within the inner Orion
Nebula. These originate from collimated outflows from very young stars, some
within the ionized portion of the nebula and others within the host molecular
cloud. We have doubled the number of Herbig-Haro objects known within the inner
Orion Nebula. We find that the best-known Herbig-Haro shocks originate from a
relatively few stars, with the optically visible X-ray source COUP 666 driving
many of them.
While some isolated shocks are driven by single collimated outflows, many
groups of shocks are the result of a single stellar source having jets oriented
in multiple directions at similar times. This explains the feature that shocks
aligned in opposite directions in the plane of the sky are usually blue shifted
because the redshifted outflows pass into the optically thick Photon Dominated
Region behind the nebula. There are two regions from which optical outflows
originate for which there are no candidate sources in the SIMBAD data base.Comment: 152 pages, 46 figures, 7 tables. Accepted by A
A Keck High Resolution Spectroscopic Study of the Orion Nebula Proplyds
We present the results of spectroscopy of four bright proplyds in the Orion
Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of
the proplyd spectra from the confusing nebular radiation, the emission line
profiles are compared with those predicted by realistic dynamic/photoionization
models of the objects. The spectral line widths show a clear correlation with
ionization potential, which is consistent with the free expansion of a
transonic, ionization-stratified, photoevaporating flow. Fitting models of such
a flow simultaneously to our spectra and HST emission line imaging provides
direct measurements of the proplyd size, ionized density and outflow velocity.
These measurements confirm that the ionization front in the proplyds is
approximately D-critical and provide the most accurate and robust estimate to
date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are
found for our spectroscopic sample, although extrapolating our results to a
larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the
proplyds as a whole. In view of the reported limits on the masses of the
circumstellar disks within the proplyds, the length of time that they can have
been exposed to ionizing radiation should not greatly exceed 10,000 years - a
factor of 30 less than the mean age of the proplyd stars. We review the various
mechanisms that have been proposed to explain this situation, and conclude that
none can plausibly work unless the disk masses are revised upwards by a
substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in
The Astronomical Journal (scheduled November 1999
Spitzer reveals what's behind Orion's Bar
We present Spitzer Space Telescope observations of 11 regions SE of the
Bright Bar in the Orion Nebula, along a radial from the exciting star
theta1OriC, extending from 2.6 to 12.1'. Our Cycle 5 programme obtained deep
spectra with matching IRS short-high (SH) and long-high (LH) aperture grid
patterns. Most previous IR missions observed only the inner few arcmin. Orion
is the benchmark for studies of the ISM particularly for elemental abundances.
Spitzer observations provide a unique perspective on the Ne and S abundances by
virtue of observing the dominant ionization states of Ne (Ne+, Ne++) and S
(S++, S3+) in Orion and H II regions in general. The Ne/H abundance ratio is
especially well determined, with a value of (1.01+/-0.08)E-4. We obtained
corresponding new ground-based spectra at CTIO. These optical data are used to
estimate the electron temperature, electron density, optical extinction, and
the S+/S++ ratio at each of our Spitzer positions. That permits an adjustment
for the total gas-phase S abundance because no S+ line is observed by Spitzer.
The gas-phase S/H abundance ratio is (7.68+/-0.30)E-6. The Ne/S abundance ratio
may be determined even when the weaker hydrogen line, H(7-6) here, is not
measured. The mean value, adjusted for the optical S+/S++ ratio, is Ne/S =
13.0+/-0.6. We derive the electron density versus distance from theta1OriC for
[S III] and [S II]. Both distributions are for the most part decreasing with
increasing distance. A dramatic find is the presence of high-ionization Ne++
all the way to the outer optical boundary ~12' from theta1OriC. This IR result
is robust, whereas the optical evidence from observations of high-ionization
species (e.g. O++) at the outer optical boundary suffers uncertainty because of
scattering of emission from the much brighter inner Huygens Region.Comment: 60 pages, 16 figures, 10 tables. MNRAS accepte
GMOS-IFU Spectroscopy of 167-317 (LV2) Proplyd in Orion
We present high spatial resolution spectroscopic observations of the proplyd
167-317 (LV2) near the Trapezium cluster in the Orion nebula, obtained during
the System Verification run of the Gemini Multi Object Spectrograph (GMOS)
Integral Field Unit (IFU) at the Gemini South Observatory. We have detected 38
forbidden and permitted emission lines associated with the proplyd and its
redshifted jet. We have been able to detect three velocity components in the
profiles of some of these lines: a peak with a 28-33 km/s systemic velocity
that is associated with the photoevaporated proplyd flow, a highly redshifted
component associated with a previously reported jet (which has receding
velocities of about 80-120 km/s with respect to the systemic velocity and is
spatially distributed to the southeast of the proplyd) and a less obvious,
approaching structure, which may possibly be associated with a faint
counter-jet with systemic velocity of (-75 +/- 15) km/s. We find evidences that
the redshifted jet has a variable velocity, with slow fluctuations as a
function of the distance from the proplyd. We present several background
subtracted, spatially distributed emission line maps and we use this
information to obtain the dynamical characteristics over the observed field.
Using a simple model and with the extinction corrected Halpha fluxes, we
estimate the mass loss rate for both the proplyd photoevaporated flow and the
redshifted microjet, obtaining (6.2 +/- 0.6) x 10^{-7} M_sun/year and (2.0 +/-
0.7) x 10^{-8} M_sun/year, respectively.Comment: 26 pages, 10 figures (6 are in colors), accepted by A
Set-valued orthogonal additivity
We study the set-valued Cauchy equation postulated for orthogonal vectors. We give its general solution as well as we look for selections of functions satisfying the equation
Photoevaporation Flows in Blister HII Regions: I. Smooth Ionization Fronts and Application to the Orion Nebula
We present hydrodynamical simulations of the photoevaporation of a cloud with
large-scale density gradients, giving rise to an ionized, photoevaporation
flow. The flow is found to be approximately steady during the large part of its
evolution, during which it can resemble a "champagne flow" or a "globule flow"
depending on the curvature of the ionization front. The distance from source to
ionization front and the front curvature uniquely determine the structure of
the flow, with the curvature depending on the steepness of the lateral density
gradient in the neutral cloud. We compare these simulations with both new and
existing observations of the Orion nebula and find that a model with a mildly
convex ionization front can reproduce the profiles of emission measure,
electron density, and mean line velocity for a variety of emitting ions on
scales of 10^{17} to 10^{18} cm. The principal failure of our model is that we
cannot explain the large observed widths of the [O I] 6300 Angstrom line that
forms at the ionization front.Comment: 21 pages, accepted for publication in The Astrophysical Journa
Photoevaporating flows from the cometary knots in the Helix nebula (NGC 7293)
We explain the Ha emission of the cometary knots in the Helix Nebula (NGC
7293) with an analytical model that describes the emission of the head of the
globules as a photoevaporated flow produced by the incident ionizing radiation
of the central star.We compare these models with the Ha emission obtained from
the HST archival images of the Helix Nebula. From a comparison of the Ha
emission with the predictions of the analytical model we obtain a rate of
ionizing photons from the central star of about 5e45 s^-1, which is consistent
with estimates based on the total Hb flux of the nebula. We also model the
tails of the cometary knots as a photoevaporated wind from a neutral shadow
region produced by the diffuse ionizing photon field of the nebula. A
comparison with the HST images allows us to obtain a direct determination of
the value of the diffuse ionizing flux. We compare the ratio of diffuse to
direct stellar flux as a function of radius inside an HII region with those
obtained from the observational data through the analytical tail and head wind
model. The agreement of this model with the values determined from the
observations of the knots is excellent.Comment: 9 pages, 5 figures, accepted for publication in Ap
Three-dimensional chemically homogeneous and bi-abundance photoionization models of the "super-metal-rich" planetary nebula NGC 6153
Deep spectroscopy of the planetary nebula (PN) NGC\,6153 shows that its heavy
element abundances derived from optical recombination lines (ORLs) are ten
times higher than those derived from collisionally excited lines (CELs), and
points to the existence of H-deficient inclusions embedded in the diffuse
nebula. In this study, we have constructed chemically homogeneous and
bi-abundance three-dimensional photoionization models, using the Monte Carlo
photoionization code {\sc mocassin}. We attempt to reproduce the multi-waveband
spectroscopic and imaging observations of NGC\,6153, and investigate the nature
and origin of the postulated H-deficient inclusions, as well as their impacts
on the empirical nebular analyses assuming a uniform chemical composition. Our
results show that chemically homogeneous models yield small electron
temperature fluctuations and fail to reproduce the strengths of ORLs from C, N,
O and Ne ions. In contrast, bi-abundance models incorporating a small amount of
metal-rich inclusions ( per cent of the total nebular mass) are able
to match all the observations within the measurement uncertainties. The
metal-rich clumps, cooled down to a very low temperature (~K) by
ionic infrared fine-structure lines, dominate the emission of heavy element
ORLs, but contribute almost nil to the emission of most CELs. We find that the
abundances of C, N, O and Ne derived empirically from CELs, assuming a uniform
chemical composition, are about 30 per cent lower than the corresponding
average values of the whole nebula, including the contribution from the
H-deficient inclusions. Ironically, in the presence of H-deficient inclusions,
the traditional standard analysis of the optical helium recombination lines,
assuming a chemically homogeneous nebula, overestimates the helium abundance by
40 per cent.Comment: 19 pages, 18 figures, accepted for publication in MNRA
Structure of Protein Interaction Networks and Their Implications on Drug Design
Protein-protein interaction networks (PINs) are rich sources of information that enable the network properties of biological systems to be understood. A study of the topological and statistical properties of budding yeast and human PINs revealed that they are scale-rich and configured as highly optimized tolerance (HOT) networks that are similar to the router-level topology of the Internet. This is different from claims that such networks are scale-free and configured through simple preferential-attachment processes. Further analysis revealed that there are extensive interconnections among middle-degree nodes that form the backbone of the networks. Degree distributions of essential genes, synthetic lethal genes, synthetic sick genes, and human drug-target genes indicate that there are advantageous drug targets among nodes with middle- to low-degree nodes. Such network properties provide the rationale for combinatorial drugs that target less prominent nodes to increase synergetic efficacy and create fewer side effects
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