136 research outputs found
Sequential SDF1a and b-induced mobility guides Medaka PGC migration
AbstractAssembly and formation of the gonad primordium are the first steps toward gonad differentiation and subsequent sex differentiation. Primordial germ cells (PGCs) give rise to the gametes that are responsible for the development of a new organism in the next generation. In many organisms, following their specification the germ cells migrate toward the location of the prospective gonadal primordium. To accomplish this, the PGCs obtain directional cues from cells positioned along their migration path. One such cue, the chemokine SDF1 (stromal cell-derived factor 1) and its receptor CXCR4 have recently been found to be critical for proper PGC migration in zebrafish, chick and mouse.We have studied the mechanisms responsible for PGC migration in Medaka. In contrast to the situation observed in zebrafish, where proper PGC positioning is the result of active migration in the direction of the source of SDF1a, Medaka PGC movements are shown to be the consequence of a combination of active SDF1a and SDF1b-guided migration. In this process both SDF1 co-orthologues show only partly overlapping expression pattern and cooperate in the correct positioning of the PGCs
Master crossover behavior of parachor correlations for one-component fluids
The master asymptotic behavior of the usual parachor correlations, expressing
surface tension as a power law of the density difference
between coexisting liquid and vapor, is analyzed for a
series of pure compounds close to their liquid-vapor critical point, using only
four critical parameters , , and ,
for each fluid.
... The main consequences of these theoretical estimations are discussed in
the light of engineering applications and process simulations where parachor
correlations constitute one of the most practical method for estimating surface
tension from density and capillary rise measurements
Water and methanol in low-mass protostellar outflows: gas-phase synthesis, ice sputtering and destruction
Water in outflows from protostars originates either as a result of gas-phase synthesis from atomic oxygen at T ≳ 200 K, or from sputtered ice mantles containing water ice. We aim to quantify the contribution of the two mechanisms that lead to water in outflows, by comparing observations of gas-phase water to methanol (a grain surface product) towards three low-mass protostars in NGC 1333. In doing so, we also quantify the amount of methanol destroyed in outflows. To do this, we make use of James Clerk Maxwell Telescope and Herschel-Heterodyne Instrument for the Far-Infrared data of H2O, CH3OH and CO emission lines and compare them to RADEX non-local thermodynamic equilibrium excitation simulations. We find up to one order of magnitude decrease in the column density ratio of CH3OH over H2O as the velocity increases in the line wings up to ∼15 km s−1. An independent decrease in X(CH3OH) with respect to CO of up to one order of magnitude is also found in these objects. We conclude that gas-phase formation of H2O must be active at high velocities (above 10 km s−1 relative to the source velocity) to re-form the water destroyed during sputtering. In addition, the transition from sputtered water at low velocities to form water at high velocities must be gradual. We place an upper limit of two orders of magnitude on the destruction of methanol by sputtering effects
Fluctuations of elastic interfaces in fluids: Theory and simulation
We study the dynamics of elastic interfaces-membranes-immersed in thermally
excited fluids. The work contains three components: the development of a
numerical method, a purely theoretical approach, and numerical simulation. In
developing a numerical method, we first discuss the dynamical coupling between
the interface and the surrounding fluids. An argument is then presented that
generalizes the single-relaxation time lattice-Boltzmann method for the
simulation of hydrodynamic interfaces to include the elastic properties of the
boundary. The implementation of the new method is outlined and it is tested by
simulating the static behavior of spherical bubbles and the dynamics of bending
waves. By means of the fluctuation-dissipation theorem we recover analytically
the equilibrium frequency power spectrum of thermally fluctuating membranes and
the correlation function of the excitations. Also, the non-equilibrium scaling
properties of the membrane roughening are deduced, leading us to formulate a
scaling law describing the interface growth, W^2(L,T)=L^3 g[t/L^(5/2)], where
W, L and T are the width of the interface, the linear size of the system and
the temperature respectively, and g is a scaling function. Finally, the
phenomenology of thermally fluctuating membranes is simulated and the frequency
power spectrum is recovered, confirming the decay of the correlation function
of the fluctuations. As a further numerical study of fluctuating elastic
interfaces, the non-equilibrium regime is reproduced by initializing the system
as an interface immersed in thermally pre-excited fluids.Comment: 15 pages, 11 figure
HERSCHEL-HIFI spectroscopy of the intermediate mass protostar NGC7129 FIRS 2
HERSCHEL-HIFI observations of water from the intermediate mass protostar
NGC7129 FIRS 2 provide a powerful diagnostic of the physical conditions in this
star formation environment. Six spectral settings, covering four H216O and two
H218O lines, were observed and all but one H218O line were detected. The four
H2 16 O lines discussed here share a similar morphology: a narrower, \approx 6
km/s, component centered slightly redward of the systemic velocity of NGC7129
FIRS 2 and a much broader, \approx 25 km/s component centered blueward and
likely associated with powerful outflows. The narrower components are
consistent with emission from water arising in the envelope around the
intermediate mass protostar, and the abundance of H2O is constrained to \approx
10-7 for the outer envelope. Additionally, the presence of a narrow
self-absorption component for the lowest energy lines is likely due to
self-absorption from colder water in the outer envelope. The broader component,
where the H2O/CO relative abundance is found to be \approx 0.2, appears to be
tracing the same energetic region that produces strong CO emission at high J.Comment: 6 pages, 4 figures, accepted by A&
Sensitive limits on the abundance of cold water vapor in the DM Tau protoplanetary disk
We performed a sensitive search for the ground-state emission lines of ortho-
and para-water vapor in the DM Tau protoplanetary disk using the Herschel/HIFI
instrument. No strong lines are detected down to 3sigma levels in 0.5 km/s
channels of 4.2 mK for the 1_{10}--1_{01} line and 12.6 mK for the
1_{11}--0_{00} line. We report a very tentative detection, however, of the
1_{10}--1_{01} line in the Wide Band Spectrometer, with a strength of
T_{mb}=2.7 mK, a width of 5.6 km/s and an integrated intensity of 16.0 mK km/s.
The latter constitutes a 6sigma detection. Regardless of the reality of this
tentative detection, model calculations indicate that our sensitive limits on
the line strengths preclude efficient desorption of water in the UV illuminated
regions of the disk. We hypothesize that more than 95-99% of the water ice is
locked up in coagulated grains that have settled to the midplane.Comment: 5 pages, 3 figures. Accepted for publication in the Herschel HIFI
special issue of A&
Hydrides in Young Stellar Objects: Radiation tracers in a protostar-disk-outflow system
Context: Hydrides of the most abundant heavier elements are fundamental
molecules in cosmic chemistry. Some of them trace gas irradiated by UV or
X-rays. Aims: We explore the abundances of major hydrides in W3 IRS5, a
prototypical region of high-mass star formation. Methods: W3 IRS5 was observed
by HIFI on the Herschel Space Observatory with deep integration (about 2500 s)
in 8 spectral regions. Results: The target lines including CH, NH, H3O+, and
the new molecules SH+, H2O+, and OH+ are detected. The H2O+ and OH+ J=1-0 lines
are found mostly in absorption, but also appear to exhibit weak emission
(P-Cyg-like). Emission requires high density, thus originates most likely near
the protostar. This is corroborated by the absence of line shifts relative to
the young stellar object (YSO). In addition, H2O+ and OH+ also contain strong
absorption components at a velocity shifted relative to W3 IRS5, which are
attributed to foreground clouds. Conclusions: The molecular column densities
derived from observations correlate well with the predictions of a model that
assumes the main emission region is in outflow walls, heated and irradiated by
protostellar UV radiation.Comment: Astronomy and Astrophysics Letters, in pres
Water in massive star-forming regions: HIFI observations of W3 IRS5
We present Herschel observations of the water molecule in the massive
star-forming region W3 IRS5. The o-H17O 110-101, p-H18O 111-000, p-H2O 22
202-111, p-H2O 111-000, o-H2O 221-212, and o-H2O 212-101 lines, covering a
frequency range from 552 up to 1669 GHz, have been detected at high spectral
resolution with HIFI. The water lines in W3 IRS5 show well-defined
high-velocity wings that indicate a clear contribution by outflows. Moreover,
the systematically blue-shifted absorption in the H2O lines suggests expansion,
presumably driven by the outflow. No infall signatures are detected. The p-H2O
111-000 and o-H2O 212-101 lines show absorption from the cold material (T ~ 10
K) in which the high-mass protostellar envelope is embedded. One-dimensional
radiative transfer models are used to estimate water abundances and to further
study the kinematics of the region. We show that the emission in the rare
isotopologues comes directly from the inner parts of the envelope (T > 100 K)
where water ices in the dust mantles evaporate and the gas-phase abundance
increases. The resulting jump in the water abundance (with a constant inner
abundance of 10^{-4}) is needed to reproduce the o-H17O 110-101 and p-H18O
111-000 spectra in our models. We estimate water abundances of 10^{-8} to
10^{-9} in the outer parts of the envelope (T < 100 K). The possibility of two
protostellar objects contributing to the emission is discussed.Comment: Accepted for publication in the A&A HIFI special issu
Water in Star-Forming Regions with the Herschel Space Observatory (WISH): Overview of key program and first results
`Water In Star-forming regions with Herschel' (WISH) is a key program on the
Herschel Space Observatory designed to probe the physical and chemical
structure of young stellar objects using water and related molecules and to
follow the water abundance from collapsing clouds to planet-forming disks.
About 80 sources are targeted covering a wide range of luminosities and
evolutionary stages, from cold pre-stellar cores to warm protostellar envelopes
and outflows to disks around young stars. Both the HIFI and PACS instruments
are used to observe a variety of lines of H2O, H218O and chemically related
species. An overview of the scientific motivation and observational strategy of
the program is given together with the modeling approach and analysis tools
that have been developed. Initial science results are presented. These include
a lack of water in cold gas at abundances that are lower than most predictions,
strong water emission from shocks in protostellar environments, the importance
of UV radiation in heating the gas along outflow walls across the full range of
luminosities, and surprisingly widespread detection of the chemically related
hydrides OH+ and H2O+ in outflows and foreground gas. Quantitative estimates of
the energy budget indicate that H2O is generally not the dominant coolant in
the warm dense gas associated with protostars. Very deep limits on the cold
gaseous water reservoir in the outer regions of protoplanetary disks are
obtained which have profound implications for our understanding of grain growth
and mixing in disks.Comment: 71 pages, 10 figures, PASP, in pres
Water in low-mass star-forming regions with Herschel: HIFI spectroscopy of NGC1333
'Water In Star-forming regions with Herschel' (WISH) is a key programme
dedicated to studying the role of water and related species during the
star-formation process and constraining the physical and chemical properties of
young stellar objects. The Heterodyne Instrument for the Far-Infrared (HIFI) on
the Herschel Space Observatory observed three deeply embedded protostars in the
low-mass star-forming region NGC1333 in several H2-16O, H2-18O, and CO
transitions. Line profiles are resolved for five H16O transitions in each
source, revealing them to be surprisingly complex. The line profiles are
decomposed into broad (>20 km/s), medium-broad (~5-10 km/s), and narrow (<5
km/s) components. The H2-18O emission is only detected in broad 1_10-1_01 lines
(>20 km/s), indicating that its physical origin is the same as for the broad
H2-16O component. In one of the sources, IRAS4A, an inverse P Cygni profile is
observed, a clear sign of infall in the envelope. From the line profiles alone,
it is clear that the bulk of emission arises from shocks, both on small (<1000
AU) and large scales along the outflow cavity walls (~10 000 AU). The H2O line
profiles are compared to CO line profiles to constrain the H2O abundance as a
function of velocity within these shocked regions. The H2O/CO abundance ratios
are measured to be in the range of ~0.1-1, corresponding to H2O abundances of
~10-5-10-4 with respect to H2. Approximately 5-10% of the gas is hot enough for
all oxygen to be driven into water in warm post-shock gas, mostly at high
velocities.Comment: Accepted for publication in the A&A HIFI special issu
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