417 research outputs found
An ellipsometric study of protein adsorption at the saliva-air interface
At the liquid-air interface of human saliva a protein layer is adsorbed. From ellipsometric measurements it was found that the thickness of the surface layer ranged from 400 to 3600 Å and the amount of protein material adsorbed was 9–340 mg/m2. Based on the concentration of protein in the layer the samples could be classified into two groups: a low concentration (ca. 0.15 g/ml) and a high concentration (0.7–1.1 g/ml). In the low concentration group the surface layers appeared to be thin (500–600 Å) while those in the high concentration group appeared to be much thicker (1000–3500 Å). A correlation between the bulk pH and the thickness of the surface layer could be established
Rheological properties of human saliva
From measurements with a Couette-type viscometer provided with a guard ring it was shown that at the saliva-air interface a protein layer is adsorbed. Measurements of the surface shear modulus of this layer on saliva of 7 healthy subjects were performed at a frequency of about 70 Hz and a temperature of 25 °C. For a surface age of about 1.5 h the surface shear modulus and the surface viscosity were in the order of 1 Nm−1 and 10−3 Nm−1 s, respectively. From ellipsometric measurements it was found that the thickness of the protein layer was approx. 100nm and, using this value, it could be concluded that the shear modulus and the dynamic viscosity were in the order of 107 Pa and 104 Pa s, respectively. The layer appeared to be fragile. Even shear deformation amplitudes of 4 × 10−5 are too high to assure linearity. The complex viscosity (η = η′ − iη′′) of the bulk liquid of human submandibular saliva below the absorbed layer was measured in the frequency range 70 Hz–200 kHz with 3 torsional resonators, each for a different frequency, and a Ni-tube resonator. It was concluded, that the real part of the complex viscosity (η′) decreases from 1.1 mPa s at 70 Hz to a value of 0.95 mPa s at high frequencies. Except at the lowest frequency (70 Hz), the value of η′′ was too small to be detected
Growing and moving low-mass planets in non-isothermal disks
We study the interaction of a low-mass planet with a protoplanetary disk with
a realistic treatment of the energy balance by doing radiation-hydrodynamical
simulations. We look at accretion and migration rates and compare them to
isothermal studies. We used a three-dimensional version of the hydrodynamical
method RODEO, together with radiative transport in the flux-limited diffusion
approach. The accretion rate, as well as the torque on the planet, depend
critically on the ability of the disk to cool efficiently. For densities
appropriate to 5 AU in the solar nebula, the accretion rate drops by more than
an order of magnitude compared to isothermal models, while at the same time the
torque on the planet is positive, indicating outward migration. It is necessary
to lower the density by a factor of 2 to recover inward migration and more than
2 orders of magnitude to recover the usual Type I migration. The torque appears
to be proportional to the radial entropy gradient in the unperturbed disk.
These findings are critical for the survival of protoplanets, and they should
ultimately find their way into population synthesis models.Comment: Accepted for publication in Astronomy and Astrophysic
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
Planets opening dust gaps in gas disks
We investigate the interaction of gas and dust in a protoplanetary disk in
the presence of a massive planet using a new two-fluid hydrodynamics code. In
view of future observations of planet-forming disks we focus on the condition
for gap formation in the dust fluid. While only planets more massive than 1
Jupiter mass (MJ) open up a gap in the gas disk, we find that a planet of 0.1
MJ already creates a gap in the dust disk. This makes it easier to find
lower-mass planets orbiting in their protoplanetary disk if there is a
significant population of mm-sized particles.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Observational Study of the Multistructured Planetary Nebula NGC 7354
We present an observational study of the planetary nebula (PN) NGC 7354
consisting of narrowband Halpha and [NII]6584 imaging as well as low- and
high-dispersion long-slit spectroscopy and VLA-D radio continuum. According to
our imaging and spectroscopic data, NGC 7354 has four main structures: a quite
round outer shell and an elliptical inner shell, a collection of low-excitation
bright knots roughly concentrated on the equatorial region of the nebula, and
two symmetrical jet-like features, not aligned either with the shells' axes, or
with each other. We have obtained physical parameters like electron temperature
and electron density as well as ionic and elemental abundances for these
different structures. Electron temperature and electron density slightly vary
throughout the nebula. The local extinction coefficient c_Hbeta shows an
increasing gradient from south to north and a decreasing gradient from east to
west consistent with the number of equatorial bright knots present in each
direction. Abundance values show slight internal variations but most of them
are within the estimated uncertainties. In general, abundance values are in
good agreement with the ones expected for PNe. Radio continuum data are
consistent with optically thin thermal emission. We have used the interactive
three-dimensional modeling tool SHAPE to reproduce the observed morphokinematic
structures in NGC 7354 with different geometrical components. Our SHAPE model
is in very good agreement with our imaging and spectroscopic observations.
Finally, after modeling NGC 7354 with SHAPE, we suggest a possible scenario for
the formation of the nebula.Comment: Accepted for publication in AJ, 12 pages, 8 figure
RODEO: a new method for planet-disk interaction
In this paper we describe a new method for studying the hydrodynamical
problem of a planet embedded in a gaseous disk. We use a finite volume method
with an approximate Riemann solver (the Roe solver), together with a special
way to integrate the source terms. This new source term integration scheme
sheds new light on the Coriolis instability, and we show that our method does
not suffer from this instability. The first results on flow structure and gap
formation are presented, as well as accretion and migration rates. For Mpl <
0.1 M_J and Mpl > 1.0 M_J (M_J = Jupiter's mass) the accretion rates do not
depend sensitively on numerical parameters, and we find that within the disk's
lifetime a planet can grow to 3-4 M_J. In between these two limits numerics
play a major role, leading to differences of more than 50 % for different
numerical parameters. Migration rates are not affected by numerics at all as
long as the mass inside the Roche lobe is not considered. We can reproduce the
Type I and Type II migration for low-mass and high-mass planets, respectively,
and the fastest moving planet of 0.1 M_J has a migration time of only 2.0 10^4
yr.Comment: Accepted for publication in A&
Stellar Outflows Driven by Magnetized Wide-Angle Winds
We present two-dimensional, cylindrically symmetric simulations of
hydrodynamic and magnetohydrodynamic (MHD) wide-angle winds interacting with a
collapsing environment. These simulations have direct relevance to young
stellar objects (YSOs). The results may also be of use in the study of
collimated outflows from proto-planetary and planetary nebulae. We study a
range of wind configurations consistent with asymptotic MHD wind collimation.
The degree of collimation is parameterized by the ratio of the wind density at
the pole to that of the equator. We find that a toroidal magnetic field can
have a significant influence on the resulting outflow, giving rise to a very
dense, jet-like flow in the post-shock region. The properties of the flow in
this region are similar to the asymptotic state of a collimated MHD wind. We
conclude that wide-angle MHD winds are quite likely capable of driving
molecular outflows. Due to difficulty in treating MHD winds ab-initio in
simulations we choose magnetic field strengths in the wind consistent slow
magnetic rotators. While MHD launched winds will be in the fast rotator regime
we discuss how our results, which rely on toroidal pinch effects, will hold for
stronger field strengths
Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882
We present images and high-resolution spectra of the planetary nebulae IC
2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are
composed of a markedly elongated inner shell, and of a less aspherical outer
shell expanding at a considerably higher velocity than the inner one. Embedded
in the outer shells of both nebulae are found several low-ionization knots. In
IC 2553, the knots show a point-symmetric distribution with respect to the
central star: one possible explanation for their formation is that they are the
survivors of pre-existing point-symmetric condensations in the AGB wind, a fact
which would imply a quite peculiar mass-loss geometry from the giant
progenitor. In the case of NGC 5882, the lack of symmetry in the distribution
of the observed low-ionization structures makes it possible that they are the
result of in situ instabilities.Comment: 20 pages including 1 table and 6 figures. ApJ accepted. Also
available at http://andromeda.roque.ing.iac.es/~sanchez/ingpub/index2000.htm
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