598 research outputs found
Liquid-vapor oscillations of water in hydrophobic nanopores
Water plays a key role in biological membrane transport. In ion channels and
water-conducting pores (aquaporins), one dimensional confinement in conjunction
with strong surface effects changes the physical behavior of water. In
molecular dynamics simulations of water in short (0.8 nm) hydrophobic pores the
water density in the pore fluctuates on a nanosecond time scale. In long
simulations (460 ns in total) at pore radii ranging from 0.35 nm to 1.0 nm we
quantify the kinetics of oscillations between a liquid-filled and a
vapor-filled pore. This behavior can be explained as capillary evaporation
alternating with capillary condensation, driven by pressure fluctuations in the
water outside the pore. The free energy difference between the two states
depends linearly on the radius. The free energy landscape shows how a
metastable liquid state gradually develops with increasing radius. For radii
larger than ca. 0.55 nm it becomes the globally stable state and the vapor
state vanishes. One dimensional confinement affects the dynamic behavior of the
water molecules and increases the self diffusion by a factor of two to three
compared to bulk water. Permeabilities for the narrow pores are of the same
order of magnitude as for biological water pores. Water flow is not continuous
but occurs in bursts. Our results suggest that simulations aimed at collective
phenomena such as hydrophobic effects may require simulation times longer than
50 ns. For water in confined geometries, it is not possible to extrapolate from
bulk or short time behavior to longer time scales.Comment: 20 pages, 4 figures, 3 tables; to be published in Proc. Natl. Acad.
Sci. US
Formation of Pillars at the Boundaries between H II Regions and Molecular Clouds
We investigate numerically the hydrodynamic instability of an ionization
front (IF) accelerating into a molecular cloud, with imposed initial
perturbations of different amplitudes. When the initial amplitude is small, the
imposed perturbation is completely stabilized and does not grow. When the
initial perturbation amplitude is large enough, roughly the ratio of the
initial amplitude to wavelength is greater than 0.02, portions of the IF
temporarily separate from the molecular cloud surface, locally decreasing the
ablation pressure. This causes the appearance of a large, warm HI region and
triggers nonlinear dynamics of the IF. The local difference of the ablation
pressure and acceleration enhances the appearance and growth of a multimode
perturbation. The stabilization usually seen at the IF in the linear regimes
does not work due to the mismatch of the modes of the perturbations at the
cloud surface and in density in HII region above the cloud surface. Molecular
pillars are observed in the late stages of the large amplitude perturbation
case. The velocity gradient in the pillars is in reasonably good agreement with
that observed in the Eagle Nebula. The initial perturbation is imposed in three
different ways: in density, in incident photon number flux, and in the surface
shape. All cases show both stabilization for a small initial perturbation and
large growth of the second harmonic by increasing amplitude of the initial
perturbation above a critical value.Comment: 21 pages, 8 figures, accepted for publication in ApJ. high resolution
figures available upon reques
Photoemission of BiSe with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?
Topological insulators are characterized by Dirac cone surface states with
electron spins aligned in the surface plane and perpendicular to their momenta.
Recent theoretical and experimental work implied that this specific spin
texture should enable control of photoelectron spins by circularly polarized
light. However, these reports questioned the so far accepted interpretation of
spin-resolved photoelectron spectroscopy. We solve this puzzle and show that
vacuum ultraviolet photons (50-70 eV) with linear or circular polarization
probe indeed the initial state spin texture of BiSe while circularly
polarized 6 eV low energy photons flip the electron spins out of plane and
reverse their spin polarization. Our photoemission calculations, considering
the interplay between the varying probing depth, dipole selection rules and
spin-dependent scattering effects involving initial and final states explain
these findings, and reveal proper conditions for light-induced spin
manipulation. This paves the way for future applications of topological
insulators in opto-spintronic devices.Comment: Submitted for publication (2013
Re-defining the Empirical ZZ Ceti Instability Strip
We use the new ZZ Ceti stars (hydrogen atmosphere white dwarf variables;
DAVs) discovered within the Sloan Digital Sky Survey (Mukadam et al. 2004) to
re-define the empirical ZZ Ceti instability strip. This is the first time since
the discovery of white dwarf variables in 1968 that we have a homogeneous set
of spectra acquired using the same instrument on the same telescope, and with
consistent data reductions, for a statistically significant sample of ZZ Ceti
stars. The homogeneity of the spectra reduces the scatter in the spectroscopic
temperatures and we find a narrow instability strip of width ~950K, from
10850--11800K. We question the purity of the DAV instability strip as we find
several non-variables within. We present our best fit for the red edge and our
constraint for the blue edge of the instability strip, determined using a
statistical approach.Comment: 14 pages, 5 pages, ApJ paper, accepte
Discovery of 9 Ly alpha emitters at redshift z~3.1 using narrow-band imaging and VLT spectroscopy
Narrow-band imaging surveys aimed at detecting the faint emission from the
5007 [O III] line of intracluster planetary nebulae in Virgo also probe high
redshift z=3.1 Ly alpha emitters. Here we report on the spectroscopic
identification of 9 Ly alpha emitters at z=3.13, obtained with the FORS
spectrograph at Unit 1 of the ESO Very Large Telescope (VLT UT1). The spectra
of these high redshift objects show a narrow, isolated Ly alpha emission with
very faint (frequently undetected) continuum, indicating a large equivalent
width. No other features are visible in our spectra. Our Ly alpha emitters are
quite similar to those found by Hu (1998), Cowie & Hu (1998) and Hu et al.
(1998). Using simple population synthesis models, on the assumption that these
sources are regions of star formation, we conclude that the nebulae are nearly
optically thick and must have a very low dust content, in order to explain the
high observed Ly alpha equivalent widths. For the cosmological and star
formation parameters we adopted, the total stellar mass produced would seem to
correspond to the formation of rather small galaxies, some of which are perhaps
destined to merge. The implied star formation density in our sampled comoving
volume is probably somewhat smaller than, but of the same order of magnitude as
the star formation density at z=3 derived by other authors from Lyman-break
galaxy surveys. This result agrees with the expectation that the Ly alpha
emitters are a low-metallicity (or low-dust) tail in a distribution of star
forming regions at high redshifts. Finally, the Ly alpha emitters may
contribute as many H-ionizing photons as QSOs at z=3.Comment: 26 pages, 17 Postscript figures, ApJ in pres
Feedback in the Antennae Galaxies (NGC 4038/9): I. High-Resolution Infrared Spectroscopy of Winds from Super Star Clusters
We present high-resolution (R ~ 24,600) near-IR spectroscopy of the youngest
super star clusters (SSCs) in the prototypical starburst merger, the Antennae
Galaxies. These SSCs are young (3-7 Myr old) and massive (10^5 - 10^7 M_sun for
a Kroupa IMF) and their spectra are characterized by broad, extended Br-gamma
emission, so we refer to them as emission-line clusters (ELCs) to distinguish
them from older SSCs. The Brgamma lines of most ELCs have supersonic widths
(60-110 km/s FWHM) and non-Gaussian wings whose velocities exceed the clusters'
escape velocities. This high-velocity unbound gas is flowing out in winds that
are powered by the clusters' massive O and W-R stars over the course of at
least several crossing times. The large sizes of some ELCs relative to those of
older SSCs may be due to expansion caused by these outflows; many of the ELCs
may not survive as bound stellar systems, but rather dissipate rapidly into the
field population. The observed tendency of older ELCs to be more compact than
young ones is consistent with the preferential survival of the most
concentrated clusters at a given age.Comment: Accepted to Ap
An Investigation of Diffuse Interstellar Gas toward a Large, Low Extinction Window into the Inner Galaxy
Halpha and Hbeta spectroscopy with the Wisconsin H-Alpha Mapper (WHAM)
reveals a strong concentration of high velocity emission in a ~ 5 by 5 deg area
centered near (l.b) = (27,-3), known as the Scutum Cloud. The high velocities
imply that we are detecting optical emission from near the plane of the Galaxy
out to the tangent point at heliocentric distances of D \gtrsim 6 kpc, assuming
the gas participates in circular Galactic rotation. The ratio of the Halpha to
Hbeta emission as a function of velocity suggests that dust along these lines
of sight produces a total visual extinction of A_v ~ 3 at D ~ 6 kpc. This makes
it possible to use optical emission lines to explore the physical conditions of
ionized gas in the inner Galaxy. At a Galactocentric distance R_G ~ 4 kpc, for
example, we find that the H^+ has an rms midplane density of ~ 1 cm^-3 with a
vertical scale height of ~ 300 pc. We also find evidence for an increase in the
flux of Lyman continuum photons and an increase in the ratio of ionized to
neutral hydrogen toward the inner Galaxy. We have extended the measurements of
E(B-V) in this direction to distances far beyond what has been accessible
through stellar photometry and find E(B-V)/N_H to be near the local mean of 1.7
x 10^-22 cm^2 mag, with evidence for an increase in this ratio at R_G ~ 4 kpc.
Finally, our observations of [NII] 6583, [SII] 6716, and [OIII] 5007 toward the
window reveal that in the inner Galaxy the temperature of the gas and the
ionization state of oxygen increase with increasing height from the midplane.Comment: ApJ, accepted. 28 pages, 13 figures, 1 tabl
Lyman alpha line formation in starbursting galaxies I. Moderately thick, dustless, and static HI media
We investigate the Lyman alpha line transfer in nearby and high redshift
starbursting galaxies, where the effect of high optical depths and the role of
dust in the scattering medium are expected to be conspicuous and should be
treated in a very careful manner. We present our first results in dustless,
static, and uniform HI media with moderate Lyman alpha line center optical
depths \tau_0=10^{3-6}. We assume that the temperatures of media to be
T=10^{1-4}K, and use a Monte Carlo technique. We investigate the basic
processes of the line transfer and confirm the criterion of a\tau_0>10^3 for
the validity of diffusion approximation suggested by Neufeld in 1990, where
is the Voigt parameter. Adopting the model suggested by Tenorio-Tagle et al.,
we performed calculations on the Lyman alpha line formation for each
evolutionary stage of an expanding supershell. The emergent Lyman alpha
profiles are characterized by the double peaks and the absorption trough at the
line center. We found that the absorption troughs expected in most of the
evolutionary stages are not wide enough to be observed with current
instruments. However, the absorption trough in the Lyman alpha emission profile
from an expanding recombining supershell can be marginally detected.Comment: 38 pages, 12 figures, Accepted for publication in the Ap
Factors Influencing the Participation of Older People in Clinical Trials : Data Analysis from the MAVIS Trial
Peer reviewedPostprin
Escape probability methods versus "exact" transfer for modelling the X-ray spectrum of Active Galactic Nuclei and X-ray binaries
In the era of XMM-Newton and Chandra missions, it is crucial to use codes
able to compute correctly the line spectrum of X-ray irradiated thick media
(Thomson thickness of the order of unity) to build models for the structure and
the emission of the central regions of AGN or X-ray binaries. In all
photoionized codes except in our code Titan, the line intensities are computed
with the "escape probability approximation". In its last version, Titan solves
the transfer of a thousand lines and of the continuum with the ``Accelerated
Lambda Iteration" method, which is one of the most efficient and most secure
for line transfer. We find that for conditions typical of the AGN or X-ray
binary emission medium, all escape approximations commonly used lead to an
overestimation of the soft X-ray lines which can reach one order of magnitude
for intense lines.Comment: 19 pages, 14 figures, accepted in A&
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