2,266 research outputs found
Thermal expansion within a chain of magnetic colloidal particles
We study the thermal expansion of chains formed by self-assembly of magnetic
colloidal particles in a magnetic field. Using video-microscopy, complete
positional data of all the particles of the chains is obtained. By changing the
ionic strength of the solution and the applied magnetic field, the interaction
potential can be tuned. We analyze the thermal expansion of the chain using a
simple model of a one dimensional anharmonic crystal of finite size.Comment: 5 pages and 3 figure
A high-sensitivity OH 5-cm line survey in late-type stars
We have undertaken a comprehensive search for 5-cm excited OH maser emission
from evolved stars representative of various stages of late stellar evolution.
Observed sources were selected from known 18-cm OH sources. This survey was
conducted with the 100-m Effelsberg telescope to achieve high signal to noise
ratio observations and a sensitivity limit of about 0.05 to 0.1 Jy. A total of
64 stellar sources were searched for both main line and satellite line
emission. We confirm the previous detection of 5 cm OH in Vy 2-2, do not
confirm emission from NML-Cyg and do not report any other new detection within
the above sensitivity limit.
Implications of these results on the pumping mechanism of the OH radical in
circumstellar envelopes are briefly discussed.Comment: 8 pages, 5 figures, A&A in pres
Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
Recent studies of methanol and ground-state OH masers at very high spectral
resolution have shed new light on small-scale maser processes. The nearby
source W3(OH), which contains numerous bright masers in several different
transitions, provides an excellent laboratory for high spectral resolution
techniques. We present a model of W3(OH) based on EVN observations of the
rotationally-excited 6030 and 6035 MHz OH masers taken at 0.024 km/s spectral
resolution. The 6.0 GHz masers are becoming brighter with time and show
evidence for tangential proper motions. We confirm the existence of a region of
magnetic field oriented toward the observer to the southeast and find another
such region to the northeast in W3(OH), near the champagne flow. The 6.0 GHz
masers trace the inner edge of a counterclockwise rotating torus feature.
Masers at 6030 MHz are usually a factor of a few weaker than at 6035 MHz but
trace the same material. Velocity gradients of nearby Zeeman components are
much more closely correlated than in the ground state, likely due to the
smaller spatial separation between Zeeman components. Hydroxyl maser peaks at
very long baseline interferometric resolution appear to have structure on
scales both smaller than that resolvable as well as on larger scales.Comment: 21 pages using emulateapj.cls including 16 figures and 2 tables,
accepted to Ap
Effelsberg Observations of Excited-State (6.0 GHz) OH in Supernova Remnants and W3(OH)
While masers in the 1720 MHz transition of OH are detected toward many
supernova remnants (SNRs), no other OH transition is seen as a maser in SNRs.
We present a search for masers at 6049 MHz, which has recently been predicted
to produce masers by pure collisional excitation at conditions similar to that
required for 1720 MHz masing. The Effelsberg 100 m telescope was used to
observe the excited-state 6016, 6030, 6035, and 6049 MHz lines of OH toward
selected SNRs, most of which have previously-detected bright 1720 MHz masers.
No excited-state masers are found toward SNRs, consistent with previous
observations of the 6049 MHz and other excited-state transitions. We do not see
clear evidence of absorption toward SNR target positions, although we do see
evidence of absorption in the molecular cloud at +50 km/s near Sgr A East. Weak
absorption is detected at 6016 MHz toward W3(OH), while stronger, narrower
emission is seen at 6049 MHz, suggesting that the 6049 MHz emission is a
low-gain maser. We conclude that conditions in SNRs are not conducive to
excited-state maser emission, especially in excited-state satellite lines.Comment: 4 pages using emulateapj.cls including 2 tables and 1 figure,
accepted to ApJ
Nanorheology : an Investigation of the Boundary Condition at Hydrophobic and Hydrophilic Interfaces
t has been shown that the flow of a simple liquid over a solid surface can
violate the so-called no-slip boundary condition. We investigate the flow of
polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a
hydrophobic surface made of a Self-Assembled Monolayer of OTS
(octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus
(DSFA) which allows one to study the flow of a liquid film confined between two
surfaces with a nanometer resolution. No-slip boundary conditions are found for
both fluids on hydrophilic surfaces only. Significant slip is found on the
hydrophobic surfaces, with a typical length of one hundred nanometers.Comment: 8 pages, 7 figures, 2 tables. Accepted for European Physical Journal
E - Sofr Mate
VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser
The OH 1667 MHz maser in the circumstellar shell around the Mira variable U
Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6
epochs, spread over 4 years. Using phase referencing techniques the position of
the most blue-shifted maser spot was monitored with respect to two
extra-galactic radio sources. The absolute radio positions of the maser can be
compared with the stellar optical position measured by the Hipparcos satellite
to 15 mas accuracy. This confirms the model in which one of the maser spots
corresponds to the stellar continuum, amplified by the maser. The stellar
proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.Comment: 6 pages, 4 figures; to be published in A&
EVLA Observations of OH Masers in ON 1
This Letter reports on initial Expanded Very Large Array (EVLA) observations
of the 6035 MHz masers in ON 1. The EVLA data are of good quality, lending
confidence in the new receiver system. Nineteen maser features, including six
Zeeman pairs, are detected. The overall distribution of 6035 MHz OH masers is
similar to that of the 1665 MHz OH masers. The spatial resolution is sufficient
to unambiguously determine that the magnetic field is strong (~ -10 mG) at the
location of the blueshifted masers in the north, consistent with Zeeman
splitting detected in 13441 MHz OH masers in the same velocity range. Left and
right circularly polarized ground-state features dominate in different regions
in the north of the source, which may be due to a combination of magnetic field
and velocity gradients. The combined distribution of all OH masers toward the
south is suggestive of a shock structure of the sort previously seen in W3(OH).Comment: 4 pages using emulateapj.cls including 2 tables and 2 color figure
Dynamics of cholesteric structures in an electric field
Motivated by Lehmann-like rotation phenomena in cholesteric drops we study
the transverse drift of two types of cholesteric fingers, which form rotating
spirals in thin layers of cholesteric liquid crystal in an ac or dc electric
field. We show that electrohydrodynamic effects induced by Carr-Helfrich charge
separation or flexoelectric charge generation can describe the drift of
cholesteric fingers. We argue that the observed Lehmann-like phenomena can be
understood on the same basis.Comment: 4 pages, 4 figures, submitted to PR
Landscape grammar: a method to analyse and design hedgerows and networks.
In Europe, rural landscapes are characterized by the presence of hedgerows and networks. In recent decades changes in agricultural systems, due in particular to the intensification of agricultural practices, have caused a transformation of hedgerows and networks, thereby reducing their qualities and changing their ecological and social functions. Yet, no global framework to analyze and design hedgerows and networks is available. Our paper is a step forward in combining the ecological and social dimension of hedgerows and hedgerow networks for both analytical and planning purposes. We propose to use a landscape grammar for deciphering the structural and functional aspects of hedgerow units and networks and to formalize rules for design and management based on scientific evidence. In the case of hedgerows, landscape grammar consists of letters, or single units of trees and shrubs of different species and their different shapes related to management practices. Appropriate or meaningful combinations of letters create words and sentences, hence forming hedgerow networks. In order to test the suitability of the grammar for reading or understanding and consequently writing or planning hedgerows in different landscapes, two study areas were chosen: Pleine-Fougères in Brittany (France) and Pianura Padana in Piedmont (Italy). The basic units, the aggregated units and the network were analyzed respectively as the letters, the words and the syntax of our landscape grammar. This metaphor provides an analytical framework for understanding hedgerows, from the individual tree to the landscape. Our model anticipates the concerns of both researchers and policymakers throughout the hedgerow network planning process
- …