880 research outputs found
Expansion of W 3(OH)
A direct measurement of the expansion of W 3(OH) is made by comparing Very
Large Array images taken about 10 yr apart. The expansion is anisotropic with a
typical speed of 3 to 5 km/s, indicating a dynamical age of only 2300 yr. These
observations are inconsistent with either the freely expanding shell model or a
simple bow shock model. The most favored model is a slowly expanding shell-like
HII region, with either a fast rarefied flow or another less massive diffuse
ionized region moving towards the observer. There is also a rapidly evolving
source near the projected center of emission, perhaps related to the central
star.Comment: LaTeX file, 28 pages, includes 8 figures. To appear in ApJ in
December 10 (1998) issue. Also available at
http://www.submm.caltech.edu/~kawamura/w3oh_pp.p
5cm OH masers as diagnostics of physical conditions in star-forming regions
We demonstrate that the observed characteristics of the 5 cm OH masers in
star-forming regions can be explained with the same model and the same
parameters as the 18 cm and the 6 cm OH masers. In our already published study
of the 18 cm and the 6 cm OH masers in star-forming regions we had examined the
pumping of the 5 cm masers, but did not report the results we had found because
of some missing collision rate coefficients, which in principle could be
important. The recently published observations on the 5 cm masers of OH
encourage us to report our old calculations along with some new ones that we
have performed. These calculations, in agreement with the observations, reveal
the main lines at 5 cm as strong masers, the 6049 MHz satellite line as a weak
maser, and the 6017 MHz satellite line as never inverted for reasonable values
of the parametersComment: TeX 15 pages, 30 postscript figures, accepted by Ap
Formation of disclination lines near a free nematic interface
We have studied the nucleation and the physical properties of a -1/2 wedge
disclination line near the free surface of a confined nematic liquid crystal.
The position of the disclination line has been related to the material
parameters (elastic constants, anchoring energy and favored anchoring angle of
the molecules at the free surface). The use of a planar model for the structure
of the director field (whose predictions have been contrasted to those of a
fully three-dimensional model) has allowed us to relate the experimentally
observed position of the disclination line to the relevant properties of the
liquid crystals. In particular, we have been able to observe the collapse of
the disclination line due to a temperature-induced anchoring angle transition,
which has allowed us to rule out the presence of a real disclination line near
the nematic/isotropic front in directional growth experiments.
61.30.Jf,61.30.G
Agricultural land-use change and ash (Fraxinus excelsior L.) colonization in Pyrenean landscapes: an interdisciplinary case study
 ONLINE FIRSTInternational audienceChanges in agricultural land use are responsible for significant modifications in mountain landscapes. This study is part of an interdisciplinary research on the processes and consequences of spontaneous afforestation of Pyrenean landscapes by ash, and the possibilities for its management. We address the relationships between vegetation dynamics and land-use change from the combination of an agricultural study of change in farm management and an ecological study of grassland colonization by ash. In the framework of a village case study, we characterized parcels management and land-use histories, and analyzed the dynamics of the composition of grassland vegetation communities. From a joint analysis of the results obtained in each discipline, we discuss the limitations and comple-mentarities of the two approaches for the interdisciplinary assessment of the afforestation process
A computational framework to emulate the human perspective in flow cytometric data analysis
Background: In recent years, intense research efforts have focused on developing methods for automated flow cytometric data analysis. However, while designing such applications, little or no attention has been paid to the human perspective that is absolutely central to the manual gating process of identifying and characterizing cell populations. In particular, the assumption of many common techniques that cell populations could be modeled reliably with pre-specified distributions may not hold true in real-life samples, which can have populations of arbitrary shapes and considerable inter-sample variation.
<p/>Results: To address this, we developed a new framework flowScape for emulating certain key aspects of the human perspective in analyzing flow data, which we implemented in multiple steps. First, flowScape begins with creating a mathematically rigorous map of the high-dimensional flow data landscape based on dense and sparse regions defined by relative concentrations of events around modes. In the second step, these modal clusters are connected with a global hierarchical structure. This representation allows flowScape to perform ridgeline analysis for both traversing the landscape and isolating cell populations at different levels of resolution. Finally, we extended manual gating with a new capacity for constructing templates that can identify target populations in terms of their relative parameters, as opposed to the more commonly used absolute or physical parameters. This allows flowScape to apply such templates in batch mode for detecting the corresponding populations in a flexible, sample-specific manner. We also demonstrated different applications of our framework to flow data analysis and show its superiority over other analytical methods.
<p/>Conclusions: The human perspective, built on top of intuition and experience, is a very important component of flow cytometric data analysis. By emulating some of its approaches and extending these with automation and rigor, flowScape provides a flexible and robust framework for computational cytomics
Nitrogen hydrides in the cold envelope of IRAS16293-2422
Nitrogen is the fifth most abundant element in the Universe, yet the
gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen
hydrides are key molecules of nitrogen chemistry. Their abundance ratios place
strong constraints on the production pathways and reaction rates of
nitrogen-bearing molecules. We observed the class 0 protostar IRAS16293-2422
with the heterodyne instrument HIFI, covering most of the frequency range from
0.48 to 1.78~THz at high spectral resolution. The hyperfine structure of the
amidogen radical o-NH2 is resolved and seen in absorption against the continuum
of the protostar. Several transitions of ammonia from 1.2 to 1.8~THz are also
seen in absorption. These lines trace the low-density envelope of the
protostar. Column densities and abundances are estimated for each hydride. We
find that NH:NH2:NH3=5:1:300. {Dark clouds chemical models predict steady-state
abundances of NH2 and NH3 in reasonable agreement with the present
observations, whilst that of NH is underpredicted by more than one order of
magnitude, even using updated kinetic rates. Additional modelling of the
nitrogen gas-phase chemistry in dark-cloud conditions is necessary before
having recourse to heterogen processes
Lattice Boltzmann simulations in microfluidics: probing the no-slip boundary condition in hydrophobic, rough, and surface nanobubble laden microchannels
In this contribution we review recent efforts on investigations of the effect
of (apparent) boundary slip by utilizing lattice Boltzmann simulations. We
demonstrate the applicability of the method to treat fundamental questions in
microfluidics by investigating fluid flow in hydrophobic and rough
microchannels as well as over surfaces covered by nano- or microscale gas
bubbles.Comment: 11 pages, 6 figure
Starburst at the Expanding Molecular Superbubble in M82: Self-Induced Starburst at the Inner Edge of the Superbubble
We present high spatial resolution (2.3"x1.9" or 43 pc x 36 pc at D = 3.9
Mpc) 100 GHz millimeter-wave continuum emission observations with the Nobeyama
Millimeter Array toward an expanding molecular superbubble in the central
region of M82. The 100 GHz continuum image, which is dominated by free-free
emission, revealed that the four strongest peaks are concentrated at the inner
edge of the superbubble along the galactic disk. The production rates of Lyman
continuum photons calculated from 100 GHz continuum flux at these peaks are an
order of magnitude higher than those from the most massive star forming regions
in our Galaxy. At these regions, high velocity ionized gas (traced by H41a and
[Ne II]) can be seen, and H2O and OH masers are also concentrated. The center
of the superbubble, on the other hand, is weak in molecular and free-free
emissions and strong in diffuse hard X-ray emission. These observations suggest
that a strong starburst produced energetic explosions and resultant plasma and
superbubble expansions, and induced the present starburst regions traced by our
100 GHz continuum observations at the inner edge of the molecular superbubble.
These results, therefore, provide the first clear evidence of self-induced
starburst in external galaxies. Starburst at the center of the superbubble, on
the other hand, begins to cease because of a lack of molecular gas. This kind
of intense starburst seems to have occurred several times within 10^6-10^7
years in the central region of M82.Comment: 30 pages, 6 figures. To be appeared in ApJ, 617, 20 December 2004
issu
- âŠ