8,850 research outputs found
Ultracold collisions of metastable helium atoms
We report scattering lengths for the singlet Sigma g +, triplet Sigma u + and
quintet Sigma g + adiabatic molecular potentials relevant to collisions of two
metastable (n=2 triplet S) helium atoms as a function of the uncertainty in
these potentials. These scattering lengths are used to calculate experimentally
observable scattering lengths, elastic cross sections and inelastic rates for
any combination of states of the colliding atoms, at temperatures where the
Wigner threshold approximation is valid.Comment: 20 pages, 8 figures, RevTeX, epsf. Small additions of tex
The galactic magnetic field in the quasar 3C216
Multifrequency polarimetric observations made with the Very Long Baseline
Array of the quasar 3C216 reveal the presence of Faraday rotation measures
(RMs) in excess of 2000 rad/m**2 in the source rest frame, in the arc of
emission located at ~ 140 mas from the core. Rotation measures in the range
-300 - +300 rad/m**2 are detected in the inner 5 mas (~30 parsecs). while the
rotation measures near the core can be explained as due to a magnetic field in
the narrow line region, we favor the interpretation for the high RM in the arc
as due to a ``local'' Faraday screen, produced in a shock where the jet is
deflected by the interstellar medium of the host galaxy. Our results indicate
that a galacit magnetic field of the order of 50 microGauss on a scale greater
than 100 pc must be present in the galactic medium.Comment: 23 pages, 3 tables, 11 figures. To appear on The Astronomical
Journal, November 1999 Issu
B2 1144+35: A Giant Low Power Radio Galaxy with Superluminal Motion
We report on centimeter VLA and VLBI observations of the giant, low power
radio galaxy 1144+35. These observations are sensitive to structures on scales
from less than 1 parsec to greater than 1 megaparsec. Diffuse steep spectrum
lobes on the megaparsec scale are consistent with an age of 10
years. On the parsec scale, a complex jet component is seen to move away from
the center of activity with an apparent velocity 2.7 h c. It shows
a central spine -- shear layer morphology. A faint parsec scale counterjet is
detected and an intrinsic jet velocity of 0.95 c and angle to the line of sight
of 25 are derived, consistent with an intrinsically symmetric ejection.
The central spine in the parsec scale jet is expected to move at a higher
velocity and a Lorentz factor 15 has been estimated near the
core.The age of this inner VLBI structure is 300 years. Assuming a
constant angle to the line-of-sight, the jet velocity is found to decrease from
0.95 c at 20 mas (32 pc on the plane of the sky) to 0.02 c at 15 arcsec (24 kpc
on the plane of the sky). These findings lend credence to the claim that (1)
even the jets of low power radio galaxies start out relativistic; and (2) these
jets are decelerated to subrelativistic velocities by the time they reach
kiloparsec scales.Comment: 21 pages, 16 separated figures. A version with figures and table in
the text is available at: ftp://terra.bo.cnr.it/papers/journals - it is a ps
gzipped file, named giovannini_apr99.gz (792kb) - ApJ in pres
Star Formation and AGN in the Core of the Shapley Supercluster: A VLA Survey of A3556, A3558, SC1327-312, SC1329-313, and A3562
The core of the Shapley supercluster (A3556, A3558, SC1327-312, SC1329-313,
and A3562) is an ideal region in which to study the effects of cluster mergers
on the activity of individual galaxies. This paper presents the most
comprehensive radio continuum investigation of the region, relying on a
63-pointing mosaic obtained with the Very Large Array yielding an areal
coverage of nearly 7 square degrees. The mosaic provides a typical sensitivity
of about 80 uJy at a resolution of 16", enabling detection of galaxies with
star formation rates as low as 1 solar mass per year. The radio data are
complemented by optical imaging in B and R, producing a catalog of 210
radio-detected galaxies with m_R <= 17.36 (M_R <= -19). At least 104 of these
radio-detected galaxies are members of the supercluster on the basis of public
velocity measurements. Across the entire core of the supercluster, there
appears to be a significant deficit of radio galaxies at intermediate optical
magnitudes (M_R between -21 and -22). This deficit is offset somewhat by an
increase in the frequency with which brighter galaxies (M_R less than -22) host
radio sources. More dramatic is the highly significant increase in the
probability for fainter galaxies (M_R between -20 and -21) in the vicinity of
A3562 and SC1329-313 to be associated with radio emission. The radio and
optical data for these sources strongly suggest that these active galaxies are
powered by star formation. In conjunction with recent X-ray analysis, this is
interpreted as young starbursts related to the recent merger of SC1329-313 with
A3562 and the rest of the supercluster.Comment: Accepted by AJ; 50 pages, including 16 figures (for full resolution
PDF, see http://mywebpages.comcast.net/nealamiller2/Shapley_pp.pdf
The VLT/MUSE view of the central galaxy in Abell 2052. Ionized gas swept by the expanding radio source
We report observations of the radio galaxy 3C317 (at z=0.0345) located at the
center of the Abell cluster A2052, obtained with the VLT/MUSE integral field
spectrograph. The Chandra images of this cluster show cavities in the X-ray
emitting gas, which were produced by the expansion of the radio lobes inflated
by the active galactic nucleus (AGN). Our exquisite MUSE data show with
unprecedented detail the complex network of line emitting filaments enshrouding
the northern X-ray cavity. We do not detect any emission lines from the
southern cavity, with a luminosity asymmetry between the two regions higher
than about 75. The emission lines produced by the warm phase of the
interstellar medium (WIM) enable us to obtain unique information on the
properties of the emitting gas. We find dense gas (up to 270 cm-3) that makes
up part of a global quasi spherical outflow that is driven by the radio source,
and obtain a direct estimate of the expansion velocity of the cavities (265 km
s-1). The emission lines diagnostic rules out ionization from the AGN or from
star-forming regions, suggesting instead ionization from slow shocks or from
cosmic rays. The striking asymmetric line emission observed between the two
cavities contrasts with the less pronounced differences between the north and
south sides in the hot gas; this represents a significant new ingredient for
our understanding of the process of the exchange of energy between the
relativistic plasma and the external medium. We conclude that the expanding
radio lobes displace the hot tenuous phase of the interstellar medium (ISM),
but also impact the colder and denser ISM phases. These results show the
effects of the AGN on its host and the importance of radio mode feedback.Comment: 7 pages, 9 figures, A&A in pres
Vacuum energy and spectral function sum rules
We reformulate the problem of the cancellation of the ultraviolet
divergencies of the vacuum energy, particularly important at the cosmological
level, in terms of a saturation of spectral function sum rules which leads to a
set of conditions on the spectrum of the fundamental theory. We specialize the
approach to both Minkowski and de Sitter space-times and investigate some
examples.Comment: 11 pages, revtex4, no figures, version to be published on PR
Arginine/Nanocellulose membranes for carbon capture applications
The present study investigates the influence of the addition of L-arginine to a matrix of carboxymethylated nanofibrillated cellulose (CMC-NFC), with the aim of fabricating a mobile carrier facilitated transport membrane for the separation of CO2. Self-standing films were prepared by casting an aqueous suspension containing different amounts of amino acid (15\u201330\u201345 wt.%) and CMC-NFC. The permeation properties were assessed in humid conditions (70\u201398% relative humidity (RH)) at 35 \ub0C for CO2 and N2 separately and compared with that of the non-loaded nanocellulose films. Both permeability and ideal selectivity appeared to be improved by the addition of L-arginine, especially when high amino-acid loadings were considered. A seven-fold increment in carbon dioxide permeability was observed between pure CMC-NFC and the 45 wt.% blend (from 29 to 220 Barrer at 94% RH), also paired to a significant increase of ideal selectivity (from 56 to 185). Interestingly, while improving the separation performance, water sorption was not substantially affected by the addition of amino acid, thus confirming that the increased permeability was not related simply to membrane swelling. Overall, the addition of aminated mobile carriers appeared to provide enhanced performances, advancing the state of the art for nanocellulose-based gas separation membranes
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