2,504 research outputs found
Decomposition of NO studied by infrared emission and CO laser absorption
A diagnostic technique for monitoring the concentration of NO using absorption of CO laser radiation was developed and applied in a study of the decomposition kinetics of NO. Simultaneous measurements of infrared emission by NO at 5.3 microns were also made to validate the laser absorption technique. The data were obtained behind incident shocks in NO-N2O-Ar (or Kr) mixtures, with temperatures in the range 2400-4100 K. Rate constants for dominant reactions were inferred from comparisons with computer simulations of the reactive flow
An Automatic and Symbolic Parallelization System for Distributed Memory Parallel Computers
This paper describes ASPAR (Automatic and Symbolic PARallelization) which consists of a source-to-source parallelizer and a set of interactive graphic tools. While the issues of data dependency have already been explored and used in many parallel computer systems such as vector and shared memory machines, distributed memory parallel computers require, in addition, explicit data decomposition. New symbolic analysis and data-dependency analysis methods are used to determine an explicit data decomposition scheme. Automatic parallelization models using high level communications are also described in this paper. The target applications are of the âregular-mesh" type typical of many scientific calculations.
The system has been implemented for the language C, and is designed for easy modification for other languages such as Fortran
Detection of FeO towards SgrB2
We have observed the J=5-4 ground state transition of FeO at a frequency of
153 GHz towards a selection of galactic sources.
Towards the galactic center source SgrB2, we see weak absorption at
approximately the velocity of other features towards this source (62 km
s LSR).
Towards other sources, the results were negative as they were also for
MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward
SgrB2 is due to FeO in the hot ( 500 K) relatively low density absorbing
gas known to be present in this line of sight.
This is the first (albeit tentative) detection of FeO or any iron--containing
molecule in the interstellar gas. Assuming the observed absorption to be due to
FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and
[FeO]/[H] of order . This is compatible with our negative
results in other sources.
Our results suggest that the iron liberated from grains in the shocks
associated with SgrB2 remains atomic and is not processed into molecular form.Comment: 1 postscrit figure,10 page
Impossibility of spontaneously breaking local symmetries and the sign problem
Elitzur's theorem stating the impossibility of spontaneous breaking of local
symmetries in a gauge theory is reexamined. The existing proofs of this theorem
rely on gauge invariance as well as positivity of the weight in the Euclidean
partition function. We examine the validity of Elitzur's theorem in gauge
theories for which the Euclidean measure of the partition function is not
positive definite. We find that Elitzur's theorem does not follow from gauge
invariance alone. We formulate a general criterion under which spontaneous
breaking of local symmetries in a gauge theory is excluded. Finally we
illustrate the results in an exactly solvable two dimensional abelian gauge
theory.Comment: Latex 6 page
The Structure, Kinematics and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253
We present 5.2" x 2.6" resolution interferometry of CO J=1-0 emission from
the starburst galaxy NGC 253. The high spatial resolution of these new data, in
combination with recent high resolution maps of 13CO, HCN and near-infrared
emission, allow us for the first time to link unambiguously the gas properties
in the central starburst of NGC 253 with its bar dynamics. We confirm that the
star formation results from bar-driven gas flows as seen in "twin peaks"
galaxies. Two distinct kinematic features are evident from the CO map and
position-velocity diagram: a group of clouds rotating as a solid body about the
kinematic center of the galaxy, and a more extended gas component associated
with the near-infrared bar. We model the line intensities of CO, HCN and 13CO
to infer the physical conditions of the gas in the nucleus of NGC 253. The
results indicate increased volume densities around the radio nucleus in a
twin-peaks morphology. Compared with the CO kinematics, the gas densities
appear highest near the radius of a likely inner Linblad resonance, and
slightly lead the bar minor axis. This result is similar to observations of the
face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst
generation due to gas inflow along a bar.Comment: To appear in the ApJ, 28 pages, 12 figure file
Simultaneous Embeddability of Two Partitions
We study the simultaneous embeddability of a pair of partitions of the same
underlying set into disjoint blocks. Each element of the set is mapped to a
point in the plane and each block of either of the two partitions is mapped to
a region that contains exactly those points that belong to the elements in the
block and that is bounded by a simple closed curve. We establish three main
classes of simultaneous embeddability (weak, strong, and full embeddability)
that differ by increasingly strict well-formedness conditions on how different
block regions are allowed to intersect. We show that these simultaneous
embeddability classes are closely related to different planarity concepts of
hypergraphs. For each embeddability class we give a full characterization. We
show that (i) every pair of partitions has a weak simultaneous embedding, (ii)
it is NP-complete to decide the existence of a strong simultaneous embedding,
and (iii) the existence of a full simultaneous embedding can be tested in
linear time.Comment: 17 pages, 7 figures, extended version of a paper to appear at GD 201
A pair of planets around HD 202206 or a circumbinary planet?
Long-term precise Doppler measurements with the CORALIE spectrograph reveal
the presence of a second planet orbiting the solar-type star HD202206. The
radial-velocity combined fit yields companion masses of m_2\sini = 17.4 M_Jup
and 2.44 M_Jup, semi-major axes of a = 0.83 AU and 2.55 AU, and eccentricities
of e = 0.43 and 0.27, respectively. A dynamical analysis of the system further
shows a 5/1 mean motion resonance between the two planets. This system is of
particular interest since the inner planet is within the brown-dwarf limits
while the outer one is much less massive. Therefore, either the inner planet
formed simultaneously in the protoplanetary disk as a superplanet, or the outer
Jupiter-like planet formed in a circumbinary disk. We believe this singular
planetary system will provide important constraints on planetary formation and
migration scenarios.Comment: 9 pages, 14 figures, accepted in A&A, 12-May-200
Recombination Line vs. Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae (PNs)
have been found to give abundances that are much larger in some cases than
abundances from collisionally-excited forbidden lines (CELs). The origins of
this abundance discrepancy are highly debated. We present new spectroscopic
observations of O II and C II recombination lines for six planetary nebulae.
With these data we compare the abundances derived from the optical
recombination lines with those determined from collisionally-excited lines.
Combining our new data with published results on RLs in other PNs, we examine
the discrepancy in abundances derived from RLs and CELs. We find that there is
a wide range in the measured abundance discrepancy Delta(O+2) = log O+2(RL) -
log O+2(CEL), ranging from approximately 0.1 dex up to 1.4 dex. Most RLs yield
similar abundances, with the notable exception of O II multiplet V15, known to
arise primarily from dielectronic recombination, which gives abundances
averaging 0.6 dex higher than other O II RLs. We compare Delta(O+2) against a
variety of physical properties of the PNs to look for clues as to the mechanism
responsible for the abundance discrepancy. The strongest correlations are found
with the nebula diameter and the Balmer surface brightness. An inverse
correlation of Delta(O+2) with nebular density is also seen. Similar results
are found for carbon in comparing C II RL abundances with ultraviolet
measurements of C III].Comment: 48 pages, 14 figures, accepted for publication in the Astrophysical
Journal Supplemen
Warm Molecular Gas Traced with CO J=7->6 in the Galaxy's Central 2 Parsecs: Dynamical Heating of the Circumnuclear Disk
We present an 11 arcsec resolution map of the central two parsecs of the
Galaxy in the CO J =7->6 rotational transition. The CO emission shows rotation
about Sgr A*, but also evidence for non-circular turbulent motion and a clumpy
morphology. We combine our dataset with available CO measurements to model the
physical conditions in the disk. We find that the molecular gas in the region
is both warm and dense, with T~200-300 K, n_H2~50,000-70,000 cm^-3. The mass of
warm molecular gas we measure in the central two parsecs is at least 2000
M_solar, about 20 times the UV-excited atomic gas mass, ruling out an UV
heating scenario for the molecular material. We compare the available spectral
tracers with theoretical models and conclude that molecular gas is heated with
magneto-hydrodynamic shocks with v~10-20 kms and B~0.3-0.5 mG. Using the
conditions derived with the CO analysis, we include the other important
coolants--neutral oxygen and molecular hydrogen--to estimate the total cooling
budget of the molecular material. We derive a mass to luminosity ratio of 2-3
M_solar/ L_solar, which is consistent with the total power dissipated via
turbulent decay in 0.1 pc cells with v_rms~15 kms. These size and velocity
scales are comparable to the observed clumping scale and the velocity
dispersion. At this rate, the material near Sgr A* its dissipating its orbital
energy on an orbital timescale, and cannot last for more than a few orbits. Our
conclusions support a scenario in which the features near Sgr A* such as the
CND and northern arm are generated by infalling clouds with low specific
angular momentum.Comment: 31 pages, including 5 figures, accepted for publication in Ap
Systematic Molecular Differentiation in Starless Cores
(Abridged) We present evidence that low-mass starless cores, the simplest
units of star formation, are systematically differentiated in their chemical
composition. Molecules including CO and CS almost vanish near the core centers,
where the abundance decreases by one or two orders of magnitude. At the same
time, N2H+ has a constant abundance, and the fraction of NH3 increases toward
the core center. Our conclusions are based on a study of 5 mostly-round
starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have
mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and
(2,2), and the 1.2 mm continuum. For each core we have built a model that fits
simultaneously the radial profile of all observed emission and the central
spectrum for the molecular lines. The observed abundance drops of CO and CS are
naturally explained by the depletion of these molecules onto dust grains at
densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to
densities of several 10^5, while the NH3 abundance may be enhanced by reactions
triggered by the disappearance of CO from the gas phase. With the help of our
models, we show that chemical differentiation automatically explains the
discrepancy between the sizes of CS and NH3 maps, a problem which has remained
unexplained for more than a decade. Our models, in addition, show that a
combination of radiative transfer effects can give rise to the previously
observed discrepancy in the linewidth of these two tracers. Although this
discrepancy has been traditionally interpreted as resulting from a systematic
increase of the turbulent linewidth with radius, our models show that it can
arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap
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