762 research outputs found
The ortho-to-para ratio of ammonia in the L1157 outflow
We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of
the L1157 outflow by observing the six metastable inversion lines from (J, K) =
(1, 1) to (6, 6). The highly excited (5, 5) and (6, 6) lines were first
detected in the low-mass star forming regions. The rotational temperature
derived from the ratio of four transition lines from (3, 3) to (6, 6) is
130-140 K, suggesting that the blueshifted gas is heated by a factor of ~10 as
compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in
the blueshifted gas is estimated to be 1.3--1.7, which is higher than the
statistical equilibrium value. This ratio provides us with evidence that the
NH3 molecules have been evaporated from dust grains with the formation
temperature between 18 and 25 K. It is most likely that the NH3 molecules on
dust grains have been released into the gas phase through the passage of strong
shock waves produced by the outflow. Such a scenario is supported by the fact
that the ammonia abundance in the blueshifted gas is enhanced by a factor of ~5
with respect to the dense quiescent gas.Comment: 16 pages, including 3 PS figures. To appear in the ApJ (Letters).
aastex macro
Elasticity of Diamond at High Pressures and Temperatures
We combine density functional theory within the local density approximation,
the quasiharmonic approximation, and vibrational density of states to calculate
single crystal elastic constants, and bulk and shear moduli of diamond at
simultaneous high pressures and temperatures in the ranges of 0-500 GPa and
0-4800 K. Comparison with experimental values at ambient pressure and high
temperature shows an excellent agreement for the first time with our
first-principles results validating our method. We show that the anisotropy
factor of diamond increases to 40% at high pressures and becomes temperature
independent.Comment: 10 pages, 3 figures, 1 tabl
Synthesizing Diverse Lung Nodules Wherever Massively: 3D Multi-Conditional GAN-Based CT Image Augmentation for Object Detection
Accurate Computer-Assisted Diagnosis, relying on large-scale annotated
pathological images, can alleviate the risk of overlooking the diagnosis.
Unfortunately, in medical imaging, most available datasets are
small/fragmented. To tackle this, as a Data Augmentation (DA) method, 3D
conditional Generative Adversarial Networks (GANs) can synthesize desired
realistic/diverse 3D images as additional training data. However, no 3D
conditional GAN-based DA approach exists for general bounding box-based 3D
object detection, while it can locate disease areas with physicians' minimum
annotation cost, unlike rigorous 3D segmentation. Moreover, since lesions vary
in position/size/attenuation, further GAN-based DA performance requires
multiple conditions. Therefore, we propose 3D Multi-Conditional GAN (MCGAN) to
generate realistic/diverse 32 X 32 X 32 nodules placed naturally on lung
Computed Tomography images to boost sensitivity in 3D object detection. Our
MCGAN adopts two discriminators for conditioning: the context discriminator
learns to classify real vs synthetic nodule/surrounding pairs with noise
box-centered surroundings; the nodule discriminator attempts to classify real
vs synthetic nodules with size/attenuation conditions. The results show that 3D
Convolutional Neural Network-based detection can achieve higher sensitivity
under any nodule size/attenuation at fixed False Positive rates and overcome
the medical data paucity with the MCGAN-generated realistic nodules---even
expert physicians fail to distinguish them from the real ones in Visual Turing
Test.Comment: 9 pages, 6 figures, accepted to 3DV 201
Transformation kinetics of alloys under non-isothermal conditions
The overall solid-to-solid phase transformation kinetics under non-isothermal
conditions has been modeled by means of a differential equation method. The
method requires provisions for expressions of the fraction of the transformed
phase in equilibrium condition and the relaxation time for transition as
functions of temperature. The thermal history is an input to the model. We have
used the method to calculate the time/temperature variation of the volume
fraction of the favored phase in the alpha-to-beta transition in a zirconium
alloy under heating and cooling, in agreement with experimental results. We
also present a formulation that accounts for both additive and non-additive
phase transformation processes. Moreover, a method based on the concept of path
integral, which considers all the possible paths in thermal histories to reach
the final state, is suggested.Comment: 16 pages, 7 figures. To appear in Modelling Simul. Mater. Sci. En
Herschel/HIFI deepens the circumstellar NH3 enigma
Circumstellar envelopes (CSEs) of a variety of evolved stars have been found
to contain ammonia (NH3) in amounts that exceed predictions from conventional
chemical models by many orders of magnitude. The observations reported here
were performed in order to better constrain the NH3 abundance in the CSEs of
four, quite diverse, oxygen-rich stars using the NH3 ortho J_K = 1_0 - 0_0
ground-state line. We used the Heterodyne Instrument for the Far Infrared
aboard Herschel to observe the NH3 J_K = 1_0 - 0_0 transition near 572.5 GHz,
simultaneously with the ortho-H2O J_Ka,Kc = 1_1,0 -1_0,1 transition, toward VY
CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative
transfer modeling with the goal to derive the NH3 abundance in these objects'
CSEs. For the latter two stars, Very Large Array imaging of NH3
radio-wavelength inversion lines were used to provide further constraints,
particularly on the spatial extent of the NH3-emitting regions. Results. We
find remarkably strong NH3 emission in all of our objects with the NH3 line
intensities rivaling those obtained for the ground state H2O line. The NH3
abundances relative to H2 are very high and range from 2 x 10-7 to 3 x 10-6 for
the objects we have studied. Our observations confirm and even deepen the
circumstellar NH3 enigma. While our radiative transfer modeling does not yield
satisfactory fits to the observed line profiles, it leads to abundance
estimates that confirm the very high values found in earlier studies. New ways
to tackle this mystery will include further Herschel observations of more NH3
lines and imaging with the Expanded Very Large Array.Comment: 4+2 page
Understanding the nature of "superhard graphite"
Numerous experiments showed that on cold compression graphite transforms into
a new superhard and transparent allotrope. Several structures with different
topologies have been proposed for this phase. While experimental data are
consistent with these models, the only way to solve this puzzle is to find
which structure is kinetically easiest to form. Using state-of-the-art
molecular-dynamics transition path sampling simulations, we investigate kinetic
pathways of the pressure-induced transformation of graphite to various
superhard candidate structures. Unlike hitherto applied methods for elucidating
nature of superhard graphite, transition path sampling realistically models
nucleation events necessary for physically meaningful transformation kinetics.
We demonstrate that nucleation mechanism and kinetics lead to -carbon as the
final product. -carbon, initially competitor to -carbon, is ruled out by
phase growth. Bct-C structure is not expected to be produced by cold
compression due to less probable nucleation and higher barrier of formation
Kinetic temperatures toward X1/X2 orbit interceptions regions and Giant Molecular Loops in the Galactic center region
Context: It is well known that the kinetic temperatures, Tkin, of the
molecular clouds in the Galactic center region are higher than in typical disk
clouds. However, the Tkin of the molecular complexes found at higher latitudes
towards the giant molecular loops in the central region of the Galaxy is so far
unknown. The gas of these high latitude molecular clouds (hereafter referred to
as halo clouds) is located in a region where the gas in the disk may interact
with the gas in the halo in the Galactic center region.
Aims: To derive Tkin in the molecular clouds at high latitude and understand
the physical process responsible for the heating of the molecular gas both in
the Central Molecular Zone (the concentration of molecular gas in the inner 500
pc) and in the giant molecular loops.
Methods: We measured the metastable inversion transitions of NH3 from (1,1)
to (6,6) toward six positions selected throughout the Galactic central disk and
halo. We used rotational diagrams and large velocity gradient modeling to
estimate the kinetic temperatures toward all the sources. We also observed
other molecules like SiO, HNCO, CS, C34S, C18O, and 13CO, to derive the
densities and to trace different physical processes (shocks, photodissociation,
dense gas) expected to dominate the heating of the molecular gas.
Results: We derive for the first time Tkin of the high latitude clouds
interacting with the disk in the Galactic center region. We find high
rotational temperatures in all the observed positions. We derive two kinetic
temperature components (150 K and 40 K) for the positions in the Central
Molecular Zone, and only the warm kinetic temperature component for the clouds
toward the giant molecular loops. The fractional abundances derived from the
different molecules suggest that shocks provide the main heating mechanism
throughout the Galactic center, also at high latitudesComment: accepted for publication in A&A 06/09/201
AMI Large Array radio continuum observations of Spitzer c2d small clouds and cores
We perform deep 1.8 cm radio continuum imaging towards thirteen protostellar
regions selected from the Spitzer c2d small clouds and cores programme at high
resolution (25") in order to detect and quantify the cm-wave emission from
deeply embedded young protostars. Within these regions we detect fifteen
compact radio sources which we identify as radio protostars including two
probable new detections. The sample is in general of low bolometric luminosity
and contains several of the newly detected VeLLO sources. We determine the 1.8
cm radio luminosity to bolometric luminosity correlation, L_rad -L_bol, for the
sample and discuss the nature of the radio emission in terms of the available
sources of ionized gas. We also investigate the L_rad-L_IR correlation and
suggest that radio flux density may be used as a proxy for the internal
luminosity of low luminosity protostars.Comment: submitted MNRA
Observation of nu_tau appearance in the CNGS beam with the OPERA experiment
The OPERA experiment is searching for nu_mu -> nu_tau oscillations in
appearance mode i.e. via the direct detection of tau leptons in nu_tau charged
current interactions. The evidence of nu_mu -> nu_tau appearance has been
previously reported with three nu_tau candidate events using a sub-sample of
data from the 2008-2012 runs. We report here a fourth nu_tau candidate event,
with the tau decaying into a hadron, found after adding the 2012 run events
without any muon in the final state to the data sample. Given the number of
analysed events and the low background, nu_mu -> nu_tau oscillations are
established with a significance of 4.2sigma.Comment: Submitted to Progress of Theoretical and Experimental Physics (PTEP
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