4,345 research outputs found
ALMA Temporal Phase Stability and the Effectiveness of Water Vapor Radiometer
Atacama Large Millimeter/submillimeter Array (ALMA) will be the world largest
mm/submm interferometer, and currently the Early Science is ongoing, together
with the commissioning and science verification (CSV). Here we present a study
of the temporal phase stability of the entire ALMA system from antennas to the
correlator. We verified the temporal phase stability of ALMA using data, taken
during the last two years of CSV activities. The data consist of integrations
on strong point sources (i.e., bright quasars) at various frequency bands, and
at various baseline lengths (up to 600 m). From the observations of strong
quasars for a long time (from a few tens of minutes, up to an hour), we derived
the 2-point Allan Standard Deviation after the atmospheric phase correction
using the 183 GHz Water Vapor Radiometer (WVR) installed in each 12 m antenna,
and confirmed that the phase stability of all the baselines reached the ALMA
specification. Since we applied the WVR phase correction to all the data
mentioned above, we also studied the effectiveness of the WVR phase correction
at various frequencies, baseline lengths, and weather conditions. The phase
stability often improves a factor of 2 - 3 after the correction, and sometimes
a factor of 7 improvement can be obtained. However, the corrected data still
displays an increasing phase fluctuation as a function of baseline length,
suggesting that the dry component (e.g., N2 and O2) in the atmosphere also
contributes the phase fluctuation in the data, although the imperfection of the
WVR phase correction cannot be ruled out at this moment.Comment: Proc. SPIE 8444-125, in press (7 pages, 4 figures, 1 table
Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study
With a view to the design of hard magnets without rare earths we explore the
possibility of large magnetocrystalline anisotropy energies in Heusler
compounds that are unstable with respect to a tetragonal distortion. We
consider the Heusler compounds FeYZ with Y = (Ni, Co, Pt), and CoYZ
with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find
that for the CoNiZ, CoPtZ, and FePtZ families the cubic phase is
always, at , unstable with respect to a tetragonal distortion, while, in
contrast, for the FeNiZ and FeCoZ families this is the case for only 2
compounds -- FeCoGe and FeCoSn. For all compounds in which a tetragonal
distortion occurs we calculate the MAE finding remarkably large values for the
Pt containing Heuslers, but also large values for a number of the other
compounds (e.g. CoNiGa has an MAE of -2.11~MJ/m). The tendency to a
tetragonal distortion we find to be strongly correlated with a high density of
states at the Fermi level in the cubic phase. As a corollary to this fact we
observe that upon doping compounds for which the cubic structure is stable such
that the Fermi level enters a region of high DOS, a tetragonal distortion is
induced and a correspondingly large value of the MAE is then observed.Comment: 8 pages, 5 figure
SMA/PdBI multiple line observations of the nearby Seyfert2 galaxy NGC 1068: Shock related gas kinematics and heating in the central 100pc?
We present high angular resolution (0.5-2.0") observations of the mm
continuum and the 12CO(J=3-2), 13CO(J=3-2), 13CO(J=2-1), C18O(J=2-1),
HCN(J=3-2), HCO+(J=4-3) and HCO+(J=3-2) line emission in the circumnuclear disk
(r=100pc) of the proto-typical Seyfert type-2 galaxy NGC1068, carried out with
the Submillimeter Array. We further include in our analysis new 13CO(J=1-0) and
improved 12CO(J=2-1) observations of NGC1068 at high angular resolution
(1.0-2.0") and sensitivity, conducted with the IRAM Plateau de Bure
Interferometer. Based on the complex dynamics of the molecular gas emission
indicating non-circular motions in the central ~100pc, we propose a scenario in
which part of the molecular gas in the circumnuclear disk of NGC1068 is
radially blown outwards as a result of shocks. This shock scenario is further
supported by quite warm (Tkin>=200K) and dense (nH2=10^4cm^-3) gas constrained
from the observed molecular line ratios. The HCN abundance in the circumnuclear
disk is found to be [HCN]/[12CO]=10^-3.5. This is slightly higher than the
abundances derived for galactic and extragalactic starforming/starbursting
regions. This results lends further support to X-ray enhanced HCN formation in
the circumnuclear disk of NGC1068, as suggested by earlier studies. The HCO+
abundance ([HCO+]/[12CO]=10^-5) appears to be somewhat lower than that of
galactic and extragalactic starforming/starbursting regions. When trying to fit
the cm to mm continuum emission by different thermal and non-thermal processes,
it appears that electron-scattered synchrotron emission yields the best results
while thermal free-free emission seems to over-predict the mm continuum
emission.Comment: accepted for publication by ApJ; 35pages, 22 figures and 6 tables (at
the end of the file); 3 figures have been decreased in quality to match size
limi
Computed Tomography Appearance of Hem-O-Lok Clips in Patients Who Have Undergone Laparoscopic Nephrectomy or Nephroureterectomy
Hem-O-Lok clips are radiopaque on computed tomography in patients who have undergone laparoscopic nephrectomy and nephroureterectomy
Magnetic Properties of 2-Dimensional Dipolar Squares: Boundary Geometry Dependence
By means of the molecular dynamics simulation on gradual cooling processes,
we investigate magnetic properties of classical spin systems only with the
magnetic dipole-dipole interaction, which we call dipolar systems. Focusing on
their finite-size effect, particularly their boundary geometry dependence, we
study two finite dipolar squares cut out from a square lattice with
and , where is an angle between the direction of the lattice axis
and that of the square boundary. Distinctly different results are obtained in
the two dipolar squares. In the square, the ``from-edge-to-interior
freezing'' of spins is observed. Its ground state has a multi-domain structure
whose domains consist of the two among infinitely (continuously) degenerated
Luttinger-Tisza (LT) ground-state orders on a bulk square lattice, i.e., the
two antiferromagnetically aligned ferromagnetic chains (af-FMC) orders directed
in parallel to the two lattice axes. In the square, on the other
hand, the freezing starts from the interior of the square, and its ground state
is nearly in a single domain with one of the two af-FMC orders. These geometry
effects are argued to originate from the anisotropic nature of the
dipole-dipole interaction which depends on the relative direction of sites in a
real space of the interacting spins.Comment: 21 pages, 13 figures, submitted to Journal of Physical Society Japa
On the disappearance of a cold molecular torus around the low-luminosity active galactic nucleus of NGC 1097
We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the
CO(3-2) and the underlying continuum emissions around the type 1 low-luminosity
active galactic nucleus (LLAGN; bolometric luminosity
erg~s) of NGC 1097 at pc resolution. These observations
revealed a detailed cold gas distribution within a pc of this LLAGN.
In contrast to the luminous Seyfert galaxy NGC 1068, where a pc cold
molecular torus was recently revealed, a distinctively dense and compact torus
is missing in our CO(3-2) integrated intensity map of NGC 1097. Based on the
CO(3-2) flux, the gas mass of the torus of NGC 1097 would be a factor of
less than that found for NGC 1068 by using the same CO-to-H
conversion factor, which implies less active nuclear star formation and/or
inflows in NGC 1097. Our dynamical modeling of the CO(3-2) velocity field
implies that the cold molecular gas is concentrated in a thin layer as compared
to the hot gas traced by the 2.12 m H emission in and around the
torus. Furthermore, we suggest that NGC 1097 hosts a geometrically thinner
torus than NGC 1068. Although the physical origin of the torus thickness
remains unclear, our observations support a theoretical prediction that
geometrically thick tori with high opacity will become deficient as AGNs evolve
from luminous Seyferts to LLAGNs.Comment: 9 pages, 5 figures. Accepted for publication in ApJ
Lagrangian fibrations of holomorphic-symplectic varieties of K3^[n]-type
Let X be a compact Kahler holomorphic-symplectic manifold, which is
deformation equivalent to the Hilbert scheme of length n subschemes of a K3
surface. Let L be a nef line-bundle on X, such that the 2n-th power of c_1(L)
vanishes and c_1(L) is primitive. Assume that the two dimensional subspace
H^{2,0}(X) + H^{0,2}(X), of the second cohomology of X with complex
coefficients, intersects trivially the integral cohomology. We prove that the
linear system of L is base point free and it induces a Lagrangian fibration on
X. In particular, the line-bundle L is effective. A determination of the
semi-group of effective divisor classes on X follows, when X is projective. For
a generic such pair (X,L), not necessarily projective, we show that X is
bimeromorphic to a Tate-Shafarevich twist of a moduli space of stable torsion
sheaves, each with pure one dimensional support, on a projective K3 surface.Comment: 34 pages. v3: Reference [Mat5] and Remark 1.8 added. Incorporated
improvement to the exposition and corrected typos according to the referees
suggestions. To appear in the proceedings of the conference Algebraic and
Complex Geometry, Hannover 201
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