1,699 research outputs found
Lagrangian turbulence in the Adriatic Sea as computed from drifter data: effects of inhomogeneity and nonstationarity
The properties of mesoscale Lagrangian turbulence in the Adriatic Sea are
studied from a drifter data set spanning 1990-1999, focusing on the role of
inhomogeneity and nonstationarity. A preliminary study is performed on the
dependence of the turbulent velocity statistics on bin averaging, and a
preferential bin scale of 0.25 is chosen. Comparison with independent estimates
obtained using an optimized spline technique confirms this choice. Three main
regions are identified where the velocity statistics are approximately
homogeneous: the two boundary currents, West (East) Adriatic Current, WAC
(EAC), and the southern central gyre, CG. The CG region is found to be
characterized by symmetric probability density function of velocity,
approximately exponential autocorrelations and well defined integral quantities
such as di usivity and time scale. The boundary regions, instead, are
significantly asymmetric with skewness indicating preferential events in the
direction of the mean flow. The autocorrelation in the along mean flow
direction is characterized by two time scales, with a secondary exponential
with slow decay time of 11-12 days particularly evident in the EAC region.
Seasonal partitioning of the data shows that this secondary scale is especially
prominent in the summer-fall season. Possible physical explanations for the
secondary scale are discussed in terms of low frequency fluctuations of
forcings and in terms of mean flow curvature inducing fluctuations in the
particle trajectories. Consequences of the results for transport modelling in
the Adriatic Sea are discussed.Comment: 45 pages, 18 figure
The influence of twin boundaries on the Flux Line Lattice structure in YBaCuO: a study by Small Angle Neutron Scattering
The influence of Twin Boundaries (TB) on the Flux Line Lattice(FLL) structure
was investigated by Small Angle Neutron Scattering (SANS). YBaCuO single
crystals possessing different TB densities were studied. The SANS experiments
show that the TB strongly modify the structure of the FLL. The flux lines
meander as soon as the magnetic field makes an angle with the TB direction.
According to the value of this angle but also to the ratio of the flux lines
density over the TB density, one observes that the FLL exhibits two different
unit cells in the plane perpendicular to the magnetic field. One is the
classical hexagonal and anisotropic cell while the other is affected by an
additional deformation induced by the TB. We discuss a possible relation
between this deformation and the increase of the critical current usually
observed in heavily twinned samples.Comment: accepted for publication in Phys Rev
Fluoride glasses : synthesis and properties
The discovery of heavy metal fluoride glasses has opened new prospects for fiber optics operating beyond 2 µm with expected losses less than 10(-2) dB/Km. The main interest of fluoride glasses lies in their infrared transmission up to 8 µm in the bulk form and 4.5 µm for optical fibers. We have reported here the preparation, the glass forming systems and properties of heavy metal fluoride glasses
The SBF Survey of Galaxy Distances. I. Sample Selection, Photometric Calibration, and the Hubble Constant
We describe a program of surface brightness fluctuation (SBF) measurements
for determining galaxy distances. This paper presents the photometric
calibration of our sample and of SBF in general. Basing our zero point on
observations of Cepheid variable stars, we find that the absolute SBF magnitude
in the Kron-Cousins I band correlates well with the mean (V-I)o color of a
galaxy according to
M_Ibar = (-1.74 +/- 0.07) + (4.5 +/- 0.25) [ (V-I)o - 1.15 ]
for 1.0 < (V-I) < 1.3. This agrees well with theoretical estimates from
stellar population models. Comparisons between SBF distances and a variety of
other estimators, including Cepheid variable stars, the Planetary Nebula
Luminosity Function (PNLF), Tully-Fisher (TF), Dn-sigma, SNII, and SNIa,
demonstrate that the calibration of SBF is universally valid and that SBF error
estimates are accurate. The zero point given by Cepheids, PNLF, TF (both
calibrated using Cepheids), and SNII is in units of Mpc; the zero point given
by TF (referenced to a distant frame), Dn-sigma and SNIa is in terms of a
Hubble expansion velocity expressed in km/s. Tying together these two zero
points yields a Hubble constant of H_0 = 81 +/- 6 km/s/Mpc. As part of this
analysis, we present SBF distances to 12 nearby groups of galaxies where
Cepheids, SNII, and SNIa have been observed.Comment: 29 pages plus 8 figures; LaTeX (AASTeX) uses aaspp4.sty (included);
To appear in The Astrophysical Journal, 1997 February 1 issue; Compressed
PostScript available from ftp://mars.tuc.noao.edu/sbf
How Dry Are Red Mergers?
The focus of current research in galaxy evolution has increasingly turned to
understanding the effect that mergers have on the evolution of systems on the
red sequence. For those interactions purported to occur dissipationlessly (so
called "dry mergers"), it would appear that the role of gas is minimal.
However, if these mergers are not completely dry, then even low levels of gas
may be detectable. The purpose of our study is to test whether early type
galaxies with HI in or around them, or "wet" ellipticals, would have been
selected as dry mergers by the criteria in van Dokkum (2005, AJ, 130, 2647). To
that end, we examine a sample of 20 early types from the HI Rogues Gallery with
neutral hydrogen in their immediate environs. Of these, the 15 brightest and
reddest galaxies match the optical dry merger criteria, but in each case, the
presence of HI means that they are not truly dry.Comment: 8 pages plus 1 table and 5 figures; accepted for publication in A
Hygroscopic behavior of atmospherically relevant water-soluble carboxylic salts and their influence on the water uptake of ammonium sulfate
The hygroscopic behavior of atmospherically relevant water-soluble carboxylic salts and their effects on ammonium sulfate were investigated using a hygroscopicity tandem differential mobility analyzer (H-TDMA). No hygroscopic growth is observed for disodium oxalate, while ammonium oxalate shows slight growth (growth factor = 1.05 at 90%). The growth factors at 90% RH for sodium acetate, disodium malonate, disodium succinate, disodium tartrate, diammonium tartrate, sodium pyruvate, disodium maleate, and humic acid sodium salt are 1.79, 1.78, 1.69, 1.54, 1.29, 1.70, 1.78, and 1.19, respectively. The hygroscopic growth of mixtures of organic salts with ammonium sulfate, which are prepared as surrogates of atmospheric aerosols, was determined. A clear shift in deliquescence relative humidity to lower RH with increasing organic mass fraction was observed for these mixtures. Above 80% RH, the contribution to water uptake by the organic salts was close to that of ammonium sulfate for the majority of investigated compounds. The observed hygroscopic growth of the mixed particles at RH above the deliquescence relative humidity of ammonium sulfate agreed well with that predicted using the Zdanovskii-Stokes-Robinson (ZSR) mixing rule. Mixtures of ammonium sulfate with organic salts are more hygroscopic than mixtures with organic acids, indicating that neutralization by gas-phase ammonia and/or association with cations of dicarbonxylic acids may enhance the hygroscopicity of the atmospheric particles
The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions
Using the surface brightness fluctuations in HST WFPC-2 images, we determine
that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC
4476 lies in the background at ~21 Mpc. We also examine the globular clusters
of M87 using archived HST fields. We detect the bimodal color distribution, and
find that the amplitude of the red peak relative to the blue peak is greatest
near the center. This feature is in good agreement with the merger model of
elliptical galaxy formation, where some of the clusters originated in
progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure
Internal tides in the central Mediterranean Sea: observational evidence and numerical studies
Internal tides are studied in the central Mediterranean Sea using observational data and numerical experiments. Both numerical results and observations indicate that the baroclinic variability in this area is dominated by the K1 diurnal tide. In agreement with previous studies, the diurnal internal tides have the characteristics of Kelvin-like bottom trapped waves. They are mainly generated by the interaction of the induced barotropic tidal flow with the steep bathymetric gradient connecting the Ionian Sea with the shallow Sicily Channel. The bathymetric gradient appears to be the major forcing shaping the propagation paths of the internal tides. The most energetic internal tides follow the steep bathymetric gradient, propagating southward and tending to dissipate rapidly. Other waves cross the continental shelf south of Malta and then split with one branch moving toward the southern coast of Sicily and the other moving toward the west. Internal tides propagate with a variable phase velocity of about 1 ms(-1) and a wavelength of the order of 100 km. During their journey, the internal waves appear to be subject to local processes that can modify their characteristics. The induced vertical shear strongly dominates the vertical turbulence and generates vertical mixing that alters the properties of the water masses traversing the area. Barotropic and internal tides remove heat from the ocean surface, increasing atmospheric heating, and redistributing energy through increased lateral heat fluxes. Lateral heat fluxes are significantly greater in the presence of internal tides due to the simultaneous increase in volume fluxes and water temperatures
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
- …