7,152 research outputs found
Understanding the spiral structure of the Milky Way using the local kinematic groups
We study the spiral arm influence on the solar neighbourhood stellar
kinematics. As the nature of the Milky Way (MW) spiral arms is not completely
determined, we study two models: the Tight-Winding Approximation (TWA) model,
which represents a local approximation, and a model with self-consistent
material arms named PERLAS. This is a mass distribution with more abrupt
gravitational forces. We perform test particle simulations after tuning the two
models to the observational range for the MW spiral arm properties. We explore
the effects of the arm properties and find that a significant region of the
allowed parameter space favours the appearance of kinematic groups. The
velocity distribution is mostly sensitive to the relative spiral arm phase and
pattern speed. In all cases the arms induce strong kinematic imprints for
pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no
substructure is induced close to corotation. The groups change significantly if
one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The
appearance time of each group is different, ranging from 0 to more than 1 Gyr.
Recent spiral arms can produce strong kinematic structures. The stellar
response to the two potential models is significantly different near the Sun,
both in density and kinematics. The PERLAS model triggers more substructure for
a larger range of pattern speed values. The kinematic groups can be used to
reduce the current uncertainty about the MW spiral structure and to test
whether this follows the TWA. However, groups such as the observed ones in the
solar vicinity can be reproduced by different parameter combinations. Data from
velocity distributions at larger distances are needed for a definitive
constraint.Comment: 18 pages, 21 figures, 4 tables; acccepted for publication in MNRA
Redescription of Orculella aragonica (Westerlund 1897), an Iberian species different from O. Bulgarica (Hesse 1915) (Gastropoda: Pulmonata: Orculidae)
Orculella bulgarica (Hesse 1915) has been recorded from Bulgaria and western Asia, including Turkey, but has also been reported from Spain by several authors. Most studies on this species have been on subfossil shells. Recent findings of living populations in Turkey and Spain have allowed us to report on the reproductive system morphology and mtDNA sequences of this taxon. Despite the apparent lack of conchological differences between specimens from these two geographical areas, this new information revealed the presence of two species. Review of the literature pertaining to Iberian orculids, led us to conclude that the examined population in Spain species must be assigned to Orculella aragonica (Westerlund 1897), and all previous reports of O. bulgarica from the Iberian Peninsula should be ascribed to the former. This species is redescribed and diagnosed herein, highlighting differences between it and O. bulgarica. Some notes about its conservation status and biogeographic origin are also provided
Substructure in clusters containing wide-angle tailed radio galaxies. I. New redshifts
We present new redshifts and positions for 635 galaxies in nine rich clusters
containing Wide-Angle Tailed (WAT) radio galaxies. Combined with existing data,
we now have a sample of 18 WAT-containing clusters with more than 10 redshifts.
This sample contains a substantial portion of the WAT clusters in the VLA 20 cm
survey of Abell clusters, including 75% of WAT clusters in the complete survey
(z0.09. It is a representative sample
which should not contain biases other than selection by radio morphology. We
graphically present the new data using histograms and sky maps. A
semi-automated procedure is used to search for emission lines in the spectra in
order to add and verify galaxy redshifts. We find that the average apparent
fraction of emission line galaxies is about 9% in both the clusters and the
field. We investigate the magnitude completeness of our redshift surveys with
CCD data for a test case, Abell 690. This case indicates that our galaxy target
lists are deeper than the detection limit of a typical MX exposure, and they
are 82% complete down to R=19.0. The importance of the uniformity of the
placement of fibers on targets is posited, and we evaluate this in our
datasets. We find some cases of non-uniformities which may influence dynamical
analyses. A second paper will use this database to look for correlations
between the WAT radio morphology and the cluster's dynamical state.Comment: 15 pages, 5 figures, 7 tables. To appear in the Astronomical Journa
William (Bill) Peterson's contributions to ocean science, management, and policy
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Schwing, F. B., Sissenwine, M. J., Batchelder, H., Dam, H. G., Gomez-Gutierrez, J., Keister, J. E., Liu, H., & Peterson, J. O. William (Bill) Peterson's contributions to ocean science, management, and policy. Progress in Oceanography, 182, (2020): 102241, doi:10.1016/j.pocean.2019.102241.In addition to being an esteemed marine ecologist and oceanographer, William T. (Bill) Peterson was a dedicated public servant, a leader in the ocean science community, and a mentor to a generation of scientists. Bill recognized the importance of applied science and the need for integrated “big science” programs to advance our understanding of ecosystems and to guide their management. As the first US GLOBEC program manager, he was pivotal in transitioning the concept of understanding how climate change impacts marine ecosystems to an operational national research program. The scientific insight and knowledge generated by US GLOBEC informed and advanced the ecosystem-based management approaches now being implemented for fishery management in the US. Bill held significant leadership roles in numerous international efforts to understand global and regional ecological processes, and organized and chaired a number of influential scientific conferences and their proceedings. He was passionate about working with and training young researchers. Bill’s academic affiliations, notably at Stony Brook and Oregon State Universities, enabled him to advise, train, and mentor a host of students, post-doctoral researchers, and laboratory technicians. Under his collegial guidance they became critical independent thinkers and diligent investigators. His former students and colleagues carry on Bill Peterson’s legacy of research that helps us understand marine ecosystems and informs more effective resource stewardship and conservation
Shell to shell energy transfer in MHD, Part II: Kinematic dynamo
We study the transfer of energy between different scales for forced
three-dimensional MHD turbulent flows in the kinematic dynamo regime. Two
different forces are examined: a non-helical Taylor Green flow with magnetic
Prandtl number P_M=0.4, and a helical ABC flow with P_M=1. This analysis allows
us to examine which scales of the velocity flow are responsible for dynamo
action, and identify which scales of the magnetic field receive energy directly
from the velocity field and which scales receive magnetic energy through the
cascade of the magnetic field from large to small scales. Our results show that
the turbulent velocity fluctuations are responsible for the magnetic field
amplification in the small scales (small scale dynamo) while the large scale
field is amplified mostly due to the large scale flow. A direct cascade of the
magnetic field energy from large to small scales is also present and is a
complementary mechanism for the increase of the magnetic field in the small
scales. Input of energy from the velocity field in the small magnetic scales
dominates over the energy that is cascaded down from the large scales until the
large-scale peak of the magnetic energy spectrum is reached. At even smaller
scales, most of the magnetic energy input is from the cascading process.Comment: Submitted to PR
Correlation of banana productivity levels and soil morphological properties using regularized optimal scaling regression
Soil morphological properties described in the field, such as texture, consistence or structure, provide a valuable tool for the evaluation of soil productivity potential. In this study, we developed a regression model between the soil morphological variables of banana plantations and a crop Productivity Index (PI) previously developed for the same areas in Venezuela. For this, we implemented categorical regression, an optimal scaling procedure in which the morphological variables are transformed into a numerical scale, and can thus be entered in a multiple regression analysis. The model was developed from data from six plantations growing “Gran Nain” bananas, each with two productivity levels (high and low), in two 4-ha experimental plots, one for each productivity level. Sixty-three A horizons in thirty-six soils were described using 15 field morphological variables on a nominal scale for structure type, texture and hue, and an ordinal scale for the rest (structure grade, structure size, wet and dry consistence, stickiness, plasticity, moist value, chroma, root abundance, root size, biological activity and reaction to HCl). The optimum model selected included biological activity, texture, dry consistence, reaction to HCl and structure type variables. These variables explained the PI with an R2 of 0.599, an expected prediction error (EPE) of 0.645 and a standard error (SE) of 0.135 using bootstrapping, and EPE of 0.662 with a SE of 0.236 using 10-fold cross validation. Our study showed how soil quality is clearly related to productivity on commercial banana plantations, and developed a way to correlate soil quality indicators to yield by using indicators based on easily measured soil morphological parameters. The methodology used in this study might be further expanded to other banana-producing areas to help identify the soils most suitable for its cultivation, thereby enhancing its environmental sustainability and profitability
Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations
are reported for the interior of a rotating, perfectly-conducting, rigid
spherical shell that is insulator-coated on the inside. A previously-reported
spectral method is used which relies on a Galerkin expansion in
Chandrasekhar-Kendall vector eigenfunctions of the curl. The new ingredient in
this set of computations is the rigid rotation of the sphere. After a few
purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial
waves), attention is focused on selective decay and the MHD dynamo problem. In
dynamo runs, prescribed mechanical forcing excites a persistent velocity field,
usually turbulent at modest Reynolds numbers, which in turn amplifies a small
seed magnetic field that is introduced. A wide variety of dynamo activity is
observed, all at unit magnetic Prandtl number. The code lacks the resolution to
probe high Reynolds numbers, but nevertheless interesting dynamo regimes turn
out to be plentiful in those parts of parameter space in which the code is
accurate. The key control parameters seem to be mechanical and magnetic
Reynolds numbers, the Rossby and Ekman numbers (which in our computations are
varied mostly by varying the rate of rotation of the sphere) and the amount of
mechanical helicity injected. Magnetic energy levels and magnetic dipole
behavior are exhibited which fluctuate strongly on a time scale of a few eddy
turnover times. These seem to stabilize as the rotation rate is increased until
the limit of the code resolution is reached.Comment: 26 pages, 17 figures, submitted to New Journal of Physic
Adaptive Mesh Refinement for Characteristic Grids
I consider techniques for Berger-Oliger adaptive mesh refinement (AMR) when
numerically solving partial differential equations with wave-like solutions,
using characteristic (double-null) grids. Such AMR algorithms are naturally
recursive, and the best-known past Berger-Oliger characteristic AMR algorithm,
that of Pretorius & Lehner (J. Comp. Phys. 198 (2004), 10), recurses on
individual "diamond" characteristic grid cells. This leads to the use of
fine-grained memory management, with individual grid cells kept in
2-dimensional linked lists at each refinement level. This complicates the
implementation and adds overhead in both space and time.
Here I describe a Berger-Oliger characteristic AMR algorithm which instead
recurses on null \emph{slices}. This algorithm is very similar to the usual
Cauchy Berger-Oliger algorithm, and uses relatively coarse-grained memory
management, allowing entire null slices to be stored in contiguous arrays in
memory. The algorithm is very efficient in both space and time.
I describe discretizations yielding both 2nd and 4th order global accuracy.
My code implementing the algorithm described here is included in the electronic
supplementary materials accompanying this paper, and is freely available to
other researchers under the terms of the GNU general public license.Comment: 37 pages, 15 figures (40 eps figure files, 8 of them color; all are
viewable ok in black-and-white), 1 mpeg movie, uses Springer-Verlag svjour3
document class, includes C++ source code. Changes from v1: revised in
response to referee comments: many references added, new figure added to
better explain the algorithm, other small changes, C++ code updated to latest
versio
Uso de pasta de algodón (Gossypium barbadense L.) de bajo nivel de gosipol en la alimentación de terneras Holstein
The effect of three levels of inclusion of cotton seed meal (PA) of low level of gossypol in the concentrate of 36 replacement Holstein calves was evaluated. Three treatments were used: T0: 0% PA; T1: 10% PA and T2: 20% PA with 0, 150 and 300 ppm of free gossypol respectively. The diets were supplied from birth to weaning (60 days). No significant differences were observed between treatments for feed consumption, body weight gain, final body weight or size. On the contrary, the activity of the serum sorbitol dehydrogenase enzyme was 29.5, 44.7 and 59.5 U/l for T0, T1 and T2, respectively (p<0.001), exceeding the value considered normal (14.7 ± 1.3 U/l); however, there were no visible negative effects on calves.Se evaluó el efecto de tres niveles de inclusión de pasta de algodón (PA) de nivel bajo de gosipol en el concentrado de 36 terneras Holstein de reemplazo, Se trabajó con tres tratamientos: T0: 0% PA; T1: 10% PA y T2: 20% PA con 0, 150 y 300 ppm de gosipol libre, respectivamente. Las dietas fueron suministradas dese el nacimiento hasta el destete (60 días). No se observaron diferencias significativas entre tratamientos para consumo de alimento, ganancia de peso, peso vivo final ni altura a la cruz. Por el contrario, la actividad de la enzima sorbitol deshidrogenasa sérica fue de 29.5, 44.7 y 59.5 U/l para T0, T1 y T2, respectivamente (p<0.001), superando el valor considerado como normal (14.7 ± 1.3 U/l); no obstante, no hubo efectos negativos visibles en las terneras
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