926 research outputs found
WFPC2 Observations of NGC 454: an Interacting Pair of Galaxies
We present WFPC2 images in the F450W, F606W and F814W filters of the
interacting pair of galaxies NGC 454. Our data indicate that the system is in
the early stages of interaction. A population of young star-clusters has formed
around the late component, and substantial amounts of gas have sunk into the
center of the earlier component, where it has not yet produced significant
visible star formation or nuclear activity. We have photometric evidence that
the star-clusters have strong line emission, which indicate the presence of a
substantial component of hot, massive stars which formed less than 5-10 Myrs
ago.Comment: 14 pages, 4 figures, Latex (AAS macros), ApJL in pres
Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering
LiFeAs is unique among the broad family of FeAs-based superconductors,
because it is superconducting with a rather large K under
ambient conditions although it is a stoichiometric compound. We studied the
electrical transport on a high-quality single crystal. The resistivity shows
quadratic temperature dependence at low temperature giving evidence for strong
electron-electron scattering and a tendency towards saturation around room
temperature. The Hall constant is negative and changes with temperature, what
most probably arises from a van Hove singularity close to the Fermi energy in
one of the hole-like bands. Using band structure calculations based on angular
resolved photoemission spectra we are able to reproduce all the basic features
of both the resistivity as well as the Hall effect data.Comment: 6 pages, 3 figures included; V2 has been considerably revised and
contain a more detailed analysis of the Hall effect dat
Phase diagram and isotope effect in (PrEu)_0.7Ca_0.3CoO_3 cobaltites exhibiting spin-state transitions
We present the study of magnetization, thermal expansion, specific heat,
resistivity, and a.c. susceptibility of
(PrEu)CaCoO cobaltites. The measurements were
performed on ceramic samples with and . Based on these
results, we construct the phase diagram, including magnetic and spin-state
transitions. The transition from the low- to intermediate-spin state is
observed for the samples with , whereas for a lower Eu-doping level,
there are no spin-state transitions, but a crossover between the ferromagnetic
and paramagnetic states occurs. The effect of oxygen isotope substitution along
with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope
substitution (O by O) is found to shift both the magnetic and
spin-state phase boundaries to lower Eu concentrations. The isotope effect on
the spin-state transition temperature () is rather strong, but it is
much weaker for the transition to a ferromagnetic state (). The
ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by
the Co ions, which favor the formation of the intermediate-spin state of
neighboring Co ions.Comment: 13 pages, including 11 figures, to be published in Phys. Rev.
Narrow Na and K Absorption Lines Toward T Tauri Stars: Tracing the Atomic Envelope of Molecular Clouds
We present a detailed analysis of narrow Na i and K i absorption resonance lines toward nearly 40 T Tauri stars in Taurus with the goal of clarifying their origin. The Na i λ5889.95 line is detected toward all but one source, while the weaker K i λ7698.96 line is detected in about two-thirds of the sample. The similarity in their peak centroids and the significant positive correlation between their equivalent widths demonstrate that these transitions trace the same atomic gas. The absorption lines are present toward both disk and diskless young stellar objects, which excludes cold gas within the circumstellar disk as the absorbing material. A comparison of Na i and CO detections and peak centroids demonstrates that the atomic gas and molecular gas are not co-located, the atomic gas being more extended than the molecular gas. The width of the atomic lines corroborates this finding and points to atomic gas about an order of magnitude warmer than the molecular gas. The distribution of Na i radial velocities shows a clear spatial gradient along the length of the Taurus molecular cloud filaments. This suggests that absorption is associated with the Taurus molecular cloud. Assuming that the gradient is due to cloud rotation, the rotation of the atomic gas is consistent with differential galactic rotation, whereas the rotation of the molecular gas, although with the same rotation axis, is retrograde. Our analysis shows that narrow Na i and K i absorption resonance lines are useful tracers of the atomic envelope of molecular clouds. In line with recent findings from giant molecular clouds, our results demonstrate that the velocity fields of the atomic and molecular gas are misaligned. The angular momentum of a molecular cloud is not simply inherited from the rotating Galactic disk from which it formed but may be redistributed by cloud–cloud interactions
Constraints on the ICM velocity power spectrum from the X-ray lines width and shift
Future X-ray observations of galaxy clusters by high spectral resolution
missions will provide spatially resolved measurements of the energy and width
for the brightest emission lines in the intracluster medium (ICM) spectrum. In
this paper we discuss various ways of using these high resolution data to
constrain velocity power spectrum in galaxy clusters. We argue that variations
of these quantities with the projected distance R in cool core clusters contain
important information on the velocity field length scales in the ICM. The
effective length along the line of sight, which provides dominant
contribution to the line flux, increases with R, allowing one to probe the
amplitude of the velocity variations at different spatial scales. In
particular, we show that the width of the line as a function of R is closely
linked to the structure function of the 3D velocity field. Yet another easily
obtainable proxy of the velocity field length scales is the ratio of the
amplitude of the projected velocity field (line energy) variations to the
dispersion of the velocity along the line of sight (line width). Finally the
projected velocity field can be easily converted into 3D velocity field,
especially for clusters like Coma with an extended flat core in the surface
brightness. Under assumption of a homogeneous isotropic Gaussian 3D velocity
field we derived simple expressions relating the power spectrum of the 3D
velocity field (or structure function) and the observables. The uncertainties
in the observables, caused by stochastic nature of the velocity field, are
estimated by making multiple realizations of the random Gaussian velocity field
and evaluating the scatter in observables. If large scale motions are present
in the ICM these uncertainties may dominate the statistical errors of line
width and shift measurements.Comment: 15 pages, 10 figures, accepted to MNRA
Sow body condition at weaning and reproduction performance in organic piglet production
The objective was to investigate the variation in backfat at weaning and its relations to reproduction results in organic sow herds in Denmark. The study included eight herds and 573 sows. The average backfat at weaning mean�13 mm; SD�4.2 mm) ranging from 10.5 to 17.3 mm among herds shows that it is possible to avoid poor body condition at weaning even with a lactation length of seven weeks or more. No main effect of backfat at weaning on reproduction performance was found, but the probability of a successful reproduction after weaning tended to decrease with decreasing backfat for first parity sows, whereas the opposite was the case for multiparous sows
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to Physical Units
Ambipolar diffusion is important in redistributing magnetic flux and in
damping Alfven waves in molecular clouds. The importance of ambipolar diffusion
on a length scale is governed by the ambipolar diffusion Reynolds
number, \rad=\ell/\lad, where \lad is the characteristic length scale for
ambipolar diffusion. The logarithmic mean of the AD Reynolds number in a sample
of 15 molecular clumps with measured magnetic fields (Crutcher 1999) is 17,
comparable to the theoretically expected value. We identify several regimes of
ambipolar diffusion in a turbulent medium, depending on the ratio of the flow
time to collision times between ions and neutrals; the clumps observed by
Crutcher (1999) are all in the standard regime of ambipolar diffusion, in which
the neutrals and ions are coupled over a flow time. We have carried out
two-fluid simulations of ambipolar diffusion in isothermal, turbulent boxes for
a range of values of \rad. The mean Mach numbers were fixed at \calm=3 and
\ma=0.67; self-gravity was not included. We study the properties of
overdensities--i.e., clumps--in the simulation and show that the slope of the
higher-mass portion of the clump mass spectrum increases as \rad decreases,
which is qualitatively consistent with Padoan et al. (2007)'s finding that the
mass spectrum in hydrodynamic turbulence is significantly steeper than in ideal
MHD turbulence. For a value of \rad similar to the observed value, we find a
slope that is consistent with that of the high-mass end of the Initial Mass
Function for stars. However, the value we find for the spectral index in our
ideal MHD simulation differs from theirs, presumably because our simulations
have different initial conditions. This suggests that the mass spectrum of the
clumps in the Padoan et al. (2007) turbulent fragmentation model for the IMF
depends on the environment, which would conflict with evidence ...Comment: 33 pages, 7 figure
Molecular gas, CO, and star formation in galaxies: emergent empirical relations, feedback, and the evolution of very gas-rich systems
We use time-varying models of the coupled evolution of the HI, H_2 gas phases
and stars in galaxy-sized numerical simulations to: a) test for the emergence
of the Kennicutt-Schmidt (K-S) and the H_2-pressure relation, b) explore a
realistic H_2-regulated star formation recipe which brings forth a neglected
and potentially significant SF-regulating factor, and c) go beyond typical
galactic environments (for which these galactic empirical relations are
deduced) to explore the early evolution of very gas-rich galaxies. In this work
we model low mass galaxies (M_{\rm baryon} \le 10^9 \msun), while
incorporating an independent treatment of CO formation and destruction, the
most important tracer molecule of H2 in galaxies, along with that for the H2
gas itself. We find that both the K-S and the H_2-pressure empirical relations
can robustly emerge in galaxies after a dynamic equilibrium sets in between the
various ISM states, the stellar component and its feedback. (abridged)Comment: 32 pages, 9 figures, accepted for publication in Ap
Using NuRadioMC to study the performance of UHE radio neutrino detectors
NuRadioMC is an open-source, Python-based simulation and reconstruction framework for radio detectors of ultra-high energy neutrinos and cosmic rays. Its modular design makes NuRadioMC suitable for use with a range of past, current and future detectors. In addition, the recent deployment
of a complete documentation as well as a pip release make NuRadioMC relatively easy to learn and use. Here, we outline the features currently available and under development in NuRadioMC, with a focus on its usage to simulate and study in-ice radio neutrino detectors
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