1,060 research outputs found
Odnowa miast w świetle zrównoważonego rozwoju
W artykule przedstawiono współczesne problemy, z jakimi musi się zmierzyć planowanie przestrzenne i urbanistykę oraz podejmowane rozwiązania przy uwzględnieniu idei reurbanizacji i zrównoważonego rozwoju
Automated spectroscopic abundances of A and F-type stars using echelle spectrographs I. Reduction of ELODIE spectra and method of abundance determination
This paper presents an automated method to determine detailed abundances for
A and F-type stars. This method is applied on spectra taken with the ELODIE
spectrograph. Since the standard reduction procedure of ELODIE is optimized to
obtain accurate radial velocities but not abundances, we present a more
appropriate reduction procedure based on IRAF. We describe an improvement of
the method of Hill & Landstreet (1993) for obtaining Vsini, microturbulence and
abundances by fitting a synthetic spectrum to the observed one. In particular,
the method of minimization is presented and tested with Vega and the Sun. We
show that it is possible, in the case of the Sun, to recover the abundances of
27 elements well within 0.1 dex of the commonly accepted values.Comment: 12 pages, 10 figures, accepted for publication in A&
Experimental evaluation of sub-sampling IQ detection for low-level RF control in particle accelerator systems
The low-level radio frequency (LLRF) control system is one of the fundamental parts of a particle accelerator, ensuring the stability of the electro-magnetic (EM) field inside the resonant cavities. It leverages on the precise measurement of the field by in-phase/quadrature (IQ) detection of an RF probe signal from the cavities, usually performed using analogue downconversion. This approach requires a local oscillator (LO) and is subject to hardware non-idealities like mixer nonlinearity and long-term temperature drifts. In this work, we experimentally evaluate IQ detection by direct sampling for the LLRF system of the Polish free electron laser (PolFEL) now under development at the National Centre for Nuclear Research (NCBJ) in Poland. We study the impact of the sampling scheme and of the clock phase noise for a 1.3-GHz input sub-sampled by a 400-MSa/s analogue-to-digital converter (ADC), estimating amplitude and phase stability below 0.01% and nearly 0.01◦, respectively. The results are in line with state-of-the-art implementations, and demonstrate the feasibility of direct sampling for GHz-range LLRF systems
A multiwavelength approach to the SFR estimation in galaxies at intermediate redshifts
We use a sample of 7 starburst galaxies at intermediate redshifts (z ~ 0.4
and z ~ 0.8) with observations ranging from the observed ultraviolet to 1.4
GHz, to compare the star formation rate (SFR) estimators which are used in the
different wavelength regimes. We find that extinction corrected Halpha
underestimates the SFR, and the degree of this underestimation increases with
the infrared luminosity of the galaxies. Galaxies with very different levels of
dust extinction as measured with SFR(IR)/SFR(Halpha, uncorrected for
extinction) present a similar attenuation A[Halpha], as if the Balmer lines
probed a different region of the galaxy than the one responsible for the bulk
of the IR luminosity for large SFRs. In addition, SFR estimates derived from
[OII]3727 match very well those inferred from Halpha after applying the
metallicity correction derived from local galaxies. SFRs estimated from the UV
luminosities show a dichotomic behavior, similar to that previously reported by
other authors in galaxies at z <~ 0.4. Here we extend this result up to z ~
0.8. Finally, one of the studied objects is a luminous compact galaxy (LCG)
that may be suffering similar dust-enshrouded star formation episodes. These
results highlight the relevance of quantifying the actual L(IR) of LCGs, as
well as that of a much larger and generic sample of luminous infrared galaxies,
which will be possible after the launch of SIRTF.Comment: Accepted for publication in The Astrophysical Journa
Non-LTE Model Atmospheres for Late-Type Stars II. Restricted NLTE Calculations for a Solar-Like Atmosphere
We test our knowledge of the atomic opacity in the solar UV spectrum. Using
the atomic data compiled in Paper I from modern, publicly available, databases,
we perform calculations that are confronted with space-based observations of
the Sun. At wavelengths longer than about 260 nm, LTE modeling can reproduce
quite closely the observed fluxes; uncertainties in the atomic line data
account fully for the differences between calculated and observed fluxes. At
shorter wavelengths, departures from LTE appear to be important, as our LTE and
restricted NLTE calculations differ. Analysis of visible-near infrared Na I and
O I lines, two species that produce a negligible absorption in the UV, shows
that observed departures from LTE for theses species can be reproduced very
accurately with restricted (fixed atmospheric structure) NLTE calculations.Comment: 13 pages, 11 figures, to appear in Ap
Evolution of Li, Be and B in the Galaxy
In this paper we study the production of Li, Be and B nuclei by Galactic
cosmic ray spallation processes. We include three kinds of processes: (i)
spallation by light cosmic rays impinging on interstellar CNO nuclei (direct
processes); (ii) spallation by CNO cosmic ray nuclei impinging on interstellar
p and 4He (inverse processes); and (iii) alpha-alpha fusion reactions. The
latter dominate the production of 6Li and 7Li. We calculate production rates
for a closed-box Galactic model, verifying the quadratic dependence of the Be
and B abundances for low values of Z. These are quite general results and are
known to disagree with observations. We then show that the multi-zone
multi-population model we used previously for other aspects of Galactic
evolution produces quite good agreement with the linear trend observed at low
metallicities without fine tuning. We argue that reported discrepancies between
theory and observations do not represent a nucleosynthetic problem, but instead
are the consequences of inaccurate treatments of Galactic evolution.Comment: 26 pages, 5 figures, LaTeX. The Astrophysical Journal, in pres
Viscosities of the Gay-Berne nematic liquid crystal
We present molecular dynamics simulation measurements of the viscosities of
the Gay-Berne phenomenological model of liquid crystals in the nematic and
isotropic phases. The temperature dependence of the rotational and shear
viscosities, including the nonmonotonic behavior of one shear viscosity are in
good agreement with experimental data. The bulk viscosities are significantly
larger than the shear viscosities, again in agreement with experiment.Comment: 11 pages, 4 Postscript figures, Revte
Phase Mixing of Alfvén Waves Near a 2D Magnetic Null Point
The propagation of linear Alfvén wave pulses in an inhomogeneous plasma near a 2D coronal null point is investigated. When a uniform plasma density is considered, it is seen that an initially planar Alfvén wavefront remains planar, despite the varying equilibrium Alfvén speed, and that all the wave collects at the separatrices. Thus, in the non-ideal case, these Alfvénic disturbances preferentially dissipate their energy at these locations. For a non-uniform equilibrium density, it is found that the Alfvén wavefront is significantly distorted away from the initially planar geometry, inviting the possibility of dissipation due to phase mixing. Despite this however, we conclude that for the Alfvén wave, current density accumulation and preferential heating still primarily occur at the separatrices, even when an extremely non-uniform density profile is considered
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