27,040 research outputs found
Nonlinear gas oscillations in pipes. Part 1. Theory
The problem of forced acoustic oscillations in a pipe is studied theoretically. The oscillations are produced by a moving piston in one end of the pipe, while a variety of boundary conditions ranging from a completely closed to a completely open mouth at the other end are considered. All these boundary conditions are modelled by two parameters: a length correction and a reflexion coefficient equivalent to the acoustic impedance.
The linear theory predicts large amplitudes near resonance and nonlinear effects become crucially important. By expanding the equations of motion in a series in the Mach number, both the amplitude and wave form of the oscillation are predicted there.
In both the open- and closed-end cases the need for shock waves in some range of parameters is found. The amplitude of the oscillation is different for the two cases, however, being proportional to the square root of the piston amplitude in the closed-end case and to the cube root for the open end
Vertical motions in the disk of NGC 5668 as seen with optical Fabry-Perot spectroscopy
We have observed the nearly face-on spiral galaxy NGC 5668 with the TAURUS II
Fabry-Perot interferometer at the William Herschel Telescope using the
line to study the kinematics of the ionized gas. From the extracted
data cube we construct intensity, velocity and velocity dispersion maps. We
calculate the rotation curve in the innermost 2 arcmin and we use the residual
velocity field to look for regions with important vertical motions. By
comparing the geometry of these regions in the residual velocity field with the
geometry in the intensity and velocity dispersion maps we are able to select
some regions which are very likely to be shells or chimneys in the disk. The
geometry and size of these regions are very similar to the shells or chimneys
detected in other galaxies by different means. Moreover, it is worth noting
than this galaxy has been reported to have a population of neutral hydrogen
high velocity clouds (Schulman et al. 1996) which, according to these
observations, could have been originated by chimneys similar to those reported
in this paper.Comment: 7 pages with 9 figures. LaTeX file using A&A v4.0 macro
F stars, metallicity, and the ages of red galaxies at z > 1
We explore whether the rest-frame near-UV spectral region, observable in
high-redshift galaxies via optical spectroscopy, contains sufficient
information to allow the degeneracy between age and metallicity to be lifted.
We do this by testing the ability of evolutionary synthesis models to reclaim
the correct metallicity when fitted to the near-UV spectra of F stars of known
(sub-solar and super-solar) metallicity. F stars are of particular interest
because the rest-frame near-UV spectra of the oldest known elliptical galaxies
at z > 1 appear to be dominated by F stars near to the main-sequence turnoff.
We find that, in the case of the F stars, where the HST ultraviolet spectra
have high signal:noise, model-fitting with metallicity allowed to vary as a
free parameter is rather successful at deriving the correct metallicity. As a
result, the estimated turnoff ages of these stars yielded by the model fitting
are well constrained. Encouraged by this we have fitted these same variable-
metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio
galaxies 53W091 and 53W069 obtained with the Keck telescope. While the
age-metallicity degeneracy is not so easily lifted for these galaxies, we find
that even when metallicity is allowed as a free parameter, the best estimates
of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly
excluded. Furthermore, we find that a search of the entire parameter space of
metallicity and star formation history using MOPED (Heavens et al., 2000) leads
to the same conclusion. Our results therefore continue to argue strongly
against an Einstein-de Sitter universe, and favour a lambda-dominated universe
in which star formation in at least these particular elliptical galaxies was
completed somewhere in the redshift range z = 3 - 5.Comment: 10 pages, LaTeX, uses MNRAS style file, incorporates 14 postscript
figures, submitted to MNRAS. Changes include: inclusion of single stellar
atmosphere model fits; more rigorous calculation of confidence regions; some
re-structurin
A computational approach to the D-module of meromorphic functions
Let be a divisor in . We present methods to compare the
-module of the meromorphic functions to some
natural approximations. We show how the analytic case can be treated with
computations in the Weyl algebra.Comment: 11 page
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