1,386 research outputs found
The Effect of Corotation on the Radial Gradient of Metallicity of Spiral Galaxies
The corotation radius in a spiral galaxy is the radius where the spiral
pattern speed has the same velocity of the rotation curve. By compiling results
from the literature for 20 spiral galaxies we verified a strong correlation
between the radius of the minima or inflections of the metallicity distribution
and the corotation radius.Comment: 3 pages, 1 figur
Bimodal chemical evolution of the Galactic disk and the Barium abundance of Cepheids
In order to understand the Barium abundance distribution in the Galactic disk
based on Cepheids, one must first be aware of important effects of the
corotation resonance, situated a little beyond the solar orbit. The thin disk
of the Galaxy is divided in two regions that are separated by a barrier
situated at that radius. Since the gas cannot get across that barrier, the
chemical evolution is independent on the two sides of it. The barrier is caused
by the opposite directions of flows of gas, on the two sides, in addition to a
Cassini-like ring void of HI (caused itself by the flows). A step in the
metallicity gradient developed at corotation, due to the difference in the
average star formation rate on the two sides, and to this lack of communication
between them. In connection with this, a proof that the spiral arms of our
Galaxy are long-lived (a few billion years) is the existence of this step. When
one studies the abundance gradients by means of stars which span a range of
ages, like the Cepheids, one has to take into account that stars, contrary to
the gas, have the possibility of crossing the corotation barrier. A few stars
born on the high metallicity side are seen on the low metallicity one, and
vice-versa. In the present work we re-discuss the data on Barium abundance in
Cepheids as a function of Galactic radius, taking into account the scenario
described above. The [Ba/H] ratio, plotted as a function of Galactic radius,
apparently presents a distribution with two branches in the external region
(beyond corotation). One can re-interpret the data and attribute the upper
branch to the stars that were born on the high metallicity side. The lower
branch, analyzed separately, indicates that the stars born beyond corotation
have a rising Barium metallicity as a function of Galactic radius.Comment: 6 pages, 7 figures, Proceedings of IAU Symposium 29
NGC 2782: a merger remnant with young stars in its gaseous tidal tail
We have searched for young star-forming regions around the merger remnant NGC
2782. By using GALEX FUV and NUV imaging and HI data we found seven UV sources,
located at distances greater than 26 kpc from the center of NGC 2782, and
coinciding with its western HI tidal tail. These regions were resolved in
several smaller systems when Gemini/GMOS r-band images were used. We compared
the observed colors to stellar population synthesis models and we found that
these objects have ages of ~1 to 11 Myr and masses ranging from 10^3.9 to
10^4.6 Msun. By using Gemini/GMOS spectroscopic data we confirm memberships and
derive high metallicities for three of the young regions in the tail
(12+log(O/H)=8.74\pm0.20, 8.81\pm0.20 and 8.78\pm0.20). These metallicities are
similar to the value presented by the nuclear region of NGC 2782 and also
similar to the value presented for an object located close to the main body of
NGC 2782. The high metallicities measured for the star-forming regions in the
gaseous tidal tail of NGC 2782 could be explained if they were formed out of
highly enriched gas which was once expelled from the center of the merging
galaxies when the system collided. An additional possibility is that the tail
has been a nursery of a few generations of young stellar systems which
ultimately polluted this medium with metals, further enriching the already
pre-enriched gas ejected to the tail when the galaxies collided.Comment: 11 pages, 5 figures. Accepted for publication in MNRA
Decomposition of time-resolved tomographic PIV
International audienceAn experimental study has been conducted on a transitional water jet at a Reynolds number of Re = 5000. Flow fields have been obtained by means of time-resolved tomographic particle image velocimetry (TR-TOMO PIV) capturing all relevant spatial and temporal scales. The measured three-dimensional flow fields have then been postprocessed by the dynamic mode decomposition (DMD) which identifies coherent structures that contribute significantly to the dynamics of the jet. Where the jet exhibits a primary axisymmetric instability followed by a pairing of the vortex rings, dominant dynamic modes have been extracted together with their amplitude distribution. These modes represent a basis for the low-dimensional description of the dominant flow features
Aeroacoustics of sawtooth trailing-edge serrations under aerodynamic loading
The impact of aerodynamic loading on a serrated trailing edge is studied experimentally. Aerodynamic and acoustic measurements are conducted on a sawtooth-shaped trailing edge, retrofitted to a flat plate featuring a trailing-edge flap, and placed at incidence to the free-stream flow. The turbulent flow across the trailing edge is inspected by time-resolved three-dimensional velocity field measurements obtained from 4D-PIV, while the wall-pressure fluctuations are measured with surface-embedded microphones. Results discuss the relation between the velocity fluctuations over the serrations, the surface pressure fluctuations, and the far-field noise spectra. The aerodynamic analysis discusses the effect of counter-rotating vortex pairs, generated by the pressure imbalance across the edges of the serrations under loading. It is shown that the interaction of these vortices with the incoming turbulence affects the intensity of the wall-pressure spectrum at the outer rim of the serration surface. On the suction side, the intensity of the pressure fluctuations from the incoming boundary layer dominates over that induced by the vortex pairs. On the pressure side, instead, the velocity gradient prescribed by the vortex pairs produces a significant increase of the pressure fluctuations around the edges. The resulting spatial distribution of the wall-pressure fluctuations directly affects the far-field noise. Scattering predictions carried out with the wall-pressure fluctuations in the centre and root (on the suction side) exhibit good agreement with the measured noise in the low-frequency range, whereas using the surface pressure data at the tip of the serration (on the pressure side) yields a better prediction in the high-frequency range
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