4,130 research outputs found
On the star formation history of IZw 18
It has been suggested that a continuous low star formation rate has been the
dominant regime in IZw 18 and in dwarf galaxies for the lifetime of these
objects (Legrand et al. 1999). Here, we discuss and model various star-forming
histories for IZw 18. Particularly, we show that if the metallicity observed in
IZw 18 results from starburst events only, the observed colors constrain the
fraction of the metals ejected from the galaxy to be less than 50-70 %. We
demonstrate that the continuous star formation scenario reproduces the observed
parameters of IZw 18. A continuous star formation rate (SFR) of about 10E-4
Msol/yr during 14 Gyr reproduces precisely the observed abundances. This SFR is
comparable with the lowest SFR observed in low surface brightness galaxies (van
Zee et al. 1997). Generalized to all galaxies, the low continuous SFR scenario
accounts for various facts: the presence of star formation in quiescent dwarfs
and LSBG, the metallicity increase with time in the most underabundant DLA
systems, and the metal content extrapolations to the outskirts of spiral
galaxies. Also the apparent absence of galaxies with a metallicity lower than
IZw 18, the apparent absence of HI clouds without optical counterparts, and the
homogeneity of abundances in dwarfs galaxies are natural outcomes of the
scenario. This implies that, even if starbursts are strong and important events
in the life of galaxies, their more subdued but continuous star formation
regime cannot be ignored when accounting for their chemical evolution.Comment: 10 pages, 5 postscript figures. Accepted for publication in Astronomy
and Astrophysics main journa
Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation
We present V and I photometry of the resolved stars in the most
metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using
Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest
ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude
diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young
stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and
red supergiants (10 Myr<age<100 Myr), but also an older evolved population of
asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance
to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars,
with preferred values in the higher range. The red giant branch (RGB) stars are
conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our
imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved
stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young
galaxy. Several star formation episodes can be inferred from the CMDs of the
main body and the C component. There have been respectively three and two
episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main
body, the younger MS and massive post-MS stars are distributed over a larger
area than the older AGB stars, suggesting that I Zw 18 is still forming from
the inside out. In the C component, different star formation episodes are
spatially distinct, with stellar population ages decreasing from the northwest
to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the
Astrophysical Journa
Particle-wall collision statistics in the open circular billiard
In the open circular billiard particles are placed initially with a uniform
distribution in their positions inside a planar circular vesicle. They all have
velocities of the same magnitude, whose initial directions are also uniformly
distributed. No particle-particle interactions are included, only specular
elastic collisions of the particles with the wall of the vesicle. The particles
may escape through an aperture with an angle . The collisions of the
particles with the wall are characterized by the angular position and the angle
of incidence. We study the evolution of the system considering the probability
distributions of these variables at successive times the particle reaches
the border of the vesicle. These distributions are calculated analytically and
measured in numerical simulations. For finite apertures , a
particular set of initial conditions exists for which the particles are in
periodic orbits and never escape the vesicle. This set is of zero measure, but
the selection of angular momenta close to these orbits is observed after some
collisions, and thus the distributions of probability have a structure formed
by peaks. We calculate the marginal distributions up to , but for
a solution is found for arbitrary . The escape probability as
a function of decays with an exponent 4 for and
evidences for a power law decay are found for lower apertures as well.Comment: 11 pages, 14 figures. Typos corrected and two new figures added,
figure captions changed and additional discussions added. Version accepted
for publication in Physica
A spectroscopic study of component C and the extended emission around I Zw 18
Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H and H emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H, H
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H,
H emission lines and the other on H, H absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the
Astrophysical Journa
Quasimodes of a chaotic elastic cavity with increasing local losses
We report non-invasive measurements of the complex field of elastic
quasimodes of a silicon wafer with chaotic shape. The amplitude and phase
spatial distribution of the flexural modes are directly obtained by Fourier
transform of time measurements. We investigate the crossover from real mode to
complex-valued quasimode, when absorption is progressively increased on one
edge of the wafer. The complexness parameter, which characterizes the degree to
which a resonance state is complex-valued, is measured for non-overlapping
resonances and is found to be proportional to the non-homogeneous contribution
to the line broadening of the resonance. A simple two-level model based on the
effective Hamiltonian formalism supports our experimental results
Statistics of the electromagnetic response of a chaotic reverberation chamber
This article presents a study of the electromagnetic response of a chaotic
reverberation chamber (RC) in the presence of losses. By means of simulations
and of experiments, the fluctuations in the maxima of the field obtained in a
conventional mode-stirred RC are compared with those in a chaotic RC in the
neighborhood of the Lowest Useable Frequency (LUF). The present work
illustrates that the universal spectral and spatial statistical properties of
chaotic RCs allow to meet more adequately the criteria required by the Standard
IEC 61000-4-21 to perform tests of electromagnetic compatibility.Comment: 6 pages, 9 figure
A continuous low star formation rate in IZw 18 ?
Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18
obtained with the CFH 3.6 m Telescope are presented. The very low value of
oxygen abundance previously reported is confirmed and a very homogeneous
abundance distribution is found (no variation larger than 0.05 dex) over the
whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al.
(1997) that the observed abundance level cannot result from the material
ejected by the stars formed in the current burst, and propose that the observed
metals were formed in a previous star formation episode. Metals ejected in the
current burst of star formation remain most probably hidden in a hot phase and
are undetectable using optical spectroscopy. We discuss different scenarios of
star formation in IZw 18. Combining various observational facts, for instance
the faint star formation rate observed in low surface brightness galaxies van
Zee et al. (1997), it is proposed that a low and continuous rate of star
formation occurring during quiescent phases between bursts could be a
significant source of metal enrichment of the interstellar medium.Comment: 10 pages, 4 Postscript figures, to be published in Astronomy and
Astrophysics main journa
Magnetism: the Driving Force of Order in CoPt. A First-Principles Study
CoPt or FePt equiatomic alloys order according to the tetragonal L10
structure which favors their strong magnetic anisotropy. Conversely magnetism
can influence chemical ordering. We present here {\it ab initio} calculations
of the stability of the L10 and L12 structures of Co-Pt alloys in their
paramagnetic and ferromagnetic states. They show that magnetism strongly
reinforces the ordering tendencies in this system. A simple tight-binding
analysis allows us to account for this behavior in terms of some pertinent
parameters
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