7,626 research outputs found
The Stellar Content and the Star Formation History of the Local Group Dwarf Galaxy LGS 3
The star formation history (SFH) and the properties of the dwarf galaxy LGS3
are analyzed using color-magnitude (CM) diagrams plotted from VRI photometry of
736 stars. The distance to the galaxy is estimated through the position of the
tip or the red giant branch. Two acceptable results have been obtained:
0.77+/-0.07 Mpc and 0.96+/-0.07 Mpc, although the first value is favored by
complementary considerations on the stellar content of the galaxy. Both values
make LGS3 a possible satellite of M31 or of M33. The SFH is investigated for
each of the two adopted distances comparing the observed CM diagrams with model
CM diagrams computed for different star formation rates (psi(t)) and chemical
enrichment laws (Z(t)).
The results are compatible with LGS3 having been forming stars since an early
epoch, 15-12 Gyr ago, at an almost constant rate if distance is 0.77 Mpc or at
an exponentially decreasing rate if distance is 0.96 Mpc. According to our
models, the current metallicity would range from Z~0.0007 to Z~0.002. Other
results are the current psi(t): (0.55+/-0.04)x10^(-10) Mo yr^(-1) pc^(-2) or
(0.47+/-0.07)x10^(-10) Mo yr^(-1) pc^(-2), depending of the distance, and its
average for the entire life of the galaxy, =(1.4+/-0.1)x10^(-10) Mo
yr^(-1) pc^(-2). At the present psi(t), the probability of LGS3 having an HII
region is 0.2, which is compatible with the fact that no HII regions have been
found in the galaxy. Its fraction of gas relative to the mass intervening in
the chemical evolution is about 0.40 and its percentage of dark matter (that
which cannot be explained as stellar remnants or by extrapolation of the used
IMF to low masses) is 95%. The results for psi(t) and Z(t) for d=0.77 Mpc are
compatible with a moderate outflow of well mixed material (lambda=3), but largeComment: 23 pages, 3 tables, 14 figures. AJ in pres
Model color-magnitude diagrams for Hubble Space Telescope observations of Local Group dwarf galaxies
In this paper, we discuss a method to conduct a quantitative study of the
star formation history (SFH) of Local Group (LG) galaxies using (HST) data.
This method has proven to be successful in the analysis of the SFH of the same
kind of galaxies using ground-based observations. It is based on the comparison
of observed CMDs with a set of model CMDs. The latter are computed assuming
different evolutionary scenarios, and include a detailed simulation of
observational effects. HST CMDs are ~3 mags deeper than typical ground-based
CMDs, allowing the observation, for all LG galaxies, of a part of the CMD that
up till now had remained accessible only for the very nearest galaxies. A very
important feature that will become accessible is the HB+red-clump. The
distribution of stars along this structure is quite sensitive to age and
metallicity and should provide a very important improvement in the time
resolution of the SFH for stars older than ~2-3 Gyr. We show and discuss four
model CMDs which would be comparable with CMDs from deep HST observations.
These model CMDs represent the following evolutionary scenarios corresponding
to a wide range of dwarf galaxy sub-types from dI to dE: A) a constant SFR from
15Gyr ago to the present time; B) as A), but with the SFR stopped 0.5 Gyr ago;
C) a constant SFR in the age range 10-9Gyr and D) as C) but in the age range
15-12 Gyr. In all four cases a range of metallicity from Z=0.0001 to Z=0.004
has been assumed. The present analysis is just a first qualitative approach to
what one may expect to find in the CMDs of LG galaxies. However a complete set
of model CMDs must be computed to analize the data for each galaxy, using the
crowding effects derived for that particular galaxy.Comment: 2 fi
Alpha Channeling with High-field Launch of Lower Hybrid Waves
Although lower hybrid waves are effective at driving currents in present-day
tokamaks, they are expected to interact strongly with high-energy particles in
extrapolating to reactors. In the presence of a radial alpha particle birth
gradient, this interaction can take the form of wave amplification rather than
damping. While it is known that this amplification more easily occurs when
launching from the tokamak high-field side, the extent of this amplification
has not been made quantitative. Here, by tracing rays launched from the high-
field-side of a tokamak, the required radial gradients to achieve amplification
are calculated for a temperature and density regime consistent with a
hot-ion-mode fusion reactor. These simulations, while valid only in the linear
regime of wave amplification, nonetheless illustrate the possibilities for wave
amplification using high-field launch of the lower hybrid wave.Comment: 7 pages, 7 figure
Theory of Stellar Population Synthesis with an application to N-Body simulations
Aims. We present here a new theoretical approach to population synthesis. The
aim is to predict colour magnitude diagrams (CMDs) for huge numbers of stars.
With this method we generate synthetic CMDs for N-body simulations of galaxies.
Sophisticated hydrodynamic N-body models of galaxies require equal quality
simulations of the photometric properties of their stellar content. The only
prerequisite for the method to work is very little information on the star
formation and chemical enrichment histories, i.e. the age and metallicity of
all star-particles as a function of time. The method takes into account the gap
between the mass of real stars and that of the star-particles in N-body
simulations, which best correspond to the mass of star clusters with different
age and metallicity, i.e. a manifold of single stellar sopulations (SSP).
Methods. The theory extends the concept of SSP to include the phase-space
(position and velocity) of each star. Furthermore, it accelerates the building
up of simulated CMD by using a database of theoretical SSPs that extends to all
ages and metallicities of interest. Finally, it uses the concept of
distribution functions to build up the CMD. The technique is independent of the
mass resolution and the way the N-body simulation has been calculated. This
allows us to generate CMDs for simulated stellar systems of any kind: from open
clusters to globular clusters, dwarf galaxies, or spiral and elliptical
galaxies. Results. The new theory is applied to an N-body simulation of a disc
galaxy to test its performance and highlight its flexibility.Comment: accepted for publication in A&
Cepheid Masses: FUSE Observations of S Mus
S Mus is the Cepheid with the hottest known companion. The large ultraviolet
flux means that it is the only Cepheid companion for which the velocity
amplitude could be measured with the echelle mode of the HST GHRS.
Unfortunately, the high temperature is difficult to constrain at wavelengths
longer than 1200 \AA because of the degeneracy between temperature and
reddening. We have obtained a FUSE spectrum in order to improve the
determination of the temperature of the companion. Two regions which are
temperature sensitive near 16,000 K but relatively unaffected by H
absorption (940 \AA, and the Ly wings) have been identified. By
comparing FUSE spectra of S Mus B with spectra of standard stars, we have
determined a temperature of 17,000 500 K. The resultant Cepheid mass is
6.0 0.4 M. This mass is consistent with main sequence
evolutionary tracks with a moderate amount of convective overshoot.Comment: accepted to Ap
Probing the Galaxy I. The galactic structure towards the galactic pole
Observations of (B-V) colour distributions towards the galactic poles are
compared with those obtained from synthetic colour-magnitude diagrams to
determine the major constituents in the disc and spheroid. The disc is
described with four stellar sub-populations: the young, intermediate, old, and
thick disc populations, which have respectively scale heights of 100 pc, 250
pc, 0.5 kpc, and 1.0 kpc. The spheroid is described with stellar contributions
from the bulge and halo. The bulge is not well constrained with the data
analyzed in this study. A non-flattened power-law describes the observed
distributions at fainter magnitudes better than a deprojected R^{1/4}-law.
Details about the age, metallicity, and normalizations are listed in Table 1.
The star counts and the colour distributions from the stars in the intermediate
fields towards the galactic anti-centre are well described with the stellar
populations mentioned above. Arguments are given that the actual solar offset
is about 15 pc north from the galactic plane.Comment: 11 pages TeX, 4 separate pages with additional figures, accepted for
publication in A&
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