4,751 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
The stellar content of the Local Group dwarf galaxy Phoenix
We present new deep ground-based photometry of the Local Group dwarf
galaxy Phoenix. Our results confirm that this galaxy is mainly dominated by red
stars, with some blue plume stars indicating recent (100 Myr old) star
formation in the central part of the galaxy. We have performed an analysis of
the structural parameters of Phoenix based on an ESO/SRC scanned plate, in
order to search for differentiated component. The results were then used to
obtain the color-magnitude diagrams for three different regions of Phoenix in
order to study the variation of the properties of its stellar population. The
young population located in the central component of Phoenix shows a clear
asymmetry in its distribution, that could indicate a propagation of star
formation across the central component. The HI cloud found at 6 arcmin
Southwest by Young & Lo (1997) could have been involved in this process.
We also find the presence of a substantial intermediate-age population in the
central region of Phoenix that would be less abundant or absent in its outer
regions. This result is also consistent with the gradient found in the number
of horizontal branch stars, whose frequency relative to red giant branch stars
increases towards the outer part of the galaxy. These results, together with
those of our morphological study, suggest the existence of an old, metal-poor
population with a spheroidal distribution surrounding the younger inner
component of Phoenix. This two-component structure may resemble the halo-disk
structure observed in spirals, although more data, in particular on kinematics,
are necessary to confirm this.Comment: 46 pages, 21 figures, 9 Tables, to be published in AJ, August 9
Thermal phase transitions for Dicke-type models in the ultra-strong coupling limit
We consider the Dicke model in the ultra-strong coupling limit to investigate
thermal phase transitions and their precursors at finite particle numbers
for bosonic and fermionic systems. We derive partition functions with
degeneracy factors that account for the number of configurations and derive
explicit expressions for the Landau free energy. This allows us to discuss the
difference between the original Dicke (fermionic) and the bosonic case. We find
a crossover between these two cases that shows up, e.g., in the specific heat.Comment: 4 pages Brief Report styl
A virtual robot solution to support programming
Computer programming is a valuable skill. But learning programming is sometimes difficult, time-consuming, even frustrating. In this context, there is a need for new approaches to help in the process of learning programming. Here it is presented a solution based on a virtual robot with progressive system and supported in a generic and portable language. Those features answer to some needs. First, the virtual robot allows for certain level tangibility to the results. Then, the implementation as a progressive system allows for the support of the learning process. Finally, the use of a generic and portable language allows for the installation of the system on many platforms.info:eu-repo/semantics/acceptedVersio
Gamification: software usage ecology
Gamification is applied in academia and industry and is being studied in the last years. Gamification is nowadays applied to several areas, such as learning, computer science, business, marketing, tourism, among others. Research performed till now suggests that its usage may improve students' and workers' motivation and increase study, productivity and sales. Nevertheless, there are many forms of implementing, according to each industry. Supported in the literature, we identified the game components, game mechanics, and game dynamics, used as part of any gamification implementation. Then, we selected some specific cases in e-learning, computer program learning, open source software development, and crowdsourcing. Finally, we list some of the main software employed in this fields, identifying also implemented features. This allowed us to find the great complexity and variety of implementations. It also enabled us to find ether practitioners or researcher use the word gamification for a broad range of approaches.info:eu-repo/semantics/publishedVersio
Developing small web based systems
In this paper it is proposed a website engineering process. The proposed process is largely supported in the concept of usability. But it incorporates concepts of software engineering, specifically simplified. This simplification results from the need of applying to small site development. The approach was applied to a site used by an accounting team
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