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

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$_2$ absorption (940 \AA, and the Ly $\beta$ 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 $\pm$ 500 K. The resultant Cepheid mass is 6.0 $\pm$ 0.4 M$_\odot$. 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&

GAIA: AGB stars as tracers of star formation histories in the Galaxy and beyond

We discuss the tracing of star formation histories with ESA's space astrometry mission GAIA, emphasizing the advantages of AGB stars for this purpose. GAIA's microarcsecond-level astrometry, multi-band photometry and spectroscopy will provide individual distances, motions, effective temperatures, gravities and metallicities for vast numbers of AGB stars in the Galaxy and beyond. Reliable ages of AGB stars can be determined to distances of \~200 kpc in a wide range of ages and metallicities, allowing star formation histories to be studied in a diversity of astrophysical environments.Comment: 4 pages, 1 figure, to be appear in 'Mass-Losing Pulsating Stars and their Circumstellar Matter', eds. Y. Nakada, M. Honma & M. Seki, Kluwer ASSL series, vol. 28

Lateral gene exchanges shape the genomes of amoeba-resisting microorganisms.

Based on Darwin's concept of the tree of life, vertical inheritance was thought to be dominant, and mutations, deletions, and duplication were streaming the genomes of living organisms. In the current genomic era, increasing data indicated that both vertical and lateral gene inheritance interact in space and time to trigger genome evolution, particularly among microorganisms sharing a given ecological niche. As a paradigm to their diversity and their survival in a variety of cell types, intracellular microorganisms, and notably intracellular bacteria, were considered as less prone to lateral genetic exchanges. Such specialized microorganisms generally have a smaller gene repertoire because they do rely on their host's factors for some basic regulatory and metabolic functions. Here we review events of lateral gene transfer (LGT) that illustrate the genetic exchanges among intra-amoebal microorganisms or between the microorganism and its amoebal host. We tentatively investigate the functions of laterally transferred genes in the light of the interaction with their host as they should confer a selective advantage and success to the amoeba-resisting microorganisms (ARMs)

The Resolved Outer Population of NGC6822 with WFPC2

We present F336W (U), F439W (B), F555W (V), and F675W (R) Wide Field Planetary Camera 2 (WFPC2) photometry of two outer regions of the Local Group dwarf irregular galaxy NGC6822. The NE region is ~13 arcmin from the galaxy centre, while the W region lies 10 arcmin out, and within the wispy low surface brightness outer regions of the galaxy. The fields are not crowded and contain few NGC 6822 stars. We discuss errors and uncertainties and find that the W region contains a main sequence that extends to stars of about 2 solar masses, with an age of about 200 Myr. The NE region has no main sequence or stars younger than 1 Gyr, but does contain some luminous red stars that are not matched in the W field. These stars are not clumped in the field. The results suggest that the W region may be a trace of a tidal event that triggered the current star-formation in this isolated galaxy.Comment: 12 pages including 2 tables, plus 4 figures (#1 omitted) To appear in PAS