3,893 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
Photometric catalog of nearby globular clusters (I)
We present the first part of the first large and homogeneous CCD
color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular
clusters (GGC) imaged in the V and I bands using only two telescopes (one for
each hemisphere). The observed clusters represent 75% of the known Galactic
globulars with (m-M)v<=16.15 mag, cover most of the globular cluster
metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances
from ~1.2 to ~18.5 kpc.
In this paper, the CMDs for the 39 GGCs observed in the southern hemisphere
are presented. The remaining 13 northern hemisphere clusters of the catalog are
presented in a companion paper. For four clusters (NGC 4833, NGC 5986, NGC
6543, and NGC 6638) we present for the first time a CMD from CCD data. The
typical CMD spans from the 22nd V magnitude to the tip of the red giant branch.
Based on a large number of standard stars, the absolute photometric calibration
is reliable to the ~0.02 mag level in both filters.
This catalog, because of its homogeneity, is expected to represent a useful
data base for the measurement of the main absolute and relative parameters
characterizing the CMD of GGCs.Comment: 34 pages, 108 figures, Astronomy & Astrophysics Supplement Series, in
press. Full resolution figures can be obtained from the authors upon reques
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
Rotation of Hot Horizontal Branch Stars in Galactic Globular Clusters
We present high resolution UVES+VLT spectroscopic observations of 61 stars in
the extended blue horizontal branches of the Galactic globular clusters NGC
1904 (M79), NGC 2808, NGC 6093 (M80), and NGC 7078 M15). Our data reveal for
the first time the presence in NGC 1904 of a sizable population of fast (v
sin(i) >= 20 km/s) horizontal branch (HB) rotators, confined to the cool end of
the EHB, similar to that found in M13. We also confirm the fast rotators
already observed in NGC 7078. The cooler stars (T_eff < 11,500 K) in these
three clusters show a range of rotation rates, with a group of stars rotating
at ~ 15 km/s or less, and a fast rotating group at ~ 30 km/s. Apparently, the
fast rotators are relatively more abundant in NGC 1904 and M13, than in NGC
7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the
stars hotter than T_eff ~ 11,500 K have projected rotational velocities vsini<
12 km/s. The connection between photometric gaps in the HB and the change in
the projected rotational velocities is not confirmed by the new data. However,
our data are consistent with a relation between this discontinuity and the HB
jump.Comment: 2 pages, 1 figure, A.S.P. Conf. Ser., in press in Vol. 296, 200
Nota sobre las investigaciones que se están efectuando sobre los cambios del nivel del Mediterráneo
Abstract not availabl
Genome-wide mapping of ORC and Mcm2p binding sites on tiling arrays and identification of essential ARS consensus sequences in S. cerevisiae
BACKGROUND: Eukaryotic replication origins exhibit different initiation efficiencies and activation times within S-phase. Although local chromatin structure and function influences origin activity, the exact mechanisms remain poorly understood. A key to understanding the exact features of chromatin that impinge on replication origin function is to define the precise locations of the DNA sequences that control origin function. In S. cerevisiae, Autonomously Replicating Sequences (ARSs) contain a consensus sequence (ACS) that binds the Origin Recognition Complex (ORC) and is essential for origin function. However, an ACS is not sufficient for origin function and the majority of ACS matches do not function as ORC binding sites, complicating the specific identification of these sites. RESULTS: To identify essential origin sequences genome-wide, we utilized a tiled oligonucleotide array (NimbleGen) to map the ORC and Mcm2p binding sites at high resolution. These binding sites define a set of potential Autonomously Replicating Sequences (ARSs), which we term nimARSs. The nimARS set comprises 529 ORC and/or Mcm2p binding sites, which includes 95% of known ARSs, and experimental verification demonstrates that 94% are functional. The resolution of the analysis facilitated identification of potential ACSs (nimACSs) within 370 nimARSs. Cross-validation shows that the nimACS predictions include 58% of known ACSs, and experimental verification indicates that 82% are essential for ARS activity. CONCLUSION: These findings provide the most comprehensive, accurate, and detailed mapping of ORC binding sites to date, adding to the emerging picture of the chromatin organization of the budding yeast genome
- …