269 research outputs found
Young Star Clusters in the Dwarf Irregular Galaxy, UGC 7636, Interacting with the Giant Elliptical Galaxy NGC 4472
We present integrated Washington CT1 photometry of 18 bright blue objects
discovered in the dwarf galaxy UGC 7636 which is located 5'.5 southeast of the
giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster.
Several lines of evidence indicate that UGC 7636 is interacting violently with
NGC 4472. These objects are very blue with colors of -0.4 < (C-T1) < 0.6, and
their magnitudes are in the range of 20.6 < T1 < 22.9 mag which corresponds to
absolute magnitudes of -10.6 < M(T1) < -8.3 mag for a distance modulus of
(m-M)o = 31.2. These objects are grouped spatially in three regions: the
central region of UGC 7636, the tidal tail region, and the HI cloud region. No
such objects were found in the counter tail region. It is concluded that these
objects are probably young star clusters which formed < 0.1Gyr ago during the
interaction between UGC 7636 and NGC 4472. Surface photometry of UGC 7636 (r <
83") shows that there is a significant excess of blue light along the tidal
tail region compared with other regions. The star clusters are bluer than the
stellar light in the tidal tail region, indicating that these clusters might
have formed later than most stars in the tidal tail region which were formed
later than most stars in the main body of the galaxy.Comment: 18 pages (AASLaTeX), 6 Postscript figures, Accepted for publication
in the Astronomical Journal, Nov. 30th, 199
Globular Clusters as Tracers of Stellar Bimodality in Elliptical Galaxies: The Case of NGC 1399
Globular cluster systems (GCS) frequently show a bi-modal distribution of the
cluster integrated colours. This work explores the arguments to support the
idea that the same feature is shared by the diffuse stellar population of the
galaxy they are associated with. In the particular case of NGC 1399 the results
show that the galaxy brightness profile and colour gradient as well as the
behaviour of the cumulative globular cluster specific frequency, are compatible
with the presence of two dominant stellar populations, associated with the so
called "blue" and "red" globular cluster families. These globular families are
characterized by different intrinsic specific frequencies (defined in terms of
each stellar population): Sn=3.3 +/- 0.3 in the case of the red globulars and
Sn=14.3 +/- 2.5 for the blue ones. We stress that this result is not
necessarily conflicting with recent works that point out a clear difference
between the metallicity distribution of (resolved) halo stars and globulars
when comparing their number statistics. The inferred specific frequencies imply
that, in terms of their associated stellar populations, the formation of the
blue globulars took place with an efficiency about 6 times higher than that
corresponding to their red counterparts. The similarity of the spatial
distribution of the blue globulars with that inferred for dark matter, as well
as with that of the X ray emiting hot gas associated with NGC 1399, is
emphasized. The impact of a relatively unconspicuous low metallicity
population, that shares the properties of the blue globulars, as a possible
source of chemical enrichment early in the formation history of the galaxy is
also briefly discussed.Comment: 15 pages; MNRAS (accepted; October 2004
Wide Field CCD Surface Photometry of the Giant Elliptical Galaxy NGC 4472 in the Virgo Cluster
We present deep wide field (16'.4 x 16'.4) Washington CT1 CCD surface
photometry of the giant elliptical galaxy NGC 4472, the brightest member in the
Virgo cluster. Our data cover a wider field than any previous CCD photometry as
well as going deeper. Surface brightness profiles of NGC 4472 are not well fit
by a single King model, but they can be fit approximately by two King models:
with separate models for the inner and outer regions. Surface brightness
profiles for the outer region can also be fit approximately by a deVaucouleurs
law. There is clearly a negative color gradient within 3' of NGC 4472, in the
sense that the color gets bluer with increasing radius. The slope of the color
gradient for this region is derived to be = -0.08 mag
arcsec for , which corresponds to a metallicity
gradient of [Fe/H] dex. However, the surface color gets
redder slowly with increasing radius beyond 3'. A comparison of the structural
parameters of NGC 4472 in C and T1 images has shown that there is little
difference in the ellipse shapes between isochromes and isophotes. In addition,
photometric and structural parameters of NGC 4472 have been determined.Comment: 8 pages(mnrasLaTeX), 8 Postscript figures, Accepted for publication
in MNRAS, 2000
The Metallicity of the Open Cluster Tombaugh 2
We investigate the nature of the chemical composition of the outer disc open
cluster Tombaugh 2, that a recent study by Frinchaboy et al. (2008) suggested
to possess an intrinsic metal abundance dispersion. We aim to investigate such
claims by high resolution spectra obtained for a number of stars in the
Tombaugh 2 field, together with independent UBVIc photometry. The spectra,
together with input atmospheric parameters and model atmospheres, are used to
determine detailed chemical abundances for a variety of elements in 13 members
having good spectra. We find the mean metallicity to be [Fe/H]=-0.31+-0.02 with
no evidence for an intrinsic abundance dispersion, in contrary to the recent
results of Frinchaboy et al. (2008). We find Ca and Ba to be slightly enhanced
while Ni and Sc are solar. The r-process element Eu was found to be enhanced,
giving an average [Eu/Ba]=+0.17. The Li abundance decreases with Teff on the
upper giant branch and maintains a low level for red clump stars. The mean
metallicity we derive is in good agreement with that expected from the radial
abundance gradient in the disc for a cluster at its Galactocentric distance.
The surprising result found by Frinchaboy et al. (2008), that is the presence
of 2 distinct abundance groups within the cluster, implying either a completely
unique open cluster with an intrinsic metallicity spread, or a very unlikely
superposition of a cold stellar stream and a very distant open cluster, is not
supported by our new result.Comment: 11 pages, 5 eps figures, in press on A&
A sample of relatively unstudied star clusters in the Large Magellanic Cloud: fundamental parameters determined from Washington photometry
To enlarge our growing sample of well-studied star clusters in the Large
Magellanic Cloud (LMC), we present CCD Washington CT1 photometry to T1 ~ 23 in
the fields of twenty-three mostly unstudied clusters located in the inner disc
and outer regions of the LMC. We estimated cluster radii from star counts.
Using the cluster Washington (T1,C-T1) colour-magnitude diagrams, statistically
cleaned from field star contamination, we derived cluster ages and
metallicities from a comparison with theoretical isochrones of the Padova
group. Whenever possible, we also derived ages using delta_T1 - the magnitude
difference between the red giant clump and the main sequence turn off - and
estimated metallicities from the standard giant branch procedure. We enlarged
our sample by adding clusters with published ages and metallicities determined
on a similar scale by applying the same methods. We examined relationships
between their positions in the LMC, ages and metallicities. We find that the
two methods for age and metallicity determination agree well with each other.
Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with
[Fe/H] values ranging from -0.4 to -0.7. The remaining nine clusters turn out
to be younger than 1 Gyr, with metallicities between 0.0 and -0.4. Our 23
clusters represent an increase of ~ 30% in the current total amount number of
well-studied LMC clusters using Washington photometry. In agreement with
previous studies, we find no evidence for a metallicity gradient. We also find
that the younger clusters were formed closer to the LMC centre than the older
ones.Comment: 11 pages, 10 figures. A&A, in pres
RR Lyrae Variables in M33. I. Evidence For a Field Halo Population
We present observations of RR Lyrae variables in the Local Group late-type
spiral galaxy M33. Using the Advanced Camera for Surveys on the Hubble Space
Telescope, we have identified 64 ab-type RR Lyraes in M33. We have estimated
reddenings for these stars based on their minimum light V-I colors and
metallicities based on their periods. From the distributions of these
properties, we conclude that the RR Lyraes belong to two populations - one
associated with the halo of M33 and the other with its disk. Given that RR
Lyraes are produced by populations older than ~10 Gyr, this suggests that not
only does the field halo of M33 contain an old component, but so does its disk.
This is one of the best pieces of evidence for the existence of a halo field
component in M33. Using a relation between RR Lyrae absolute magnitude and
metallicity (Mv(RR) = 0.23[Fe/H] + 0.93), we estimate a mean distance modulus
of = 24.67 +/- 0.08 for M33. This places M33 approximately 70 kpc
beyond M31 in line-of-sight distance.Comment: 20 pages, 17 figures, accepted for publication in The Astronomical
Journa
Probing the Large Magellanic Cloud's recent chemical enrichment history through its star clusters
We present Washington system colour-magnitude diagrams (CMDs) for 17
practically unstudied star clusters located in the bar as well as in the inner
disc and outer regions of the Large Magellanic Cloud (LMC). Cluster sizes were
estimated from star counts distributed throughout the entire observed fields.
Based on the best fits of theoretical isochrones to the cleaned
CMDs, as well as on the parameter and the standard giant branch
method, we derive ages and metallicities for the cluster sample. Four objects
are found to be intermediate-age clusters (1.8-2.5 Gyr), with [Fe/H] ranging
from -0.66 to -0.84. With the exception of SL263, a very young cluster (
16 Myr), the remaining 12 objects are aged between 0.32 and 0.89 Gyr, with
their [Fe/H] values ranging from -0.19 to -0.50. We combined our results with
those for other 231 clusters studied in a similar way using the Washington
system. The resulting age-metallicity relationship shows a significant
dispersion in metallicities, whatever age is considered. Although there is a
clear tendency for the younger clusters to be more metal-rich than the
intermediate ones, we believe that none of the chemical evolution models
currently available in the literature reasonably well represents the recent
chemical enrichment processes in the LMC clusters. The present sample of 17
clusters is part of our ongoing project of generating a database of LMC
clusters homogeneously studied using the Washington photometric system and
applying the same analysis procedureComment: 11 pages, 20 figures, 3 tables, Accepted for publication in MNRA
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