940 research outputs found
AL 3 (BH 261): a new globular cluster in the Galaxy
AL~3 (BH 261), previously classified as a faint open cluster candidate, is
shown to be a new globular cluster in the Milky Way, by means of B, V and I
Color-Magnitude Diagrams. The main feature of AL~3 is a prominent blue extended
Horizontal Branch. Its Color-Magnitude Diagrams match those of the intermediate
metallicity cluster M~5. The cluster is projected in a rich bulge field, also
contaminated by the disk main sequence. The globular cluster is located in the
Galactic bulge at a distance from the Sun d = 6.00.5 kpc. The
reddening is E(B-V)=0.360.03 and the metallicity is estimated to be [Fe/H]
-1.30.25. AL~3 is probably one of the least massive globular
clusters of the Galaxy.Comment: 6 figures. Astrophysical Journal Letters, in pres
The old metal-poor open cluster ESO 92-SC05: accreted from a dwarf galaxy?
The study of old open clusters outside the solar circle can bring constraints
on formation scenarios of the outer disk. In particular, accretion of dwarf
galaxies has been proposed as a likely mechanism in the area. We use BVI
photometry for determining fundamental parameters of the faint open cluster ESO
92-SC05. Colour-Magnitude Diagrams are compared with Padova isochrones, in
order to derive age, reddening and distance. We derive a reddening E(B-V)=
0.17, and an old age of 6.0 Gyr.
It is one of the rare open clusters known to be older than 5 Gyr. A
metallicity of Z0.004 or [M/H]-0.7 is found. The rather low
metallicity suggests that this cluster might be the result of an accretion
episode of a dwarf galaxy.Comment: 11 figures: 1, 2a,b,c, 3a,b, 4a,b, 5, 6, 7 6 pages to compile with
mn2e.cls. Monthly Notices of the Royal Astronomical Society, in pres
Pfleiderer2: identification of a new globular cluster in the Galaxy
We provide evidence that indicate the star cluster Pfleiderer 2, which is
projected in a rich field, as a newly identified Galactic globular cluster.
Since it is located in a crowded field, core extraction and decontamination
tools were applied to reveal the cluster sequences in B, V and I
Color-Magnitude Diagrams (CMDs). The main CMD features of Pfleiderer 2 are a
tilted Red Giant Branch, and a red Horizontal Branch, indicating a high
metallicity around solar. The reddening is E(B-V)=1.01. The globular cluster is
located at a distance from the Sun d = 162 kpc.
The cluster is located at 2.7 kpc above the Galactic plane and at a distance
from the Galactic center of R=9.7 kpc, which is unusual for a
metal-rich globular cluster.Comment: Accepted by The Astronomical Journa
Detection of Ks-excess stars in the 14Myr open cluster NGC4755
We derive the structure, distribution of MS and PMS stars and dynamical state
of the young open cluster NGC 4755. We explore the possibility that, at the
cluster age, some MS and PMS stars still present infrared excesses related to
dust envelopes and proto-planetary discs. The radial density profile follows
King's law with a core radius \rm\rc=0.7\pm0.1 pc and a limiting radius
\rm\rl=6.9\pm0.1 pc; the cluster age is . Field-star
decontamination reveals a low-MS limit at \rm\approx1.4 \ms. The core MF
() is flatter than the halo's (). NGC 4755
contains candidate PMS stars of age , and a few
evolved stars. The mass locked up in PMS, MS and evolved stars amounts to
\rm\sim1150 \ms. Proper motions show that \ks-excess MS and PMS stars are
cluster members. \ks-excess fractions in PMS and MS stars are
and respectively, consistent with the cluster age. The core is
deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in
the halo but they are scarce in the core. Compared to open clusters in
different dynamical states studied with similar methods, NGC 4755 fits
relations involving structural and dynamical parameters in the expected locus
for its age and mass. On the other hand, the flatter core MF probably
originates from primordial processes related to parent molecular cloud
fragmentation and mass segregation over . Star formation in NGC
4755 began ago and proceeded for about the same length of
time. Detection of \ks-excess emission in member MS stars suggests that some
circumstellar dust discs survived for , occurring both in some
MS and PMS stars for the age and spread observed in NGC 4755.Comment: 10 figs. Astronomy & Astrophysics, in pres
HST NICMOS Photometry of the reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan 4
We present results from NICMOS Hubble Space Telescope observations of the
reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan
4, obtained through the filters F110W and F160W (nearly equivalent to J and H).
For the first time the turnoff region of Liller 1 and the main sequence of
Terzan 5 and Terzan 4 are reached, as well as the horizontal branch of UKS 1.
The magnitude difference between the turnoff and the red horizontal branch
is used as an age indicator. From
comparisons with new isochrones in the NICMOS photometric system, we conclude
that the two metal-rich clusters NGC 6528 and Terzan 5 are coeval within
uncertainties () with 47 Tucanae. Liller 1 and UKS 1 are confirmed as
metal-rich globular clusters. Terzan 4 is confirmed as an interesting case of a
metal-poor cluster in the bulge with a blue horizontal branch.Comment: 7 pages, 6 figures, accepted for publication in A&
NTT infrared imaging of star cluster candidates towards the central parts of the Galaxy
We address the issue whether the central parts of the Galaxy harbour young
clusters other than Arches, Quintuplet and the Nuclear Young Cluster. A large
sample of centrally projected cluster candidates has been recently identified
from the 2MASS J, H and Ks Atlas. We provide a catalogue of higher angular
resolution and deeper images for 57 2MASS cluster candidates, obtained with the
near-IR camera SOFI at the ESO NTT telescope. We classify 10 objects as star
clusters, some of them deeply embedded in gas and/or dust clouds. Three other
objects are probably star clusters, although the presence of dust in the field
does not exclude the possibility of their being field stars seen through
low-absorption regions. Eleven objects are concentrations of stars in areas of
little or no gas, and are classified as dissolving cluster candidates. Finally,
31 objects turned out to be the blend of a few bright stars, not resolved as
such in the low resolution 2MASS images. By combining the above results with
other known objects we provide an updated sample of 42 embedded clusters and
candidates projected within 7 degrees. As a first step we study Object 11 of
Dutra & Bica (2000) projected at approximately 1 degree from the nucleus. We
present H and Ks photometry and study the colour-magnitude diagram and
luminosity function. Object 11 appears to be a less massive cluster than Arches
or Quintuplet, and it is located at a distance from the Sun d=8 kpc, with a
visual absorption Av=15.Comment: accepted to A&A, 9 pages, 10 figure
V, J, H and K Imaging of the Metal Rich Globular Cluster NGC 6528
New near-infrared observations of NGC6528 are presented. The JHK observations
complement a previous HST/NICMOS data set by Ortolani et al. (2001), in that
they sample a larger area, contain a more numerous sample of red giant stars,
and include the K band. Also, archival HST data sets (separated by 6.093 years)
were used to proper-motion decontaminate the near-infrared sample and extract a
clean VJHK catalogue. Using the present wide colour baseline, we compared the
cleaned colour-magnitude diagrams of NGC6528 with those of NGC 6553 and NGC104
and derived new estimates of reddening and distance, E(B-V)=0.55 and
(m-M)o=14.44 (7.7 kpc). Moreover, the morphology and location of the cleaned
red giant branch were used to derive a photometric estimate of the cluster
metallicity. The average of 10 metallicity indicators yields a mean value of
[M/H] ~ 0.0, and [Fe/H] ~-0.20 and +0.08 on the Zinn & West (1984) and Carretta
& Gratton (1997) revised metallicity scale, respectively. The best isochrone
fit to the cleaned K,V-K diagram is obtained for a 12.6 Gyr and Z=0.02
isochrone, i.e. the derived metallicity of NGC6528 turns out to be very close
to the mean of stars in the Baade's Window. Five AGB variable star candidates,
whose membership has to be confirmed spectroscopically, are bolometrically as
bright as the known long period variable stars in NGC6553. As discussed in
Guarnieri et al. (1997) for NGC6553, this may indicate that an `intermediate
age' population is not needed to account for the brightest stars in external
galaxies such as M32.Comment: 11 pages, 9 figures, A&A accepte
Bulge Globular Clusters
The properties of the globular clusters located within 20° à 20° of the Galactic Center are discussed. In particular their spatial distribution, metallicities and ages are presented and discussed in the context of different scenarios of bulge formation
The Optical Gravitational Lensing Experiment. Is Interstellar Extinction Toward the Galactic Center Anomalous?
Photometry of the Galactic bulge, collected during the OGLE-II microlensing
search, indicates high and non-uniform interstellar extinction toward the
observed fields. We use the mean I-band magnitude and V-I color of red clump
stars as a tracer of interstellar extinction toward four small regions of the
Galactic bulge with highly variable reddening. Similar test is performed for
the most reddened region observed in the LMC.
We find that the slope of the location of red clump stars in the
color-magnitude diagrams (CMDs) in the Galactic bulge is significantly smaller
than the slope of the reddening line following the standard extinction law
(R_V=3.1) for approximations of the extinction curve by both Cardelli, Clayton
and Mathis (1989, CCM89) and Fitzpatrick (1999, F99). The differences are much
larger for the CCM89 approximation which, on the other hand, indicates the same
slopes for the control field in the LMC, contrary to the F99 approximation. We
discuss possible systematic effects that could cause the observed discrepancy.
Anomalous extinction toward the Galactic bulge seems to be the most natural
explanation. Our data indicate that, generally, the ratio of the total to
selective absorption, R_VI, is much smaller toward the Galactic bulge than the
value corresponding to the standard extinction curve (R_V=3.1). However, R_VI
varies from one line-of-sight to another.
Our results explain why the red clump and RR Lyr stars in the Baade's window
dereddened with standard value of R_VI are redder compared to those of the
local population.Comment: 16 pages. Accepted for publication in ApJ. Major changes include:
comparison of the OGLE-II photometry with other data, additional comparison
of the observed reddening line with that resulting from approximation of the
standard extinction curve by Fitzpatrick (1999
An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553
We provide a high dispersion line-by-line abundance analysis of five red HB
stars in the extremely metal rich galactic globular cluster NGC 6553. These red
HB stars are significantly hotter than the very cool stars near the tip of the
giant branch in such a metal rich globular cluster and hence their spectra are
much more amenable to an abundance analysis than would be the case for red
giants.
We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the
mean abundance in the galactic bulge found by McWilliam & Rich (1994) and
considerably higher than that obtained from an analysis of two red giants in
this cluster by Barbuy etal (1999). The relative abundance for the best
determined alpha process element (Ca) indicates an excess of alpha process
elements of about a factor of two. The metallicity of NGC 6553 reaches the
average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap
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