761 research outputs found
Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellar content
of 103 old open clusters and 147 globular clusters of the Milky Way with those
derived from DIRBE/IRAS 100 micron dust emission in the same directions. Star
clusters at |b|> 20 show comparable reddening values between the two methods,
in agreement with the fact that most of them are located beyond the disk dust
layer. For very low galactic latitude lines of sight, differences occur in the
sense that DIRBE/IRAS reddening values can be substantially larger, suggesting
effects due to the depth distribution of the dust. The differences appear to
arise from dust in the background of the clusters consistent with a dust layer
where important extinction occurs up to distances from the Plane of ~ 300 pc.
For 3 % of the sample a significant background dust contribution might be
explained by higher dust clouds. We find evidence that the Milky Way dust lane
and higher dust clouds are similar to those of several edge-on spiral galaxies
recently studied in detail by means of CCD imaging.Comment: manuscript in LATEX with 14 pages, 8 figures .ps Accepted for
Astronomy and Astrophysics main journal on 13.04.200
Characterization of open cluster remnants
Despite progress in the theoretical knowledge of open cluster remnants and
the growing search for observational identifications in recent years, open
questions still remain. The methods used to analyze open cluster remnants and
criteria to define them as physical systems are not homogeneous. In this work
we present a systematic method for studying these objects that provides a view
of their properties and allows their characterization. Eighteen remnant
candidates are analyzed by means of photometric and proper motion data. These
data provide information on objects and their fields. We establish criteria for
characterizing open cluster remnants, taking observational uncertainties into
account. 2MASS J and H photometry is employed (i) to study structural
properties of the objects by means of radial stellar density profiles, (ii) to
test for any similarity between objects and fields with a statistical
comparison method applied to the distributions of stars in the CMDs, and (iii)
to obtain ages, reddening values, and distances from the CMD, taking an index
of isochrone fit into account. The UCAC2 proper motions allowed an objective
comparison between objects and large solid angle offset fields. The objective
analysis based on the present methods indicates 13 open-cluster remnants in the
sample. Evidence of the presence of binary stars is found, as expected for
dynamically evolved systems. Finally, we infer possible evolutionary stages
among remnants from the structure, proper motion, and CMD distributions. The
low stellar statistics for individual objects is overcome by means of the
construction of composite proper motion and CMD diagrams. The distributions of
remnants in the composite diagrams resemble the single-star and unresolved
binary star distributions of open clusters.Comment: 16 pages, 15 figures, A&A accepte
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
Discovery of three optical open clusters in the Galaxy
We report the discovery of three optical open clusters in the Milky Way. Two
clusters are in Scutum (Cluster1 at l=18.44 degrees and b=-0.42 degrees, and
Cluster2 at l=19.60 degrees and b=-1.02 degrees), thus projected not far from
the Galactic center direction, and the other is in Canis Major (Cluster3 at
l=235.61 degrees and b=-4.10 degrees), near the anti-center direction. Cluster3
is less populous than Clusters 1 and 2, but presents evidence of being a
physical system. The objects were found optically by inspecting maps obtained
from the Guide Star Catalogue and images from the Digitized Sky Survey. No
previous identification of cluster has been reported in each area so far. The
analysis was carried out with 2MASS photometry in J and H. For Cluster1 we
derive an age of 25 Myr, a reddening E(B-V)=2.18 and a distance from the Sun
1.64kpc; for Cluster2, age of 500Myr, E(B-V)=0.91 and distance 2.19kpc; finally
for Cluster3, age 32-100Myr, E(B-V)=0.94 and distance of 3.93kpc. Luminosity
and mass functions are derived for Clusters1 and 2 which, in turn, allowed us
to estimate their observed masses as 147 and 89 solar masses, respectively.
Estimated total masses, by extrapolating the mass functions to 0.08 solar mass,
amount to 382 and 614 solar masses, for the two clusters. Cluster3 has an
observed mass of 55 solar masses. The present results indicate that further
searches in the optical might still reveal new open clusters, and more so in
infrared bands.Comment: accepted to Astronomy and Astrophysics, 9 figure
The embedded cluster or association Trumpler 37 in IC1396: a search for evolutionary constraints
It is currently widely accepted that open star clusters and stellar
associations result from the evolution of embedded star clusters. Parameters
such star formation efficiency, time-scale of gas removal and velocity
dispersion can be determinants of their future as bound or unbound systems.
Finding objects at an intermediate evolution state can provide constraints to
model the embedded cluster evolution. In the HII region IC1396, Trumpler 37 is
an extended young cluster that presents characteristics of an association. We
employed the Two Micron All Sky Survey (2MASS) photometry to analysing its
structure and stellar content, and determining its astrophysical parameters. We
also analysed 11 bright-rimmed clouds in IC1396 in order to search for young
infrared star clusters, and the background open star cluster Teutsch 74, to
verify whether it has any contribution to the observed stellar density profile
of Trumpler 37. The derived parameters and comparison with template objects
from other studies lead us to conclude that Trumpler 37, rather than as a star
cluster, will probably emerge from its molecular cloud as an OB association.Comment: 13 pages, 10 figures, 6 table
Near-infrared study of new embedded clusters in the Carina complex
We analyse the nature of a sample of stellar overdensities that we found
projected on the Carina complex. This study is based on 2MASS photometry and
involves the photometry decontamination of field stars, elaboration of
intrinsic colour-magnitude diagrams J(J-Ks), colour-colour diagrams
(J-H)(H-Ks) and radial density profiles, in order to determine the
structure and the main astrophysical parameters of the best candidates. The
verification of an overdensity as an embedded cluster requires a CMD consistent
with a PMS content and MS stars, if any. From these results, we are able to
verify if they are, in fact, embedded clusters. The results were, in general,
rewarding: in a sample of 101 overdensities, the analysis provided 15
candidates, of which three were previously catalogued as clusters
(CCCP-Cl16, Treasure Chest and FSR1555), and the 12 remaining are
discoveries that provided significant results, with ages not above 4.5Myr
and distances compatible with the studied complex. The resulting values for the
differential reddening of most candidates were relatively high, confirming that
these clusters are still (partially or fully) embedded in the surrounding gas
and dust, as a rule within a shell. Histograms with the distribution of the
masses, ages and distances were also produced, to give an overview of the
results. We conclude that all the 12 newly found embedded clusters are related
to the Carina complex.Comment: 10 pages, 14 figures, accepted for publication in MNRA
Probing embedded star clusters in the HII complex NGC 6357 with VVV
NGC 6357 is an active star-forming region located in the Sagittarius arm
displaying several star clusters, which makes it a very interesting target to
investigate star formation and early cluster evolution. We explore NGC 6357
with the "VISTA Variables in the V\'ia a L\'actea" (VVV) photometry of seven
embedded clusters (ECs), and one open cluster (OC) projected in the outskirts
of the complex.Photometric and structural properties (age, reddening, distance,
core and total radii) of the star clusters are derived. VVV saturated stars are
replaced by their 2MASS counterparts. Field-decontaminated VVV photometry is
used to analyse Colour-Magnitude Diagrams (CMDs), stellar radial density
profiles (RDPs) and determine astrophysical parameters. We report the discovery
of four ECs and one intermediate-age cluster in the complex area. We derive a
revised distance estimate for NGC 6357 of 1.780.1 kpc based on the cluster
CMD morphologies. Among the ECs, one contains the binary star the WR 93, while
the remaining ones are dominated by pre-main sequence (PMS) stars,
young-stellar objects (YSO) and/or and have a developed main sequence. These
features reflect a significant age spread among the clusters. Evidence is found
that the relatively populous cluster Pismis 24 hosts two subclusters.Comment: This article will be published in the A&A. 11 pages, 15 figures and 3
table
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