856 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
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
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
The Dusty Starburst Nucleus of M33
We have thoroughly characterized the ultraviolet to near-infrared (0.15 - 2.2
micron) spectral energy distribution (SED) of the central parsec of the M33
nucleus through new infrared photometry and optical/near-infrared spectroscopy,
combined with ultraviolet/optical observations from the literature and the HST
archive. The SED shows evidence for a significant level of attenuation, which
we model through a Monte Carlo radiative transfer code as a shell of clumpy
Milky Way-type dust (tau_V ~ 2 +/- 1). The discovery of Milky Way-type dust
(with a strong 2175 A bump) internal to the M33 nucleus is different from
previous work which has found SMC-like dust (no bump) near starburst regions.
The amount by which dust can be processed may be related to the mass and age of
the starburst as well as the extent to which the dust can shield itself. Our
starburst models include the effects of this dust and can fit the SED if the
nucleus was the site of a moderate (~10^8 L_sun at 10 Myrs) episode of coeval
star formation about 70 Myrs ago. This result is quite different from previous
studies which resorted to multiple stellar populations (between 2 and 7)
attenuated by either no or very little internal dust. The M33 nuclear starburst
is remarkably similar to an older version (70 Myr versus 10 Myr) of the
ultra-compact starburst in the center of the Milky Way.Comment: 29 pages, 9 embedded figures, ApJ, in pres
A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where are the X-ray Binaries?
We present results from a 34 ks Chandra/ACIS-S observation of the nearby
(d=3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually
low X-ray luminosity. We detect eleven X-ray point sources within the the
optical boundary of the galaxy (93% of the light), one third to one
half of which are likely to be background AGN. One source is coincident with
the optical nucleus and may be a low-luminosity AGN. Only two sources with an
X-ray luminosity greater than 10 ergs s in the 0.5-5.0 keV band
were detected, one of which is statistically likely to be a background AGN. We
expected to detect 6 such luminous sources if the XRB population scales
linearly with optical magnitude of the host galaxy. NGC 5102 has an unusually
low number of XRBs. NGC 5102 is unusually blue for its morphological type, and
has undergone at least two recent bursts of star formation. We present the
results of optical/UV spectral synthesis analysis and demonstrate that a
significant fraction (50%) of the stars in this galaxy are comparatively
young ( years old). If the lack of X-ray binaries is related to
the relative youth of most of the stars, this would support models of LMXB
formation and evolution that require wide binaries to shed angular momentum on
a timescale of Gyrs. We find that NGC 5102 has an unusually low specific
frequency of globular clusters (0.4), which could also explain the
lack of LMXBs. We also detect diffuse X-ray emission in the central 1 kpc
of the galaxy. This hot gas is most likely a superbubble created by multiple
supernovae of massive stars born during the most recent star burst, and is
driving the shock into the ISM which was inferred from optical observations.Comment: 33 pages, 7 figures, 6 tables - Accepted for publication in the
Astrophysical Journa
A catalogue of dust clouds in the Galaxy
In this study 21 catalogues of dust clouds in the Galaxy were cross-identified by taking into account available properties such as position, angular dimensions, opacity class and velocity. An initial list of 6500 entries was condensed into a cross-identified all-sky catalogue containing 5004 dust clouds. In particular, the transition zone between high and low Galactic latitude studies was also cross-identified. The unified catalogue contains 525 high-latitude clouds. The catalogue deals primarily with optical dark nebulae and globules, but it includes as well substantial information from their molecular counterparts. Some previously uncatalogued clouds were detected on optical images and FIR maps. Finally, we address recent results and prospective work based on NIR imaging, especially for clouds detected in the 2MASS Atlas
Discovery of a stellar system in the background of 47 Tucanae
We report on the discovery of a stellar system in the background of the
Galactic globular cluster 47 Tucanae (NGC 104), located 14.8' North-West of the
cluster center. The object, whose apparent diameter is D~30'', is partially
resolved into stars on the available CCD images, reaching a limiting magnitude
of V~22.5, and is detected as a significant (more than 5 sigma) overdensity of
blue stars (B-V<0.7). The color magnitude diagram of the system, its
characteristic projected size and its position in the sky suggest that it is an
intermediate-old age cluster belonging to the Small Magellanic Cloud, whose
outskirts lie in the background of 47 Tuc. Although less likely, the
possibility that the object is an unknown dwarf galaxy in the outskirts of the
Local Group cannot completely be ruled out by the present data.Comment: 5 pages, 5 figures, accepted for publication in A&
New infrared star clusters and candidates in the Galaxy detected with 2MASS
A sample of 42 new infrared star clusters, stellar groups and candidates was found throughout the Galaxy in the 2MASS J, H and especially K Atlases. In the Cygnus X region 19 new clusters, stellar groups and candidates were found as compared to 6 such objects in the previous literature. Colour-Magnitude Diagrams using the 2MASS Point Source Catalogue provided preliminary distance estimates in the range 1.0 d 1.8 kpc for 7 Cygnus X clusters. Towards the central parts of the Galaxy 7 new IR clusters and candidates were found as compared to 61 previous objects. A search for prominent dark nebulae in K was also carried out in the central bulge area. We report 5 dark nebulae, 2 of them are candidates for molecular clouds able to generate massive star clusters near the Nucleus, such as the Arches and Quintuplet clusters
New star clusters projected close to the Galactic Centre
We carried out a systematic search for new star clusters in a field of
5 centred close to the Galactic Nucleus using the
infrared JHK 2MASS Survey archive. In addition we searched for embedded
clusters in the directions of HII regions and dark clouds for
. As a result we present a list of 58 IR star clusters or
candidates. We provide positions, sizes and reddening estimated from 100 m
dust emission. Their angular distribution together with previously catalogued
objects in the region and possible relation with star forming complexes are
also discussed.Comment: manuscript in LATEX with 4 pages, 4 figures .ps Accepted for
Astronomy and Astrophysics main journa
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