275 research outputs found
Stellar Associations and their Field East of LMC 4 in the Large Magellanic Cloud
We report about the stellar content and the luminosity and mass functions of
three stellar associations and their field located on the north-east edge of
the super-bubble LMC 4 in the Large Magellanic Cloud.Comment: To be appeared in the meeting Proceedings of ``Modes of Star
Formation and the Origin of Field Populations'', Heidelberg, Germany, October
2000; to be published in the ASP Conference Series, edited by E. K. Grebel
and W. Brandne
Spatial distribution of stellar populations in the Magellanic Clouds: Implementation to Gaia
The main goal of our project is to investigate the spatial distribution of
different stellar populations in the Magellanic Clouds. The results from
modelling the Magellanic Clouds can be useful, among others, for simulations
during the Gaia mission preparation. Isodensity contour maps have been used in
order to trace the morphology of the different stellar populations and estimate
the size of these structures. Moreover, star density maps are constructed
through star counts and projected radial density profiles are obtained. Fitting
exponential disk and King law curves to the spatial distribution allows us to
derive the structural parameters that describe these profiles. The
morphological structure and spatial distributions of various stellar components
in the Magellanic Clouds (young and intermediate age stars, carbon stars) along
with the overall spatial distribution in both Clouds are provided.Comment: 12 pages, 9 figures, to be published in Astronomy & Astrophysics;
typos and language correcte
HST WFPC2 Observations of the Peculiar Main Sequence of the Double Star Cluster NGC 2011 in the Large Magellanic Cloud
We report the serendipitous discovery of a peculiar main sequence in archived
Hubble Space Telescope WFPC2 observations of the young star cluster NGC 2011 in
the Large Magellanic Cloud. The bright part of this main sequence exhibits a
prominent double, fork-like feature, as if it consists of twin main sequences,
one of them being redder. The color-magnitude diagram, constructed from the
stars found in the only available WFPC2 field of the cluster, is used to
distinguish the stars according to their membership to each of these sequences
and to study their spatial distribution. We find that there are two well
distinguished populations in the sense that the redder main sequence is
dominated by stars that belong to the main body of the cluster, while the stars
of the bluer main sequence belong to the surrounding region. Providing that NGC
2011 is a verified binary cluster, with the second companion unfortunately not
observed, and taking into account the general region where this cluster is
located, we discuss the possible scenarios from both star formation, and early
dynamical evolution point-of-view that might explain this unique discovery.Comment: Astrophysical Journal Letters, in press. Figures in higher resolution
available upon request to D. Gouliermis ([email protected]
Structure of the SMC - Stellar component distribution from 2MASS data
The spatial distribution of the SMC stellar component is investigated from
2MASS data. The morphology of the different age populations is presented. The
center of the distribution is calculated and compared with previous
estimations. The rotation of the stellar content and possible consequence of
dark matter presence are discussed. The different stellar populations are
identified through a CMD diagram of the 2MASS data. Isopleth contour maps are
produced in every case, to reveal the spatial distribution. The derived density
profiles are discussed. The older stellar population follows an exponential
profile at projected diameters of about 5 kpc (~5 deg) for the major axis and
~4 kpc for the minor axis, centred at RA: 0h:51min, Dec: -73deg 7' (J2000.0).
The centre coordinates are found the same for all the different age population
maps and are in good accordance with the kinematical centre of the SMC. However
they are found considerably different from the coordinates of the centre of the
gas distribution. The fact that the older population found on an exponential
disk, gives evidence that the stellar content is rotating, with a possible
consequence of dark matter presence. The strong interactions between the MCs
and the MilkyWay might explain the difference in the distributions of the
stellar and gas components. The lack in the observed velocity element, that
implies absence of rotation, and contradicts with the consequences of
exponential profile of the stellar component, may also be a result of the
gravitational interactions.Comment: 7 Pages, 6 figures, accepted for publication in A&
Luminous AGB stars in nearby galaxies. A study using Virtual Observatory tools
Aims. This study focuses on very luminous Mbol<-6.0 mag AGB stars with
J-Ks>1.5 mag and H-Ks>0.4 mag in the LMC, SMC, M31, and M33 from 2MASS data.
Methods.The data were taken from the 2MASS All-Sky Point Source catalogue
archive. We used Virtual Observatory tools and took advantage of its
capabilities at various stages in the analysis.
Results. It is well known that stars with the colors we selected correspond
mainly to carbon stars. Although the most luminous AGBs detected here contain a
large number of carbon stars,they are not included in existing catalogues
produced from data in the optical domain, where they are not visible since they
are dust-enshrouded. A comparison of the AGB stars detected with combined near
and mid-infrared data from MSX and 2MASS in the LMC shows that 10% of the
bright AGB stars are bright carbon stars never detected before and that the
other 50% are OH/IR oxygen rich stars, whereas the 40% that remain were not
cross-matched.
Conclusions. The catalogues of the most luminous AGB stars compiled here are
an important complement to existing data. In the LMC, these bright AGB stars
are centrally located, whereas they are concentrated in an active
star-formation ring in M31. In the SMC and M33, there are not enough of them to
draw definite conclusions, although they tend to be centrally located. Their
luminosity functions are similar for the four galaxies we studied.Comment: 16 pages, 12 figures, 4 tables (Appendix A), accepted in A&
The Star Clusters of the Small Magellanic Cloud: Structural Parameters
We present structural parameters for 204 stellar clusters in the Small
Magellanic Cloud derived from fitting King and Elson, Fall, & Freeman model
profiles to the V-band surface brightness profiles as measured from the
Magellanic Clouds Photometric Survey images. Both King and EFF profiles are
satisfactory fits to the majority of the profiles although King profiles are
generally slightly superior to the softened power-law profiles of Elson, Fall,
and Freeman and provide statistically acceptable fits to ~90% of the sample. We
find no correlation between the preferred model and cluster age. The only
systematic deviation in the surface brightness profiles that we identify is a
lack of a central concentration in a subsample of clusters, which we designate
as "ring" clusters. In agreement with previous studies, we find that the
clusters in the SMC are significantly more elliptical than those in the Milky
Way. However, given the mean age difference and the rapid destruction of these
systems, the comparison between SMC and MW should not directly be interpreted
as either a difference in the initial cluster properties or their subsequent
evolution. We find that cluster ellipticity correlates with cluster mass more
strongly than with cluster age. We identify several other correlations (central
surface brightness vs. local background density, core radius vs. tidal force,
size vs. distance) that can be used to constrain models of cluster evolution in
the SMC.Comment: 14 pgs., accepted for publication in A
OB stellar associations in the large magellanic cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (2000, Paper I), for the detection
and classification of stellar systems in the LMC, was used for the
identification of stellar associations and open clusters in the central area of
the LMC. This method was applied on the stellar catalog produced from a scanned
1.2m UK Schmidt Telescope Plate in U with a field of view almost 6.5 deg x 6.5
deg, centered on the Bar of this galaxy. The survey of the identified systems
is presented here followed by the results of the investigation on their spatial
distribution and their structural parameters, as were estimated according to
our proposed methodology in Paper I. The detected open clusters and stellar
associations show to form large filamentary structures, which are often
connected with the loci of HI shells. The derived mean size of the stellar
associations in this survey was found to agree with the average size found
previously by other authors, for stellar associations in different galaxies.
This common size of about 80 pc might represent a universal scale for the star
formation process, whereas the parameter correlations of the detected loose
systems support the distinction between open clusters and stellar associations
Measuring Sizes of Marginally Resolved Young Globular Clusters with HST
We present a method to derive sizes of marginally resolved star clusters from
HST/WFPC2 observations by fitting King models to observations. We describe
results on both simulated images and on observations of young compact clusters
in NGC 3597 and NGC 1275. From the simulations, we find that we can measure
King model concentrations (c) to an accuracy of about a factor of two for all
combinations of c and King radius (r_0) of interest if the data have high S/N
(>~ 500 for the integrated brightness). If the concentration is accurately
measured, we can measure the King radius accurately. For lower S/N, marginally
resolved King profiles suffer from a degeneracy; different values of the
concentration give different r_0 but have comparable reduced chi-squared
values. In this case, neither the core radius nor the concentration can be
constrained individually, but the half-light radius can be recovered
accurately.
In NGC 3597, we can only differentiate between concentrations for the very
brightest clusters; these suggest a concentration of ~ 2. Assuming a
concentration of 2 for the rest of the objects, we find an average King radius
for the clusters in NGC 3597 of 0.7 pc, while the clusters in NGC 1275 have an
average radius of 1.1 pc. These are similar to the average core radii for
Galactic globular clusters, 0.92 pc. We find average half-light radii of 5.4 pc
and 6.2 pc for the young clusters in NGC 3597 and NGC 1275, respectively, while
the average half-light radii of Galactic globulars is 3.4 pc. The spread in the
derived radial parameters in each cluster system is comparable to that observed
in the Galactic globular cluster system.Comment: 38 pages, 19 figures, accepted for publication in PAS
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