337 research outputs found
The sub-solar Initial Mass Function in the Large Magellanic Cloud
The Magellanic Clouds offer a unique variety of star forming regions seen as
bright nebulae of ionized gas, related to bright young stellar associations.
Nowadays, observations with the high resolving efficiency of the Hubble Space
Telescope allow the detection of the faintest infant stars, and a more complete
picture of clustered star formation in our dwarf neighbors has emerged. I
present results from our studies of the Magellanic Clouds, with emphasis in the
young low-mass pre-main sequence populations. Our data include imaging with the
Advanced Camera for Surveys of the association LH~95 in the Large Magellanic
Cloud, the deepest observations ever taken with HST of this galaxy. I discuss
our findings in terms of the Initial Mass Function, which we constructed with
an unprecedented completeness down to the sub-solar regime, as the outcome of
star formation in the low-metallicity environment of the LMC.Comment: To appear in the Proceedings of IAU Symposium 256 "The Magellanic
System: Stars, Gas, and Galaxies", 6 pages, 3 figure
Assessment of Stellar Stratification in Three Young Star Clusters in the Large Magellanic Cloud
(abridged) We present a comprehensive study of stellar stratification in
young star clusters in the Large Magellanic Cloud (LMC). We apply our recently
developed effective radius method for the assessment of stellar stratification
on imaging data obtained with the Advanced Camera for Surveys of three young
LMC clusters to characterize the phenomenon and develop a comparative scheme
for its assessment in such clusters. The clusters of our sample, NGC 1983, NGC
2002 and NGC 2010, are selected on the basis of their youthfulness, and their
variety in appearance, structure, stellar content, and surrounding stellar
ambient. Our photometry is complete for magnitudes down to m_814 ~ 23 mag,
allowing the calculation of the structural parameters of the clusters, the
estimation of their ages and the determination of their stellar content. Our
study shows that each cluster in our sample demonstrates stellar stratification
in a quite different manner and at different degree from the others.
Specifically, NGC 1983 shows to be partially segregated only for the faintest
stars of the cluster, NGC 2002 shows evidence of strong stellar stratification
for both bright and faint stars, and NGC 2010 is found not to be segregated.
For the parametrization of the phenomenon of stellar stratification and its
quantitative comparison among these clusters, we propose the slope derived from
the change in the effective radius over the corresponding magnitude range as
indicative parameter of the degree of stratification in the clusters. A
positive value of this slope indicates mass segregation in the cluster, while a
negative or zero value signifies the lack of the phenomenon.Comment: To appear in the Astrophysical Journal Vol. 709 (2010), pp. 263-277
Version with low-Resolution gray-scaled figures. Version with full resolution
color figures available from
http://rapidshare.com/files/328406139/Gouliermis_2010.ApJ.709.pd
Clustered Star Formation in the Small Magellanic Cloud. A Spitzer/IRAC View of the Star-Forming Region NGC 602/N 90
We present Spitzer/IRAC photometry on the star-forming HII region N 90,
related to the young stellar association NGC 602 in the Small Magellanic Cloud.
Our photometry revealed bright mid-infrared sources, which we classify with the
use of a scheme based on templates and models of red sources in the Milky Way,
and criteria recently developed from the Spitzer Survey of the SMC for the
selection of candidate Young Stellar Objects (YSOs). We detected 57 sources in
all four IRAC channels in a 6.2' x 4.8' field-of-view centered on N 90; 22 of
these sources are classified as candidate YSOs. We compare the locations of
these objects with the position of optical sources recently found in the same
region with high-resolution HST/ACS imaging of NGC 602, and we find that 17
candidate YSOs have one or more optical counterparts. All of these optical
sources are identified as pre-main sequence stars, indicating, thus, ongoing
clustered star formation events in the region. The positions of the detected
YSOs and their related PMS clusters give a clear picture of the current star
formation in N 90, according to which the young stellar association
photo-ionizes the surrounding interstellar medium, revealing the HII nebula,
and triggering sequential star formation events mainly along the eastern and
southern rims of the formed cavity of the parental molecular cloud.Comment: Accepted fro Publication in ApJ. 8 pages, 6 figures, 3 color figures
submitted as JP
The Star-Forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations I. Photometry
We present a photometric study of the star-forming region NGC 346 and its
surrounding field in the Small Magellanic Cloud, using data taken with the
Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).
The data set contains both short and long exposures for increased dynamic
range, and photometry was performed using the ACS module of the stellar
photometry package DOLPHOT. We detected almost 100,000 stars over a magnitude
range of V ~ 11 to V ~ 28 mag, including all stellar types from the most
massive young stars to faint lower main sequence and pre-main sequence stars.
We find that this region, which is characterized by a plethora of stellar
systems and interesting objects, is an outstanding example of mixed stellar
populations. We take into account different features of the color-magnitude
diagram of all the detected stars to distinguish the two dominant stellar
systems: The stellar association NGC 346 and the old spherical star cluster BS
90. These observations provide a complete stellar sample of a field about 5
arcmin x 5 arcmin around the most active star-forming region in this galaxy.
Considering the importance of these data for various investigations in the
area, we provide the full stellar catalog from our photometry. This paper is
the first part of an ongoing study to investigate in detail the two dominant
stellar systems in the area and their surrounding field.Comment: 8 pages, 5 figures, Accepted for publication in The Astrophysical
Journal Supplement Series, A high-resolution version can be found at:
http://www.astro.uni-bonn.de/~dgoulier/Science/Gouliermis+_NGC346_I.pd
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
A New Method for the Assessment of Age and Age-Spread of Pre-Main Sequence Stars in Young Stellar Associations of the Magellanic Clouds
We present a new method for the evaluation of the age and age-spread among
pre-main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds,
accounting simultaneously for photometric errors, unresolved binarity,
differential extinction, stellar variability, accretion and crowding. The
application of the method is performed with the statistical construction of
synthetic color-magnitude diagrams using PMS evolutionary models. We convert
each isochrone into 2D probability distributions of artificial PMS stars in the
CMD by applying the aforementioned biases that dislocate these stars from their
original CMD positions. A maximum-likelihood technique is then applied to
derive the probability for each observed star to have a certain age, as well as
the best age for the entire cluster. We apply our method to the photometric
catalog of ~2000 PMS stars in the young association LH 95 in the LMC, based on
the deepest HST/ACS imaging ever performed toward this galaxy, with a detection
limit of V~28, corresponding to M~0.2 Msun. Our treatment shows that the age
determination is very sensitive to the considered grid of evolutionary models
and the assumed binary fraction. The age of LH 95 is found to vary from 2.8 Myr
to 4.4 Myr, depending on these factors. Our analysis allows us to disentangle a
real age-spread from the apparent CMD-broadening caused by the physical and
observational biases. We find that LH 95 hosts an age-spread well represented
by a gaussian distribution with a FWHM of the order of 2.8 Myr to 4.2 Myr
depending on the model and binary fraction. We detect a dependence of the
average age of the system with stellar mass. This dependence does not appear to
have any physical meaning, being rather due to imperfections of the PMS
evolutionary models, which tend to predict lower ages for the intermediate
masses, and higher ages for low-mass stars.Comment: 19 pages, 16 figures, accepted for publication by the Astrophysical
Journa
Surface Brightness Fluctuations: A Case for Extremely Large Telescopes
The Surface Brightness Fluctuations (SBF) Method for distance determinations
of elliptical galaxies is been modeled in order to investigate the effect of
the Point Spread Function (PSF). We developed a method to simulate observations
of SBF of galaxies having various properties and located at different
distances. We will use this method in order to test the accuracy on the
estimates of the extra-galactic distances for PSFs representing typical seeing
conditions, Adaptive Optics (AO) systems and for future observations with
Extremely Large Telescopes (ELTs) close to the diffraction limit.Comment: 3 pages, 2 figures, to appear in the proceedings of the ESO Workshop
"Science with Adaptive Optics", eds. W. Brandner & M. Kasper, Springer-Verlag
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