65 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
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
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
Discovery of the Pre-Main Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope ACS Observations
We report the discovery of an extraordinary number of pre-main sequence (PMS)
stars in the vicinity of the stellar association LH 95 in the Large Magellanic
Cloud (LMC). Using the {\em Advanced Camera for Surveys} on-board the {\em
Hubble} Space Telescope in wide-field mode we obtained deep high-resolution
imaging of the main body of the association and of a nearby representative LMC
background field. These observations allowed us to construct the
color-magnitude diagram (CMD) of the association in unprecedented detail, and
to decontaminate the CMD for the average LMC stellar population. The most
significant result is the direct detection of a substantial population of PMS
stars and their clustering properties with respect to the distribution of the
higher mass members of the association. Although LH 95 represents a rather
modest star forming region, our photometry, with a detection limit \lsim 28
mag, reveals in its vicinity more than 2,500 PMS stars with masses down to
M{\solar}. Our observations offer, thus, a new perspective of a
typical LMC association: The stellar content of LH 95 is found to extend from
bright OB stars to faint red PMS stars, suggesting a fully populated Initial
Mass Function (IMF) from the massive blue giants down to the sub-solar mass
regime.Comment: Accepted for Publication in ApJ Letters - 4 Pages ApJ paper format -
3 figures in low-resolution/grayscal
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
The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90
We present the results of our investigation of the intermediate-age star
cluster BS 90, located in the vicinity of the HII region N 66 in the SMC,
observed with HST/ACS. The high-resolution data provide a unique opportunity
for a very detailed photometric study performed on one of the rare
intermediate-age rich SMC clusters. The complete set of observations is
centered on the association NGC 346 and contains almost 100,000 stars down to V
~28 mag. In this study we focus on the northern part of the region, which
covers almost the whole stellar content of BS 90. We construct its stellar
surface density profile and derive structural parameters. Isochrone fits on the
CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function
is constructed and the present-day mass function of BS 90 has been obtained
using the mass-luminosity relation, derived from the isochrone models. We found
a slope between -1.30 and -0.95, comparable or somewhat shallower than a
typical Salpeter IMF. Examination of the radial dependence of the mass function
shows a steeper slope at larger radial distances, indicating mass segregation
in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that
the cluster is mass segregated due to its dynamical evolution. From the
isochrone model fits we derive a metallicity for BS 90 of [Fe/H]=-0.72, which
adds an important point to the age-metallicity relation of the SMC. We discuss
our findings on this relation in comparison to other SMC clusters.Comment: Accepted for Publication in ApJ, 12 pages emulateapj TeX style, 10
figure
The clustered nature of star formation. Pre--main-sequence clusters in the star-forming region NGC 602/N90 in the Small Magellanic Cloud
Located at the tip of the wing of the Small Magellanic Cloud (SMC), the
star-forming region NGC602/N90 is characterized by the HII nebular ring N90 and
the young cluster of pre--main-sequence (PMS) and early-type main sequence
stars NGC602. We present a thorough cluster analysis of the stellar sample
identified with HST/ACS camera in the region. We show that apart from the
central cluster, low-mass PMS stars are congregated in thirteen additional
small compact sub-clusters at the periphery of NGC602. We find that the spatial
distribution of the PMS stars is bimodal, with an unusually large fraction
(~60%) of the total population being clustered, while the remaining is
diffusely distributed in the inter-cluster area. From the corresponding
color-magnitude diagrams we disentangle an age-difference of ~2.5Myr between
NGC602 and the compact sub-clusters which appear younger. The diffuse PMS
population appears to host stars as old as those in NGC602. Almost all detected
PMS sub-clusters appear to be centrally concentrated. When the complete PMS
stellar sample, including both clustered and diffused stars, is considered in
our cluster analysis, it appears as a single centrally concentrated stellar
agglomeration, covering the whole central area of the region. Considering also
the hot massive stars of the system, we find evidence that this agglomeration
is hierarchically structured. Based on our findings we propose a scenario,
according to which the region NGC602/N90 experiences an active clustered star
formation for the last ~5Myr. The central cluster NGC602 was formed first and
rapidly started dissolving into its immediate ambient environment, possibly
ejecting also massive stars found away from its center. Star formation
continued in sub-clusters of a larger stellar agglomeration, introducing an
age-spread of the order of 2.5Myr among the PMS populations.Comment: Accepted for publication by The Astrophysical Journal. 14 pages, 11
figures, 1 table, 2-columns forma
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