60 research outputs found
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
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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
Pre--Main-Sequence stellar populations across Shapley Constellation III. I. Photometric Analysis and Identification
We present our investigation of pre--main-sequence (PMS) stellar populations
in the Large Magellanic Cloud (LMC) from imaging with Hubble Space Telescope
WFPC2 camera. Our targets of interest are four star-forming regions located at
the periphery of the super-giant shell LMC 4 (Shapley Constellation III). The
PMS stellar content of the regions is revealed through the differential Hess
diagrams and the observed color-magnitude diagrams (CMDs). Further statistical
analysis of stellar distributions along cross-sections of the faint part of the
CMDs allowed the quantitative assessment of the PMS stars census, and the
isolation of faint PMS stars as the true low-mass stellar members of the
regions. These distributions are found to be well represented by a double
Gaussian function, the first component of which represents the main-sequence
field stars and the second the native PMS stars of each region. Based on this
result, a cluster membership probability was assigned to each PMS star
according to its CMD position. The higher extinction in the region LH 88 did
not allow the unambiguous identification of its native stellar population. The
CMD distributions of the PMS stars with the highest membership probability in
the regions LH 60, LH 63 and LH 72 exhibit an extraordinary similarity among
the regions, suggesting that these stars share common characteristics, as well
as common recent star formation history. Considering that the regions are
located at different areas of the edge of LMC 4, this finding suggests that
star formation along the super-giant shell may have occurred almost
simultaneously.Comment: Accepted for publication in the Astrophysical Journal. 19 pages, 19
figures (three omitted due to size limitations, without affecting the
comprehension of the manuscript
The Massive Stellar Population in the Young Association LH 95 in the LMC
We present a spectroscopic study of the most massive stars in the young (4
Myr old) stellar cluster LH 95 in the Large Magellanic Cloud. This analysis
allows us to complete the census of the stellar population of the system,
previously investigated by us down to 0.4 solar masses with deep HST Advanced
Camera for Surveys photometry. We perform spectral classification of the five
stars in our sample, based on high resolution optical spectroscopy obtained
with 2.2m MPG/ESO FEROS. We use complementary ground-based photometry,
previously performed by us, to place these stars in the Hertzsprung-Russel
diagram. We derive their masses and ages by interpolation from evolutionary
models. The average ages and age spread of the most massive stars are found to
be in general comparable with those previously derived for the cluster from its
low mass PMS stars. We use the masses of the 5 sample stars to extend to the
high-mass end the stellar initial mass function of LH 95 previously established
by us. We find that the initial mass function follows a Salpeter relation down
to the intermediate-mass regime at 2 Msun. The second most massive star in LH
95 shows broad Balmer line emission and infrared excess, which are compatible
with a classical Be star. The existence of such a star in the system adds a
constrain to the age of the cluster, which is well covered by our age and age
spread determinations. The most massive star, a 60-70 Msun O2 giant is found to
be younger (<1 Myr) than the rest of the population. Its mass in relation to
the total mass of the system does not follow the empirical relation of the
maximum stellar mass versus the hosting cluster mass, making LH 95 an exception
to the average trend.Comment: 15 pages, 9 figures, MNRAS accepte
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
The Complete Initial Mass Function Down to the Sub-Solar Regime in the Large Magellanic Cloud with Hubble Space Telescope ACS Observations
In this photometric study of the stellar association LH 95 in the Large
Magellanic Cloud (LMC) we focus on the pre-main Sequence (PMS) population in
order to construct, for the first time, the sub-solar initial mass function
(IMF) in the LMC. We use the deepest photometry ever performed in the LMC with
the Advanced Camera for Surveys (ACS) on-board the Hubble Space Telescope(HST).
We carry out a Monte Carlo technique to subtract the contribution of the
general field of LMC and we isolate the central region in the observed area of
the association. We study the mass function of its field-subtracted population.
For this purpose, we introduce a new set of evolutionary models, derived from
the calculations on the evolution of PMS stars by Siess et al. We use these
models with our observations of LH 95 to derive the IMF of the system, which is
reliably constructed down to 0.43 M_sun, the lowest mass ever observed within
reasonable completeness in the Magellanic Clouds. Consequently, its
construction offers an outstanding improvement in our understanding of the
low-mass star formation in the LMC. The system IMF of LH 95 shows a definite
change in its slope at 1 M_sun, where it becomes more shallow. In general, the
shape of this IMF agrees very well with the average Galactic IMF, down to the
sub-solar regime. As far as the slope of this system IMF is concerned, it is
found to be somewhat more shallow than the corresponding classical Galactic IMF
in the sub-solar regime, probably due to unresolved binarity, while for stars
with M>1M_sun it becomes slightly steeper. We do not find significant
differences in the shape of the overall IMF of LH 95 from that of each of the
three individual sub-clusters, suggesting that the IMF of LH 95 is not subject
to local variability.Comment: 19 pages, 13 figures. Accepted for publication to Ap
Photometric determination of the mass accretion rates of pre-main sequence stars. II. NGC346 in the Small Magellanic Cloud
[Abridged] We have studied the properties of the stellar populations in the
field of the NGC346 cluster in the Small Magellanic Cloud, using a novel
self-consistent method that allows us to reliably identify pre-main sequence
(PMS) objects actively undergoing mass accretion, regardless of their age. The
method does not require spectroscopy and combines broad-band V and I photometry
with narrow-band Halpha imaging to identify all stars with excess Halpha
emission and derive the accretion luminosity Lacc and mass accretion rate Macc
for all of them. The application of this method to existing HST/ACS photometry
of the NGC346 field has allowed us to identify and study 680 bona-fide PMS
stars with masses from ~0.4 to ~4 Msolar and ages in the range from ~1 to ~30
Myr. This is the first study to reveal that, besides a young population of PMS
stars (~ 1 Myr old), in this field there is also an older population of PMS
objects with a median age of ~20 Myr. We provide for all of them accurate
physical parameters. We study the evolution of the mass accretion rate as a
function of stellar parameters and find that logMacc ~ -0.6 Log t + Log m + c,
where t is the age of the star, m its mass and c a quantity that is higher at
lower metallicity. The high mass accretion rates that we find suggest that a
considerable fraction of the stellar mass is accreted during the PMS phase and
that PMS evolutionary models that do not account for this effect will
systematically underestimate the true age when compared with the observations.Comment: Accepted for publication in the ApJ. 14 pages, 11 figures. Corrected
typos and reference
Clues to the star formation in NGC 346 across time and space
We have studied the properties of the stellar populations in the field of the
NGC 346 cluster in the Small Magellanic Cloud, using the results of a novel
self-consistent method that provides a reliable identification of pre-main
sequence (PMS) objects actively undergoing mass accretion, regardless of their
age. The 680 identified bona-fide PMS stars show a bimodal age distribution,
with two roughly equally numerous populations peaked respectively at ~1 Myr,
and ~20 Myr. We use the age and other physical properties of these PMS stars to
study how star formation has proceeded across time and space in NGC 346. We
find no correlation between the locations of young and old PMS stars, nor do we
find a correspondence between the positions of young PMS stars and those of
massive OB stars of similar age. Furthermore, the mass distribution of stars
with similar age shows large variations throughout the region. We conclude
that, while on a global scale it makes sense to talk about an initial mass
function, this concept is not meaningful for individual star-forming regions.
An interesting implication of the separation between regions where massive
stars and low-mass objects appear to form is that high-mass stars might not be
"perfect" indicators of star formation and hence a large number of low-mass
stars formed elsewhere might have so far remained unnoticed. For certain low
surface density galaxies this way of preferential low-mass star formation may
be the predominant mechanism, with the consequence that their total mass as
derived from the luminosity may be severely underestimated and that their
evolution is not correctly understood.Comment: 12 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Low-mass pre--main-sequence stars in the Magellanic Clouds
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar
stars form in very large numbers. Most attractive places for catching low-mass
star formation in the act are young stellar clusters and associations, still
(half-)embedded in star-forming regions. The low-mass stars in such regions are
still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature
of these objects and the contamination of their samples by the evolved
populations of the Galactic disk impose demanding observational techniques for
the detection of complete numbers of PMS stars in the Milky Way. The Magellanic
Clouds, the companion galaxies to our own, demonstrate an exceptional star
formation activity. The low extinction and stellar field contamination in
star-forming regions of these galaxies imply a more efficient detection of
low-mass PMS stars than in the Milky Way, but their distance from us make the
application of special detection techniques unfeasible. Nonetheless, imaging
with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS
stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of
such objects are identified as the low-mass stellar content of their
star-forming regions, changing completely our picture of young stellar systems
outside the Milky Way, and extending the extragalactic stellar IMF below the
persisting threshold of a few solar masses. This review presents the recent
developments in the investigation of PMS stars in the Magellanic Clouds, with
special focus on the limitations by single-epoch photometry that can only be
circumvented by the detailed study of the observable behavior of these stars in
the color-magnitude diagram. The achieved characterization of the low-mass PMS
stars in the Magellanic Clouds allowed thus a more comprehensive understanding
of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4
figures. Accepted for publication in Space Science Review
The properties, origin and evolution of stellar clusters in galaxy simulations and observations
Published onlineWe investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one ‘cluster’, for the isolated galaxies we are able to model features we term ‘clusters’ with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myr) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas on to the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback.We thank the referee for a useful report. The calculations for this paper were performed primarily on the DiRAC machine ‘Complexity’, as well as the supercomputer at Exeter, which is jointly funded by STFC, the Large Facilities Capital Fund of BIS, and the University of Exeter. We would like to thank Michele Fumagalli for work putting together the LEGUS cluster catalogues. CLD and CGF acknowledge funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. CGF thanks Ben Thompson for performing data reduction. DG kindly acknowledges financial support by the German Research Foundation (DFG) through grant GO 1659/3-2. Figures in this paper were produced using splash (Price 2007)
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