177 research outputs found
Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as a benchmark
We present a novel approach to derive the age of very young star clusters, by
using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram
(CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS
luminosity function (LF) of the cluster, the TOn is identified as a peak
followed by a dip. We propose that by combining the CMD analysis with the
monitoring of the spatial distribution of MS stars it is possible to reliably
identify the TOn in extragalactic star forming regions. Compared to alternative
methods, this technique is complementary to the turn-off dating and avoids the
systematic biases affecting the PMS phase. We describe the method and its
uncertainties, and apply it to the star forming region NGC346, which has been
extensively imaged with the Hubble Space Telescope (HST). This study extends
the LF approach in crowded extragalactic regions and opens the way for future
studies with HST/WFC3, JWST and from the ground with adaptive optics.Comment: 6 pages, 4 figures. Accepted for publication in ApJ Letter
History and modes of star formation in the most active region of the Small Magellanic Cloud, NGC 346
We discuss the star formation history of the SMC region NGC 346 based on
Hubble Space Telescope images. The region contains both field stars and cluster
members. Using a classical synthetic CMD procedure applied to the field around
NGC 346 we find that there the star formation pace has been rising from a quite
low rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last
100 Myr. This value is significantly higher than in other star forming regions
of the SMC. For NGC 346 itself, we compare theoretical and observed
Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in
the region, and we derive their basic evolution parameters. We find that NGC
346 experienced different star formation regimes, including a dominant and
focused "high density mode", with the sub-clusters hosting both pre-main
sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density
mode", as indicated by the presence of low-mass PMS sub-clusters.
Quantitatively, the star formation in the oldest sub-clusters started about 6
Myr ago with remarkable synchronization, it continued at high rate (up to 2
\times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a
lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars
seem to experience now the first episode of star formation, following
multi-seeded spatial patterns instead of resulting from a coherent trigger. Two
speculative scenarios are put forth to explain the deficiency of UMS stars: the
first invokes under-threshold conditions of the parent gas; the second
speculates that the initial mass function (IMF) is a function of time, with the
youngest sub-clusters not having had sufficient time to form more massive
stars.Comment: 17 pages. Accepted for publication in A
Uniform detection of the pre-main sequence population in the 5 embedded clusters related to the H\,II region NGC\,2174 (Sh2-252)
We investigate 5 embedded clusters (ECs) and the extended stellar group
itself of the prominent H\,II region NGC\,2174 (Sh2-252), which presents scarce
and heterogeneous information, coming from the optical and infrared.
Considering the discrepant values of distance and age, the clusters and the
H\,II region appear to be physically unrelated. The analysis is based on
field-star decontaminated 2MASS photometry, which allows sampling the pre-main
sequence (PMS). We find that Sh2-252A, C, E, NGC\,2175s, and Teu\,136 are small
ECs (radius within \,pc) characterised by a similar age
(\,Myr), reddening (\aV\sim1), distance from the Sun
(\ds\sim1.4\,kpc), and low mass (60-200\,\ms). This age is consistent with
the H\,II region, the presence of O and B stars still in the MS, and the
dominance (\ga95% in number) of PMS stars in colour-magnitude diagrams
(CMDs). NGC\,2175 is not a star cluster, but an extended stellar group that
encompasses the ECs Sh2-252\,A and C. It contains of the member stars
(essentially PMS) in the area, with the remaining belonging to the 2 ECs. CMDs
of the overall star-forming region and the ECs provide \ds=1.4\pm0.4\,kpc for
the NGC\,2174 complex, consistent with the value estimated for the
physically-related association Gem\,OB1. Our uniform approach shows that
NGC\,2174 and its related ECs (except, perhaps, for Teu\,136) are part of a
single star-forming complex. CMD similarities among the ECs and the overall
region suggest a coeval (to within \,Myr) star-forming event extending
for several Myr. At least 4 ECs originated in the event, together with the
off-cluster star formation that probably gave rise to the scattered stars of
NGC\,2175.Comment: Accepted by MNRA
The Initial Mass Function of the Stellar Association NGC 602 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations
We present our photometric study of the stellar association NGC 602 in the
wing of the Small Magellanic Cloud (SMC). The data were taken in the filters
F555W and F814W using the Advanced Camera for Surveys (ACS) on-board the Hubble
Space Telescope (HST). Photometry was performed using the ACS module of the
stellar photometry package DOLPHOT. We detected more than 5,500 stars with a
magnitude range of 14 \lsim m_{555} \lsim 28 mag. Three prominent stellar
concentrations are identified with star counts in the observed field, the
association NGC 602 itself, and two clusters, one of them not being currently
in any known catalog. The Color-Magnitude Diagrams (CMDs) of both clusters show
features typical for young open clusters, while that of the association reveals
bright main sequence (MS) and faint pre-main sequence (PMS) stars as the
members of the system. We construct the initial mass spectrum (IMS) of the
association by applying an age-independent method of counting the PMS stars
within evolutionary tracks, while for the bright MS stars we transform their
magnitudes to masses with the use of mass-luminosity relations. The IMS of NGC
602 is found to be well represented by a single-power law, corresponding to an
Initial Mass Function (IMF) of slope \Gamma\approx -1.2 for 1 \lsim M/M{\solar}
\lsim 45. This indicates that the shape of the IMF of a star forming system in
the SMC for stars with masses higher than 1 M{\solar} seems to be quite similar
to the field IMF in the solar neighborhood.Comment: Accepted for publication in ApJ, 13 pages, 14 figures, emulateapj.cls
LaTeX style, full resolution version available on
http://www.astro.uni-bonn.de/~dgoulier/Science/NGC602/ms.pd
NGC 346 in The Small Magellanic Cloud. IV. Triggered Star Formation in the HII Region N66
Stellar feedback, expanding HII regions, wind-blown bubbles, and supernovae
are thought to be important triggering mechanisms of star formation. Stellar
associations, being hosts of significant numbers of early-type stars, are the
loci where these mechanisms act. In this part of our photometric study of the
star-forming region NGC346/N66 in the Small Magellanic Cloud, we present
evidence based on previous and recent detailed studies, that it hosts at least
two different events of triggered star formation and we reveal the complexity
of its recent star formation history. In our earlier studies of this region
(Papers I, III) we find that besides the central part of N66, where the bright
OB stellar content of the association NGC346 is concentrated, an arc-like
nebular feature, north of the association, hosts recent star formation. This
feature is characterized by a high concentration of emission-line stars and
Young Stellar Objects, as well as embedded sources seen as IR-emission peaks
that coincide with young compact clusters of low-mass pre-main sequence stars.
All these objects indicate that the northern arc of N66 encompasses the most
current star formation event in the region. We present evidence that this star
formation is the product of a different mechanism than that in the general area
of the association, and that it is triggered by a wind-driven expanding HII
region (or bubble) blown by a massive supernova progenitor, and possibly other
bright stars, a few Myr ago. We propose a scenario according to which this
mechanism triggered star formation away from the bar of N66, while in the bar
of N66 star formation is introduced by the photo-ionizing OB stars of the
association itself.Comment: Astrophysical Journal, In Press. 10 pages, 4 figures, emulateapj
LaTeX style. Figures with Scaled-down resolution. Related Press Releases:
http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20081008.html and
http://www.eso.org/public/outreach/press-rel/pr-2008/pr-34-08.htm
NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HII Region N 66
In the third part of our photometric study of the star-forming region NGC
346/N~66 and its surrounding field in the Small Magellanic Cloud (SMC), we
focus on the large number of low-mass pre-main sequence (PMS) stars revealed by
the Hubble Space Telescope Observations with the Advanced Camera for Surveys.
We investigate the origin of the observed broadening of the pre-main sequence
population in the , CMD. The most likely explanations are either the
presence of differential reddening or an age spread among the young stars.
Assuming the latter, simulations indicate that we cannot exclude the
possibility that stars in NGC 346 might have formed in two distinct events
occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are
not homogeneously distributed across NGC 346, but instead are grouped in at
least five different clusters. On spatial scales from 0.8 to 8 (0.24 to
2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed
by a two-point angular correlation function is self-similar with a power law
slope . The clustering properties are quite similar to
Milky Way star forming regions like Orion OB or Oph. Thus molecular
cloud fragmentation in the SMC seems to proceed on the same spatial scales as
in the Milky Way. This is remarkable given the differences in metallicity and
hence dust content between SMC and Milky Way star forming regions.Comment: Accepted for publication in ApJ. 16 pages, 13 (low-resolution)
figures, emulateapj.cls LaTeX styl
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
A Hubble View of Star Forming Regions in the Magellanic Clouds
The Magellanic Clouds (MCs) offer an outstanding variety of young stellar
associations, in which large samples of low-mass stars (with masses less than 1
solar mass) currently in the act of formation can be resolved and explored
sufficiently with the Hubble Space Telescope. These pre-main sequence (PMS)
stars provide a unique snapshot of the star formation process, as it is being
recorded for the last 20 Myr, and they give important information on the
low-mass Initial Mass Function (IMF) of their host environments. We present the
latest results from observations with the Advanced Camera for Surveys (ACS) of
such star-forming regions in the MCs, and discuss the importance of Hubble}for
a comprehensive collection of substantial information on the most recent
low-mass star formation and the low-mass IMF in the MCs.Comment: To appear in the proceedings of the 41st ESLAB Symposium: The Impact
of HST on European Astronomy, 4 pages, LaTeX ESA Publications style, 5
Figure
Why Do Stars Form In Clusters? An Analytic Model for Stellar Correlation Functions
Recently, we have shown that if the ISM is governed by super-sonic turbulent
flows, the excursion-set formalism can be used to calculate the statistics of
self-gravitating objects over a wide range of scales. On the largest
self-gravitating scales ('first crossing'), these correspond to GMCs, and on
the smallest non-fragmenting self-gravitating scales ('last crossing'), to
protostellar cores. Here, we extend this formalism to rigorously calculate the
auto and cross-correlation functions of cores (and by extension, young stars)
as a function of spatial separation and mass, in analogy to the cosmological
calculation of halo clustering. We show that this generically predicts that
star formation is very strongly clustered on small scales: stars form in
clusters, themselves inside GMCs. Outside the binary-star regime, the projected
correlation function declines as a weak power-law, until a characteristic scale
which corresponds to the characteristic mass scale of GMCs. On much larger
scales the clustering declines such that star formation is not strongly biased
on galactic scales, relative to the actual dense gas distribution. The precise
correlation function shape depends on properties of the turbulent spectrum, but
its qualitative behavior is quite general. The predictions agree well with
observations of young star and core autocorrelation functions over ~4 dex in
radius. Clustered star formation is a generic consequence of supersonic
turbulence if most of the power in the velocity field, hence the contribution
to density fluctuations, comes from large scales. The distribution of
self-gravitating masses near the sonic length is then imprinted by fluctuations
on larger scales. We similarly show that the fraction of stars formed in
'isolated' modes should be small (\lesssim10%).Comment: 8 pages, 3 figures, accepted to MNRAS (minor revisions to match
accepted version
Star formation history in the SMC: the case of NGC602
Deep HST/ACS photometry of the young cluster NGC 602, located in the remote
low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main
sequence stars as well as young stars on the main sequence. The resolved
stellar content thus provides a basis for studying the star formation history
into recent times and constraining several stellar population properties, such
as the present day mass function, the initial mass function and the binary
fraction. To better characterize the pre-main sequence population, we present a
new set of model stellar evolutionary tracks for this evolutionary phase with
metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a
stellar population synthesis code, which takes into account a full range of
stellar evolution phases to derive our best estimate for the star formation
history in the region by comparing observed and synthetic color-magnitude
diagrams. The derived present day mass function for NGC 602 is consistent with
that resulting from the synthetic diagrams. The star formation rate in the
region has increased with time on a scale of tens of Myr, reaching in the last 2.5 Myr, comparable to what is
found in Galactic OB associations. Star formation is most complete in the main
cluster but continues at moderate levels in the gas-rich periphery of the
nebula.Comment: 24 pages. Accepted for publication in A
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