24 research outputs found
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 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
Candidate LBV stars in galaxy NGC 7793 found via HST photometry + MUSE spectroscopy
Only about 19 Galactic and 25 extragalactic bonafide luminous blue variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and Hâα luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have MV †â7 and a combination of: Hâα with a P-Cygni profile; no [OâI]λ6300 emission; weak or no [OâIII]λ5007 emission; large [NâII]/Hâα relative to HâII regions; and [SâII]λ6716/[SâII]λ6731âŒ1â . It is not clear if these five cLBVs are isolated from O-type stars, which would favour the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies
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
Star Formation Histories of the LEGUS Spiral Galaxies. I. the Flocculent Spiral NGC 7793
We present a detailed study of the flocculent spiral galaxy NGC 7793, part of the Sculptor group. By analyzing the resolved stellar populations of the galaxy, located at a distance of ~3.7 Mpc, we infer for the first time its radial star formation history (SFH) from Hubble Space Telescope photometry, thanks to both archival and new data from the Legacy ExtraGalactic UV Survey. We determine an average star formation rate (SFR) for the galaxy portion covered by our F555W and F814W data of 0.23 ± 0.02 M â yrâ1 over the whole Hubble time, corresponding to a total stellar mass of (3.09 ± 0.33) Ă 109 M â, in agreement with previous determinations. Thanks to the new data extending to the F336W band, we are able to analyze the youngest stellar populations with a higher time resolution. Most importantly, we recover the resolved SFH in different radial regions of the galaxy; this shows an indication of a growing trend of the present-to-past SFR ratio, increasing from internal to more external regions, supporting previous findings of the inside-out growth of the galaxy.Support for this program was provided by
NASA through grants from the Space Telescope Science
Institute. M.C. and M.T. acknowledge funding from the INAF
PRIN-SKA 2017 program 1.05.01.88.04. A.A. acknowledges
the support of the Swedish Research Council, VetenskapsrÄdet
and the Swedish National Space Agency (SNSA)
An Open System for Social Computation
Part of the power of social computation comes from using the collective intelligence of humans to tame the aggregate uncertainty of (otherwise) low veracity data obtained from human and automated sources. We have witnessed a surge in development of social computing systems but, ironically, there have been few attempts to generalise across this activity so that creation of the underlying mechanisms themselves can be made more social. We describe a method for achieving this by standardising patterns of social computation via lightweight formal specifications (we call these social artifacts) that can be connected to existing internet architectures via a single model of computation. Upon this framework we build a mechanism for extracting provenance meta-data across social computations
Exploring the IMF of star clusters: a joint SLUG and LEGUS effort
We present the implementation of a Bayesian formalism within the Stochastically Lighting Up Galaxies (SLUG) stellar population synthesis code, which is designed to investigate variations in the initial mass function (IMF) of star clusters. By comparing observed cluster photometry to large libraries of clusters simulated with a continuously varying IMF, our formalism yields the posterior probability distribution function (PDF) of the cluster mass, age and extinction, jointly with the parameters describing the IMF. We apply this formalism to a sample of star clusters from the nearby galaxy NGC 628, for which broad-band photometry in five filters is available as part of the Legacy ExtraGalactic UV Survey (LEGUS). After allowing the upper-end slope of the IMF (α3) to vary, we recover PDFs for the mass, age and extinction that are broadly consistent with what is found when assuming an invariant Kroupa IMF. However, the posterior PDF for α3 is very broad due to a strong degeneracy with the cluster mass, and it is found to be sensitive to the choice of priors, particularly on the cluster mass. We find only a modest improvement in the constraining power of α3 when adding Hα photometry from the companion Hα-LEGUS survey. Conversely, Hα photometry significantly improves the age determination, reducing the frequency of multi-modal PDFs. With the aid of mock clusters, we quantify the degeneracy between physical parameters, showing how constraints on the cluster mass that are independent of photometry can be used to pin down the IMF properties of star clusters.GA acknowledges support from the Science and Technology Facilities Council (ST/L00075X/1 and ST/M503472/1). MF acknowledges support by the Science and Technology Facilities Council
[grant number ST/L00075X/1]. DAG kindly acknowledges financial support by the German Research Foundation (DFG) through
programme GO 1659/3-2
The Panchromatic Hubble Andromeda Treasury. VIII. A Wide-area, High-resolution Map of Dust Extinction in M31
We map the distribution of dust in M31 at 25 pc resolution using stellar photometry from the Panchromatic Hubble Andromeda Treasury survey. The map is derived with a new technique that models the near-infrared colorâmagnitude diagram (CMD) of red giant branch (RGB) stars. The model CMDs combine an unreddened foreground of RGB stars with a reddened background population viewed through a log-normal column density distribution of dust. Fits to the model constrain the median extinction, the width of the extinction distribution, and the fraction of reddened stars in each 25 pc cell. The resulting extinction map has a factor of âł 4 times better resolution than maps of dust emission, while providing a more direct measurement of the dust column. There is superb morphological agreement between the new map and maps of the extinction inferred from dust emission by Draine et al. However, the widely used Draine & Li dust models overpredict the observed extinction by a factor of ~2.5, suggesting that M31's true dust mass is lower and that dust grains are significantly more emissive than assumed in Draine et al. The observed factor of ~2.5 discrepancy is consistent with similar findings in the Milky Way by the Plank Collaboration et al., but we find a more complex dependence on parameters from the Draine & Li dust models. We also show that the the discrepancy with the Draine et al. map is lowest where the current interstellar radiation field has a harder spectrum than average. We discuss possible improvements to the CMD dust mapping technique, and explore further applications in both M31 and other galaxies