185 research outputs found
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
Contrast-Enhanced Ultrasonograpic Findings in Pancreatic Tumors
OBJECTIVE: The purpose of this article is to present the potentials and limits of contrast-enhanced ultrasonography (CEUS) in the characterization of pancreatic tumors, usually hypoechoic or cystic at B-mode ultrasound
The Relationship Between Molecular Gas and Star Formation in Low-Mass E/S0 Galaxies
We consider the relationship between molecular-gas and star-formation surface
densities in 19 morphologically defined E/S0s with stellar mass <~ 4x10^10
M_sun, paying particular attention to those found on the blue sequence in color
vs. stellar mass parameter space, where spiral galaxies typically reside. While
some blue-sequence E/S0s must be young major-merger remnants, many low-mass
blue-sequence E/S0s appear much less disturbed, and may be experiencing the
milder starbursts associated with inner-disk building as spirals (re)grow. For
a sample of eight E/S0s (four blue-, two mid-, and two red-sequence) whose
CARMA CO(1-0), Spitzer MIPS 24um, and GALEX FUV emission distributions are
spatially resolved on a 750pc scale, we find roughly linear relationships
between molecular-gas and star-formation surface densities within all galaxies,
with power law indices N = 0.6-1.9 (median 1.2). Adding 11 more blue-sequence
E/S0s whose CO(1-0) emission is not as well resolved, we find that most of our
E/S0s have global 1-8 kpc aperture-averaged molecular-gas surface densities
overlapping the range spanned by the disks and centers of spiral galaxies.
While many of our E/S0s fall on the same Schmidt-Kennicutt relation as local
spirals, ~80% (predominantly on the blue sequence) are offset towards
apparently higher molecular-gas star formation efficiency (i.e., shorter
molecular gas depletion time). Possible interpretations of the elevated
efficiencies include bursty star formation similar to that in local dwarf
galaxies, H2 depletion in advanced starbursts, or simply a failure of the
CO(1-0) emission to trace all of the molecular gas.Comment: ApJL, accepted, 16 pages with 3 color figures, 2 table
The Complex Star Formation History of NGC 1569
We present new results on the star formation history of the dwarf irregular
galaxy NGC 1569. The data were obtained with Hubble Space Telescope's
NICMOS/NIC2 in the F110W (J) and F160W (H) near-infrared (NIR) filters and
interpreted with the synthetic color-magnitude diagram method. The galaxy
experienced a complex star formation (SF) activity. The best fit to the data is
found by assuming three episodes of activity in the last 1-2 Gyr. The most
recent and strong episode constrained by these NIR data started ~37 Myr ago and
ended ~13 Myr ago, although we cannot exclude the possibility that up to three
SF episodes occurred in this time interval. The average star-formation rate
(SFR) of the episode is 3.2 Msun yr-1 kpc-2, in agreement with literature data.
A previous episode produced stars between 150 Myr and 40 Myr ago, with a mean
SFR about 2/3 lower than the mean SFR of the youngest episode. An older SF
episode occurred about 1 Gyr ago. All these SFRs are 2-3 orders of magnitude
higher than those derived for late-type dwarfs of the Local Group. In all cases
an initial mass function similar to Salpeter's allows for a good reproduction
of the data, but we cannot exclude flatter mass functions. These results have
been obtained adopting a distance of 2.2 Mpc and a reddening E(B-V)=0.56. A
larger distance would require younger episodes and higher SFRs. We have
explored some possible scenarios using the astrated mass in the best fit model,
in order to constrain the past star formation history. We cannot rule out a low
rate in the past SF but we can safely conclude that the last 1-2 Gyr have been
peculiar.Comment: Accepted by AJ, 2005, vol 129, 14 pages, 9 figure
Star formation history and environment of the dwarf galaxy UGCA 92
We present a quantitative star formation history of the nearby dwarf galaxy
UGCA 92. This irregular dwarf is situated in the vicinity of the Local Group of
galaxies in a zone of strong Galactic extinction (IC 342 group of galaxies).
The galaxy was resolved into stars with HST/ACS including old red giant branch.
We have constructed a model of the resolved stellar populations and measured
the star formation rate and metallicity as function of time. The main star
formation activity period occurred about 8 - 14 Gyr ago. These stars are mostly
metal-poor, with a mean metallicity [Fe/H] ~ -1.5 -- -2.0 dex. About 84 per
cent of the total stellar mass was formed during this event. There are also
indications of recent star formation starting about 1.5 Gyr ago and continuing
to the present. The star formation in this event shows moderate enhancement
from ~ 200 Myr to 300 Myr ago. It is very likely that the ongoing star
formation period has higher metallicity of about -0.6 -- -0.3 dex. UGCA 92 is
often considered to be the companion to the starburst galaxy NGC 1569.
Comparing our star formation history of UGCA 92 with that of NGC 1569 reveals
no causal or temporal connection between recent star formation events in these
two galaxies. We suggest that the starburst phenomenon in NGC 1569 is not
related to the galaxy's closest dwarf neighbours and does not affect their star
formation history.Comment: 11 pages, 9 figures, accepted to MNRA
Past and present star formation in the SMC: NGC 346 and its neighborhood
In the quest of understanding how star formation occurs and propagates in the
low metallicity environment of the Small Magellanic Cloud (SMC), we acquired
deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star
forming region NGC 346. These images and their photometric analysis provide us
with a snapshot of the star formation history of the region. We find evidence
for star formation extending from ~10 Gyr in the past until ~150 Myr in the
field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated
with the NGC 346 cluster. It includes a rich component of low mass pre-main
sequence stars mainly concentrated in a number of sub-clusters, spatially co-
located with CO clumps previously detected by Rubio et al. (2000). Within our
analysis uncertainties, these sub-clusters appear coeval with each other. The
most massive stars appear concentrated in the central sub-clusters, indicating
possible mass segregation. A number of embedded clusters are also observed.
This finding, combined with the overall wealth of dust and gas, could imply
that star formation is still active. An intermediate age star cluster, BS90,
formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of
the SMC has supported star formation with varying levels of intensity over much
of the cosmic time.Comment: 38 pages, 13 figures, 3 tables; AJ accepte
Magnetic resonance imaging in locally advanced rectal cancer : quantitative evaluation of the complete response to neoadjuvant therapy
Purpose: To assess the diagnostic performance of diffusion-weighted imaging (DWI) for the discrimination of complete responder (CR) from the non-complete responder (n-CR) in patients with locally advanced rectal cancer (LARC) undergoing chemotherapy and radiation (CRT). Material and methods: Between December 2009 and January 2014, 32 patients (33 lesions: one patient had two synchronous lesions) were enrolled in this retrospective study. All patients underwent a pre- and post-CRT conventional MRI study completed with DWI. For both data sets (T2-weighted and DWI), the pre- and post-CRT tumour volume (VT2; VDWI) and the tumour volume reduction ratio (ÎV%) were determined as well as pre- and post-CRT apparent diffusion coefficient (ADC) and ADC change (ÎADC%). Histopathological findings were the standard of reference. Receiver operating characteristic (ROC) curves were generated to compare performance of T2-weighted and DWI volumetry, as well as ADC. Results: The area under the ROC curve (AUC) revealed a good accuracy of pre- and post-CRT values of VT2 (0.86; 0.91) and VDWI (0.82; 1.00) as well as those of ÎVT2% (0.84) and ÎVDWI% (1.00) for the CR assessment, with no statistical difference. The AUC of pre- and post-CRT ADC (0.53; 0.54) and that of ÎADC% (0.58) were significantly lower. Conclusions: Both post-CRT VDWI and ÎVDWI% (AUC = 1) are very accurate for the assessment of the CR, in spite of no significant differences in comparison to the conventional post-CRT VT2 (AUC = 0.91) and ÎVT2% (AUC = 0.84). On the contrary, both ADC and ÎADC% values are not reliable
Dynamical and chemical evolution of NGC1569
Blue Compact Dwarf and Dwarf Irregular galaxies are generally believed to be
unevolved objects, due to their blue colors, compact appearance and large gas
fractions. Many of these objects show an ongoing intense burst of star
formation or have experienced it in the recent past. By means of 2-D
hydrodynamical simulations, coupled with detailed chemical yields originating
from SNeII, SNeIa, and intermediate-mass stars, we study the dynamical and
chemical evolution of model galaxies with structural parameters similar to
NGC1569, a prototypical starburst galaxy. A burst of star formation with short
duration is not able to account for the chemical and morphological properties
of this galaxy. The best way to reproduce the chemical composition of this
object is by assuming long-lasting episodes of star formation and a more recent
burst, separated from the previous episodes by a short quiescent period. The
last burst of star formation, in most of the explored cases, does not affect
the chemical composition of the galaxy, since the enriched gas produced by
young stars is in a too hot phase to be detectable with the optical
spectroscopy. Models assuming the infall of a big cloud towards the center of
the galaxy reproduce the chemical composition of the NGC1569, but the pressure
exercised by the cloud hampers the expansion of the galactic wind, at variance
with what observed in NGC1569.Comment: 16 pages, 10 figures, A&A accepte
Synthetic miniprion PrP106.
Elucidation of structure and biological properties of the prion protein scrapie (PrP(Sc)) is fundamental to an understanding of the mechanism of conformational transition of cellular (PrP(C)) into disease-specific isoforms and the pathogenesis of prion diseases. Unfortunately, the insolubility and heterogeneity of PrP(Sc) have limited these studies. The observation that a construct of 106 amino acids (termed PrP106 or miniprion), derived from mouse PrP and containing two deletions (Delta 23-88, Delta 141-176), becomes protease-resistant when expressed in scrapie-infected neuroblastoma cells and sustains prion replication when expressed in PrP(0/0) mice prompted us to generate a corresponding synthetic peptide (sPrP106) to be used for biochemical and cell culture studies. sPrP106 was obtained successfully with a straightforward procedure, which combines classical stepwise solid phase synthesis with a purification strategy based on transient labeling with a lipophilic chromatographic probe. sPrP106 readily adopted a beta-sheet structure, aggregated into branched filamentous structures without ultrastructural and tinctorial properties of amyloid, exhibited a proteinase K-resistant domain spanning residues 134-217, was highly toxic to primary neuronal cultures, and induced a remarkable increase in membrane microviscosity. These features are central properties of PrP(Sc) and make sPrP106 an excellent tool for investigating the molecular basis of the conformational conversion of PrP(C) into PrP(Sc) and prion disease pathogenesis
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