118 research outputs found
A population of binaries in the Asymptotic Giant Branch of 47 Tucanae?
We have used a set of archived Hubble Space Telescope/ACS images to probe the
evolved populations of the globular cluster 47 Tucanae. We find an excess of
Asymptotic Giant Branch (AGB) stars in the cluster core. We interpret this
feature as the signature of an extra-population likely made by the progeny of
massive stars originated by the evolution of binary systems. Indeed the
comparison with theoretical tracks suggests that the AGB population of 47 Tuc
can be significantly contaminated by more massive stars currently experiencing
the first ascending Red Giant Branch.Comment: 14 pages, 4 figures, in press on ApJ Letter
A complete census of emitters in NGC 6397
We used a dataset of archival Hubble Space Telescope images obtained through
the F555W, F814W and F656N filters, to perform a complete search for objects
showing emission in the globular cluster NGC 6397. As photometric
diagnostic, we used the color excess in the
- color-color diagram. In the analysed field of view,
we identified 53 emitters. In particular, we confirmed the optical
counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars
and 11 active binaries) and identified 33 previously unknown sources, thus
significantly enlarging the population of known active binaries in this
cluster. We report the main characteristics for each class of objects.
Photometric estimates of the equivalent width of the emission line,
were derived from the -excess and, for the first time, compared
to the spectroscopic measurements obtained from the analysis of MUSE spectra.
The very good agreement between the spectroscopic and photometric measures
fully confirmed the reliability of the proposed approach to measure the
emission. The search demonstrated the efficiency of this novel
approach to pinpoint and measure -emitters, thus offering a powerful
tool to conduct complete census of objects whose formation and evolution can be
strongly affected by dynamical interactions in star clusters.Comment: Accepted for publication by ApJ; 14 pages, 8 Figures, 1 Tabl
Deep multi-telescope photometry of NGC 5466. II. The radial behaviour of the mass function slope
We use a combination of data acquired with the Advanced Camera for Survey
(ACS) on board the Hubble Space Telescope and the Large Binocular Camera
(LBC-blue) mounted on the Large Binocular Telescope, to sample the main
sequence stars of the globular cluster NGC~5466 in the mass range
. We derive the cluster's Luminosity Function in several
radial regions, from the center of the cluster out to the tidal radius. After
corrections for incompleteness and field-contamination, this has been compared
to theoretical Luminosity Functions, obtained by multiplying a simple power law
Mass Function in the form dN/dm by the derivative of the
mass-luminosity relationship of the best-fit isochrone. We find that
varies from -0.6 in the core region to -1.9 in the outer region. This fact
allows us to observationally prove that the stars in NGC 5466 have experienced
the effects of mass segregation. We compare the radial variation of
from the center out to 5 core radii (r) in NGC 5466 and the globular
cluster M10, finding that the gradient of in the first 5r is more
than a factor of 2 shallower in NGC 5466 than in M10, in line with the
differences in the clusters' relaxation timescales. NGC 5466 is dynamically
younger than M10, with two-body relaxation processes only recently starting to
shape the distribution of main sequence stars. This result fully agrees with
the conclusion obtained in our previous works on the radial distribution of
Blue Straggler Stars, further confirming that this can be used as an efficient
clock to measure the dynamical age of stellar systems.Comment: Accepted for publications on Ap
Modelling the Observed Stellar Mass Function and its Radial Variation in Galactic Globular Clusters
We measure how the slope of the stellar mass function (MF) changes
as a function of clustercentric distance in five Galactic globular clusters
and compare to predictions from direct -body star cluster
simulations. Theoretical studies predict that (which traces the
degree of mass segregation in a cluster) should steepen with time as a cluster
undergoes two-body relaxation and that the amount by which the global MF can
evolve from its initial state due to stellar escape is directly linked to
. We find that the amount of mass segregation in M10, NGC 6218, and
NGC 6981 is consistent with their dynamical ages, but only the global MF of M10
is consistent with its degree of mass segregation as well. NGC 5466 and NGC
6101 on the other hand appear to be less segregated than their dynamical ages
would indicate. Furthermore, despite the fact that the escape rate of stars in
non-segregated clusters is independent of stellar mass, both NGC 5466 and NGC
6101 have near-flat MFs. We discuss various mechanisms which could produce
non-segregated clusters with near-flat MFs, including higher mass-loss rates
and black hole retention, but argue that for some clusters (NGC 5466 and NGC
6101) explaining the present-day properties might require either a
non-universal IMF or a much more complex dynamical history.Comment: 12 pages, 9 figures, Accepted for publication in MNRA
The central Blue Straggler population in four outer-halo globular clusters
Using HST/WFPC2 data, we have performed a comparative study of the Blue
Straggler Star (BSS) populations in the central regions of the globular
clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC >
50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC
2419) the most distant clusters in the Halo. We determine their
colour-magnitude diagrams and centres of gravity. The four clusters turn out to
have similar ages (10.5-11 Gyr), significantly smaller than those of the
inner-Halo globulars, and similar metallicities. By exploiting wide field
ground based data, we build the most extended radial density profiles from
resolved star counts ever published for these systems. These are well
reproduced by isotropic King models of relatively low concentration. BSSs
appear to be significantly more centrally segregated than red giants in all
globular clusters, in agreement with the estimated core and half-mass
relaxation times which are smaller than the cluster ages. Assuming that this is
a signature of mass segregation, we conclude that AM 1 and Eridanus are
slightly dynamically more evolved than Pal 3 and Pal 4.Comment: Accepted for publication in ApJ, references update
UV Properties of Galactic Globular Clusters with GALEX I. The Color-Magnitude Diagrams
We present GALEX data for 44 Galactic globular clusters obtained during 3
GALEX observing cycles between 2004 and 2008. This is the largest homogeneous
data set on the UV photometric properties of Galactic globular clusters ever
collected. The sample selection and photometric analysis are discussed, and
color-magnitude diagrams are presented. The blue and intermediate-blue
horizontal branch is the dominant feature of the UV color-magnitude diagrams of
old Galactic globular clusters. Our sample is large enough to display the
remarkable variety of horizontal branch shapes found in old stellar
populations. Other stellar types that are obviously detected are blue
stragglers and post core-He burning stars. The main features of UV
color-magnitude diagrams of Galactic globular clusters are briefly discussed.
We establish the locus of post-core He burning stars in the UV color-magnitude
diagram and present a catalog of candidate AGB-manqu \'e, post early-AGB, and
post-AGB stars within our cluster sample.Comment: Accepted for publication by The Astronomical Journal. 46 pages,
including 21 Figures and 3 tables. All data will be made publicly available
by the time the article is published. In the meantime, please contact the
authors for data requests. Revised version fixed error with figure numbers
and caption
Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process
In this paper we report on the discovery of a double blue straggler star
(BSS) sequence in the core of the core-collapsed cluster M15 (NGC 7078). We
performed a detailed photometric analysis of the extremely dense core of the
cluster using a set of images secured with the Advanced Camera for Survey in
the High Resolution Channel mode on-board the Hubble Space Telescope. The
proper combination of the large number of single frames in the near-UV (F220W),
and blue (F435W) filters allowed us to perform a superb modeling of the Point
Spread Function and an accurate deblending procedure. The Color-Magnitude
diagram revealed the presence of two distinct parallel sequences of blue
stragglers. In particular, the blue BSS sequence is characterized by the
intriguing presence of two different branches. The first branch appears
extremely narrow, it extends up to 2.5 magnitudes brighter than the cluster
main-sequence turnoff (MS-TO) point, and it is nicely reproduced by a 2 Gyr-old
collisional isochrone. The second branch extends up to 1.5 magnitudes from the
MS-TO and it is reproduced by a 5.5 Gyr-old collisional isochrone. Our
observations suggest that each of these branches is mainly constituted by a
population of nearly coeval collisional BSS of different masses generated
during two episodes of high collisional activity. We discuss the possibility
that the oldest episode corresponds to the core-collapse event (occurred about
5.5 Gyr ago), while the most recent one (occurred about 2 Gyr ago) is
associated with a core oscillation in the post-core collapse evolution. The
discovery of these features provides further strong evidence in support of the
connection between the BSS properties and GC dynamical evolution, and it opens
new perspectives on the study of core-collapse and post core-collapse
evolution.Comment: Accepted for publication on Ap
Three new bricks in the wall: Berkeley 23, Berkeley 31, and King 8
A comprehensive census of Galactic open cluster properties places unique
constraints on the Galactic disc structure and evolution. In this framework we
investigate the evolutionary status of three poorly-studied open clusters,
Berkeley 31, Berkeley 23 and King 8, all located in the Galactic anti-centre
direction. To this aim, we make use of deep LBT observations, reaching more
than 6 mag below the main sequence Turn- Off. To determine the cluster
parameters, namely age, metallicity, distance, reddening and binary fraction,
we compare the observational colour-magnitude diagrams (CMDs) with a library of
synthetic CMDs generated with different evolutionary sets (Padova, FRANEC and
FST) and metallicities. We find that Berkeley 31 is relatively old, with an age
between 2.3 and 2.9 Gyr, and rather high above the Galactic plane, at about 700
pc. Berkeley 23 and King 8 are younger, with best fitting ages in the range
1.1-1.3 Gyr and 0.8-1.3 Gyr, respectively. The position above the Galactic
plane is about 500- 600 pc for the former, and 200 pc for the latter. Although
a spectroscopic confirmation is needed, our analysis suggests a sub-solar
metallicity for all three clusters.Comment: 17 Pages, Accepted for publication in MNRA
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Angiogenic potential of human dental pulp stromal (stem) cells.
Dental pulp is a heterogeneous microenviroment where unipotent progenitor and pluripotent mesenchymal stem cells cohabit. In this study we investigated whether human Dental Pulp Stromal (Stem) Cells (DP-SCs) committed to the angiogenic fate. DP-SCs showed the specific mesenchymal immunophenotypical profile positive for CD29, CD44, CD73, CD105, CD166 and negative for CD14, CD34, CD45, in accordance with that reported for bone marrow-derived SCs. The Oct-4 expression in DP-SCs, evaluated through RT-PCR analysis, increased in relation with the number of the passages in cell culture and decreased after angiogenic induction. In agreement with their multipotency, DP-SCs differentiated toward osteogenic and adipogenic commitments. In angiogenic experiments, differentiation of DP-SCs, through Vascular Endothelial Growth Factor (VEGF) induction, was evaluated by in vitro matrigel assay and by cytometric analysis. Accordingly, endothelial-specific markers like Flt-1 and KDR were basally expressed and they increased after exposure to VEGF together with the occurrence of ICAM-1 and von Willebrand Factor positive cells. In addition, VEGF-induced DP-SCs maintained endothelial cell-like features when cultured in a 3-D fibrin mesh, displaying focal organization into capillary-like structures. The DP-SC angiogenic potential may prove a remarkable tool for novel approaches to developing tissue-engineered vascular grafts which are useful when vascularization of ischemic tissues is required
Term amniotic membrane is a high throughput source for multipotent mesenchymal stem cells with the ability to differentiate into endothelial cells in vitro
BACKGROUND: Term Amniotic membrane (AM) is a very attractive source of Mesenchymal Stem Cells (MSCs) due to the fact that this fetal tissue is usually discarded without ethical conflicts, leading to high efficiency in MSC recovery with no intrusive procedures. Here we confirmed that term AM, as previously reported in the literature, is an abundant source of hMSCs; in particular we further investigated the AM differentiation potential by assessing whether these cells may also be committed to the angiogenic fate. In agreement with the recommendation of the International Society for Cellular Therapy, the mesenchymal cells herein investigated were named Amniotic Membrane-human Mesenchymal Stromal Cells (AM-hMSC). RESULTS: The recovery of hMSCs and their in vitro expansion potential were greater in amniotic membrane than in bone marrow stroma. At flow cytometry analysis AM-hMSCs showed an immunophenotypical profile, i.e., positive for CD105, CD73, CD29, CD44, CD166 and negative for CD14, CD34, CD45, consistent with that reported for bone marrow-derived MSCs. In addition, amniotic membrane-isolated cells underwent in vitro osteogenic (von Kossa stain), adipogenic (Oil Red-O stain), chondrogenic (collagen type II immunohistochemichal detection) and myogenic (RT-PCR MyoD and Myogenin expression as well as desmin immunohistochemical detection) differentiation. In angiogenic experiments, a spontaneous differentiation into endothelial cells was detected by in vitro matrigel assay and this behaviour has been enhanced through Vascular Endothelial Growth Factor (VEGF) induction. According to these findings, VEGF receptor 1 and 2 (FLT-1 and KDR) were basally expressed in AM-hMSCs and the expression of endothelial-specific markers like FLT-1 KDR, ICAM-1 increased after exposure to VEGF together with the occurrence of CD34 and von Willebrand Factor positive cells. CONCLUSION: The current study suggests that AM-hMSCs may emerge as a remarkable tool for the cell therapy of multiple diseased tissues. AM-hMSCs may potentially assist both bone and cartilage repair, nevertheless, due to their angiogenic potential, they may also pave the way for novel approaches in the development of tissue-engineered vascular grafts which are useful when vascularization of ischemic tissues is required
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