79 research outputs found
Statistical Study of the Blue Straggler Properties in Galactic Globular Clusters
In this paper we report on the most significant results from a statistical
analysis of the main properties of globular cluster blue straggler stars (BSS)
extracted from the HST snapshot database of photometrically homogeneous CMDs
(Piotto et al. 2002). The BSS relative frequency presents a significant
anticorrelation with the collisional rate and with the cluster total absolute
luminosity.Comment: 2 pages, 1 figure, A.S.P. Conf. Ser., in press in Vol. 296, 200
Blue straggler production in globular clusters
Recent HST observations of a large sample of globular clusters reveal that
every cluster contains between 40 and 400 blue stragglers. The population does
not correlate with either stellar collision rate (as would be expected if all
blue stragglers were formed via collisions) or total mass (as would be expected
if all blue stragglers were formed via the unhindered evolution of a subset of
the stellar population). In this paper, we support the idea that blue
stragglers are made through both channels. The number produced via collisions
tends to increase with cluster mass. In this paper we show how the current
population produced from primordial binaries decreases with increasing cluster
mass; exchange encounters with third, single, stars in the most massive
clusters tend to reduce the fraction of binaries containing a primary close to
the current turn-off mass. Rather their primaries tend to be somewhat more
massive (~1-3 M_sun) and have evolved off the main sequence, filling their
Roche lobes in the past, often converting their secondaries into blue
stragglers (but more than 1 Gyr or so ago and thus they are no longer visible
as blue stragglers). We show that this decline in the primordial blue straggler
population is likely to be offset by the increase in the number of blue
stragglers produced via collisions. The predicted total blue straggler
population is therefore relatively independent of cluster mass, thus matching
the observed population. This result does not depend on any particular assumed
blue straggler lifetime.Comment: 7 pages, 6 figures; MNRAS in pres
The MODEST questions: challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellar
cluster research, including young clusters, globular clusters, and galactic
nuclei. Topics considered include: primordial mass segregation and runaway
mergers, expulsion of gas from clusters, the production of stellar exotica seen
in some clusters (eg blue stragglers and extreme horizontal--branch stars),
binary populations within clusters, the black--hole population within stellar
clusters, the final parsec problem, stellar dynamics around a massive black
hole, and stellar collisions. The Modest Questions posed here are the outcome
of discussions which took place at the Modest-6A workshop held in Lund, Sweden,
in December, 2005. Modest-6A was organised as part of the activities of the
Modest Collaboration (see www.manybody.org for further details)Comment: 24 pages, no figures, accepted for publication in New Astronom
Tracing the Dynamical History of the Globular Cluster 47 Tucanae
We use two stellar populations in the globular cluster 47 Tucanae to trace
its dynamical history: blue stragglers and low mass main sequence stars. We
assumed that the blue stragglers were formed through stellar collisions in all
regions of the cluster. We find that in the core of the cluster, models of
collisional blue stragglers agree well with the observations as long as blue
stragglers are still continuing to form and the mass function in the cluster is
extremely biased towards massive stars (x=-8 where a Salpeter mass function has
x=+1.35). We show that such an extreme mass function is supported by direct
measurements of the luminosity function of main sequence stars in the centre of
the cluster. In the middle region of our dataset (25'' to 130'' from the
cluster centre), blue straggler formation seems to have stopped about half a
Gyr ago. In the outskirts of the cluster, our models are least successful at
reproducing the blue straggler data. Taken at face value, they indicate that
blue straggler formation has been insignificant over the past billion years,
and that a Salpeter mass function applies. However, it is more likely that the
dominant formation mechanism in this part of the cluster is not the collisional
one, and that our models are not appropriate for this region of the cluster. We
conclude that blue stragglers can be used as tracers of dynamics in globular
clusters, despite our incomplete understanding of how and where they were
formed.Comment: Accepted by Ap
Synthesis, computational studies and assessment of in vitro inhibitory activity of umbelliferon-based compounds against tumour-associated carbonic anhydrase isoforms IX and XII
Coumarins are widely diffused secondary metabolites possessing a plethora of biological activities. It has been established that coumarins represent a peculiar class of human carbonic anhydrase (hCA) inhibitors having a distinct mechanism of action involving a non-classical binding with amino acid residues paving the entrance of hCA catalytic site. Herein, we report the synthesis of a small series of new coumarin derivatives 7-11, 15, 17 prepared via classical Pechmann condensation starting from resorcinol derivatives and suitable ÎČ-ketoesters. The evaluation of inhibitory activity revealed that these compounds possessed nanomolar affinity and high selectivity towards tumour-associated hCA IX and XII over cytosolic hCA I and hCA II isoforms. To investigate the binding mode of these new coumarin-inspired inhibitors, the most active compounds 10 and 17 were docked within hCA XII catalytic cleft
Relative Frequencies of Blue Stragglers in Galactic Globular Clusters: Constraints for the Formation Mechanisms
We discuss the main properties of the Galactic globular cluster (GC) blue
straggler stars (BSS), as inferred from our new catalog containing nearly 3000
BSS. The catalog has been extracted from the photometrically homogeneous V vs.
(B-V) color-magnitude diagrams (CMD) of 56 GCs, based on WFPC2 images of their
central cores. In our analysis we used consistent relative distances based on
the same photometry and calibration. The number of BSS has been normalized to
obtain relative frequencies (F_{BSS}) and specific densities (N_S) using
different stellar populations extracted from the CMD. The cluster F_{BSS} is
significantly smaller than the relative frequency of field BSS. We find a
significant anti-correlation between the BSS relative frequency in a cluster
and its total absolute luminosity (mass). There is no statistically significant
trend between the BSS frequency and the expected collision rate. F_{BSS} does
not depend on other cluster parameters, apart from a mild dependence on the
central density. PCC clusters act like normal clusters as far as the BSS
frequency is concerned. We also show that the BSS luminosity function for the
most luminous clusters is significantly different, with a brighter peak and
extending to brighter luminosities than in the less luminous clusters. These
results imply that the efficiency of BSS production mechanisms and their
relative importance vary with the cluster mass.Comment: 12 pages, 3 figures. accepted for publication in ApJ
Clouds, Streams and Bridges: Redrawing the blueprint of the Magellanic System with Gaia DR1
We present the discovery of stellar tidal tails around the Large and the
Small Magellanic Clouds in the Gaia DR1 data. In between the Clouds, their
tidal arms are stretched towards each other to form an almost continuous
stellar bridge. Our analysis relies on the exquisite quality of the Gaia's
photometric catalogue to build detailed star-count maps of the Clouds. We
demonstrate that the Gaia DR1 data can be used to detect variable stars across
the whole sky, and in particular, RR Lyrae stars in and around the LMC and the
SMC. Additionally, we use a combination of Gaia and Gale to follow the
distribution of Young Main Sequence stars in the Magellanic System. Viewed by
Gaia, the Clouds show unmistakable signs of interaction. Around the LMC, clumps
of RR Lyrae are observable as far as ~20 degrees, in agreement with the most
recent map of Mira-like stars reported in Deason et al (2016). The SMC's outer
stellar density contours show a characteristic S-shape, symptomatic of the
on-set of tidal stripping. Beyond several degrees from the center of the dwarf,
the Gaia RR Lyrae stars trace the Cloud's trailing arm, extending towards the
LMC. This stellar tidal tail mapped with RR Lyrae is not aligned with the
gaseous Magellanic Bridge, and is shifted by some ~5 degrees from the Young
Main Sequence bridge. We use the offset between the bridges to put constraints
on the density of the hot gaseous corona of the Milky Way.Comment: Submitted to MNRA
Discovery of New Potential Anti-Infective Compounds Based on Carbonic Anhydrase Inhibitors by Rational Target-Focused Repurposing Approaches
In academia, compound recycling represents an alternative drug discovery strategy to identify new pharmaceutical targets from a library of chemical compounds available in house. Herein we report the application of a rational target-based drug-repurposing approach to find diverse applications for our in-house collection of compounds. The carbonic anhydrase (CA, EC 4.2.1.1) metalloenzyme superfamily was identified as a potential target of our compounds. The combination of a thoroughly validated docking screening protocol, together with in vitro assays against various CA families and isoforms, allowed us to identify two unprecedented chemotypes as CA inhibitors. The identified compounds have the capacity to preferentially bind pathogenic (bacterial/protozoan) CAs over human isoforms and represent excellent hits for further optimization in hit-to-lead campaigns
Close encounters in young stellar clusters: implications for planetary systems in the solar neighbourhood
The stars that populate the solar neighbourhood were formed in stellar
clusters. Through N-body simulations of these clusters, we measure the rate of
close encounters between stars. By monitoring the interaction histories of each
star, we investigate the singleton fraction in the solar neighbourhood. A
singleton is a star which formed as a single star, has never experienced any
close encounters with other stars or binaries, or undergone an exchange
encounter with a binary. We find that, of the stars which formed as single
stars, a significant fraction are not singletons once the clusters have
dispersed. If some of these stars had planetary systems, with properties
similar to those of the solar system, the planets orbits may have been
perturbed by the effects of close encounters with other stars or the effects of
a companion star within a binary. Such perturbations can lead to strong
planet-planet interactions which eject several planets, leaving the remaining
planets on eccentric orbits. Some of the single stars exchange into binaries.
Most of these binaries are broken up via subsequent interactions within the
cluster, but some remain intact beyond the lifetime of the cluster. The
properties of these binaries are similar to those of the observed binary
systems containing extra-solar planets. Thus, dynamical processes in young
stellar clusters will alter significantly any population of solar-system-like
planetary systems. In addition, beginning with a population of planetary
systems exactly resembling the solar system around single stars, dynamical
encounters in young stellar clusters may produce at least some of the
extra-solar planetary systems observed in the solar neighbourhood.Comment: 11 pages, 9 figures, 1 table. Accepted for publication in MNRA
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