118 research outputs found
Assessment of Stellar Stratification in Three Young Star Clusters in the Large Magellanic Cloud
(abridged) We present a comprehensive study of stellar stratification in
young star clusters in the Large Magellanic Cloud (LMC). We apply our recently
developed effective radius method for the assessment of stellar stratification
on imaging data obtained with the Advanced Camera for Surveys of three young
LMC clusters to characterize the phenomenon and develop a comparative scheme
for its assessment in such clusters. The clusters of our sample, NGC 1983, NGC
2002 and NGC 2010, are selected on the basis of their youthfulness, and their
variety in appearance, structure, stellar content, and surrounding stellar
ambient. Our photometry is complete for magnitudes down to m_814 ~ 23 mag,
allowing the calculation of the structural parameters of the clusters, the
estimation of their ages and the determination of their stellar content. Our
study shows that each cluster in our sample demonstrates stellar stratification
in a quite different manner and at different degree from the others.
Specifically, NGC 1983 shows to be partially segregated only for the faintest
stars of the cluster, NGC 2002 shows evidence of strong stellar stratification
for both bright and faint stars, and NGC 2010 is found not to be segregated.
For the parametrization of the phenomenon of stellar stratification and its
quantitative comparison among these clusters, we propose the slope derived from
the change in the effective radius over the corresponding magnitude range as
indicative parameter of the degree of stratification in the clusters. A
positive value of this slope indicates mass segregation in the cluster, while a
negative or zero value signifies the lack of the phenomenon.Comment: To appear in the Astrophysical Journal Vol. 709 (2010), pp. 263-277
Version with low-Resolution gray-scaled figures. Version with full resolution
color figures available from
http://rapidshare.com/files/328406139/Gouliermis_2010.ApJ.709.pd
The Extended Main-Sequence Turn-off Clusters of the Large Magellanic Cloud - Missing links in Globular Cluster Evolution
Recent observations of intermediate age (1 - 3 Gyr) massive star clusters in
the Large Magellanic Cloud (LMC) have revealed that the majority possess
bifurcated or extended main-sequence turn-off (EMSTO) morphologies. This effect
can be understood to arise from subsequent star formation amongst the stellar
population with age differences between constituent stars amounting to 50 - 300
Myr. Age spreads of this order are similarly invoked to explain the light
element abundance variations witnessed in ancient globular clusters. In this
paper we explore the proposition that the clusters exhibiting the EMSTO
phenomenon are a general phase in the evolution of massive clusters, one that
naturally leads to the particular chemical properties of the ancient globular
cluster population.
We show that the isolation of EMSTO clusters to intermediate ages is the
consequence of observational selection effects. In our proposed scenario, the
EMSTO phenomenon is identical to that which establishes the light element
abundance variations that are ubiquitous in the ancient globular cluster
population. Our scenario makes a strong prediction: EMSTO clusters will exhibit
abundance variations in the light elements characteristic of the ancient GC
population.Comment: ApJ accepted. 33 pages, 5 figure
The merger history, AGN and dwarf galaxies of Hickson Compact Group 59
Compact group galaxies often appear unaffected by their unusually dense
environment. Closer examination can, however, reveal the subtle, cumulative
effects of multiple galaxy interactions. Hickson Compact Group (HCG) 59 is an
excellent example of this situation. We present a photometric study of this
group in the optical (HST), infrared (Spitzer) and X-ray (Chandra) regimes
aimed at characterizing the star formation and nuclear activity in its
constituent galaxies and intra-group medium. We associate five dwarf galaxies
with the group and update the velocity dispersion, leading to an increase in
the dynamical mass of the group of up to a factor of 10 (to 2.8e13 Msun), and a
subsequent revision of its evolutionary stage. Star formation is proceeding at
a level consistent with the morphological types of the four main galaxies, of
which two are star-forming and the other two quiescent. Unlike in some other
compact groups, star-forming complexes across HCG 59 closely follow mass-radius
scaling relations typical of nearby galaxies. In contrast, the ancient globular
cluster populations in galaxies HCG 59A and B show intriguing irregularities,
and two extragalactic HII regions are found just west of B. We age-date a faint
stellar stream in the intra-group medium at ~1 Gyr to examine recent
interactions. We detect a likely low-luminosity AGN in HCG 59A by its ~10e40
erg/s X-ray emission; the active nucleus rather than star formation can account
for the UV+IR SED. We discuss the implications of our findings in the context
of galaxy evolution in dense environments.Comment: 38 pages, 17 figures. Please visit "http://tinyurl.com/isk-hcg59" for
a full-resolution PDF. Accepted for publication in the Astrophysical Journa
Patterns of natural selection acting on the mitochondrial genome of a locally adapted fish species
Elucidation of the ATP7B N-Domain Mg2+-ATP Coordination Site and Its Allosteric Regulation
The diagnostic of orphan genetic disease is often a puzzling task as less attention is paid to the elucidation of the pathophysiology of these rare disorders at the molecular level. We present here a multidisciplinary approach using molecular modeling tools and surface plasmonic resonance to study the function of the ATP7B protein, which is impaired in the Wilson disease. Experimentally validated in silico models allow the elucidation in the Nucleotide binding domain (N-domain) of the Mg2+-ATP coordination site and answer to the controversial role of the Mg2+ ion in the nucleotide binding process. The analysis of protein motions revealed a substantial effect on a long flexible loop branched to the N-domain protein core. We demonstrated the capacity of the loop to disrupt the interaction between Mg2+-ATP complex and the N-domain and propose a role for this loop in the allosteric regulation of the nucleotide binding process
Positive Darwinian Selection in the Piston That Powers Proton Pumps in Complex I of the Mitochondria of Pacific Salmon
The mechanism of oxidative phosphorylation is well understood, but evolution of the proteins involved is not. We combined phylogenetic, genomic, and structural biology analyses to examine the evolution of twelve mitochondrial encoded proteins of closely related, yet phenotypically diverse, Pacific salmon. Two separate analyses identified the same seven positively selected sites in ND5. A strong signal was also detected at three sites of ND2. An energetic coupling analysis revealed several structures in the ND5 protein that may have co-evolved with the selected sites. These data implicate Complex I, specifically the piston arm of ND5 where it connects the proton pumps, as important in the evolution of Pacific salmon. Lastly, the lineage to Chinook experienced rapid evolution at the piston arm
Filament Depolymerization Can Explain Chromosome Pulling during Bacterial Mitosis
Chromosome segregation is fundamental to all cells, but the force-generating mechanisms underlying chromosome translocation in bacteria remain mysterious. Caulobacter crescentus utilizes a depolymerization-driven process in which a ParA protein structure elongates from the new cell pole, binds to a ParB-decorated chromosome, and then retracts via disassembly, pulling the chromosome across the cell. This poses the question of how a depolymerizing structure can robustly pull the chromosome that disassembles it. We perform Brownian dynamics simulations with a simple, physically consistent model of the ParABS system. The simulations suggest that the mechanism of translocation is βself-diffusiophoreticβ: by disassembling ParA, ParB generates a ParA concentration gradient so that the ParA concentration is higher in front of the chromosome than behind it. Since the chromosome is attracted to ParA via ParB, it moves up the ParA gradient and across the cell. We find that translocation is most robust when ParB binds side-on to ParA filaments. In this case, robust translocation occurs over a wide parameter range and is controlled by a single dimensionless quantity: the product of the rate of ParA disassembly and a characteristic relaxation time of the chromosome. This time scale measures the time it takes for the chromosome to recover its average shape after it is has been pulled. Our results suggest explanations for observed phenomena such as segregation failure, filament-length-dependent translocation velocity, and chromosomal compaction
Divergence across mitochondrial genomes of sympatric members of the Schistosoma indicum group and clues into the evolution of Schistosoma spindale
Schistosoma spindale and Schistosoma indicum are ruminant-infecting trematodes of the Schistosoma indicum group that are widespread across Southeast Asia. Though neglected, these parasites can cause major pathology and mortality to livestock leading to significant welfare and socio-economic issues, predominantly amongst poor subsistence farmers and their families. Here we used mitogenomic analysis to determine the relationships between these two sympatric species of schistosome and to characterise S. spindale diversity in order to identify possible cryptic speciation. The mitochondrial genomes of S. spindale and S. indicum were assembled and genetic analyses revealed high levels of diversity within the S. indicum group. Evidence of functional changes in mitochondrial genes indicated adaptation to environmental change associated with speciation events in S. spindale around 2.5 million years ago. We discuss our results in terms of their theoretical and applied implications
Region-Specific Expression of Mitochondrial Complex I Genes during Murine Brain Development
Mutations in the nuclear encoded subunits of mitochondrial complex I (NADH:ubiquinone oxidoreductase) may cause circumscribed cerebral lesions ranging from degeneration of the striatal and brainstem gray matter (Leigh syndrome) to leukodystrophy. We hypothesized that such pattern of regional pathology might be due to local differences in the dependence on complex I function. Using in situ hybridization we investigated the relative expression of 33 nuclear encoded complex I subunits in different brain regions of the mouse at E11.5, E17.5, P1, P11, P28 and adult (12 weeks). With respect to timing and relative intensity of complex I gene expression we found a highly variant pattern in different regions during development. High average expression levels were detected in periods of intense neurogenesis. In cerebellar Purkinje and in hippocampal CA1/CA3 pyramidal neurons we found a second even higher peak during the period of synaptogenesis and maturation. The extraordinary dependence of these structures on complex I gene expression during synaptogenesis is in accord with our recent findings that gamma oscillations β known to be associated with higher cognitive functions of the mammalian brain β strongly depend on the complex I activity. However, with the exception of the mesencephalon, we detected only average complex I expression levels in the striatum and basal ganglia, which does not explain the exquisite vulnerability of these structures in mitochondrial disorders
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