49 research outputs found
Progenitors of GRB originated in the dense star cluster
The evidences are given for the origin of the GRB progenitors from the ready
stellar remnants in result of the dynamical interaction in the dense star
clusters. In this way the close binaries of compact objects may form and then
merge, often after ejecting from the parent cluster. The GRB may trigger star
formation in the gaseous partiall sphere and occurence of the GRB afterglows
near the star formation regions may be connected with the successive GRB
origins in the same parent cluster. The observed distributions of GRB
afterglows in Z and in distance to the galactic centers are compatible with the
origin of many GRBs in the (present day classical) globular clusters, which
were younger than 2 Gyr at the time of many GRBs events.Comment: Paper presented to the Second Roma Workshop on GRB in the afterglow
era, October 200
A Young Globular Cluster in the Galaxy NGC 6946
A globular cluster ~15 My old that contains 5x10^5 Msun of stars inside an 11
pc radius has been found in the nearby spiral galaxy NGC 6946, surrounded by
clouds of dust and smaller young clusters inside a giant circular bubble 300 pc
in radius. At the edge of the bubble is an arc of regularly-spaced clusters
that could have been triggered during the bubble's formation. The region is at
the end of a spiral arm, suggesting an origin by the asymmetric collapse of
spiral arm gas. The globular is one of the nearest examples of a cluster that
is similar to the massive old globulars in the Milky Way. We consider the
energetics of the bubble and possible formation mechanisms for the globular
cluster, including the coalescence of smaller clusters.Comment: 20 pages, 7 figures, accepted for Astrophysical Journal Vol 535, June
1 200
Snake Cytotoxins Bind to Membranes via Interactions with Phosphatidylserine Head Groups of Lipids
The major representatives of Elapidae snake venom, cytotoxins (CTs), share similar three-fingered fold and exert diverse range of biological activities against various cell types. CT-induced cell death starts from the membrane recognition process, whose molecular details remain unclear. It is known, however, that the presence of anionic lipids in cell membranes is one of the important factors determining CT-membrane binding. In this work, we therefore investigated specific interactions between one of the most abundant of such lipids, phosphatidylserine (PS), and CT 4 of Naja kaouthia using a combined, experimental and modeling, approach. It was shown that incorporation of PS into zwitterionic liposomes greatly increased the membrane-damaging activity of CT 4 measured by the release of the liposome-entrapped calcein fluorescent dye. The CT-induced leakage rate depends on the PS concentration with a maximum at approximately 20% PS. Interestingly, the effects observed for PS were much more pronounced than those measured for another anionic lipid, sulfatide. To delineate the potential PS binding sites on CT 4 and estimate their relative affinities, a series of computer simulations was performed for the systems containing the head group of PS and different spatial models of CT 4 in aqueous solution and in an implicit membrane. This was done using an original hybrid computational protocol implementing docking, Monte Carlo and molecular dynamics simulations. As a result, at least three putative PS-binding sites with different affinities to PS molecule were delineated. Being located in different parts of the CT molecule, these anion-binding sites can potentially facilitate and modulate the multi-step process of the toxin insertion into lipid bilayers. This feature together with the diverse binding affinities of the sites to a wide variety of anionic targets on the membrane surface appears to be functionally meaningful and may adjust CT action against different types of cells
Hubble Space Telescope imaging of a peculiar stellar complex in NGC 6946
The stellar populations in a stellar complex in NGC 6946 are analyzed on
images taken with the Wide Field Planetary Camera 2 on board the Hubble Space
Telescope. The complex is peculiar by its very high density of stars and
clusters and semicircular shape. Its physical dimensions are about the same as
for the local Gould Belt, but the stellar density is 1 - 2 orders of magnitude
higher. In addition to an extremely luminous, 15 Myr old cluster discussed in
an earlier paper, accounting for about 17% of the integrated V-band light, we
identify 18 stellar clusters within the complex with luminosities similar to
the brightest open clusters in the Milky Way. The color-magnitude diagram of
individual stars in the complex shows a paucity of red supergiants compared to
model predictions in the 10-20 Myr age range for a uniform star formation rate.
We thus find tentative evidence for a gap in the dispersed star formation
history, with a concentration of star formation into a young globular cluster
during this gap. Confirmation of this result must, however, await a better
understanding of the late evolution of stars in the corresponding mass range (>
12 Msun). A reddening map based on individual reddenings for 373 early-type
stars is presented, showing significant variations in the absorption across the
complex. These may be responsible for some of the arc-like structures
previously identified on ground-based images. We finally discuss various
formation scenarios for the complex and the star clusters within it.Comment: 51 pages, including 19 figures and 4 tables. Accepted for publication
in Ap