Excess of Soft Dielectrons and Photons

Spectra of unbound electron–positron pairs (dielectrons, in brief) and photons from decays of parapositronia produced in ultraperipheral collisions of electrically charged objects are calculated. Their shapes at energies of the NICA collider are demonstrated. Soft dielectrons and photons are abundantly produced. The relevance of these processes to the astrophysical problem of cooling electron–positron pairs and the intense emission of 511 keV photons from the Galactic center is discussed.

MODEST: Modeling Stellar Evolution and (Hydro)dynamics

Simulations of dense stellar systems currently face two major hurdles, one astrophysical and one computational. The astrophysical problem lies in the fact that several major stages in binary evolution, such as common envelope evolution, are still poorly understood. The best we can do in these cases is to parameterize our ignorance, in a way that is reminiscent of the introduction of a mixing length to describe convection in a single star, or an alpha parameter in modeling an accretion disk. The hope is that by modeling a whole star cluster in great detail, and comparing the results to the wealth of observational data currently available, we will be able to constrain the parameters that capture the unknown physics. The computational problem is one of composition: while we have accurate computer codes for modeling stellar dynamics, stellar hydrodynamics, and stellar evolution, we currently have no good way to put all this knowledge together in a single software environment. A year ago, a loosely-knit organization was founded to address these problems, MODEST for Modeling DEnse STellar systems, with nine working groups and a series of meetings that are held every half year. This report reviews the first year of this initiative. Much more detail can be found on the MODEST web site http://www.manybody.org/modest.html.

Revealing the Physical Changes Taking Place in Z-Track Motion in Low Mass X-Ray Binaries

The explanation of the strong physical changes clearly taking place in the Z-track class of Low Mass X-ray Binaries has so far not been obtained, and this remains a significant astrophysical problem, without which we cannot claim to understand accretion in LMXB. We have for the first time applied the Birmingham emission model (2,3) to this problem to attempt to obtain a solution from the spectral evolution along the Z-track in the source GX 340+0 observed with Rossi- ХTE. In this model, X-ray emission consists of blackbody from the neutron star, plus Comptonized emission from an extended ADC.

Rotation Curves of 38 Galaxies in Clusters from Scanning Fabry-Perot observations

A major astrophysical problem is related to the fact that rotation curves (RCs)) of galaxies are flat. The presence of a dark halo is most often invoked to explain that. Some controversies exist concerning the existence of a dark halo for galaxies in very hostile environments like in center of clusters. Using a scanning Fabry-Perot interferometer, we have observed Hα velocity fields of a sample of 38 galaxies located in 7 different clusters of galaxies. From this sample, our conclusion is that spirals located in the central part of clusters do not have decreasing RCs within the optical radius.

Wide Field Star Count Mapping of Large Scale Areas of Star Formation in the Large Magellanic Cloud

Star formation in galaxies is a major astrophysical problem which can be investigated in several ways. The distribution and loci of all kinds of young objects, including OB associations, young clusters, HII regions, GMCs, Bok globules, dark clouds, dust lanes, protostars, as well as YSOs detected in NIR and FIR surveys constitute the principal signposts for this investigation. The individual nature of all these objects has been and is still continously studied. However it is also extremely interesting to associate the coexistence of these objects, and their relation to the structure of the parent galaxy. Such studies have been carried out by several investigators and are frequently summarized when star formation processes are examined.