2012: April March

April 2012

Abstract: Stereology allows shifting from the 3D distribution of the volumes of Poissonian Voronoi Diagrams to their 2D cross-sections. The basic assumption is that the 3D statistics of the volumes of the voids in the local Universe has a distribution function of the gamma-type. The standard rule of conversion from 3D volumes to 2D circles, adopting the standard rules of stereology, produces a new probability density function of the radii which contains the Meijer G-function. A non-Poissonian distribution of volumes is also considered. The distribution of the 3D radii of the Sloan Digital Sky Survey Data Release 7 is best fitted by a non-Poissonian distribution in volumes as given by the Kiang function with argument of about two.

Abstract: Ultracompact HII regions are in a regime of approximate pressure balance with the surrounding, dense molecular clouds. In this paper, we apply a newly developed “thick shell” formalism to the case of an expanding HII region energized by both the photoionizing radiation and the wind from the central stellar source. The resulting model leads to analytic and quasi-analytic solutions that have a transition to a regime (absent in the classical “wind-driven bubble” analytic solutions) in which the hot, stellar wind bubble expands quasi-statically, in approximate pressure equilibrium with the surrounding ISM. This regime is relevant for the observed ultracompact HII regions. We present the time-evolution of the radius and the thickness of the expanding HII shell for different values of a single, dimensionless parameter that determines the characteristics of the solution.

March 2012

Abstract: Oxygen abundances have been determined for a total of 15 late-type spiral galaxies. The intensities of the emission lines were determined from the spectra retrieved from the SDSS data-base. Only three galaxies have abundances previously reported in the literature. For four of them the forbidden oxygen line was detected in the spectra but the T_e determined for three of them is larger than 20,000 K. The chemical abundances for the other galaxies were determined with the semi-empirical methods. The values indicate that, in general, the oxygen abundance of dS galaxies is smaller than for Sm ones.

Abstract: We present speckle interferometric measurements of binary stars performed during September and October of 2009 with the 2.1 m telescope of the Observatorio Astronómico Nacional at SPM (Mexico). We report here the results of 200 measurements of 196 pairs with a primary limiting magnitude of V=12.3. The measured angular separations range from 0”.115 to 5”.26. Ninety seven pairs have separations less than 1”. The mean error in separation is 0”.03 and 1^∘.5 in position angle. The usual 180^∘ ambiguity was corrected for a majority of position angles by comparison with observations performed by other observers.

Abstract: We present a set of optical and infrared images combined with long-slit, medium and high dispersion spectra of the southern planetary nebula (PN) NGC 5189. The complex morphology of this PN is puzzling and has not been studied in detailed so far. Our investigation reveals the presence of a new dense and cold infrared torus (alongside the optical one) which probably generated one of the two optically seen bipolar outflows and which might be responsible for the twisted appearance of the optical torus via an interaction process. The high-resolution MES-AAT spectra clearly show the presence of filamentary and knotty structures as well as three expanding bubbles. Our findings therefore suggest that NGC 5189 is a quadrupolar nebula with multiple sets of symmetrical condensations in which the interaction of outflows has determined its complex morphology.