Multiple shell planetary nebulae (MSPNe) are those which, in addition to the main bright inner shell, show one or two additional external shells. These faint external shells often have aspherical geometries, indicating an interaction with the surrounding interstellar medium (ISM). Our aim is to investigate, first of all, if the current theoretical

The High Altitude Observatory of the National Center for Atmospheric Research (HAO/NCAR) in collaboration with the Instituto de Astrofísica de Canarias (IAC) have recently completed a two-station helioseismic network with sites at Mauna Loa, Hawaii and Izana, Tenerife. The instruments are Doppler imagers employing Magneto-Optical filters and are

When the Reynolds number is not small, the wake trailing a buoyant magnetic flux tube sheds vortex rolls therefore producing a Von Karman vortex street and an imbalance of vorticity in the tube which results in a transverse oscillation of the tube as a whole. The actual path followed by the magnetic structure is therefore directly affected by the

The ISO coronal line spectrum of a sample of the brightest known Seyfert galaxies is presented and modeled. ISO observations of [O IV] 25.9 μ, [Ne V] 14.3 μ, [Mg VIII] 3.02 μ and [Si IX] 2.58 μ lines are presented; their relationship with the ionizing spectrum from 50 to 300 eV is investigated. Pure photoionization models reproduce the line ratios

We have developed a generalization of the Zeldovich approximation that is exact in a wide variety of situations, including planar, spherical, and cylindrical symmetries. We have shown that this generalization, which we call the complete Zeldovich approximation (CZA), is exact to second order at an arbitrary point within any field. For Gaussian

The case of the possible 11 yr periodicity in the historical light curve of OJ 287 over a century of observations is examined critically largely independently of possible models that may explain it. Although it is evident that there is at least a quasiperiodicity in the light curve because of the regular spacing of the nine observed outbursts