By analogy with the minimum-mass solar nebula, we construct a surfacedensity profile using the orbits of the 26 precise-Doppler planets found in multiple-planet systems: Σ = 2200 g cm (a/1 AU), where a is the circumstellar radius and β = 2.0 ± 0.5. The minimum-mass solar nebula (β = 1.5) is consistent with this model, but the uniform-α accretion disk model (β ≈ 1) is not. In a nebula with β < 2...

We present Far Ultraviolet Spectroscopic Explorer (FUSE ) observations of the young, compact planetary nebula (PN) SwSt 1 along the line of sight to its central star HD 167362. We detect circumstellar absorption lines from several species against the continuum of the central star. The physical parameters of the nebula derived from the FUSE data differ significantly from those found from emissio...

Continuing our series of papers on the three-dimensional (3-D) structures of and accurate distances to Planetary Nebulae (PNe), we present our study of the planetary nebula NGC6781. For this object we construct a 3-D photoionization model and, using the constraints provided by observational data from the literature we determine the detailed 3-D structure of the nebula, the physical parameters o...

Continuing our series of papers on the three-dimensional (3-D) structures of and accurate distances to Planetary Nebulae (PNe), we present our study of the planetary nebula NGC6781. For this object we construct a 3-D photoionization model and, using the constraints provided by observational data from the literature we determine the detailed 3-D structure of the nebula, the physical parameters o...

We obtained deep imaging and high-resolution spectroscopy of the planetary nebula NGC 2438. In addition to the bright inner rim and the faint halo-like shell already known from previous imagery, our observations also highlight the existence of an even fainter large halo of nearly spherical shape. Analysis of the surface-brightness profiles and the line splitting indicates a clear ionization str...