The following document lists the file abstract/RRUBIN_PN.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
Recent work has shown that nebular abundances can be in error by as much as a factor of five, possibly due to unexplained temperature fluctuations (t^2). We propose a program to use ISO IR spectroscopy, cospatial optical/UV spectra, and our theoretical photoionization codes, to understand the origin of physical conditions on the microscopic scale within a few planetary nebulae (PNs). This takes advantage of heretofore unobservable IR lines, which are especially useful because of their weak dependence on electron temperature (Te) and extinction. Proposed ISO observations include 7 pairs of electron density (Ne) sensitive line ratios of different ionic species (Ar++, Ar+4, Ne++, Ne+4, O++, S++, and Mg+4) - all having similar terms for the 5-lowest energy levels (^3P_0,1,2;^1D_2;^1S_0). With these data as well as additional Ne-sensitive lines in the optical-UV, we will examine the density and ionization structure of the PNs in greater detail than has been previously possible. Cospatial observations from the ground or available from IUE of the "nebular" (from ^1D_2) and "auroral" (from ^1S_0) lines together with our ISO ground-state Ne-sensitive pair will permit the measurement of Te and t^2. The measurement of t^2 for most of these ions has not been possible previously, and will provide information on how t^2 depend on the level of ionization or excitation of the gas. With improved knowledge of the nebular density, we will be able to construct more realistic photoionization models to compare with the entire set of observational data. Our ultimate goal is to significantly improve the gas-phase abundance estimates for all the heavy elements observed. For refractory elements, we will determine abundances as a function of ionization stage (nebular position). This will allow us to determine the extent of grain destruction within the nebula and the effects on the thermal balance of the gas. All of these goals have as a common underpinning - understanding the conditions within the nebulae and the physical origin of the inferred Te variations, and finding a prescription for determining reliable nebular abundances - fundamental to understanding the chemical evolution of the Galaxy.