Angular diameters, fluxes and extinction of compact planetary nebulae: further evidence for steeper extinction towards the bulge

Abstract

We present values for angular diameter, flux and extinction for 70 Galactic planetary nebulae observed using narrow band filters. Angular diameters are derived using constant emissivity shell and photoionization line emission models. The mean of the results from these two models are presented as our best estimate. Contour plots of 36 fully resolved objects are included and the low intensity contours often reveal an elliptical structure that is not always apparent from FWHM measurements. Flux densities are determined, and for both H-alpha and O[III] there is little evidence of any systematic differences between observed and catalogued values. Observed H-alpha extinction values are determined using observed H-alpha and catalogued radio fluxes. H-alpha extinction values are also derived from catalogued H-alpha and H-beta flux values by means of an Rv dependent extinction law. Rv is then calculated in terms of observed extinction values and catalogued H-alpha and H-beta flux values. Comparing observed and catalogue extinction values for a subset of Bulge objects, observed values tend to be lower than catalogue values calculated with Rv = 3.1. For the same subset we calculate = 2.0, confirming that toward the Bulge interstellar extinction is steeper than Rv = 3.1. For the inner Galaxy a relation with the higher supernova rate is suggested, and that the low-density warm ionized medium is the site of the anomalous extinction. Lowvalues of extinction are also derived using dust models with a turnover radius of 0.08 microns.