A Search for Temperature and Density Variations in NGC 7027 with the Hubble Space Telescope

Abstract

We observed the young planetary nebula NGC 7027 with the HST Imaging Spectrograph (STIS) in long-slit mode at five slit positions along the major axis of the nebula, over the wavelength range from 3000 to ~10,000 Å. We used dereddened line ratios to determine line-of-sight average values for (1) He++ and He+ ionization fractions from He II λ4686 and He I λ5876; (2) electron density from both [S II] (λ6716/λ6731) and [Ar IV] (λ4711/λ4740); and (3) temperature from [O III] (λ4959 + λ5007)/λ4363. The He++/H+ ratio varies across the nebula, with values up to 0.06 over the central parts, falling off to 0.03 at offsets of 5''-6'' from the central star. The line-of-sight mean densities vary by factors of ~5 on angular scales as small as ~1''. The average electron temperatures are generally consistent with a constant langTerang = 13,800 ± 2000 K within the uncertainties, except for ~2% of measured pixels where Te > 18,000 K. The variations of density along the slits are not obviously correlated with variations in extinction. The ionization structure of the nebula varies on both large and small scales. [O I] λ6300, Hα, and He II λ4686 intensity profiles along each slit appear to trace ionization gradients at Hα peaks, with the highest ionization on the side toward the central star and lowest ionization on the far side. These structures may result from effects of stellar radiation, shocks from a fast stellar wind, or some combination of the two processes.