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[39.02] Inhomogeneities in the Gas and Dust of the Planetary Nebula SwSt 1

N. C. Sterling, H. L. Dinerstein (University of Texas at Austin), C. W. Bowers (GSFC-NASA), S. Redfield (University of Texas at Austin)

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of circumstellar absorption lines against the continuum of the central star of the planetary nebula (PN) SwSt 1. We find that the physical characteristics of the nebular material in the line of sight differ significantly from those of the emitting regions. In particular, we find the electron density derived from excited fine-structure levels of S III (n_e=8800⁺⁴⁸⁰⁰_-2400 cm^-3) to be lower by more than a factor of 3 of all estimates based on emission lines. Although SwSt 1 is a source of near-infrared H₂ emission, we do not detect H₂ resonance absorption lines in its UV spectrum. The measured upper limit of N(H₂)<10¹⁵ cm^-2 is \textit{at least} four orders of magnitude smaller than that estimated from emission lines. We also find the iron abundance in the sight line to be [Fe/S] = -0.35±.12, which indicates that Fe is not strongly depleted into dust. This value can be compared to the Fe abundance found from optical emission lines ([Fe/S]=-1.15±.33), derived using the same ions as the UV measurement. The marked contrast between the absorption and emission line results indicates that SwSt 1 has an inhomogeneous structure. In particular, the line of sight to the central star traces a low-density region that has a very small amount of molecular material and dust compared to the emitting regions of the nebula. Due to the small angular size of SwSt 1, we are not able to discern whether the inhomogeneities are due to a global asymmetry or small-scale clumping. It is likely that molecular material in the line of sight has been photodissociated, since the low density of this gas provides poor shielding from energetic stellar photons. Although the reason for the observed variations in the dust-to-gas ratio is not clear, we discuss a number of possible explanations, including shock and radiative dust destruction, as well as variations in the dust formation efficiency of the AGB outflow.

This work has been supported by NASA grants NAG 5-11597 and NAG 5-13724, and NSF grant 97-131156.

The author(s) of this abstract have provided an email address for comments about the abstract: sterling@astro.as.utexas.edu

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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.