Inhomogeneities in the Gas and Dust of the Planetary Nebula SwSt 1

Details Inhomogeneities in the Gas and Dust of the Planetary Nebula SwSt 1 Inhomogeneities in the Gas and Dust of the Planetary Nebula SwSt 1

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005aas...206.3902s&link_type=abstract

American Astronomical Society Meeting 206, #39.02; Bulletin of the American Astronomical Society, Vol. 37, p.493

Astronomy and Astrophysics

Astronomy

Scientific paper

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 (ne=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±0.12, 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±0.33), 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.

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