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A. K. Speck, M. Meixner, D. Fong, P. R. McCullough, D. E. Moser, T. Ueta (University of Illinois at Urbana-Champaign)
We present new observations of the ionized gas (H\alpha), molecular gas (H2) and cool dust in the Helix nebula (NGC~7293). The H\alpha observations are more sensitive than previous measurements and show the huge extent of the Helix, confirming it as at least partially density-bounded. The shape of the extended H\alpha emission is elongated in the east--west direction and shows evidence of point-symmetric structures.
High-resolution H2 observations show that the H2 emission is clumpy. The molecular emission is confined to that part of the nebula seen in the classic optical image. Furthermore, comparison of the H2 emission strength with time-dependent models for photo-dissociation regions (PDRs) shows that the emission arises from thermal excitation of the hydrogen molecules in PDRs and not from shocks.
The FIR images of dust, at 90 and 160micron, are compared with the H\alpha observation shows that the dust and ionized gas are coincident and extend to ~1100'' radius (>1pc).
Assuming that the outer layers of the circumstellar shell have spherical symmetry, radiative transfer modeling of the emission in H\alpha gives a shell mass of ~1.5 solar masses. However, the modeling does not cover the outer most part of the shell (beyond ~600'' radius) and therefore this is a lower limit for the shell mass. Moreover, the models suggest that either ~80--90% of the dust mass is in grains larger than 3.5micron or there is much less dust in the Helix than previously thought.
Furthermore, there is evidence that small dust grains close to the central star are destroyed by the high energy photons from the star.
Comparison of these new observations with previous observations shows the large scale stratification of the Helix in terms of ionized gas and dust as well as the co-existence of molecular species inside the ionized zones, where molecules survive in dense globules and cometary knots.
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The author(s) of this abstract have provided an email address for comments about the abstract: akspeck@astro.uiuc.edu