Twenty-two bow shocks in the outer Helix Nebula trace the progressive fragmentation of AGB shell clumps and imply a ~10,000 year timescale for their disruption into the ISM.
Infrared Observations of The Helix Planetary Nebula
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abstract
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PN, and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 micron IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the "cometary knots" which have been previously studied in this PN. The "tails" of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H2 models, with a small (~10%) component from photodissociation regions. In the Northeast Arc, the H2 emission is located in a shell outside of the H alpha emission.
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Numerous bow shocks in the outer Helix Nebula
Twenty-two bow shocks in the outer Helix Nebula trace the progressive fragmentation of AGB shell clumps and imply a ~10,000 year timescale for their disruption into the ISM.