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Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRACinstrument on the Spitzer Space Telescope. The Helix is one of theclosest bright PNs and therefore provides an opportunity to resolve thesmall-scale structure in the nebula. The emission from this PN in the5.8 and 8 μm IRAC bands is dominated by the pure rotational lines ofmolecular hydrogen, with a smaller contribution from forbidden lineemission such as [Ar III] in the ionized region. The IRAC images resolvethe ``cometary knots,'' which have been previously studied in this PN.The ``tails'' of the knots and the radial rays extending into the outerregions of the PN are seen in emission in the IRAC bands. IRS spectra onthe main ring and the emission in the IRAC bands are consistent withshock-excited H2 models, with a small (~10%) component fromphotodissociation regions. In the northeast arc, the H2emission is located in a shell outside the Hα emission.
| An optical absorption study of the Helix planetary nebula. Na I and Ca II lines, and a search for diffuse bands
We present the first results of an optical absorption study ofNGC 7293, the Helix planetary nebula (PN), designedto search definitively for diffuse band (DIB) absorptions perhapsarising in the carbon-rich circumstellar matter of the PN. We used theVLT-UVES spectrograph to achieve high resolution (R=50 000) spectra of 8stars located angularly close to and behind the Helix. These targetswere selected through their photometric UBV or 2MASS properties,permitting derivation of spectral types (mainly F-G dwarfs) and distancethat place several of them far (700-1500 pc) beyond the Helix (210 pc).Through a detailed analysis of the Na I and Ca II lines to the 8targets, we find that two lines of sight situated close to the nebula asmapped in CO and H I exhibit very strong Na I absorption. Thisabsorption is unlikely to arise in the foreground or backgroundinterstellar medium which has a relatively low column density, becausethe Helix is at high galactic latitude b = -57°. It ismuch more probable that it is due to the PN neutral or molecularmaterial. This circumstellar origin is reinforced by the fact that nocorresponding Ca II line is observed (as it would usually be from theISM), which is in agreement with the very high Ca depletion oftenobserved in PNs. No trace of any DIB features was found in these twocircumstellar lines of sight, nor to the other targets. The twocircumstellar sightlines discovered in this work open the way to searchfor molecular species such as C2 which are not observable atradio wavelengths, and to obtain more information on the rich neutraland molecular content of the Helix.Based on observations carried out with the European Southern ObservatoryVLT/UVES, program 70.C-0100.
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