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NGC 2022 - planetary nebula in Orion, Paul Ricker

NGC 2022 - planetary nebula in Orion

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Description

A retired colleague who for fun is writing papers on NGC objects numbered after the year suggested I go after NGC 2022, a double-shell planetary nebula in Orion. It's about 2.5 kpc away. In a double-shell planetary, the nebula (produced by the loss of most of the star's remaining hydrogen envelope, and in this case expanding at about 5-10 km/s with a radius of 0.17 pc) is compressed by a fast wind off the nascent white dwarf expanding at about 30 km/s, producing an inner shell (in this case with a radius of about 0.12 pc). The fast wind is being produced because the white dwarf still has a very thin layer of hydrogen or helium that continues to burn. In a few thousand years this layer will extinguish.

The white dwarf is still very hot (~ 100,000 K) and is producing enough UV emission to ionize the nebula. While we can see the inner shell in the optical, in the UV the nebula is still somewhat optically thick. Nevertheless, it appears to be transitioning to an optically thin state, and some UV photons are able to escape and photoionize a much larger cloud of gas with a radius of ~ 0.5 pc which probably represents the wind from the star's earlier asymptotic giant branch (AGB) phase. This 'halo' is about 1000x dimmer than the main part of the nebula and requires a great deal of integration time to capture. While it's very faintly visible in H alpha, most of the emission is in OIII. Corradi et al. (2003) show a nice OIII image from a 3.5 m telescope that makes it clear just how faint this halo is.

Registered and stacked each combination of filter+exposure time in ASTAP, then registered and stacked the stack for each filter with DSS (DSS seems to be much less tolerant of lousy stars). The stars have been healed in StarTools. For the OIII I combined 1x1 and 4x4 binned versions to get detail in the bright part of the nebula and exposure in the faint halo. The actual dates of observations were 5, 7, 19, 20, 26, and 27 February -- I didn't feel like entering every separate set. Some of the LRGB observations used 25 instead of 50 flats.

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NGC 2022 - planetary nebula in Orion, Paul Ricker