Stanghellini, Letizia; Shaw, Richard A.; Villaver, Eva (2008), ”The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale”, The Astrophysical Journal 689 (1): 194–202, doi:10.1086/592395, Bibcode: 2008ApJ...689..194S.
Pottasch, S. R.; Beintema, D. A.; Feibelman, W. A. (June 2005), ”Abundances of planetary nebulae NGC 2022, NGC 6818 and IC 4191”, Astronomy and Astrophysics 436 (3): 953–965, doi:10.1051/0004-6361:20042627, Bibcode: 2005A&A...436..953P.
Corradi, R. L. M.; Schönberner, D.; Steffen, M.; Perinotto, M. (April 2003), ”Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties”, Monthly Notices of the Royal Astronomical Society 340 (2): 417–446, doi:10.1046/j.1365-8711.2003.06294.x, Bibcode: 2003MNRAS.340..417C.
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Stanghellini, Letizia; Shaw, Richard A.; Villaver, Eva (2008), ”The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale”, The Astrophysical Journal 689 (1): 194–202, doi:10.1086/592395, Bibcode: 2008ApJ...689..194S.
Sabbadin, F.; Bianchini, A.; Hamzaoglu, E. (July 1984), ”The planetary nebulae NGC 1535 and NGC 2022.”, Astronomy and Astrophysics 136: 193–199, Bibcode: 1984A&A...136..193S.
Pottasch, S. R.; Beintema, D. A.; Feibelman, W. A. (June 2005), ”Abundances of planetary nebulae NGC 2022, NGC 6818 and IC 4191”, Astronomy and Astrophysics 436 (3): 953–965, doi:10.1051/0004-6361:20042627, Bibcode: 2005A&A...436..953P.
Corradi, R. L. M.; Schönberner, D.; Steffen, M.; Perinotto, M. (April 2003), ”Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties”, Monthly Notices of the Royal Astronomical Society 340 (2): 417–446, doi:10.1046/j.1365-8711.2003.06294.x, Bibcode: 2003MNRAS.340..417C.
