DdDm 1 (Dd 1) is a planetary nebula (PN) located in the galactic halo. DdDm 1 features a blue stellar-like appearance with an elliptical gas and dust structure. The nebula was discovered by Dolidze-Dzimselejsvili 1, named after Soviet astronomers Dolidze M. V. and Dzimselejsvili G. N. in 1966.
DdDm 1 is a metal-deficient highly carbon-poor PN and is a rare field halo PN, this nebula differs from other halo PNe because it is the only known O-rich halo PN. This shows that DdDm 1 experienced neither a third dredge-up (TDU) events nor a hot bottom burning during the AGB phase. DdDm 1 exhibits emission lines including O III, O II, O I, and Fe III.
The Fe III 5270 Å emission is noticeably more compact than that of the O III 4959 Å emission, with a major-axis measuring 0.61 arcsec and 1.03 arcsec respectively. DdDm 1 is enriched in N, O, S, Ne, and Ar, producing two emission lines, a few forbidden lines, and a lot of recombination lines. This shows that the abundances from recombination lines are larger than forbidden lines, by a factor of 10 ∼ 100.
The heliocentric radial velocity of DdDm 1 is -309.14±1.22 km sec⁻¹. Some studies suggest DdDm 1 could be an extragalactic PN because of the heliocentric velocity.
DdDm 1 and the previously mentioned high heliocentric radial velocity of roughly ~310 km/s confirms that it belongs to the galactic halo population rather than the galactic disc.
The central star has an effective temperature of 38,000-55,000 K and a luminosity of which corresponds to an initial mass of for the progenitor star on the H-R diagram. If DdDm 1 evolved from a single star with an initial mass of , which is typical of halo stars, its evolutionary timescale should be too slow to evolve into a PN. The star should end its life as a white dwarf (WD) with a mass of .
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