DdDm 1 (Dd 1) is a rare planetary nebula (PN, plural PNe) located in the galactic halo, DdDm 1 features a blue, stellar-like appearance with an elliptical gas and dust structure. DdDm 1 stands for Dolidze-Dzimselejsvili 1, named after Soviet astronomers Dolidze M. V., Dzimselejsvili G. N. This nebula was discovered in 1966. DdDm 1 is a metal-deficient, highly carbon-poor PN, As a rare field halo PN It differs from other halo PNe. DdDm 1 is the only known O-rich halo PN. This might mean DdDm 1 experienced neither efficient third dredge-up (TDU) events nor hot bottom burning during the AGB phase. This nebula 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 major-axis radii measuring 0.61 arcsec and 1.03 arcsec, respectively. DdDm 1 is enriched in N, O, S, Ne and Ar. producing two emission lines, some forbidden lines, and some recombination lines. Furthermore, 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. On the other hand DdDm 1 and the previously mentioned high heliocentric radial velocity of roughly ~310 km/s, This large velocity 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 1000-3000 L⊙, corresponding to an initial mass of ~1M⊙ for the progenitor star on the H-R diagram. If DdDm 1 evolved from a single star with an initial mass of ∼0.9 M⊙, which is typical of halo stars, its evolutionary timescale should be too slow to evolve into a PN. The ∼0.9 M⊙ star should end its life as a white dwarf (WD) with a mass of ∼0.56 M⊙.
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