The Planetary Nebula in the Galactic Cluster NGC 2818

Extract

The planetary nebula PK 261+8°1 is of special interest due to its apparent physical association with the galactic cluster NGC 2818 (Tiff, Connally, and Webb 1972, M.N.R.A.S., 158, 47). During 1975–76 we obtained spectrophotometric observations (both photographic and photoelectric) and interference filter photographs (Hβ, [OIII], [NII], and [SII]) of this object at Cerro Tololo. The nebula has a rather smooth bilaterally symmetric “hourglass” structure in the Hβ and [OlII] photographs, while several bright filaments appear in [NII] and [SII]. The spectrum of PK 261+8°1 is very rich in lines characteristic of both low ionization ([OI], [OII], [NI], [NII], [SII]) and high ionization (He II, [OIII], [NeV]) regions. Using the photo electric observations, we derive Te = 14,500 ± 1500°K from the [Olli] lines and Te = 11,000 ± 1300°K from the [OIII] lines and Te = 11,000 ± 1300°K from the [NII] lines. The λ6717/λ6730 [SII] ratio suggests that Ne ≤ 800 cm⁻³ in the regions where S+ exists. Additional information μ the density and temperature structure of the nebula derived from the deep photographic spectra will also be presented. Due to the complicated ionization structure, an accurate abundance analysis of PK 261+8°1 is difficult. Consequently, we will present the quantitative results from a variety of abundance calculation techniques. We conclude that He/H = 0.14 ± 0.01, N is substantially enriched relative to H and 0 (N/H ≥ 0/H = 4 × 10⁻⁴), while the abundances of 0, Ne, S, Ar, and Cl relative to H are similar to values typical of galactic planetary nebulae. Other characteristics of the nebula derived from its association with the cluster NGC 2818 and its spectral similarity to Henize 97 in the Large Magellanic Cloud will be discussed. This research was supported in part by a grant from the Research Corporation.