M 4-18 : a young cool planetary nebula.

Abstract

The unresolved planetary nebula M4-18 has been studied spectroscopically from the ultraviolet to the near-infrared. The nebula is found to be of very low excitation, with electron density Ne = 11,000 + or - 1,000/cu cm and electron temperature Te = 5600 + or - 1300 K, and a high O abundance. The central star is found to be among the coolest central stars known, with temperature 22,000 + or - 2,000 K. M4-18 appears to be a very young planetary nebula, possibly over 4000 years old. It is roughly 3.5 kpc distant and is highly reddened, with an ultraviolet reddening law which is anomalous. Comparisons with other young planetary nebulae and the irregular variable star V348 Sgr are discussed.