The optical/infrared counterpart(s) of IRAS 18333-2357

Abstract

Observations of the potential optical counterparts of the unusual source IRAS 18333-2357 show that this source is associated with an extraordinary planetary nebula system in the galactic globular cluster M22. Three distinct optical objects were found within 2" of the IRAS 18333-2357 position as determined by precisely locating the 20 μm infrared source. One object is a red star with m_v ≈ 14.7 mag, which appears to be an unrelated background field star that is possibly significantly reddened beyond the line-of-sight reddening to M22. The second stellar object is a very blue star with m_v ≈ 14.3 mag located about 1~3 south of the red star. Absorption lines of He 11 and possibly H are present in 4000-5000 A spectra of the stellar pair, similar to spectra of planetary nebula nuclei. The third member of this optical triple is an extended emission line nebulosity approximately 10" x 7" in size, centered about 1" east and south of the red star. The ionized gas in this nebulosity is extraordinarily oxygen-rich and neon-rich relative to both hydrogen and helium compared to the atmospheres of M22 red giants and is substantially oxygen-rich and neon-rich relative to hydrogen in comparison with typical planetary nebulae. This nebulosity is almost certainly in M22. The blue star is also very likely to be a member of M22, the source of ionizing photons for the nebulosity and probably the luminosity source for IRAS 18333-2357. We suggest that the dust responsible for the strong infrared emission of IRAS 18333-2357 is physically associated with the M22 nebulosity. In this case the total nebular mass, comprised of 3-10 x 10^(-4) M_☉ of ionized gas plus > 6 x 10^(-4) M_☉ of silicate or carbon-based grains, is possibly dominated by the dust component. The 0, Mg, Si abundances in the case of silicate grains, or carbon in the case of carbon-based grains, may be enhanced relative to hydrogen by at least a factor of 1000 compared to solar abundances. The relative abundances and mass of the M22 nebula are very unusual among known planetary nebulae. It is speculated that this system in M22 may be the result of the interaction within a close binary system containing at least one 0-Ne white dwarf component, or perhaps related to planetary nebulae like A30 and A78.