Iron Depletion in the Hot Bubbles in Planetary Nebulae

Abstract

We have searched for the emission from [Fe X] λ6374 and [Fe XIV] λ5303 that is expected from the gas emitting in diffuse X-rays in BD +30°3639, NGC 6543, NGC 7009, and NGC 7027. Neither line was detected in any object. Models that fit the X-ray spectra of these objects indicate that the [Fe X] λ6374 emission should be below our detection thresholds, but the predicted [Fe XIV] λ5303 emission exceeds our observed upper limits (1 σ) by factors of at least 3.5-12. The best explanation for the absence of [Fe XIV] λ5303 is that the X-ray plasma is depleted in iron. In principle, this result provides a clear chemical signature that may be used to determine the origin of the X-ray gas in either the nebular gas or the stellar wind. At present, although various lines of evidence appear to favor a nebular origin, the lack of atmospheric and nebular iron abundances in the objects studied here precludes a definitive conclusion.

Based on observations obtained at the Observatorio Astronómico Nacional in San Pedro Mártir, Baja California, Mexico.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.