Abstract
Planetary nebulae (PNe) are often recognized as the hallmark of compact H ii regions in the Universe. However, there exist dusty neutral regions extending beyond the central ionized region. We demonstrate that such dusty neutral regions (also known as photo-dissociation regions, or PDRs) around the central ionized region are significant parts of PNe in terms of energetics and mass. We do so by using our latest dusty photoionization model of NGC 6781 (of 13 parameters) based on one of the most comprehensive panchromatic data sets ever assembled for a PN encompassing from X-ray to radio (of 136 constraining data, including 19 flux densities, 78 line fluxes, and 37 band fluxes). We find that NGC 6781, evolved out of a 2.25–3.0 M ⊙ star located 460 pc away from us, possesses a massive concentration of neutral gas (molecular hydrogen) just beyond the central ionized region and that the amount of ionized gas in NGC 6781 is only 22% of the observationally accounted amount of matter in the circumstellar environment, which itself does not even account for the amount of mass presumably ejected by the central star during the last thermal pulse event according to the latest evolutionary models. This means that the observed nebula in this PN is only the tip of the iceberg.
Highlights
While Planetary nebulae (PNe) are famous for their spectacular circumstellar structures seen often via bright optical emission lines arising from the ionized gas component of the nebula, the ionized part of PNe is surrounded by the neutral gas and dust components of lower temperature
We derive the empirical characteristics of the central star and its nebula with a greater amount of self-consistency, and use the derived quantities as input parameters and/or constraints to construct a dusty photoionization model consisting of ionized, atomic, and molecular gas plus dust grains ([6] to which readers are encouraged to refer, as a fair amount of details is left out from this contribution)
We determine the electron density and temperature of the ionized gas component of NGC 6781 from 9 diagnostic lines based on 15 line ratios computed from 28 line fluxes of collisionally excited and recombination lines out of 81 individual lines measured from the adopted spectra of the object
Summary
The life cycle of matter in the Universe is synonymous with the stellar evolution, because the chemical evolution of galaxies is made possible by stellar mass loss that would expel nucleosynthesized matter into the interstellar environments. While PNe are famous for their spectacular circumstellar structures seen often via bright optical emission lines arising from the ionized gas component of the nebula, the ionized part of PNe is surrounded by the neutral gas and dust components of lower temperature (i.e., the photo-dissociated region, or the PDR). Each of these ionic/atomic/molecular/dust components in the circumstellar shell of a PN contains variable clues about the history of mass loss from the central star. We conduct the Herschel Planetary Nebula Survey (HerPlaNS) [1] and its follow-up archival study, HerPlaNS+, using PN data collected with the Herschel Space Observatory [2]
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