Poster Presentation
Herschel Planetary Nebula Survey (HerPLaNS): Construction of a Detailed Dusty Photoionization Model of NGC6781
Presenter: Masaaki Otsuka (ASIAA)
We performed a comprehensive analysis of the planetary nebula (PN) NGC6781 in order to investigate physical conditions of atomic gas, dust grains, and molecules in the nebula and evolution of the central star based on our own Herschel data and the rich archival data in the wavelengths from GALEX UV (0.2-um) to Herschel SPIRE far-IR (600-um). Spitzer/IRS mid-IR spectrum shows the 11.3-micron PAH band, pure rotational hydrogen molecule (H2) lines, and the featureless carbon dust continuum in ~15-40-micron. Comparison with the theoretical shock models suggests that H2 could be excited by shock interaction (shock velocity ~20 km/s) with the remnant AGB circumstellar envelope (hydrogen density 10000-20000 cc). We performed a detailed plasma diagnostics and derived nebular abundances of the nine elements. By comparing with the AGB nucleosynthesis model, the progenitor would be a 2.25-3.0 Msun star. In addition to evolutional status, elemental abundances of the H2-rich bipolar PN NGC6720 (Ring Nebula) is in excellent agreement with NGC6781, suggesting that both NGC6781 and NGC6720 have evolved from a very similar mass star and share with their evolution. Based on our own technique, we tried to determine the distance of 460pc. We constructed the photoionisation model to be self-consistent with all the observations. About 40% of the total dust mass measured in NGC6781 would be from warm-cold dust components. We found that other heating sources are necessary to explain the observed H2 line fluxes if H2 originates from PN ejecta. By introducing warm regions with a constant kinematic gas temperature ~1000K within PDRs, we obtained better fitting for the observed H2, CO, and OH line fluxes. Our case study of NGC6781 would help understanding physical conditions of unresolved dusty objects. In the future, we will extend our PN project to SMA and ALMA in order to study molecule formation in details. This work is supported by NASA and MoST grant.
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