The Herschel Planetary Nebula Survey (HerPlaNS) I. Data Overview and Analysis Demonstration with NGC 6781

Abstract

This is the first of a series of investigations into far-IR characteristics of 11 planetary nebulae (PNs) under the Herschel Space Observatory Open Time 1 program, Herschel Planetary Nebula Survey (HerPlaNS). Using the HerPlaNS data set, we look into the PN energetics and variations of the physical conditions within the target nebulae. In the present work, we provide an overview of the survey, data acquisition and processing, and resulting data products. We perform (1) PACS/SPIRE broadband imaging to determine the spatial distribution of the cold dust component in the target PNs and (2) PACS/SPIRE spectral-energy-distribution (SED) and line spectroscopy to determine the spatial distribution of the gas component in the target PNs. For the case of NGC 6781, the broadband maps confirm the nearly pole-on barrel structure of the amorphous carbon-richdust shell and the surrounding halo having temperatures of 26-40 K. The PACS/SPIRE multi-position spectra show spatial variations of far-IR lines that reflect the physical stratification of the nebula. We demonstrate that spatially-resolved far-IR line diagnostics yield the (T_e, n_e) profiles, from which distributions of ionized, atomic, and molecular gases can be determined. Direct comparison of the dust and gas column mass maps constrained by the HerPlaNS data allows to construct an empirical gas-to-dust mass ratio map, which shows a range of ratios with the median of 195+-110. The present analysis yields estimates of the total mass of the shell to be 0.86 M_sun, consisting of 0.54 M_sun of ionized gas, 0.12 M_sun of atomic gas, 0.2 M_sun of molecular gas, and 4 x 10^-3 M_sun of dust grains. These estimates also suggest that the central star of about 1.5 M_sun initial mass is terminating its PN evolution onto the white dwarf cooling track.

27 pages, 16 figures, accepted for publication in A&A

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• The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781
• Review of scientific topics for Millimetron space observatory
• Extended Structures of Planetary Nebulae Detected in H${}_2$ Emission
• An Imaging Spectroscopic Survey of the Planetary Nebula NGC 7009 with MUSE
• Herschel Planetary Nebula Survey (HerPlaNS) - First Detection of OH+ in Planetary Nebulae
• Proper Plasma Analysis Practice (PPAP), an Integrated Procedure of the Extinction Correction and Plasma Diagnostics: a Demo with an HST/WFC3 Image Set of NGC6720
• Herschel imaging of the dust in the Helix Nebula (NGC 7293)
• Herschel Planetary Nebula Survey (HerPlaNS): Hydrogen Recombination Laser Lines in Mz 3
• Surface Brightness Correction for Compact Extended Sources Observed by the AKARI Far-Infrared Surveyor (FIS) in the Slow-Scan Mode
• AKARI Mission Program: Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES) I. Far-IR Photometry
• Detection of the [C I] $\lambda$8727 emission line. Low-ionization structures in NGC 7009
• OH${}^{+}$ emission from cometary knots in planetary nebulae
• 3D models of the circumstellar environments of evolved stars: Formation of multiple spiral structures