Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio

Abstract

Airborne and space-based low-resolution spectroscopy in the 1980s discovered tantalizing quantitative relationships between the gas phase C/O abundance ratio in planetary nebulae (PNe) and the fractions of total far-infrared (FIR) luminosity radiated by the 7.7-and 11.3-µm bands (the C = C stretch and C-H bend, respectively), of polycyclic aromatic hydrocarbons (PAHs). Only a very small sample of nebulae was studied in this context, limited by airborne observations of the 7.7-µm band, or the existence of adequate IRAS Low Resolution Spectrometer data for the 11.3-µm band. To investigate these trends further, we have expanded the sample of planetaries available for this study using Infrared Space Observatory (ISO) lowresolution spectra secured with the Short Wavelength Spectrometer and the Long Wavelength Spectrometer. The new sample of 43 PNe, of which 17 are detected in PAH emission, addresses the range from C/O = 0.2-13 with the objective of trying to delineate the pathways by which carbon dust grains might have formed in planetaries. For the 7.7-µm and 11.3-µm bands, we confirm that the ratio of band strength to total infrared (IR) luminosity is correlated with the nebular C/O ratio. Expressed in equivalent width terms, the cut-on C/O ratio for the 7.7-µm band is found to be 0.6 +0.2 −0.4 , in good accord with that found from sensitive ground-based measurements of the 3.3-µ band.

A Survey of 3.3 μm PAH Emission in Planetary Nebulae

“…This shows a PAH equivalent width detection threshold at C/O ¼ 0:65 AE 0:28. This result is within the uncertainties obtained by Roche et al (1996), who performed a similar analysis, and with Cohen & Barlow (2005), who measured a C/O ratio versus 7.7 m PAH emission.…”

“…The 3.3 m feature shows relatively little variation even in very different emission sources (van Diedenhoven 2004;Cohen & Barlow 2005;Tokunaga et al 1991), while longer wavelength PAH emission shows greater variation in central wavelength and FWHM (Peeters et al 2002). In longer wavelength studies planetary nebulae are classified as class B 6 Y 9 objects, while some very young post-AGB stars exhibit class C 6 Y 9 emission.…”

“…Roche et al (1996) found a threshold C/O ratio for 3.3 m PAH emission in planetary nebulae of $0.6. A study of 7 and 8 m emission by Cohen & Barlow (2005) found a roughly consistent threshold ratio of 0:56 AE 0:3. In nebulae with lower C/O ratios, nebular carbon is expected to be incorporated in silicates and CO, while carbon-rich nebulae produce hydrocarbons including PAHs .…”

“…Emission features associated with PAHs have been observed in a wide variety of objects, including planetary nebulae, H ii regions, protostellar clouds, and star-forming galaxies ( Peeters et al 2002;. This ubiquity shows PAHs to be potentially important components of the interstellar medium ( ISM ), and has led to several studies aimed at correlating PAH emission with region characteristics (Tokunaga et al 1991;Roche et al 1996;Cohen & Barlow 2005;Peeters et al 2002;van Diedenhoven et al 2004). Recent surveys ( Peeters et al 2002;van Diedenhoven et al 2004) using space-based telescopes have shown variability in the PAH emission.…”

“…This survey has three purposes: investigate the properties of 3.3 m PAH emission in planetary nebulae by expanding the analysis performed by Roche et al (1996) and Cohen & Barlow (2005); expand the classification of 3.3 m emission to a larger sample of planetary nebulae using a higher resolution than previous surveys; and demonstrate the FLITECAM instrument's spectroscopy mode in a survey likely to be undertaken once aboard SOFIA.…”