On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars

Abstract

The mysterious 21mu emission feature seen in 12 proto-planetary nebulae (PPNe) remains unidentified since its first detection in 1989. Over a dozen of candidate materials have been proposed within the past decade, but none of them has received general acceptance. Very recently, silicon carbide (SiC) grains with impurities were suggested to be the carrier of this enigmatic feature, based on recent laboratory data that doped SiC grains exhibit a resonance at \~21mu. This proposal gains strength from the fact that SiC is a common dust species in carbon-rich circumstellar envelopes. However, SiC dust has a strong vibrational band at ~11.3mu. We show in this Letter that in order to be consistent with the observed flux ratios of the 11.3mu feature to the 21mu feature, the band strength of the 21mu resonance has to be very strong, too strong to be consistent with current laboratory measurements. But this does not yet readily rule out the SiC hypothesis since recent experimental results have demonstrated that the 21mu resonance of doped SiC becomes stronger as the C impurity increases. Further laboratory measurements of SiC dust with high fractions of C impurity are urgently needed to test the hypothesis of SiC as the carrier of the 21mu feature.

arXiv comment14 pages, 3 figures, accepted for publication in ApJL

References in corpus (9)

• Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
• The circumstellar envelope of the C-rich post-AGB star HD 56126
• The Photophysics of the Carrier of Extended Red Emission
• Temperature effects on the 15-85-micron spectra of olivines and pyroxenes
• The SiC problem: astronomical and meteoritic evidence
• The discovery of the "21" micron and "30" micron emission features in Planetary Nebulae with Wolf-Rayet central stars
• Reconsidering the origin of the 21 micron feature: Oxides in carbon-rich PPNe?
• The Molecular and Dust Envelope of HD 56126
• On Titanium Carbide Nanoparticles as the Origin of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars

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• Interstellar and Ejecta Dust in the Cas A Supernova Remnant
• Optical properties of silicon carbide for astrophysical applications I. New laboratory infrared reflectance spectra and optical constants
• A Dust Twin of Cas A: Cool Dust and 21-micron Silicate Dust Feature in the Supernova Remnant G54.1+0.3
• A JWST Survey of the Supernova Remnant Cassiopeia A
• A Spitzer Study of 21 and 30 Micron Emission in Several Galactic Carbon-rich Proto-Planetary Nebulae
• On the Carriers of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
• Peculiarities and variations in the optical spectrum of the post-AGB star V448Lac=IRAS22223+4327
• On the Relation between the Mysterious 21 Micrometer Emission Feature of Post-Asymptotic Giant Branch Stars and Their Mass Loss Rates
• A Tale of Three Mysterious Spectral Features in Carbon-Rich Evolved Stars: The 21 Micrometer, 30 Micrometer, and "Unidentified Infrared" Emission Features
• The 21 micron and 30 micron circumstellar dust features in evolved C-rich objects