Identification of three new proto-Planetary Nebulae exhibiting the unidentified feature at 21 um

TL;DR

This study reports the discovery of three new proto-Planetary Nebulae exhibiting the mysterious 21-micron feature, analyzes potential carriers like amorphous carbon and FeO, and explores correlations with other dust features to understand its nature.

Contribution

It identifies three new 21-micron feature sources and investigates their spectral characteristics and possible dust carriers, expanding knowledge of this unidentified feature in proto-PNe.

Findings

Detection of 21-micron feature in three new proto-PNe

Correlation between 21-micron flux and PAH/HAC emissions

Modeling with amorphous carbon and FeO was not fully successful

Abstract

Among its great findings, the IRAS mission showed the existence of an unidentified mid-IR feature around 21 um. Since its discovery, this feature has been detected in all C-rich proto-PNe of intermediate spectral type (A-G) and - weakly - in a few PNe and AGB stars, but the nature of its carriers remains unknown. In this paper, we show the detection of this feature in the spectra of three new stars transiting from the AGB to the PN stage obtained with the Spitzer Space Telescope. Following a recent suggestion, we try to model the SEDs of our targets with amorphous carbon and FeO, which might be responsible for the unidentified feature. The fit thus obtained is not completely satisfactory, since the shape of the feature is not well matched. In the attempt to relate the unidentified feature to other dust features, we retrieved mid-IR spectra of all the 21-um sources currently known from ISO and Spitzer on-line archives and noticed a correlation between the flux emitted in the 21-um feature and that emitted at 7 and 11 um (PAH bands and HAC broad emission). Such a correlation may point to a common nature of the carriers.