When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars

TL;DR

This study reports the detection of hydrocarbon emission features, specifically PAHs, in four S-type AGB stars, expanding understanding of hydrocarbon formation in stars without hot companions, with implications for stellar chemistry and evolution.

Contribution

First detection of PAH infrared features in S-type AGB stars, showing hydrocarbons form in stars without hot companions, and revealing a correlation with stellar temperature.

Findings

PAH features observed in four S-type AGB stars.

Correlation between star temperature and PAH feature wavelength.

Evidence of rich aliphatic hydrocarbon components.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared emission bands that have been observed in a wide range of astrophysical environments, where carbonaceous material is subjected to ultraviolet (UV) radiation. Although PAHs are expected to form in carbon-rich AGB stars, they have up to now only been observed in binary systems where a hot companion provides a hard radiation field. In this letter, we present low-resolution infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type AGB stars observed with the infrared spectrograph aboard the Spitzer satellite. The spectra of these four stars show the typical infrared features of PAH molecules. We confirm the correlation between the temperature of the central star and the centroid wavelength of the 7.9 {\mu}m feature, present in a wide variety of stars spanning a temperature range from 3 000 to 12 000 K. Three of four sources presented in this paper extend this relation towards lower temperatures. We argue that the mixture of hydrocarbons we see in these S-stars has a rich aliphatic component. The fourth star, BZ CMa, deviates from this correlation. Based on the similarity with the evolved binary TU Tau, we predict that BZ CMa has a hot companion as well.