Stellar Pulsation and the Production of Dust and Molecules in Galactic Carbon Stars

TL;DR

This study links stellar pulsations in Galactic carbon stars to dust and molecule formation in their circumstellar shells, revealing a clear boundary between semi-regular and Mira variables in dust content and chemistry, with implications across different metallicities.

Contribution

It provides new infrared spectral evidence connecting pulsation types with dust chemistry and production processes in Galactic and Magellanic carbon stars.

Findings

Semi-regulars show little dust; Miras show more dust and weaker SiC features.

A sharp boundary in dust content separates semi-regular and Mira variables.

Differences in dust and molecular features allow spectroscopic classification without long-term monitoring.

Abstract

New infrared spectra of 33 Galactic carbon stars from FORCAST on SOFIA reveal strong connections between stellar pulsations and the dust and molecular chemistry in their circumstellar shells. A sharp boundary in overall dust content, which predominantly measures the amount of amorphous carbon, separates the semi-regular and Mira variables, with the semi-regulars showing little dust in their spectra and the Miras showing more. In semi-regulars, the contribution from SiC dust increases rapidly as the overall dust content grows, but in Miras, the SiC dust feature grows weaker as more dust is added. A similar dichotomy is found with the absorption band from CS at $\sim$7.3 $\mu$m, which is generally limited to semi-regular variables. Observationally, these differences make it straightforward to distinguish semi-regular and Mira variables spectroscopically without the need for long-term photometric observations or knowledge of their distances. The rapid onset of strong SiC emission in Galactic carbon stars in semi-regulars variables points to a different dust-condensation process before strong pulsations take over. The break in the production of amorphous carbon between semi-regulars and Miras seen in the Galactic sample is also evident in Magellanic carbon stars, linking strong pulsations in carbon stars to the strong mass-loss rates which will end their lives as stars across a wide range of metallicities.