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.
تحميل البحث