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 semiregular and Mira variables, with the semiregulars showing little dust in their spectra and the Miras showing more. In semiregulars, 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 ~7.3{mu}m, which is generally limited to semiregular variables. Observationally, these differences make it straightforward to distinguish semiregular 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 semiregular variables points to a different dust-condensation process before strong pulsations take over. The break in the production of amorphous carbon between semiregulars 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.

Cone search capability for table J/ApJ/887/82/targets (SOFIA/FORCAST targets (Table 1) and dust properties (Table 2))

Cone search capability for table J/ApJ/887/82/table5 (Revised feature strengths - SWS sample (Sloan+, 2016, J/ApJ/826/44/table6))

Cone search capability for table J/ApJ/887/82/table6 (Revised feature strengths - IRS sample (Sloan+, 2016, J/ApJ/826/44/cstars))