The Chemical Enrichment of the Universe

By Kathleen Kraemer, Boston College and Gregory Sloan, Cornell University

Paper: Stellar Pulsation and the Production of Dust and Molecules in Galactic Carbon Stars
K. E. Kraemer, et al., 2019, ApJ, 887, 1.

Infrared spectra from SOFIA have uncovered a fundamental difference between the dust produced by two types of carbon-rich variable stars. New FORCAST spectra reveal a close relation to the pulsations racking these evolved stars, as revealed by their variability type as well as the dust and molecules they are producing. The Mira variables, with their strong pulsations, are producing significant quantities of amorphous carbon dust. The semi-regular variables, on the other hand, show little dust, and what dust they have is primarily silicon carbide (SiC).

A fundamental issue in stellar astrophysics is the role of low- and intermediate-mass stars in the chemical enrichment of the Universe. Dust grains can condense in the cool atmospheres of these stars, and radiation pressure on the grains drives mass-loss, with the gas dragged along by the dust. Through this process, stars that started out with about 1–8 solar masses expel the bulk of their material while on the asymptotic giant branch, ultimately ending up as small white dwarfs.

Studies of the Magellanic Clouds suggest that carbon stars may be the dominant source of dust contributed by stars to the interstellar medium. Those asymptotic giant branch stars that dredge enough freshly fused carbon from their interiors to their surfaces become carbon stars. With more carbon than oxygen in their atmospheres, carbon-rich material dominates their chemistry in both the gas phase and in condensing dust. They will quickly embed themselves in optically thick shells of amorphous carbon dust instead of the silicates seen in oxygen-rich stars.

Recent spectroscopic studies of carbon stars concentrated on the metal-poor Magellanic Clouds using Spitzer’s Infrared Spectrograph. Galactic studies have been hampered by limited samples (as with the Infrared Space Observatory, or ISO, in the 1990s). Spectral surveys could overcome this problem, but the Infrared Astronomical Satellite (IRAS) which flew in the 1980s, had limited wavelength range and low spectral resolution. To address these problems with the Galactic sample, we obtained 5–14 µm spectra with SOFIA’s FORCAST of 33 Galactic carbon stars, focusing on the underrepresented variables: semi-regulars and the longest-period Miras. 

The new spectra, when combined with the samples from ISO and Spitzer, expose a tight relationship between molecules and dust these stars are generating and their pulsation mode. Mira variables are well-known for their long pulsation periods and strong amplitudes. Semi-regular variables always have weaker amplitudes, and they usually have shorter periods. The Mira variables, with their strong pulsations, are producing copious amounts of amorphous carbon dust, while the semi-regulars, particularly those classified as “SRb,” make much less dust, and it is mostly SiC.

The figure shows this dichotomy. It plots the relative strength of the emission feature from SiC dust as a function of the [6]-[9] color, which is derived from the spectra and reddens as the star produces more amorphous carbon dust. In the Milky Way, the semi-regulars are cleanly separated from the Miras (at a [6]-[9] color of ~0.3), because they are making little or no amorphous carbon. As the total dust content increases, the strength of the SiC feature rises sharply, up to the transition to Mira variables. From that point to redder colors (higher x-axis values), the SiC feature decreases in relative strength as the total dust content rises. The carbon stars in the Magellanic Clouds also show this behavior, with the semi-regular variables and Miras largely distinguishable by their [6]-[9] colors.

The different pulsational properties of Miras and semi-regulars are almost certainly responsible for the differences in the dust. Miras are experiencing radial pulsations in the fundamental mode, so that their entire atmosphere is moving inward and outward in unison, which will push gas further away from the photosphere and enhance the condensation of dust. Semi-regulars pulsate more weakly and often in overtone modes, which will result in lower pulsation velocities and smaller changes in radius. As a result, less gas will cool to temperatures low enough for dust to condense. These weak pulsations appear to be more conducive for the formation of just SiC. Once the star shifts to the fundamental mode and the pulsations grow sufficiently in amplitude, then amorphous carbon begins to form and the mass-loss process can take off. When this phase begins, the end of its life as a star is in sight. ν

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