Abstract

Stars may develop large-scale pulsations at certain stages of evolution. These stars experience changes in size, brightness, and temperature, often at regular intervals. Study of these pulsations has led to a greater understanding of stellar interiors and evolution. Certain types of pulsating stars can be used to estimate distances, with classical Cepheids being the most used for this purpose. In recent years, a discrepancy between measurements of the expansion rate of the universe has developed. One way to measure the expansion requires distances derived from the Cepheids and, as a result, these stars have been investigated for potential uncertainties that may affect distance measurements. Much of astronomy is moving to longer wavelengths, and a deeper look at these stars in the near-infrared is necessary. This dissertation presents the largest sample of time-series near-infrared spectra of Cepheids to date which were obtained from the Astrophysical Research Consortium 3.5-meter telescope at Apache Point Observatory. By measuring the strength of absorption lines in the spectra, I showed that the carbon monoxide (CO) in the atmospheres of these stars varied with pulsation. This confirms results from theoretical models and indirect observations. In addition to CO, these stars also have cyano radical (CN) absorption that varies. Both of these molecular features are dependent on temperature, which explains varying strengths during pulsation cycles. The atomic species involved (C, N, and O) are of interest from an evolutionary perspective as their atmospheric abundances are affected by mixing that occurs at later stages of evolution. Via comparison with synthetic atmosphere models, I showed that the strength of CN confirms overabundance of nitrogen suggested by the literature. In addition to the molecular features, atomic lines were also studied, specifically those that may indicate shockwaves propagating from deep within the star. A few of the classical Cepheids in the sample display helium emission which has been linked to shocks. We have also observed this same feature in a Type II Cepheid which is known to experience shocks and a delta Scuti pulsating star, where shocks have been theorized but observational evidence has been insufficient.

Degree

PhD

College and Department

Computational, Mathematical, and Physical Sciences; Physics and Astronomy

Rights

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