The dust that formed our planet, and the elements necessary for life to evolve here were created by stars. It is common for supernova to receive credit for our life on Earth, and there is no doubt they played a crucial role, however, theirs is not the only meaningful contribution. Recent observations have shown that Asymptotic Giant Branch (AGB) stars are responsible for returning substantial amounts of enriched material to their surrounding environments. AGB stars are highly-evolved low-to-intermediate mass stars that undergo significant mass loss as they near the end of their lives. The majority of stars that have died in our Universe have done so following the AGB phase (Hofner & Oloffson 2018), indicating these stars have great influence over galactic enrichment and the creation of new solar systems and potential life. Mira variables are AGB stars that undergo regular pulsation every 200–500 days. These pulsations, combined with their cool atmospheres (2000–3000 K), make Mira variables prolific molecule and dust factories. Studying the circumstellar environments surrounding Mira variables provides astrophysicists with the opportunity to observe the processes that created the building blocks of our solar system.
The characteristic pulsations of Mira variables make their circumstellar environments quite dynamic; to study how different atmospheric layers respond to the pulsation we need multiple observations across the entire pulsational period of the star. Observing Mira atmospheres at mid-infrared wavelengths provides access to the molecule and dust forming regions; these observations are best done with space-based instruments to avoid affects of telluric absorption. The Mira variables in this work were all observed at least twice with the Spitzer Space Telescope. High-resolution spectra (R ~600) were taken approximately once a month with the Infrared Spectrograph (IRS) (Houck et al 2004); the stars in this study are all bright in the infrared, and thus the exposures were kept brief to prevent saturation of the detector. The resulting spectra have high signal-to-noise ratios that display both gaseous and solid-state (dust) features.
The full Spitzer data set contains nearly 100 spectra for 25 stars spanning all three chemical subclasses. This dissertation focuses on analyzing five ro-vibrational Q-branch bandheads of CO2 identified in the spectra of the oxygen-rich Miras (M-types,) and a previously un-observed feature at 17.62 micron that was observed across all three chemical subclasses. We have tentatively identified this new feature as Fe I. The CO2 lines were analyzed using the publicly available code, RADEX (van der Tak et al. 2007), which uses a molecular data file that includes collisions to solve the radiative transfer; RADEX also has the capability of solving the radiative transfer under non-local thermodynamic equilibrium (NLTE) conditions, which is important for Mira atmosphere, because we do not know how the pulsating atmosphere affects the CO2 gas. Files for many molecules ready for calculations are included as part of the Leiden Atomic and Molecular Database (LAMDA), however, the majority of these files were built for modeling pure rotational spectra in the radio. We built a custom molecular file of ro-vibrational transitions of CO2 that includes over 800 radiative transitions, approximately 20,000 collisions with H2, and spans temperatures from 100–1000 K. We used RADEX to calculate synthetic spectra that match the observed CO2 Q-branch bandheads. The synthetic spectra allow us to determine atmospheric conditions of the CO2 gas like column density and kinetic temperature. For the new feature at 17.62 micron we fit every observation with Gaussian line profile to track its behavior with phase; this line has a completely different character than other features, and is extremely narrow, and bright. This behavior may be caused by fluorescence, and we explored several possible pumping mechanisms.
The results of the RADEX calculations show that CO2 is highly extended throughout the M-type atmospheres. The kinetic temperatures also indicate that the CO2 gas is much cooler in regions close to the star than radiative equilibrium conditions would predict. This suggests that CO2 is in a previously theorized "refrigeration zone'' that requires a break from radiative equilibrium, and allows dust condensation within a stellar radii (Willson 2000). The behavior of the CO2 lines shows that the M-types with longer periods are behaving differently than those with shorter periods. This behavior is also seen with the 17.62 micron feature; the line strength are consistently greater across all three chemical subclasses in the Miras with periods over 300 days. These results indicate that Miras with longer periods perturb their surrounding atmospheres differently than Miras with shorter periods.
|Advisor:||Creech-Eakman, Michelle J.|
|Commitee:||Meier, David S., Sessions, Sharon L., van Wijk, Jolante|
|School:||New Mexico Institute of Mining and Technology|
|School Location:||United States -- New Mexico|
|Source:||DAI-B 82/7(E), Dissertation Abstracts International|
|Keywords:||AGB star, Circumstellar Environments, Mira variable, Spitzer, Stellar atmospheres|
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