Publications
Abstract: We present analysis of Spitzer mid-IR spectra of 13 M-type Mira variables. The data were taken with Spitzer's Infrared Spectrometer as part of a study tracking pulsation effects in Mira variable circumstellar environments. All the Miras in this study exhibit unresolved ro-vibrational Q-branch bandheads of CO 2 at 13.87, 14.98, and 16.18 μm . The features at 13.87 and 16.18 μm were always seen in emission, while the 14.98 μ m feature was seen in both emission and absorption. We adapted RADEX to model these CO 2 features by building a molecular file of ro-vibrational transitions between 10 and 20 μm . The file includes eight vibrational levels, each with up to J=50 rotational levels, and contains 818 radiative transitions and 20 000 collisional rate coefficients. Our RADEX model spectra were combined with K-band angular sizes measured with the Palomar Testbed Interferometer to locate the emitting region of the CO 2 gas within the atmosphere. We found the CO 2 gas to be highly extended and span a wide range of temperatures and column densities. Just over half of the Miras in our sample also exhibited the 13 μm feature, which allowed us to investigate phase-dependent morphology and explore its relationship to the CO 2 features.
Abstract: The conditions in Mira variable atmospheres make them wonderful laboratories to study a variety of stellar physics such as molecule–grain formation, dust production, shock chemistry, stellar winds, mass loss, opacity-driven pulsation, and shocks. We were awarded an NSF grant to analyze over a decade of synoptic observations from the Palomar Testbed Interferometer (PTI) of 106 Miras to curate a Mira Reference Dataset. The Miras included in this dataset include M-types, S-types, and C-types, and span a wide range of pulsation periods. PTI measured K-band angular sizes that when combined with a distance allow us to directly determine fundamental stellar parameters such as effective temperature, radial size, and bolometric flux. Supplementing observations with interferometric measurements of the stars opens the Mira laboratory to a wealth of different experiments. We provide two case studies to serve as examples of the power of the Mira Reference Dataset. The first case study describes combining PTI measurements with Spitzer IRS spectra of M-type Miras, which allowed us to fully characterize CO2 gas in their atmospheres. The second case study examines how PTI narrow-band data can be used to study phase-dependent pulsation effects on the stellar atmosphere. We provide a list of all the Miras (with coordinates) included in the set for anyone who would like to add them to their observing programs. All the data we produce and collate for this Mira Reference Dataset will be hosted and curated on a website open to the public so that other researchers and citizen scientists can participate in expanding the utility and body of knowledge on this set of “wonderful” stars. Keywords: Mira variables; Mira Reference Dataset; techniques: (long-baseline interferometry, high-angular resolution); instrument: Palomar Testbed Interferometer (PTI); stars: fundamental parameters (angular diameters, radii, temperatures)
Abstract: One of the fundamental topics in astrophysics is how stars enrich their environments as they reach the end of their lives. This enrichment is vital for new star and planet formation, but our understanding of molecule and dust production in circumstellar environments is still rudimentary. Mira variables are highly evolved cool stars with regular pulsations that loft enriched material into their surroundings, making them perfect laboratories for studying molecules and dust in stellar environments. I will present time-dependent analysis of mid-infrared spectra of 14 oxygen-rich (M-type) Mira variables taken with the Spitzer Infrared Spectrograph (IRS) in the high-resolution mode. Each star has multiple spectra obtained over a one-year period (from 2008-09) which allows us to analyze how pulsations affect the stellar atmospheres. We have identified several CO2 Q-branch bandheads in the M-type spectra. These lines exhibit unique, fluctuating behavior possibly tied to the pulsational phase of the star. We built a custom molecular file of ro-vibrational data to model the CO2 bandheads with the radiative transfer code RADEX. The results from the models indicate that CO2 is highly extended in Mira atmospheres. The models also show the kinetic temperature of the CO2 layers is cool enough for dust condensation to occur within a few stellar radii; this is observational evidence of “refrigeration zones” that have been theorized as part of the dust condensation process. The analysis from the CO2 features also indicates that Miras with longer periods (over 300 days) have different atmospheric behavior than those with shorter periods.
Abstract: 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 \citep{2018A&ARv..26....1H}, 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) \citep{2004SPIE.5487...62H}; 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 CO 2 2 identified in the spectra of the oxygen-rich Miras (M-types,) and a previously un-observed feature at 17.62 μ μ m that was observed across all three chemical subclasses. We have tentatively identified this new feature as Fe I. The CO 2 2 lines were analyzed using the publicly available code, RADEX \citep{2007A&A...468..627V}, 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 CO 2 2 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 CO 2 2 that includes over 800 radiative transitions, approximately 20,000 collisions with H 2 2 , and spans temperatures from 100-1000 K. We used RADEX to calculate synthetic spectra that match the observed CO 2 2 Q-branch bandheads. The synthetic spectra allow us to determine atmospheric conditions of the CO 2 2 gas like column density and kinetic temperature. For the new feature at 17.62 μ μ m 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 CO 2 2 is highly extended throughout the M-type atmospheres. The kinetic temperatures also indicate that the CO 2 2 gas is much cooler in regions close to the star than radiative equilibrium conditions would predict. This suggests that CO 2 2 is in a previously theorized \lQ refrigeration zone'' that requires a break from radiative equilibrium, and allows dust condensation within a few R ⋆ ⋆ \citep{2000ARA&A..38..573W}. The behavior of the CO 2 2 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 μ μ m 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.
Abstract: We present analysis of mid-infrared (IR) spectra of the oxygen-rich Mira variable R Tri. The data were taken with the Spitzer Infrared Spectrometer (IRS) as part of a study tracking how Mira variables' regular pulsations affect circumstellar envelopes. We detected strong emission lines at 13.87, 16.18, and 17.6 μm , and one strong absorption feature at 14.98 μm. The emission features at 13.87 and 16.18 μm are excited vibrational bands of CO2, while the absorption feature is the fundamental ν2 band. The 17.6 μm emission feature has a completely different character than the molecular lines and we report its identification as Fe I fluorescence. We used a two-slab model with the radiative transfer code RADEX to model the CO2 Q-branch bandheads. Our results indicate a slab of gas with T∼600 K located at ∼3-4 R*. The cool temperature discrepancy with the radius provides observational evidence for the previously theoretical 'refrigeration zone'.
Abstract: We present analysis of mid-infrared spectra of oxygen-rich (M-type) Mira variables. Each star has multiple spectra obtained over a one-year period (2008--09) using the Spitzer Infrared Spectrograph (IRS) in the high resolution mode (R~600). Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have identified several ro-vibrational Q-branch bandheads of CO2 that are not observable with ground based instruments because telluric features dominate at these wavelengths. Additionally, there is a narrow bright feature at 17.6μm that is present which we have identified as Fe fluorescence. The CO2 lines exhibit unique, fluctuating behavior possibly tied to the pulsational phase of the star; for example the fundamental band at 15μm is seen in both emission and absorption. We built a file of ro-vibrational data that we used to model the CO2 lines with the radiative transfer code RADEX. We present results from these CO2 models that describe the physical characteristics of the gas such as temperature and density. Using RADEX results from several M-type stars will give us a better understanding of how the CO2 gas behaves in oxygen rich Mira atmospheres.
Abstract: One of the outstanding questions in astrophysics is how stars enrich their environments as they reach the end of their lives. This enrichment is vital for new star and planet formation, but our understanding of molecule and dust production in circumstellar environments is still rudimentary. Mira variables are highly evolved cool stars with regular pulsations that loft enriched material into their surroundings, making them perfect laboratories for studying molecules and dust in stellar environments. We present analysis of mid-infrared Spitzer spectra of oxygen-rich (M-type) and carbon-rich (C-type) Mira variables. Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have spectra of 25 Mira variables, taken with phase, using the Spitzer Infrared Spectrograph (IRS) in the high-resolution mode. Each star has multiple spectra obtained over a one-year period (from 2008-09). We have identified several CO2 lines in the M-type Miras, and HCN and C2H2 lines in the C-type Miras. Our spectra show CO2 lines that are not observable with ground based instruments because telluric features dominate at these wavelengths. Additionally, there is a narrow bright feature at 17.6 μm that is present in both chemistries; we have preliminarily identified this feature as Fe fluorescence. The CO2 lines exhibit unique, fluctuating behavior possibly tied to the pulsational phase of the star; for example the fundamental band at 15 μm is seen in both emission and absorption. We built a file of ro-vibrational data that we used to model the CO2 lines with the radiative transfer code RADEX. We present results from these CO2 models that describe the physical characteristics of the gas such as temperature and density. Using RADEX results from several M-type stars will give us a better understanding of how the CO2 gas behaves in oxygen rich Mira atmospheres.
Blinded By The Lines: Mid-IR Spectra Of Mira Variables Taken With Spitzer
Abstract: We present preliminary analysis of mid-infrared spectra of M-type and C-type Mira variables. Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have spectra of 25 Mira variables, taken with phase, using the Spitzer Infrared Spectrograph (IRS) high-resolution module. Each star has multiple spectra obtained over a one-year period from 2008-09. This is a rich, unique data set due to multiple observations of each star and the high signal-to-noise ratio from quick exposure times to prevent saturation of the IRS instrument. This paper focuses on the 17.6 and 33.2 micron lines shared by M-types and C-types. These are mostly emission lines that change with phase. We discuss preliminary physical diagnostics for the atmospheres based on the lines, as well as possible line identifications such as fluorescence of metal species.