The Chemical History and Evolution of Titan’s Atmosphere as Revealed by ALMA
Saturn’s largest moon, Titan, possesses a substantial atmosphere containing significant minorities of nitrile and hydrocarbon species, predominantly due to the photodissociation of the major gases, N2 and CH4. Titan’s methane cycle, liquid lakes, and complex organic chemistry make it an intriguing target through its similarities to Earth and the allure of its astrobiological potential. Though the existence of heavy nitrile species – such as CH3C3N, HC5N, and C3H7CN – has been inferred through Cassini Ion and Neutral Mass Spectrometer (INMS) data, confirmation of these species has yet to be made spectroscopically. Other hydrocarbon species, such as C3H4 and C3H8 have been detected using Voyager’s Infrared Spectrometer (IRIS; Maguire et al., 1981) and later mapped by the Composite Infrared Spectrometer (CIRS; Nixon et al., 2013) onboard Cassini, but abundance constraints for these species in the mesosphere is poor. To fully understand the production of these species and their spatial distribution in Titan’s atmosphere, vertical abundance profiles must be produced to use with current photochemical models. Utilizing early science calibration images of Titan obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA), Cordiner et al. (2014; 2015) determined the vertical distribution of various nitriles and hydrocarbons in Titan’s atmosphere, including at least one previously undetected molecule – C2H5CN. For my dissertation project, I will calibrate and model sub-millimeter emissions from molecules in Titan’s atmosphere, and quantify variations in the spatial distribution of various species throughout its seasonal cycle by utilizing high resolution ALMA data. The main goals of this project are as follows:
1. To search for previously undetected molecules in Titan’s atmosphere through analysis of the existing public ALMA data, and/or through ALMA proposals of my own;
2. Constrain abundance profiles of detected molecular species, and provide upper abundance limits for those we cannot detect;
3. Map the spatial distribution of detected species in order to improve our understanding of Titan’s atmospheric transport and circulation;
4. Determine how these spatial distributions change over Titan’s seasonal cycle by utilizing multiple years of public ALMA data.
The majority of this work will employ the Non-linear Optimal Estimator for MultivariatE Spectral analySIS (NEMESIS) software package, developed by Oxford University (Irwin et al., 2008), to retrieve abundance and temperature information through radiative transfer models. These results will allow us to investigate the chemical evolution and history of Titan’s rich, pre-biotic atmosphere by providing valuable abundance measurements and constraints to molecular photochemical and dynamical models. We will compare our results with measurements made by the Cassini spacecraft, thereby enhancing the scientific return from both orbiter and ALMA datasets. The increased inventory of complex, organic molecules observable with ALMA’s sub-mm frequency range and high spatial resolution may also yield detections of species fundamental to the formation of living organisms, such as amino acids. Thus, by informing photochemical and dynamical models and increasing our known inventory of complex molecular species, we will also assess Titan’s potential habitability.
Extinction mapping with LEGUS
The study of star formation and galaxy evolution in nearby galaxies depends on obtaining accurate stellar photometry in those galaxies. However, dust in the galaxies hinders our ability to obtain accurate stellar photometry, particularly in star-forming galaxies that have the highest concentrations of dust. This proposal presents a thesis project to develop a method for generating extragalactic extinction maps using photometry of massive stars from the Hubble Space Telescope. This photometry spans nearly 50 galaxies observed by the Legacy Extragalactic Ultraviolet Survey (LEGUS). The derived extinction maps can be used to correct other stars and Halpha maps (from the Halpha LEGUS) for extinction, and will be used to constrain changes in the dust-to-gas ratio across the galaxy sample and in different star formation rate, metallicity and morphological environments. Previous studies have found links between galaxy metallicty and the dust-to-gas mass ratio. The relationship between these two quantities can be used to constrain chemical evolution models.
Selected galaxies will also be compared to IR-derived dust maps for comparison to recent M31 results from Dalcanton et al. (2015) which found a minimum factor of 2 inconsistency between their extinction-derived maps and emission-derived maps from Draine et al. (2014).
Utilizing Planetary Oscillations to Constrain the Interior Structure of the Jovian Planets
Seismology has been the premier tool of study for understanding the
interior structure of the Earth, the Sun, and even other stars. Yet in this
thesis proposal, we wish to utilize these tools to understand the interior
structure of the Jovian planets, Saturn in particular. Recent observations
of spiral density structures in Saturn’s rings caused by its oscillations
have provided insight into which modes exist within Saturn and at what
frequencies. Utilizing these frequencies to compare to probable mode can-
didates calculated from Saturn models will also us to ascertain the interior
profiles of state variables such as density, sound speed, rotation, etc. Using
these profiles in a Saturn model, coupled with tweaking the interior struc-
ture of the model, i.e. the inclusion of stably stratified regions, should
allow us to explain which modes are responsible for the density structures
in the rings, as well as predict where to look to find more such structures.
In doing so, we will not only have a much greater understanding of Sat-
urn’s interior structure, but will have constructed a method that can also
be applied to Jupiter once observations of its mode frequencies become
available. In addition, we seek to explain if moist convection on Jupiter is
responsible for exciting its modes. We aim to do this by modeling Jupiter
as a 2D harmonic oscillator. By creating a resonance between moist con-
vective storms and Jovian modes, we hope to match the expected mode
energies and surface displacements of Jupiter’s oscillations.
Characterizing the oscillatory response of the chromosphere during solar flares
Laurel Farris; NMSU Astronomy Department
Quasi-periodic pulsations (QPPs) are observed in the emission of solar flares over a wide range of wavelengths,
particularly in the radio and hard x-ray regimes where non-thermal emission dominates. These pulsations are
considered to be an intrinsic feature of flares, yet the exact mechanism that triggers them remains unclear.
There have been reports of an increase in the oscillatory power at 3-minute periods (the local acoustic
cutoff frequency) in the solar chromosphere associated with flaring events. I propose to investigate the
chromospheric response to flares by inspecting the spatial and temporal onset and evolution of the 3-minute
oscillatory power, along with any QPP patterns that may appear in chromospheric emission. The analysis
will be extended to multiple flares, and will include time before, during, and after the main event. To test
initial methods, the target of interest was the well-studied 2011 February 15 X-class flare. Data from two
instruments on board the Solar Dynamics Observatory (SDO) were used in the preliminary study, including
continuum images from the Helioseismic and Magnetic Imager (HMI) and UV images at 1600 and 1700
Angstroms from the Atmospheric Imaging Assembly (AIA). Later, spectroscopic data from the Interface
Region Imaging Spectrometer (IRIS) will be used to examine velocity patterns in addition to intensity.
The Circumstellar Disks and Binary Companions of Be Stars
Drew Chojnowski, NMSU
Tremendous progress has been made over the past two decades toward understanding Be stars, but a number of key aspects of them remain enigmatic. The unsolved mysteries include identification of the mechanism responsible for disk formation, the reason this mechanism occasionally turns off or on unexpectedly, the source of viscosity in the circumstellar disks, and the cause of slowly precessing density perturbations in the disks of many or most Be stars. On a deeper level, the origin of Be stars’ near-critical rotation is unknown, with one possible explanation being spin-up due to interaction with a binary companion. A better understanding of these stars is needed, with a particular focus on high-mass binaries being warranted in the age of gravitational wave astronomy. In this dissertation, I will extend the knowledge and understanding of Be stars through a series of three projects. First, I will present and describe the largest ever homogeneous, spectroscopic sample of Be stars to date. I will then focus on investigation of a rare class of Be stars found in binary systems with hot, low mass companions. The second project will present detailed characterization and modeling of HD~55606, a newly discovered member of this class. Finally, I will discuss the results of spectroscopic monitoring of seven newly discovered systems and establish or place limits on the orbital parameters of the binary components.
Metal Absorption in the Circumgalactic Medium During the Epoch of Reionization
Caitlin Doughty, NMSU
The characteristics of metal absorption arising from the circumgalactic medium of galaxies have been demonstrated to be related to conditions in the galaxy which sourced them, as well as to the ambient ultraviolet background. I propose a three- pronged thesis in order to better understand and utilize these relationships. First, I will explore whether the spectral energy distributions of binary stars, incorporated into a custom version of GADGET-3, can explain the discrepancy between observed and simulated absorber statistics. Second, I will study the relationship between neu- tral oxygen absorbers and the neutral hydrogen fraction in simulated quasar sight- lines and relate the results to observations of neutral oxygen at z ≥ 4.0. Third, I will study the relationships between the emissive properties of galaxies, stemming from their nebular gas, and the metal absorbers which they source. Taken as a whole, this thesis will improve the ability of cosmological simulations to reproduce realistic metal absorption, probe the local progress and topology of reionization, and under- stand what emissive galaxy traits we expect at z > 5 based on observations of metal absorbers.