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.
Simulations of the interstellar medium at high redshift: What does [CII] trace?
Dr. Karen Olsen, Arizona State University
We are in an exciting era were simulations on large, cosmological scales meet modeling of the interstellar medium (ISM) on sub-parsec scales. This gives us a way to predict and interpret observations of the ISM, and in particular the star-forming gas, in high-redshift galaxies, useful for ongoing and future ALMA/VLA projects.
In this talk, I will walk you though the current state of simulations targeting the the fine structure line of [CII] at 158 microns, which has now been observed in several z>6 galaxies. [CII] can arise throughout the interstellar medium (ISM), but the brightness of the [CII] line depends strongly on local environment within a galaxy, meaning that the ISM phase dominating the [CII] emission can depend on galaxy type. This complicates the use of [CII] as a tracer of either SFR or ISM mass and calls for detailed modeling following the different ways in which [CII] can be excited.
I will present SÍGAME (Simulator of GAlaxy Millimeter/submillimeter emission) – a novel method for predicting the origin and strength of line emission from galaxies. Our method combines data from cosmological simulations with sub-grid physics that carefully calculates local radiation field strength, pressure, and ionizational/thermal balance. Preliminary results will be shown from recent modeling of [CII] emission from z~6 star-forming galaxies with SÍGAME. We find strong potential for using the total [CII] luminosity to derive the ISM and molecular gas mass of galaxies during the Epoch of Reionization (EoR).
A Faint Flux-Limited LAE Sample at z = 0.3
Isak Wold, UT Austin
Observational surveys of Lya emitters (LAEs) have proven to be an efficient method to identify and study large numbers of galaxies over a wide redshift range. To understand what types of galaxies are selected in LAE surveys – and how this evolves with redshift – it is important to establish a low-redshift reference sample that can be directly compared to high-redshift samples. The lowest redshift where a direct Lya survey is currently possible is at a redshift of z~0.3 via the Galaxy Evolution Explorer (GALEX ) FUV grism data. Using the z~0.3 GALEX sample as an anchor point, it has been suggested that at low redshifts high equivalent width (EW) LAEs become less prevalent and that the amount of escaping Lya emission declines rapidly. A number of explanations for these trends have been suggested including increasing dust content, increasing neutral column density, and/or increasing metallicity of star-forming galaxies at lower redshifts. However, the published z~0.3 GALEX sample is pre-selected from bright NUV objects. Thus, objects with strong Lya emission but faint continuum (high-EW LAEs) could be missed. In this talk, I will present my efforts to re-reduce the deepest archival GALEX FUV grism data and obtain a sample that is not biased against high-EW LAEs. I will discuss the implications of this new sample on the evolutionary trends listed above.
Asteroseismology of Red Giants: The Detailed Modeling of Red Giants in Eclipsing Binary Systems
Jean McKeever, NMSU
Asteroseismology is an invaluable tool that allows one to peer into the inside of a star and know its fundamental stellar properties with relative ease. There has been much exploration of solar-like oscillations within red giants with recent advances in technology, leading to new innovations in observing. The Kepler mission, with its 4-year observations of a single patch of sky, has opened the floodgates on asteroseismic studies. Binary star systems are also an invaluable tool for their ability to provide independent constraints on fundamental stellar parameters such as mass and radius. The asteroseismic scaling laws link observables in the light curves of stars to the physical parameters in the star, providing a unique tool to study large populations of stars quite easily. In this work we present our 4-year radial velocity observing program to provide accurate dynamical masses for 16 red giants in eclipsing binary systems. From this we find that asteroseismology overestimates the mass and radius of red giants by 15% and 5% respectively. We further attempt to model the pulsations of a few of these stars using stellar evolution and oscillation codes. The goal is to determine which masses are correct and if there is a physical cause for the discrepancy in asteroseismic masses. We find there are many challenges to modeling evolved stars such as red giants and we address a few of the major concerns. These systems are some of the best studied systems to date and further exploration of their asteroseismic mysteries is inevitable.
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.