Calendar

Jan
24
Wed
Colloquium Thesis Proposal: Laurel Farris
Jan 24 @ 2:30 pm – 3:30 pm
Colloquium Thesis Proposal: Laurel Farris @ Science Hall, Room 110

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.

Feb
23
Fri
Colloquium Thesis Proposal: Jodi Berdis
Feb 23 @ 3:15 pm – 4:15 pm
Colloquium Thesis Proposal: Jodi Berdis @ BX102

Colloquium Title

Jodi Berdis, NMSU

Abstract text

Mar
2
Fri
Colloquium PhD Thesis Defense: Gordon MacDonald
Mar 2 @ 3:15 pm – 4:15 pm
Colloquium PhD Thesis Defense: Gordon MacDonald @ BX102

Colloquium Title

Gordon MacDonald, NMSU

Abstract

Mar
15
Thu
Colloquium Thesis Proposal: Drew Chojnowski
Mar 15 @ 3:15 pm – 4:15 pm
Colloquium Thesis Proposal: Drew Chojnowski @ Domenici Hall 102

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.

Mar
28
Wed
Colloquium PhD Thesis Defense: Ethan Dederick
Mar 28 @ 3:15 pm – 4:15 pm
Colloquium PhD Thesis Defense: Ethan Dederick @ Science Hall 109

Seismic Inferences of Gas Giant Planets: Excitation & Interiors

Ethan Dederick, NMSU

Seismology has been the premier tool of study for understanding the interior structure of the Earth, the Sun, and even other stars. In this thesis we develop the framework for the first ever seismic inversion of a rapidly rotating gas giant planet. We extensively test this framework to ensure that the inversions are robust and operate within a linear regime. This framework is then applied to Saturn to solve for its interior density and sound speed profiles to better constrain its interior structure. This is done by incorporating observations of its mode frequencies derived from Linblad and Vertical Resonances in Saturn’s C-ring. We find that although the accuracy of the inversions is mitigated by the limited number of observed modes, we find that Saturn’s core density must be at least 8.97 +/- 0.01 g cm^{-3} below r/R_S = 0.3352 and its sound speed must be greater than 54.09 +/- 0.01 km s^{-1} below r/R_S = 0.2237. These new constraints can aid the development of accurate equations of state and thus help determine the composition in Saturn’s core. In addition, we investigate mode excitation and whether the \kappa-Mechanism can excite modes on Jupiter. While we find that the \kappa-Mechanism does not play a role in Jovian mode excitation, we discover a different opacity driven mechanism, The Radiative Suppression Mechanism, that can excite modes in hot giant planets orbiting extremely close to their host stars if they receive a stellar flux greater than 10^9~erg cm^{-2} s^{-1}. Finally, we investigate whether moist convection is responsible for exciting Jovian modes. Mode driving can occur if, on average, one cloud column with a 1-km radius exists per 6423 km^2 or if ~43 storms with 200 columns, each with a radius of 25 km, erupt per day. While this seems unlikely given current observations, moist convection does have enough thermal energy to drive Jovian oscillations, should it be available to them.

Apr
6
Fri
Colloquium PhD Thesis Defense: Sten Hasselquist
Apr 6 @ 3:15 pm – 4:15 pm
Colloquium PhD Thesis Defense: Sten Hasselquist @ BX102

Colloquium Title

Sten Hasselquist, NMSU

Abstract

Apr
13
Fri
Colloquium Thesis Proposal: Emma Dahl
Apr 13 @ 3:15 pm – 4:15 pm
Colloquium Thesis Proposal: Emma Dahl @ BX102

Colloquium Title

Emma Dahl, NMSU

Abstract text

May
9
Wed
Colloquium Thesis Proposal: Caitlin Doughty
May 9 @ 2:15 pm – 3:15 pm
Colloquium Thesis Proposal: Caitlin Doughty @ Science Hall 107

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.

Sep
12
Wed
Colloquium PhD Thesis Defense: Alexander Thelen (Host: Nancy Chanover)
Sep 12 @ 3:00 pm – 4:00 pm
Colloquium PhD Thesis Defense: Alexander Thelen (Host: Nancy Chanover) @ Domenici Hall Room 102

The Chemical Composition and Dynamics of Titan’s Atmosphere as Revealed by ALMA

Alexander Thelen, NMSU

Over the last century, remarkable advances in our understanding of Titan’s atmosphere have been accomplished by a campaign of ground- and space-based observations revealing a wealth of complex, organic species in the moon’s upper atmosphere. Many of Titan’s atmospheric constituents produced through the photochemistry and ionospheric interactions of N2 and CH4 exhibit significant variations with latitude and time, particularly towards the poles and within the winter circumpolar vortex. The measurement of spatial and temporal variations in Titan’s atmosphere enables us to elucidate connections between its dynamics, photochemistry, and the influence of seasonal changes. At the end of the Cassini mission in 2017, we can employ the Atacama Large Millimeter/submillimeter Array (ALMA) for future observations of Titan’s atmosphere. Here we detail the analysis of numerous short integration (~3 minute) ALMA observations from 2012 to 2015 to investigate Titan’s stratospheric composition, temporal variations, and search for new molecular species. Using the Non-linear optimal Estimator for MultivariatE spectral analySIS (NEMESIS) radiative transfer code, we retrieved vertical profiles of temperature and abundance in Titan’s lower stratosphere through mesosphere (~50–550 km) from three spatially independent regions. We modeled CO emission lines to obtain temperature measurements, and retrieved abundance profiles for HCN, HC3N, C3H4, and CH3CN. The combination of integrated flux maps and vertical atmospheric profiles from spatially resolved observations allowed us to study the circulation of Titan’s middle atmosphere during northern spring. We observed increased temperatures in Titan’s stratopause at high northern latitudes and a persistent northern enrichment of HCN, C3H4, and CH3CN during this epoch; however, increased abundances of all molecules in the southern mesosphere, particularly HCN, and spatial maps of HC3N also show evidence for subsidence at the south pole. We validated these measurements through direct comparisons with contemporaneous Cassini data, previous ground-based observations, and photochemical model results. While no new trace species were detected, ALMA has proven to be a highly capable asset to enhance the data from the final few years of the Cassini mission, and for the continued study of Titan’s atmospheric dynamics, composition, and chemistry into Titan’s northern summer.

Jan
23
Wed
Colloquium Thesis Defense: Lauren Kahre
Jan 23 @ 3:00 pm – 4:00 pm
Colloquium Thesis Defense: Lauren Kahre

Extinction Mapping and Dust-to-Gas Ratios of Nearby Galaxies

Lauren Kahre, NMSU

We present a study of the dust{to{gas ratios in 31 nearby (D >
10 Mpc) galaxies. Using Hubble Space Telescope broad band WFC3/UVIS UV and
optical images from the Treasury program LEGUS (Legacy ExtraGalactic UV
Survey) combined with archival HST/ACS data, we correct thousands of
individual stars for extinction across these galaxies using an
isochrone-matching (reddening-free Q) method. We generate extinction maps
for each galaxy from the individual stellar extinctions using both
adaptive and fixed resolution techniques, and correlate these maps with
neutral HI and CO gas maps from literature, including The HI Nearby Galaxy
Survey (THINGS) and the HERA CO-Line ExtraGalactic Survey (HERACLES). We
calculate dust-to-gas ratios and investigate variations in the dust-to-gas
ratio with galaxy metallicity. We find a power law relationship between
dust-to-gas ratio and metallicity. The single power law is consistent with
other studies of dust-to-gas ratio compared to metallicity, while the
broken power law shows a significantly shallower slope for low metallicity
galaxies than previously observed. We find a change in the relation when
H_2 is not included. This implies that underestimation of N_H2 in
low-metallicity dwarfs from a too-low CO-to-H2 conversion factor X_CO
could have produced too low a slope in the derived relationship between
dust-to-gas ratio and metallicity. We also
compare our extinctions to those derived from fitting the spectral energy
distribution (SED) using the Bayesian Extinction and Stellar Tool (BEAST)
for NGC 7793 and and systematically lower extinctions from SED-fitting as
compared to isochrone matching. Finally, we compare our extinction maps of
NGC 628 to maps of the dust obtained via IR emission from Aniano et al.
(2012) and find a factor of 2 difference in dust-to-gas ratios determined
from the two maps, consistent with previous work.