2021 Astronomy/Astrophysics
The pre-recorded talks and posters on this page showcase the work of students who received NC Space Grant research funding for the 2019-20 and 2020-21 academic years. The menu at right provides links to pre-recorded talks and posters by other funded students on additional topics.
Joel Bernstein
2020-2021 NC Space Grant Undergraduate Research Scholar
The University of North Carolina at Chapel Hill
Undergraduate Student (Junior), Physics
Stellar-Stellar Counterrotation in E/S0 Galaxies in the RESOLVE Survey
Galaxy morphologies are broadly categorized by the relative contribution of two components: the spheroidal “bulge” and the pancake-like “disk.” Disk-dominated galaxies, like the Milky Way, tend to have abundant gas and high rates of new star formation. Bulgier galaxies, on the other hand, are poorer in gas and have older stellar populations. The canonical model of structure formation in the universe predicts that galaxies should tend to become bulgier throughout their lifetimes as they collide with each other. Contrary to this expectation, disk-dominated galaxies are observed to be the most common type. A mechanism called disk regrowth, in which accreted gas results in disk formation over time, has been suggested as a way to explain this disagreement. Tracers of this process can be found in the form of stellar populations rotating counter to the rest of the galaxy. We attempt to identify these tracers in spectroscopy for a sample of spheroidal galaxies in the REsolved Spectroscopy Of a Local VolumE (RESOLVE) survey. This is achieved through the use of the stellar kinematics analysis software Penalized PiXel-Fitting (pPXF), which is able to fit multiple kinematic components to the spectrum. Through this analysis we may discern between disk regrowth and other processes as the source of the counterrotation.
Faculty Advisor: Sheila Kannappan
Reece Boston
2020-2021 NC Space Grant Graduate Research Fellow
The University of North Carolina at Chapel Hill
Graduate Student (Ph.D.), Physics
Relativistic Pulsations of White Dwarfs
The now-retired K2 and the current TESS missions are aimed primarily at observing exoplanet transitions. One side-effect of these NASA missions is an enormous leap in continuous observation times for pulsating white dwarfs (WDs). The increase in observation times leads to significantly enhanced precision when measuring the periods of WD pulsations seen in light curves. The relative uncertainties for the periods are now on the order of 10^{-4} and in some cases much smaller. This accuracy scale is important, because for a typical WD with mass around 0.6 solar masses, general relativity also introduces effects in the periods on order 10^{-4}.
This suggests that gravity in a WD is strong enough, and our measurements accurate enough, that gravitational effects beyond Newtonian physics can be detected within WD pulsation spectra. If so, then asteroseismic studies of WDs using these accurate periods from the K2 or TESS data must incorporate Einstein’s General Relativity (GR) in order to use the full accuracy of the period observations.
To study this question, we formed a set of internal wave equations using the Post-Newtonian (PN) approximation to GR. This approximation works very well in the case of weak fields and slow motion, such as most WDs. The resulting PN equations closely mirror the Newtonian equations, which means they can be easily incorporated in most pre-existing methods for finding WD frequencies. Using these equations, we performed numerical studies in both Newtonian and PN physics, comparing the frequencies. We find the relative difference is on the expected order 10^{-4}, indicating that the relativistic effects on the frequency could indeed be measured by modern missions such as TESS.
Faculty Advisor: Charles Evans
Derrick Carr
2020-2021 NC Space Grant Graduate Research Fellow
The University of North Carolina at Chapel Hill
Graduate Student (Masters), Astrophysics
Probing the Evolution of Local Nuggets in the RESOLVE Survey
Blue nuggets are compact, highly star-forming galaxies that are typically observed in the early universe. Simulations show that they form via gas-rich interactions collectively known as “compaction events.” Theory suggests that as blue nuggets reach a particular key mass, they stop forming stars and evolve into red nuggets. While nuggets are mainly studied in the early universe, there has been little exploration in nuggets that are forming nearby and how they evolve. Excitingly, our team has recently discovered that blue nuggets are still forming today. This motivates us to search for nuggets at various stages in their evolution within the REsolved Spectroscopy Of a Local VolumE (RESOLVE) survey. While we are currently working on creating a nugget sample that is guided by simulations, we created a pilot sample to verify that our science is feasible with the RESOLVE survey. Our sample shows strong evidence of being nuggets at various evolutionary stages and reaffirms that the blue-to-red nugget transition follows theoretical expectations.
Faculty Advisor: Sheila Kannappan
Ella Castelloe
2020-2021 NC Space Grant Undergraduate Research Scholar
The University of North Carolina at Chapel Hill
Undergraduate Student (Senior), Astrophysics
Finding Galaxy Groups in the Early Stages of Formation in the RESOLVE Survey
The most common galaxy group finding algorithm, Friends-of-Friends (FoF), finds settled groups that share a common dark matter halo but misses groups that are in earlier stages of formation and have not yet merged halos. We present a new algorithm that is designed to find groups like the Local Group that are gravitationally bound but do not yet share a common halo and are not identified as FoF groups. We use escape velocity to test whether settled groups are bound to other nearby settled groups. We statistically correct for projection effects that arise from using observational data with a mock catalog containing simulated three-dimensional data. We apply the boundness method to RESOLVE and ECO, two large volume limited surveys of galaxies in the local universe. Large volume limited surveys are ideal for studying galaxy groups since the population of groups that we identify should accurately reflect the population of groups in the broader universe. Using our boundness method increases the number of multiple galaxy systems that are identified from 235 to 386, and decreases the number of single galaxy systems from 832 to 694 in RESOLVE. To determine how well we find systems like the Local Group, we identify a population of Local Group analogues in the Mock Catalog. FoF finds only 6% of the Local Group analogues, whereas the boundness method finds 96%. We examine different categories of bound systems, focusing on the evolutionary connection between groups and “proto-groups”. We compare properties such as the gas content, and large scale environments of proto-groups and small settled groups (FoF groups with halo mass less than 1012 M☉). We use metrics such as virialization state, compactness, and color dispersion to quantify the evolutionary state of groups. Our results provide a foundation for studies of the evolution of groups over cosmic time.
Faculty Advisor: Sheila Kannappan
Hank Corbett
2020-2021 NC Space Grant Graduate Research Fellow
The University of North Carolina at Chapel Hill
Graduate Student (Ph.D.), Astronomy
Orbital Foregrounds for Ultra-Short Duration Transients
Reflections from objects in Earth orbit can produce sub-second, star-like optical flashes similar to astrophysical transients. Reflections have historically caused false alarms for transient surveys, but the population has not been systematically studied. We report event rates for these orbital flashes using the Evryscope Fast Transient Engine, a low-latency transient detection pipeline for the Evryscopes. We select single-epoch detections likely caused by Earth satellites and model the event rate as a function of both magnitude and sky position. We measure a rate of 1800 [+600/-280] per sky per hour, peaking at m = 13.0, for flashes morphologically degenerate with real astrophysical signals in surveys like the Evryscopes. Of these, 340 [+150/-85] per sky per hour are bright enough to be visible to the naked eye in typical suburban skies with a visual limiting magnitude of V. These measurements place the event rate of orbital flashes orders of magnitude higher than the combined rate of public alerts from all active all-sky fast-timescale transient searches, including neutrino, gravitational-wave, gamma-ray, and radio observatories. Short-timescale orbital flashes form a dominating foreground for un-triggered searches for fast transients in low-resolution, wide-angle surveys. However, events like fast radio bursts (FRBs) with arcminute-scale localization have a low probability (~10^-5) of coincidence with an orbital flash, allowing optical surveys to place constraints on their potential optical counterparts in single images. Upcoming satellite internet constellations, like SpaceX Starlink, are unlikely to contribute significantly to the population of orbital flashes in normal operations. In this talk, I will discuss the flash rate as a function of observed magnitude and sky position, and the impact of this population on both current and upcoming observatory facilities.
Faculty Advisor: Nicholas M. Law
Nathan Galliher
2020-2021 NC Space Grant Graduate Research Fellow
The University of North Carolina at Chapel Hill
Graduate Student (Ph.D.), Astrophysics
Chasing Young Star Superflares Using the Evryscopes
Using real time detections from the Evryscope-South we have begun a study of large flaring events (superflares) from young stars (<1 Gyr). These energetic flares emit in excess of 10^{33} erg, 10-1000x the energy of the largest solar flares. High energy superflare events directly impact the potential habitability of any orbiting planets. Young stars flare significantly more often than their older counterparts. Superflares from these stars could support life on planets orbiting outside of the habitable zone, or strip conventionally habitable planets of their atmospheres. Very few multi-band or spectral measurements exist of superflares from populations of young stars; this project aims to expand on these systems by at least an order of magnitude. We use the Evryscope-South to search for superflare events occurring in real time. The Evryscopes are full-sky, high-cadence telescopes that are based in both the Northern and Southern hemispheres. The two instruments are located on Cerro Tololo in Chile and Mount Laguna in California, and each monitor the full sky using 24 6.1 cm aperture telescopes on a shared mount, covering 30 million targets at a two-minute cadence. The Evryscope-South triggers on flares as they start (a 25% flux increase is easily detectable and can represent the start of a superflare). Using the real-time alerts from the Evryscope we target SOAR’s Goodman spectrograph on superflares as they happen, allowing us to capture nearly all of the flare evolution and decay processes.
Faculty Advisor: Nicholas M. Law
Benjamin Kaiser
2020-2021 NC Space Grant Graduate Research Fellow
The University of North Carolina at Chapel Hill
Graduate Student (Ph.D.), Astrophysics
Lithium Enrichment and Depletion in Extrasolar Planetesimals
Tidal disruption and subsequent accretion of planetesimals by white dwarfs can reveal the elemental abundances of rocky bodies in exoplanetary systems. Those abundances provide information on the composition of the nebula from which the systems formed and the geological processes that the rocky material experienced, which is analogous to how meteorite abundances inform our understanding of the Solar System. Multiple white dwarfs have recently been discovered to be polluted by lithium from extrasolar planetesimals, with lithium levels higher than those of rocky material from the Solar System. The origin of this apparent lithium enrichment is currently disputed with geological processes and initial nebular abundances each being proposed as the origin. We interpret these abundances from the literature and newly obtained abundance limits for white dwarf SDSS J1636+1619 to demonstrate that geological processes are not adequate to explain the lithium overabundances in all extrasolar planetesimals with lithium detections, but they can explain the non-detection of lithium in objects such as the one that pollutes SDSS J1636+1619. We present the non-detection and subsequent upper limits of lithium and potassium abundances in the planetesimal-polluted atmosphere of SDSS J1636+1619 as measured from spectroscopic observations carried out during Director’s Discretionary Time at the Gemini Observatory with the GMOS spectrograph. The abundances and relative ages of the systems support the apparent lithium overabundances resulting from the initial nebular abundances. We compare our results to predictions for Galactic nucleosynthetic enrichment models and the Big Bang nucleosynthetic predictions for lithium.
Faculty Advisor: J. Christopher Clemens
Isabel O’Brien
2020-2021 NC Space Grant Undergraduate Research Scholar
The University of North Carolina Chapel Hill
Undergraduate Student (Junior), Biology/Environmental Studies
Analysis of OJ-287 Data
OJ-287 is a binary black hole system in which the smaller black hole periodically passes through the accretion disk of the larger. These collisions result in bright, hot bubbles of gas that emit radiation detectable on Earth. In the summer of 2019, myself and other students working with Dr. Reichart used the 20 meter telescope at Green Bank Observatory to take hourly images of OJ-287 during the three months surrounding a collision event. Because of the sun’s proximity in the sky to the black holes at the time, the images contain a lot of solar interference. My work this year has been focused on analyzing this data to produce clearer images using code written by some of Dr. Reichart’s previous students. Because the interference patterns caused by the sun rotate with each consecutive image, I have been stacking the images together in a way that will hopefully mitigate the interference. My results so far have been fairly inconclusive; stacking the images has resulted in a decrease in interference, but not enough to clearly see the source of radiation that we are looking for. By adjusting RFI (radio frequency interference) values and other parameters in the code, I am hoping to clean up the images enough that we can conclusively see OJ-287 and learn more.
Faculty Advisor: Dan Reichart
Joshua Reding
2020-2021 NC Space Grant Graduate Research Fellow
The University of North Carolina at Chapel Hill
Graduate Student (Ph.D.), Astrophysics
Directly Measuring White Dwarf Rotation With High-Resolution Spectroscopy
Recent research suggests that a significant fraction of high-mass white dwarf stars may be formed in stellar mergers rather than from single-star evolution, as indicated by the presence of unusual characteristics such as strong magnetic fields and rapid rotation due to angular momentum conservation during the merger. However, measuring white dwarf rotation is an observational challenge; accurate rotation periods require the presence of a magnetic spot on the stellar surface which causes photometric variability, or the presence of pulsations where variability modes are split proportional to the stellar rotation rate. Following the work of Koester et al. (1998) and Karl et al. (2005), we collected high-resolution spectra of eight white dwarf stars with masses > 0.8 M_sun using the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.1 m Very Large Telescope (VLT) array in Chile, and fit rotationally broadened synthetic spectra to directly measure stellar rotation. We investigate the relation between white dwarf mass and rotation rate; a strong positive correlation would support the growing consensus that many high-mass white dwarfs are formed in binary mergers.
Faculty Advisor: J. Christopher Clemens