Highest Honors in Astronomy
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Item Impact of Entropy on Black Hole Astrophysics(Vanderbilt University. Dept. of Physics and Astronomy, 2024-04-18) Chen, SiyuanAs an irreversible natural process, mergers between two black holes will increase the entropy of the universe. By imposing thermodynamical constraints derived from general relativity and quasi-circular hypothesis, we showcase BRAHMA – a novel framework to infer the properties and astrophysical implications of binary black hole mergers in LIGO-Virgo-KAGRA (LVK). We apply the framework as an IMR (inspiral-merger-ringdown) consistency test to 10 heaviest binary black hole merger events reported by LVK Collaboration and perform a systematic investigation on the consistency between phenomenological waveform and ringdown models (analysis data available on Zenodo). In doing so, we obtain astrophysical insights into the origins of black holes for GW190521 and GW191109, two of the heaviest confirmed merger events. We also show the high consistency of the NRSur7dq4 waveform and Kerr221 ringdown model, providing insights into the timing of the ringdown stage. For events without QNM measurements due to low SNR of ringdown, we use the post-merger conditions inferred from phenomenological waveforms to compute the IBBH (Merger Entropy Index: measures the efficiency of entropy transfer during BBH merger) distribution across all GW events. IBBH shows high differentiability in formation channels, which can be used as a tool to classify compact binary populations. Therefore, we employ IBBH to identify compact objects in the lower mass gap (2.5 ∽ 5𝑀⊙), including the newest discovery GW230529 in the 4th observing run.Item DARK MATTER MAY NOT BE SO DARK: Testing Massive Primordial Black Hole Dark Matter with Galactic X-Ray Emission(Vanderbilt University. Dept. of Physics and Astronomy, 2024-04-16) McPhaul, NolanWere primordial black holes (PBHs) to exist, they could constitute a significant portion of dark matter in our universe which could be explored through gravitational wave emission, gravitational lensing, and electromagnetic observations. Assuming PBHs as a dark matter candidate, we determine the X-ray emission they would radiate from accreting gas in their host galaxy by modeling the dark matter particles in the Romulus25 cosmological simulation as clusters of PBHs. Upon simulating such emission, we compare our X-Ray luminosity functions to those derived from observations with the Chandra X-Ray Telescope to constrain the abundance of PBHs compared to dark matter as a function of PBH mass spanning 10^-2 − 10^5 M . Our results highly constrain the most massive PBHs but leave a vast sea of stellar and sub-stellar mass PBHs unscathed by falling under the flux limitations of Chandra. Therefore, further studies are needed in such a mass regime to explore the possibility of PBHs as a considerable source of dark matter and black holes in our universe. The implications of such could have drastic impacts on our understanding of the composition, machinations, and evolution of our universe.Item Expected Host Galaxy Properties of PTA Detectable Massive Black Hole Binaries(Vanderbilt University. Dept. of Physics and Astronomy, 2021-12-06) Cella, KatharineMassive black hole binaries (MBHB) produce gravitational waves (GW) that will be detectable with pulsar timing arrays within the next few years. We determine the properties of the host galaxies of MBHB at the time they are producing detectable GW. The population of MBHB systems we evaluate is from the Illustris cosmological simulations taken in tandem with post processing semi-analytic models of environmental factors in the evolution of binaries. Upon evolving to the frequency regime detectable by pulsar timing arrays, we calculate the detection probability of each system using a variety of different values for red and white noise. We average over multiple realizations of the universe by re-sampling the host galaxy properties using a kernel density estimator to approximate the statistical distributions of the universe. Excitingly, we find that detectable systems have host galaxies that are clearly distinct from the overall population. With conservative noise factors, we found that stellar metallicity, for example, peaks at approximately twice solar metallicity as opposed to the total population of galaxies which peaks at approximately solar metallicity. Additionally, the most detectable systems are brighter and more red in color than the overall population. These results can be used to develop effective search strategies for identifying host galaxies and electromagnetic counterparts following GW detections.Item The search for M-M Dwarf Eclipsing Binary Systems(Vanderbilt University. Dept. of Physics and Astronomy, 2016-05-18) Lubin, Jack B.; Stassun, Keivan G.Using data from the Kilodegree Extremely Little Telescope (KELT) project, we performed a survey of 2028 known M Dwarf stars in the KELT footprint for M-M Dwarf eclipsing binaries (EBs). After searching 2028 Mdwarf stars, we Follow up observation of our best target’s eclipse events confirmed our hypothesis, and further Spectroscopic and Radial Velocity (RV) data allowed us to model the system. In this paper, we overview our survey and report the discovery and analysis of a new double-lined M-M eclipsing binary system, KS20C055996. This system is the second brightest M-M EB known (V 13.9) with M1 = 0.47 M and M2 = 0.38 M , orbiting on a period of 1.11 days. KS20C055996 provides a unique opportunity to study Very Low Mass Stars (VLMS) and further compare observational results with current theoretical models of low mass stellar interiors.Item Simulating the universe with GPU-accelerated supercomputers: n-body methods, tests, and examples(Vanderbilt University. Department of Physics and Astronomy, 2013-04-22) Wibking, Benjamin; Berlind, Andreas A., 1972-We demonstrate the acceleration obtained from using GPU/CPU hybrid clusters and supercomputers for N-body simulations of gravity based in part on the author's new code development. Validation tests are shown for cosmological simulations and for galaxy simulations, along with their respective speedups compared to traditional simulations. Potential new applications for science enabled by this advance are highlighted.Item Rotation periods of the Hyades open cluster using ASAS light curves : measuring the Hyades gyro-age and benchmarking tools for gyrochronology studies with LSST(Vanderbilt University. Dept. of Physics and Astronomy, 2012-04) Kundert, Alisha; Cargile, Phillip A. (Philip Andrew)Rotation period distributions for older open clusters are difficult to obtain because of the general scarcity of clusters with ages greater than 200 Myr, as well as the challenge of measuring rotation periods for less active older stars. This has clouded our knowledge of how stars spin down as they evolve from the zero-age main sequence. A key piece of our understanding of how stellar rotation rates change over time relies on the study of the nearby (~45 pc) and old (~625 Myr) Hyades open cluster. We present the results from a detailed analysis of the rotation period distribution of the Hyades in an effort to estimate the gyro-age of this benchmark open cluster. Analyzing All Sky Automated Survey (ASAS) light curves with 14+ year baselines, we measure rotation periods for Hyades members by applying a period-finding technique that combines the Lomb-Scargle periodogram and Monte Carlo simulations to optimize period detection in long-baseline, low-cadence light curves that potentially suffer from time-dependent changes, e.g. star spot evolution. We derive 67 rotation periods for Hyades members, 40 of which are determined for the first time. These rotation periods measured from ASAS light curves allow us to characterize the rotation period distribution of the Hyades, enabling the application of current gyrochronology models to calculate a rotation-age for the cluster. Additionally, using ASAS data provides a valuable test of our ability to determine rotation periods in future low-cadence, extended duration surveys, such as the Large Synoptic Survey Telescope (LSST).Item Density profiles of dark matter halos(Vanderbilt University. Dept. of Physics and Astronomy, 2010-04) Robbins, Katherine; Berlind, Andreas A., 1972-Item Number density profiles of galaxy groups and clusters in the SDSS(Vanderbilt University. Dept. of Physics and Astronomy, 2008-04-20) Norris, Jackson M.; Berlind, AndreasWe use projected number density profiles of galaxy groups to investigate the relationship between galaxy group structure and dark matter structure within halos. We primarily study a specific model of how galaxies populate dark matter halos by comparing the number density profiles of galaxy groups from the Sloan Digital Sky Survey with number density profiles of galaxy groups taken from dark matter halo simulations. We also investigate the dependence of galaxy locations within galaxy groups on luminosity and color.Item A Search for Pre-Main Sequence Eclipsing Binary Stars in the Lagoon Nebula(Vanderbilt University. Dept. of Physics and Astronomy, 2009-04-20) Henderson, Calen B.; Stassun, KeivanWe report time-series CCD I-band photometry for the pre-main-sequence cluster NGC 6530, located within the Lagoon Nebula. The data were obtained with the 4Kx4K imager on the SMARTS 1.0m telescope at CTIO on 36 nights over the summers of 2005 and 2006. In total we have light curves for ~50,000 stars in an area ~1 deg2, with a sampling cadence of ~1 hour. Approximately 1,000 of these stars have been identified as bona fide members of the cluster with masses in the range 0.25-4.0 Msun, assuming a distance of 1.25 kpc to the cluster. Our goals are to measure rotation periods for these stars and to identify eclipsing binary candidates. Here we present light curves of the 223 stars in our sample found by us to be periodically variable, and we present the distribution of the stellar rotation periods derived from these light curves. We also present 16 potential eclipsing binary star systems that, through follow-up work, will provide direct measurements of the masses and radii of the stars for testing of stellar evolutionary models. This work has been supported by the National Science Foundation under Career grant AST-0349075.Item Structure function analysis of blazars AP Librae and 3c279(Vanderbilt University. Dept. of Physics & Astronomy, 2008-04-27) Jensen, Anders A.; Knop, Robert A.Blazars AP Librae and 3c279 are analyzed for microvariability using a technique known as structure function analysis. AP Librae was observed in April of 2005 and 3c279 was observed in April of 2007. The data for AP Librae was previously reduced by Andrew Collazzi and the author reduced the data for 3c279. Both sets of data were reduced using Robert Knop's data reduction program. The author ran structure function analysis on both sets of data. Structure function analysis is a statistical analysis run on data that is suppose to nd timescales of variability, periodicity, and the noise type of data. Previous analysis of AP Librae confirmed mircrovariability, which also shows up in the structure function of AP Librae. Blazar 3c279 was much quieter than AP Librae and showed no microvariability durning any of the nights.