Browsing by Subject "Astronomy"

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400 Years of Sunspots
(McDonald Observatory, 2011) University of Texas at Austin; McDonald Observatory
50th Anniversary Lecture: Founding & History of McDonald Observatory
(1989) University of Texas at Austin; McDonald Observatory
75th Anniversary Lecture: The Frontier & McDonald Observatory
(McDonald Observatory, 2013-10-19) University of Texas at Austin; McDonald Observatory
Age of the Milky Way
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
Astro Madness
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
Black holes and how to find them : on the detection of compact objects at several scales
(2021-08-05) Bustamante Rosell, María José; Zimmerman, Aaron; Gebhardt, Karl; Matzner, Richard; Boddy, Kimberly
This dissertation is structured in four chapters. First, the reader is introduced to current detection methods for astrophysical black holes at different scales. The following three chapters correspond to different research projects regarding black hole detection. The second chapter presents the kinematic detection of a supermassive black hole at the center of the dwarf spherical galaxy Leo I. Chapter three presents a novel idea for the detection of supermassive black hole binaries using the effect their gravitational waves produce on lower frequency gravitational waves in our vicinity. Finally, chapter four describes the spectroscopic follow-up of LIGO-Virgo-Kagra gravitational wave sources conducted using the Hobby Eberly Telescope at McDonald Observatory.
The chemical abundances of stars in the Halo (CASH) project
(2014-08) Hollek, Julie Ann; Sneden, Christopher
This dissertation is a compilation of four separate studies under the umbrella of the Chemical Abundance of Stars in the Halo (CASH) Project. The overall goal of the CASH project is to gain a better understanding of the events and processes that occurred during the early universe that shaped the composition of the stars that we observe today. In order to do so, we have conducted a comprehensive study of the abundances of the oldest observable stars. These stars have preserved the chemical signature of the material from which they formed in their atmospheres. The old, metal-poor stars that make up the stellar halo thus provide a means to study the history of the universe. We will discuss the motivation for the project in Chapter 1, introducing the field of metal-poor halo stars and providing background about the processes that have contributed to the chemical make up of the stars. The first generation of stars that created much of the material from which these stars formed are discussed, along with the low-mass evolved stars that synthesized additional elements in their interiors that are also observed in metal-poor stars today. Utilizing so-called ``snapshot'' spectra obtained with the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory, we provide abundances for 262 stars over the sample. In Chapter 2, we present Robospect, a new code to automatically measure and deblend line equivalent widths for both absorption and emission spectra. We used this code to calculate equivalent width measurements, which provide the foundation of the analysis, from our spectra. We test the accuracy of these measurements against simulated spectra as well as manual equivalent width measurements of five stellar spectra over a range of signal-to-noise values and a set of long slit emission spectra. We find that Robospect accurately matches both the synthetic and manual measurements, with scatter consistent with the expectations based on the data quality and the results of Cayrel (1988). In Chapter 3, we present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the CASH project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R ~15,000) and corresponding high-resolution (R~35,000) Magellan MIKE spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H]<-3.7. These pilot sample stars are the most metal-poor ([Fe/H]≲-3.0) of the brightest stars included in CASH and are used to calibrate a newly-developed, automated stellar parameter and abundance determination pipeline. This code is used for the entire CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]<-3. We also find four neutron-capture enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. In Chapter 4, we present stellar parameters and abundances for the full CASH sample of 263 metal-poor halo star candidates derived from snapshot spectra obtained with the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We determine abundance statistics and trends for 16 elements over the full sample. We identify astrophysically-interesting stars that merit further investigation, including carbon-enhanced metal-poor stars, neutron-capture element enhanced stars, and extremely metal-poor stars. We note one Li giant with a unique abundance pattern. In Chapter~5 we present a detailed abundance analysis of 23 elements for a newly discovered carbon-enhanced metal-poor (CEMP) star, HE 0414-0343, from the CASH sample. Its spectroscopic stellar parameters are T_eff=4863 ,K, log g=1.25, ξ=20 km/s, and [Fe/H]=-2.24. Radial velocity measurements covering seven years indicate HE 0414-0343 to be a binary. HE 0414-0343 has [C/Fe]=1.44 and is strongly enhanced in neutron-capture elements but its abundances cannot be reproduced by a solar-type s-process pattern alone. It could be classified as ``CEMP-r/s'' star but we find that no r-process component is required as explanation of this and other similar stars classified as ``CEMP-s'' and ``CEMP-r/s'' stars. Rather, based on comparisons with AGB star nucleosynthesis models, we suggest a new physically-motivated classification scheme, especially for the still poorly-understood ``CEMP-r/s'' stars. Importantly, it reflects the continuous transition between these so-far distinctly treated subgroups: CEMP-sA, CEMP-sB, and CEMP-sC. The [Y/Ba] ratio parameterizes the classification and can thus be used to classify any future such stars. Possible causes for the transition include the number of thermal pulses the AGB companion underwent and the effect of different AGB star masses on their nucleosynthetic yields. We then speculate that higher AGB masses may explain ``CEMP-r/s'' or now CEMP-sB and CEMP-sC stars, whereas less massive AGB stars would account for ``CEMP-s'' or CEMP-sA stars. Based on a limited set of AGB models, we suggest the abundance signature of HE~0414$-$0343 to have come from a >1.3 M_⊙ mass AGB star and a late-time mass transfer, thereby making it a CEMP-sC star. Finally, in Chapter 6, we summarize our results and provide future directions for the project.
Color of Stars
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
Coma Clusters of Galaxies
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
Dark Skies
(The University of Texas at Austin, 2010-06-28) University of Texas at Austin; McDonald Observatory; Star Date
Deadly Skies
(The University of Texas at Austin, 2010-07-04) The University of Texas at Austin; McDonald Observatory; Star Date
Delphinus
(The University of Texas at Austin, 2010-06-30) The University of Texas a Austin; McDonald Observatory; Star Date
Delta, Delta, Delta
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
The development of replicated optical integral field spectrographs and their application to the study of Lyman-alpha emission at moderate redshifts
(2015-08) Chonis, Taylor Steven; Hill, Gary J.; Finkelstein, Steven L; Gebhardt, Karl; Greene, Jenny E; Jaffe, Daniel T
In the upcoming era of extremely large ground-based astronomical telescopes, the design of wide-field spectroscopic survey instrumentation has become increasingly complex due to the linear growth of instrument pupil size with telescope diameter for a constant spectral resolving power. The upcoming Visible Integral field Replicable Unit Spectrograph (VIRUS), a baseline array of 150 copies of a simple integral field spectrograph that will be fed by 33,600 optical fibers on the upgraded Hobby-Eberly Telescope (HET) at McDonald Observatory, represents one of the first uses of large-scale replication to break the relationship between instrument pupil size and telescope diameter. By dividing the telescope's field of view between a large number of smaller and more manageable instruments, the total information grasp of a traditional monolithic survey spectrograph can be achieved at a fraction of the cost and engineering complexity. To highlight the power of this method, VIRUS will execute the HET Dark Energy Experiment (HETDEX) and survey ~420 square degrees of sky to an emission line flux limit of ~1e-17 erg/s/cm^2 to detect ~1e6 Lyman-alpha emitting galaxies (LAEs) as probes of large-scale structure at redshifts of 1.9
Emission-line properties of active galactic nuclei and an experiment in integrated, guided-inquiry science classes and implications for teaching astronomy
(2012-08) Ludwig, Randi Renae; Kopp, Sacha; Robinson, Edward Lewis, 1945-; Hemenway, Mary Kay; Prather, Edward; Shields, Gregory; Wills, Derek
This dissertation examines two broad topics -- emission line properties of active galactic nuclei (AGN) and the effect of hands-on, integrated science courses on student understanding of astronomy. To investigate trends in overall properties of emission lines in AGN, we apply principal component analysis (PCA) to the fluxes in the H [beta] - (O III) region of a sample of 9046 spectroscopically-identified broad-line AGN from the Sloan Digital Sky Survey (SDSS) Data Release 5 with a redshift range of 0.1 < z < 0.56. After performing independent spectral PCA on subsets defined effectively by their (O III) equivalent width (EW), we find only the weakest (O III) objects retain the optical Fe II - (O III) anticorrelation and the correlation of EW[subscript O III] with H [beta] linewidth that have previously been found in high-luminosity AGN. The objects with strongest EW[subscript O III] do not differ from the entire data set significantly in other spectral and derived properties, such as luminosity, redshift, emission line shapes, Eddington ratio, continuum slope, and radio properties. However, our findings are consistent with previous suggestions that (O III) emission is primarily a function of covering factor of the narrow-line region. To investigate the other side of the Fe II - (O III) anticorrelation, we examine the effect of changes in the gas-phase abundance of Fe on observed variation in Fe II. Using AGN spectra from the SDSS in the redshift range of 0.2 < z < 0.35, we measure the Fe/Ne abundance of the narrow-line region (NLR) using the (Fe VII)/(Ne V) line intensity ratio. We find no significant difference in the abundance of Fe relative to Ne in the NLR as a function of Fe II/H [beta]. However, the (N II)/(S II) ratio increases by a factor of 2 with increasing Fe II strength. This indicates a trend in N/S abundance ratio, and by implication in the overall metallicity of the NLR gas, with increasing Fe II strength. We propose that the wide range of Fe II strength in AGN largely results from the selective depletion of Fe into grains in the low ionization portion of the broad-line region. We utilize photoionization models to show that the strength of the optical Fe II lines varies almost linearly with gas-phase Fe abundance, while the ultraviolet Fe II strength varies more weakly, as seen observationally. After examining the emission line properties of large samples of fairly typical AGN, we investigated the newly expanded regime of low-mass AGN (M[subscript BH] [less than or approximately equal to] 10⁶ M[subscript sun]) with respect to their emission line properties at a smaller scale. We utilize the high spectral resolution and small aperture of our Keck data of 27 low-mass AGN, taken with the Echellette Spectrograph and Imager, to isolate the NLRs of these low-mass black holes. Some of these low-luminosity objects plausibly represent examples of the low-metallicity AGN described by Groves et al. (2006), based on their (N II)/H[alpha] ratios and their consistency with the Kewley & Ellison (2008) mass-metallicity relation. We also find that these low-mass AGN have steeper UV continuum slopes than more-massive AGN based on their He II/H[beta] ratio. Overall, NLR emission lines in these low-mass AGN exhibit trends similar to those seen in AGN with higher-mass BHs, such as increasing blueshifts and broadening with increasing ionization potential. Additionally, we see evidence of an intermediate line region whose intensity correlates with L/L[subscript Edd] in these objects, as seen in higher-mass AGN. We highlight the interesting trend that, at least in these low-mass BHs, the (O III) EW is highest in symmetric NLR lines with no blue wing. This trend of increasing (O III) EW with line symmetry could be explained by a high covering factor of lower ionization gas in the NLR. We also investigate effective methods for teaching astronomy and connections between astronomical topics in student learning and understanding. After developing the curriculum for a hands-on, learner-centered astronomy course (Hands-on-Science, hereafter HoS) aimed at pre-service elementary teachers, we measure student performance in HoS compared to traditional, large lecture courses (hereafter Astro101). We utilize distractor-driven multiple choice assessments in order to quantitatively assess student understanding and evaluate the persistence or correction of common misconceptions in astronomy. We find that for the topics included in the HoS curriculum, HoS students have a higher average post-test score, and higher normalized gains, than the Astro101 students. We cannot pinpoint the exact cause of this student achievement because of the multitude of nontraditional practices incorporated into the HoS implementation. Increased time-on-task, a classroom environment structured around student discussion, or focus on conceptual understanding could each be key factors in the high achievement of HoS students. We conclude that the HoS students are better prepared in astronomy for their future careers as elementary school teachers by HoS courses than they would have been in traditional, introductory astronomy courses. When we compare directly between topics covered in both HoS and Astro101, we find that HoS students have normalized gains that are a factor of 2-4 higher than those of Astro101 students. Therefore, we conclude that curricula similar to the HoS approach would benefit Astro101 students as well, particularly for topics which are most impacted by the HoS method, such as Moon phases and seasons. Lastly, a PCA of the changes in HoS student scores reveals that there is very little systematic student variation apart from the trends apparent in the mean changes in the sample. Thus, we do not find groupings of questions that some subsets of students systematically learn more readily than others. Another way to interpret this result is that the HoS curriculum and methodology indiscriminately help all kinds of pre-service elementary teachers, despite presumptive differences in their own learning styles and strengths.
Equatorial Sundial
(McDonald Observatory, 0000-00-00) University of Texas at Austin; Mcdonald Observatory
Exploring Black Holes
(McDonald Observatory, 2005) University of Texas at Austin; Mcdonald Observatory
Exploring Light: the Optics of Diffraction
(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald Observatory
A gravitational redshift determination of the mean mass of white dwarfs. DA stars.
(2010-08) Falcon, Ross Edward; Winget, Donald Earl, 1955-; Montgomery, Michael H.
We measure apparent velocities (v_app) of the H alpha and H beta Balmer line cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Assuming these WDs are nearby and comoving, we correct our velocities to the local standard of rest so that the remaining stellar motions are random. By averaging over the sample, we are left with the mean gravitational redshift, : we find = = 32.57+/-1.17 km/s. Using the mass-radius relation from evolutionary models, this translates to a mean mass of 0.647+0.013-0.014 Msun. We interpret this as the mean mass for all DAs. Our results are in agreement with previous gravitational redshift studies but are significantly higher than all previous spectroscopic determinations except the recent findings of Tremblay & Bergeron. Since the gravitational redshift method is independent of surface gravity from atmosphere models, we investigate the mean mass of DAs with spectroscopic Teff both above and below 12,000 K; fits to line profiles give a rapid increase in the mean mass with decreasing Teff. Our results are consistent with no significant change in mean mass: ^hot = 0.640+/-0.014 Msun and ^cool = 0.686+0.035-0.039 Msun.
Gravitational waves, pulsations, and more : high-speed photometry of low-mass, He-core white dwarfs
(2013-08) Hermes, James Joseph, Jr.; Winget, Donald Earl, 1955-; Montgomery, Michael Houston
This dissertation is an observational exploration of the exciting physics that can be enabled by high-speed photometric monitoring of extremely low-mass (< 0.25 M[subscript sun symbol]) white dwarf stars, which are found in some of the most compact binaries known. It includes the cleanest indirect detection of gravitational waves at visible wavelengths, the discovery of pulsations in He-core WDs, the strongest evidence for excited p-mode pulsations in a WD, the discovery of the first tidally distorted WDs and their use to constrain the low-end of the WD mass-radius relationship, and the strongest cases of Doppler beaming observed in a binary system. It is the result of the more than 220 nights spent at McDonald Observatory doing high-speed photometry with the Argos instrument on the 2.1 m Otto Struve telescope, which has led to a number of additional exciting results, including the discovery of an intermediate timescale in the evolution of cooling DA WDs and the discovery of the most massive pulsating WD, which should have an ONe-core and should be highly crystallized.