Browsing by Subject "Stars"
Now showing 1 - 20 of 22
- Results Per Page
- Sort Options
Item A chemical abundance analysis of stars believed to be metal poor members of the galactic stellar thick disk(2006-05) Simmerer, Jennifer Ann; Sneden, ChristopherGalactic formation models have long sought to reproduce the observed chemical and kinematical properties of the Milky Way's stellar halo and disk. Recently it is the so-called "intermediate population", the stellar thick disk, that is driving advances in our understanding of the formation of spiral galaxies. The thick disk is kinematically more like the thin disk than the halo, for all the thick disk has a velocity dispersion twice that of the thin disk and rotates ~40 km/s more slowly. It is generally accepted that the thick disk's metallicity distribution function peaks at a lower metallicity than the thin disk but at higher metallicity than the halo. The lower bound of the thick disk is still uncertain, as many observational studies have found only a few thick disk candidate stars or clusters that are more metal poor than (Fe/H)=1. Beers et al. (2002) have so far proposed the largest sample of metal poor thick disk candidates, presenting 9 stars at (Fe/H)= -1.2 or lower and 46 more stars at (Fe/H)= -1 or lower, all of which are believed to belong to the thick disk. Beers et al. (2002) present possible thick disk stars as metal poor as (Fe/H)~ -2.5, roughly 1 dex lower than is suggested by current Galactic formation models (Brook et al., 2005). This study is a high-resolution spectroscopic follow-up of 29 of the stars Beers et al. (2002) and Chiba & Beers (2000) identify as potential metal poor members of the thick disk and an additional 40 stars from the cannonical thick disk, halo, and thin disk. None of the very metal-poor stars identified by Beers et al. (2002) can be confirmed as members of the thick disk and many are not metal poor at all. Only two stars more metal poor than (Fe/H)= 1.2 retain their thick disk membership. These two stars exhibit some of the chemical characteristics of the cannonical thick disk: high α-element abundances and a relatively low s--/r-- process element ratio. Also of interest are six stars with thin disk kinematic signatures but thick disk α-element abundances. That only a small number of metal poor thick disk stars could be confirmed in this study indicates that the thick disk is neither as populous nor as metal poor as has been proposed by Beers et al. (2002).Item Chemical abundances of Giant Planet Host Stars(2014-12) Brugamyer, Erik John; Sneden, Christopher; Dodson-Robinson, Sarah E.The positive correlation between planet detection rate and host star iron abundance lends strong support to the core accretion theory of planet formation. However, iron is not the most significant mass contributor to the cores of giant planets. Since giant planet cores are thought to grow from silicate grains with icy mantles, the likelihood of gas giant formation should depend heavily on the oxygen and silicon abundance of the planet formation environment. Here we compare the silicon and oxygen abundances of a set of 76 planet hosts and a control sample of 80 metal-rich stars without any known giant planets. Our new, independent analysis was conducted using high resolution, high signal-to-noise data obtained at McDonald Observatory. Because we do not wish to simply reproduce the known planet-metallicity correlation, we have devised a statistical method for matching the underlying [Fe/H] distributions of our two sets of stars. We find a 99\% probability that planet detection rate depends on the silicon abundance of the host star, over and above the observed planet-metallicity correlation. We do not detect any such correlation for oxygen. Our results would thus seem to suggest that grain nucleation, rather than subsequent icy mantle growth, is the important limiting factor in forming giant planets via core accretion. Based on our results and interpretation, we predict that planet detection should correlate with host star abundance for refractory elements responsible for grain nucleation and that no such trends should exist for the most abundant volatile elements responsible for icy mantle growth.Item The chemical abundances of stars in the Halo (CASH) project(2014-08) Hollek, Julie Ann; Sneden, ChristopherThis 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.Item Cosmic fossils : the spectroscopic study of stars and comets(2023-08-17) Nelson, Tyler William; Hawkins, Keith A.; Cochran, Anita Light, 1954-; Ting, Yuan-Sen; Offner, Stella; Morley, CarolineChemical Fossils, i.e. objects which preserve their initial elemental composition, provide vital constraints on the nature and evolution of the Universe. The study of these objects is therefore fundamental for astronomy, from the scales of the Solar System, to the Galaxy and beyond. The unique focus of this dissertation is to advance our knowledge of and apply chemical fossils across a wide range of scales (cometary fossils in the Solar System and stellar fossils on a Galactic scale) to better understand the formation and assembly of our Solar System and the Galaxy in which it resides. The first half of this dissertation considers the evolution of the Universe at the scale of a single star, the Sun, and its Solar System, using comets. I concentrate on characterizing the emission spectrum from the diatomic carbon (C₂) Swan bands. This fragment species is ubiquitous in comets. It is commonly used as a proxy to measure production rates of gas as well as a taxonomic classification tool. However, its parent species and the details of its emission are not well understood. A bimodal rotational temperature has been found in the Swan bands for comet 1P/Halley (Lambert et al., 1990). The following models have been proposed to explain this phenomenon: C₂ inheriting excited states from the parent species (Jackson et al., 1996), properties inherent to C₂ through intercombinational/satellite transitions (Lambert et al., 1990), and multiple populations of C₂ present in the photochemical environment (Lambert et al., 1990). Leveraging a unique library of high resolution, high signal-to-noise optical spectra, collected at McDonald Observatory, I investigate the proposed mechanisms for this rotational temperature bimodality for comets 122P/de Vico, 153P/Ikeya-Zhang, and C/1995 O1 (Hale-Bopp). I find bimodal temperatures in all spectra studied and supersolar temperatures in C/1995 O1, which is incompatible with the models from the literature. I suggest the supersolar temperatures for Hale-Bopp are a consequence of heating from the Solar wind for material outside the cometopause. The second half of this dissertation considers cosmic fossils at the scale of the Galaxy. Photospheric abundances of stars are mostly conserved over their lifetimes, and therefore stars can act as chemical fossils for the Galaxy. I focus on the use of chemical tagging within the Milky Way. Chemical tagging of stars is one of the pillars of Galactic Archaeology, motivating numerous large scale surveys. It has dramatically reshaped our knowledge of the Galaxy over the last two decades. Chemical tagging relies upon stars which are born together, i.e. co-natal, sharing a common chemical composition. I find observational evidence for an untapped reservoir of co-natal, co-moving pairs of stars, through the application of chemical tagging. Co-natal stars provide an excellent laboratory for numerous areas of astronomy, from stellar physics, to survey calibration. A common application of chemical tagging is relating a wayward star to a possible birthplace. Hyper- velocity stars (HVSs) are gravitationally unbound to the Milky Way. However the physical mechanisms that give rise to the large velocities of late-type HVSs are poorly understood. To solve this problem, I applied chemo-dynamic tagging to a sample of HVS candidates identified in Gaia data. Since these production mechanisms are connected to specific locations or chemical environments within the Galactic neighborhood, chemical tagging can distinguish which production pathways could create these enigmatic fast stars. I present work on the chemo-dynamic tagging, i.e. using both chemical and kinematic tagging, of late-type candidate HVSs. I find conclusive evidence of one unbound late-type HVS and two marginally unbound HVSs. These stars appear to originate in the ‘in situ’ stellar halo based on their chemical composition and orbit properties. These stars are produced by some of the most extreme astrophysical phenomena in the Galaxy. The origins of these late-type HVS constrain their production mechanisms and hence the importance of these energetic processes within the Galaxy. This knowledge can then be applied to models of the Milky Way’s evolution. Furthermore, expanding the number of HVSs is useful for studying the dark matter halo of the Galaxy.Item Dark Skies(The University of Texas at Austin, 2010-06-28) University of Texas at Austin; McDonald Observatory; Star DateItem Deadly Skies(The University of Texas at Austin, 2010-07-04) The University of Texas at Austin; McDonald Observatory; Star DateItem Delphinus(The University of Texas at Austin, 2010-06-30) The University of Texas a Austin; McDonald Observatory; Star DateItem Discovery, observations and theory of over luminous supernovae and peculiar transients(2010-05) Chatzopoulos, Emmanouil; Wheeler, J. Craig; Robinson, Edward L.; Kumar, Pawan; Milosavljevic, Milos; Vinko, JoszefModern wide-field imaging transient search projects led to the discovery of a new class of rare, over-luminous stellar explosions. Events like SN 2005ap (Quimby et al. 2007a), SN 2006gy (Quimby 2006; Smith et al. 2007), SN 2006tf (Quimby, Castro \& Mondol 2007; Quimby et al. 2007b; Smith et al. 2008), SN 2008am (Chatzopoulos et al. 2010), SN 2008es (Yuan et al. 2008; Gezari et al. 2008; Miller et al. 2008) SN 2008fz (Drake et al. 2009) and SN 2008iy (Miller et al. 2010) introduced us new evidence about stellar death, since traditional ideas about the mechanisms that can power these phenomena are found to be inadequate to explain the observed properties. The Texas Supernova Search Project (TSS; Quimby et al. 2005) and its successor, the ROTSE-Supernova Verification Project (RSVP; Yuan et al. 2007) discovered most of the above mentioned over-luminous supernovae (OLSNe) over the past five years of their operation. The advantage of this project is that it is essentially free of selection bias or the limits of a targeted search; the automated wide field (~3 square degrees) ROTSE-III telescopes (Akerlof et al. 2003), scan the whole sky blindly, looking for transients down to unfiltered magnitude ~19 mag and they do not focus on pre-selected galaxies. An estimated rate for these exceptionally luminous supernovae is ~ 2.6 10^{-7} events/Mpc^{3}/yr (Quimby et al. 2009a). Current and future transient search projects such as the SDSS-II Supernova Survey (Frieman et al. 2008),the Palomar Transient Factory (PTF; Law et al. 2009), SkyMapper (Schmidt et al. 2005), PanSTARRS (Chambers 2006) and Large Synoptic Survey Telescope (Tyson \& LSST collaboration 2002) are expected to increase the number of rare over-luminous (or, under-luminous) supernove and to discover new, unusual transients.Item 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.Item M dwarf metallicities and exoplanets(2007-08) Bean, Jacob Lyle, 1980-; Benedict, G. Fritz; Sneden, ChristopherThis dissertation is primarily focused on the topic of M dwarf metallicities and their relevance to the study of extrasolar planets. I begin by describing a method for accurately determining M dwarf metallicities with spectral synthesis based on abundance analyses of visual binary stars. I then apply this technique and present the first spectroscopic metallicities of three M dwarfs with planetary mass companions. The three M dwarf planet hosts have sub-solar metallicities, a surprising departure from the trend observed in FGK-type planet hosting stars. I discuss the implications of this result for extrasolar planet searches around the most numerous stars in the galaxy. I also present measured metallicities for a select group of M dwarfs with precisely determined masses and luminosities. Comparison of these stars' V band magnitudes with the predictions of the current state-of-the-art theoretical models for low-mass stars indicate that the models are deficient, as previously thought. I discuss how the cool star model atmospheres that were developed for the metallicity analysis technique might be used to improve M dwarf structure and evolution models. In addition to M dwarf metallicities, I describe some complementary work to determine the true mass of an extrasolar planet candidate using a combined analysis of high-precision astrometry and radial velocities. I present a dynamical mass for the companion to HD 33636 that indicates it is a low-mass star instead of an exoplanet. This result demonstrates the value of follow-up astrometric observations to determine the true masses of exoplanet candidates detected with the radial velocity method. Finally, I discuss the broader implications of the results presented in this dissertation and the prospect for similar work in the future.Item Moon and Jupiter(The University of Texas at Austin, 2010-07-02) The University of Texas at Austin; McDonald Observatory; Star DateItem Presentation: Exploding Stars in an Accelerating Universe(Environmental Science Institute, 2007-10-19) Environmental Science Institute; Wheeler, J. CraigItem Presentation: Reach for the Stars(Environmental Science Institute, 2009-10-30) Environmental Science Institute; Ride, SallyItem Preserving Dark Skies(McDonald Observatory, 2014) University of Texas at Austin; McDonald ObservatoryItem Properties of White Dwarfs(McDonald Observatory, 0000-00-00) University of Texas at Austin; McDonald ObservatoryItem Seeing the Invisible - Dust in the Universe(McDonald Observatory, 2005) University of Texas at Austin; McDonald ObservatoryItem Sky Test(The University of Texas at Austin, 2010-07-03) The University of Texas at Austin; McDonald Observatory; Star DateItem Sneaky Stars(The University of Texas at Austin, 2010-07-01) The University of Texas at Austin; McDonald Observatory; Star DateItem StarMapper : an android-based application to map celestial objects(2013-12) O'Donnell, John Jason; Aziz, AdnanThis report describes StarMapper, a mobile appliation designed for the Android platform that interactively maps the celestial sky and can provide information from Wikipedia about celestial objects to the user. The stars, constellations, planets, sun, and moon are all rendered in real-time and the user can navigate the celestial map simply by pointing the device around the sky to find and identify the different celestial objects. However, if the user prefers, a manual touch-based map navigation feature is also available in StarMapper. While other Android applications currently exist for mapping the sky, such as Google's Sky Map, StarMapper aims to enhance the experience by also providing additional information about celestial objects to the user by means of a simple click on the screen. For obtaining more information about a particular constellation or other celestial object, the user only needs to click on the object's name in the map, and the device's web browser opens to the Wikipedia page of the clicked object. Through this simple mechanism, the user can learn much more about astronomy than just locations of celestial objects.Item Stars and Galaxies(McDonald Observatory, 2006) University of Texas at Austin; McDonald Observatory