Browsing by Subject "Asteroseismology"
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Item Do metal-polluted stars of the ZZ ceti instability strip have a distinct asteroseismic signature?(2015-08) Jumper, Kevin Arthur; Winget, Donald Earl, 1955-; Montgomery, Michael Houston; Sneden, ChrisCooling DA stars that pass through the ZZ instability strip, a region between temperatures of approximately 12,600 K to 11,100 K, tend to experience the driving of g-mode pulsations near their surface layers. These pulsations cause variations in the luminosities of such stars, leading them to be known as DAVs. A fraction of DAVs also have photospheres contaminated by metals, usually thought to be from the tidally disrupted remnants of planetary systems. The high resolution spectroscopy needed to make definite identifications of these metal lines is relatively demanding, whereas it is simple to obtain photometric data on the pulsation periods of DAV stars. Therefore, if known metal-polluted DAVs (DAZVs) have systematic differences in their photometric data compared to that of DAVs that lack such pollution, photometry could provide an easy way to determine which stars are likely to contain metals in their photospheres in the future. However, we find that the known DAZV population is not large enough to permit its behavior to be distinguished from that of the normal DAV population at the present time, though extremely low-mass white dwarfs may help expand the populations and improve the quality of our fits.Item Evaluation of mode identification techniques in two key white dwarf pulsators(2000-12) Nitta, Atsuko; Winget, Donald Earl, 1955-; Oliveira Filho, Kepler de SouzaThe success of asteroseismology lies in the correct identification of the normal modes of oscillation. The Whole Earth Telescope (WET) identified the normal modes of a helium white dwarf pulsator, GD358, by analyzing the period distribution of the pulsation modes. Another way to identify modes is by comparing pulsation amplitudes in the UV to the optical. To cross-calibrate the two mode identification methods, we observed GD358 in August, 1996 with the Hubble Space Telescope (HST) to obtain the UV data while observing nearly simultaneously from the ground. During our observations, GD358 went through a very drastic amplitude modulation in a time scale of hours. These short time scale amplitude changes made the direct UV to optical amplitude determinations difficult. We successfully eliminate the possibility that the 423s mode, the dominant mode at the time of these observations, is an l=3 or 4 g-mode pulsation, but we cannot unambiguously decide if it is an l=1 or 2. Theoretical calculations indicate that the massive pulsating DA white dwarf BPM 37093 has a crystallized interior (Winget et al. 1997; Kanaan 1996; Montgomery 1998). Crystallization was predicted theoretically 40 years ago (Kirshnitz 1960; Abrisokov 1961; Salpeter 1961) although uncertainties in the nature and extent of crystallization, as well as its associated effects, are the largest sources of uncertainty in calculating the ages of the coolest white dwarf stars- important chronometers of the galactic disk. The WET observed BPM 37093 in April 1998 and again in April 1999, simultaneously with the HST, in hopes of using both the period distribution and the amplitude comparison method to identify the l value of the modes and measure the first crystallized mass-fraction of a stellar interior. Here we also rule out the possibility of the observed modes being l=3 and higher and demonstrate that not all the observed modes are l-1. If all the observed modes are l-2, then we conclude that the crystallization mass fraction is between 0-80%, depending mainly on the surface H layer. In the end, we evaluate the amplitude comparison method and address advantages and problems using this method compared to other mode identification methods.Item 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 HoustonThis 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.Item Pulsational oddities at the extremes of the DA white dwarf instability strip(2017-08) Bell, Keaton John; Winget, Donald Earl, 1955-.; Montgomery, Michael Houston; Claver, Charles F.; Kepler, S. O.; Robinson, E. L.; Wheeler, J. C.White dwarf stars are the remnant products of the vast majority of Galactic stellar evolution. They are compact objects that serve as remote laboratories for studying high energy/density physics. The outer regions of hydrogen-atmosphere (DA) white dwarfs become convective and able to drive global, nonradial, gravity-mode pulsations below roughly 12,500 K. The pulsations propagate through and are affected by the interior structures of these stars. The oscillations cause a pulsating star to exhibit brightness variations at its characteristic frequencies as a physical system. These frequencies can be measured through Fourier analysis of time series photometric observations. I have focused my studies on new pulsational phenomena near the cool and low-mass edges of the DA white dwarf instability strip, using extensive space-based data from the Kepler spacecraft and the K2 mission, as well as high-speed ground-based photometry from the 2.1-meter Otto Struve Telescope at McDonald Observatory (where I have personally observed 225 nights). The extensive short-cadence (1-min exposures) light curve of the first DAV (DA variable) identified within the original Kepler field of view provided one of the most complete and sensitive records of white dwarf pulsations ever. The light curve also revealed a new, completely unexpected outburst-like phenomenon. I detected 178 instances of significant brightness enhancement in 20 months of observations of the cool DAV KIC 4552982. Recurring with a quasi-period of 2.7 days, the outbursts last 4–25 hours and increase the stellar flux by up to 17%. I estimate the energy of each outburst to be of-order 10³³ ergs. After the Kepler spacecraft suffered the loss of a second reaction wheel in May 2013, it began the K2 mission, visiting new fields along the ecliptic roughly every 80 days. This allowed us to increase the number of DAVs with extensive space-based photometry, and we quickly discovered a second, more dramatic example of this new outburst behavior in PG 1149+057 (Hermes et al. 2015b). I have led the efforts to characterize the outbursts in DAVs ever since and have detected these events in eight DAVs through K2 Campaign 10. Notably, spectroscopic effective temperature constraints place all known members of this new outbursting class of DAV near the cool (red) edge of the instability strip. With a growing outbursting class of DAV, we begin to study their ensemble outburst properties to inform a theory of their physical mechanism. Much of my work from McDonald Observatory has continued in the recent tradition of discovering and characterizing new pulsating extremely low-mass (ELM) white dwarfs. After identifying candidate ELM variables (ELMVs) from the ELM Survey catalog and parameters from model fits to the Sloan Digital Sky Survey spectroscopic data, I obtained time series photometric observations on the 2.1-meter Otto Struve telescope. I published SDSS J1618+3854 as the sixth member of this new class of variable star. However, most of the variability that I measured for this project was inconsistent with expectations for cooling track ELM white dwarfs. This includes long pulsation periods, high pulsation amplitudes, long eclipse timescales, and an overabundance of photometric variables that are not in confirmed short-period binaries from time series radial velocity measurements. Either the surface gravities of another class of star are being systematically overestimated from model fits to hydrogen line profiles in stellar spectra, or these observations are revealing an unexpectedly large population of recently formed pre-ELM white dwarfs. In total, I have discovered and characterized the variability of nine new pulsating stars in the spectroscopic parameter space of ELM white dwarfs, and I also developed an improved framework for interpreting measurements of tidally induced ellipsoidal variations in photometric binaries. Beyond these main results of my thesis on extreme pulsating white dwarfs, I have also explored the limits of the detectability of stellar pulsations in extreme photometric data sets. I analyze long-cadence (30-minute) K2 observations of two fairly typical DAVs in one such study, where the pulsations are severely undersampled. While accurate frequency determinations are nontrivial in such cases, I am able to recover the super-Nyquist frequencies of some pulsation modes with full K2 precision with the help of a few hours of ground-based observations. The space-based data, in turn, enables me to select the intrinsic frequency from the complex alias structure of multi-night ground-based data, providing a practical demonstration of the importance of carefully considering the spectral window. I apply what I have learned about undersampled data to anticipate upcoming pulsating star science in the next generation of synoptic time domain photometric surveys such as the Zwicky Transient Facility and the Large Synoptic Survey Telescope.Item A search for periodic variations in pulse arrival times in DA white dwarfs(2010-08) Hermes, James Joseph, Jr.; Winget, Donald Earl, 1955-; Montgomery, Michael H.; Robinson, Rob; Jaffe, Dan; Bromm, VolkerWe present updated observations of a pilot survey of 14 pulsating DA white dwarfs, monitored for evidence of center-of-mass motion caused by a planetary companion. We have nearly doubled the number of periodicites for which we can produce O-C diagrams that document pulse arrival times from our stars, and have implemented a method to minimize the apertures we use in our reductions in order to reduce sky noise. In addition to a previously published candidate, GD66, we have identi fed at least four more systems worthy of rigorous observational follow-up. We have also implemented a method, a generalized Lomb-Scargle periodogram, that takes into account weighted points in order to characterize any periodic behavior present in our O-C diagrams. For at least one DAV within this same sample, we have found strong observational evidence for an evolutionary time scale (via the rate of period change) that is inconsistent with cooling alone. In that star, WD0111+0018, we report for the first time measurement of the rate of period change of nonlinear combination frequencies in a pulsating white dwarf. We speculate that this may be caused by a changing rotation rate that aff ects only modes with m not equal to 0.