Browsing by Subject "White dwarf stars"
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Item Cool white dwarfs and the age of the galaxy(2006) Kilic, Mukremin; Winget, Don E.; Hippel, Ted vonWhite dwarf stars represent the most common endpoint of stellar evolution and therefore provide a reliable estimate of the star formation history and age of different Galactic populations. The major observational requirement for accurate age measurements is to have a large sample of cool, old white dwarfs. The intrinsic faintness of the coolest white dwarfs has made them difficult to observe, and the previous studies of the Galactic disk and halo suffered from small samples of cool white dwarfs. The most commonly used sample of cool white dwarfs in the Galactic disk included only 43 stars. The current best estimate for the age of the disk is about 8 billion years. Due to small number statistics, the error in this age estimate is ∼1.5 billion years. In order to reduce the age uncertainty to less than 10%, we have created a large sample of cool white dwarfs from the Sloan Digital Sky Survey. We have used a reduced proper motion diagram to effectively identify white dwarfs among many other field stars and assembled a new white dwarf luminosity function including 6000 white dwarfs. This new luminosity function is consistent with an 8 billion years old population and -when supplied with our ongoing near infrared photometric observations- will provide an accurate age estimate for the Galactic disk. In addition to being accurate cosmochronometers, white dwarfs have become important in the search for dark matter and micro-lensing objects. For many years, astronomers have been trying to understand dark matter. Claims by several investigators that they had found a large number of faint halo white dwarfs in deep field images suggested that halo white dwarfs may exist in large numbers and explain part of the missing matter. In order to test this claim, we have looked at the deepest images of the Universe taken with the Hubble Space Telescope. We have obtained proper motion measurements of the point sources in the Hubble Deep Field North and South, and showed that the observed number of white dwarfs is consistent with the standard Galactic models. We have demonstrated that the white dwarfs in these fields do not contribute to dark matter in the Galaxy. By studying possible planetary systems around white dwarfs, we can predict the future of our solar system and see if the Earth is going to survive the demise of a giant Sun. Even though planets around white dwarfs are waiting to be discovered, the existence of debris disks around several white dwarfs suggest that planets may exist around white dwarfs. Planets in previously stable orbits around a star undergoing mass loss may become unstable, and some of these systems may result in close encounters which could result in tidal stripping of a parent body that would end up in a circumstellar debris disk around a white dwarf. Until recently, there was only a single white dwarf known to have a circumstellar debris disk. We have found four more debris disks around white dwarfs, all of which turned out to be DAZs - white dwarfs with hydrogen rich atmospheres that have trace amounts of metals. Our observations strengthened the connection between the debris disk phenomenon and the observed metal abundances in cool DAZ white dwarfs. We have demonstrated that accretion from circumstellar debris disks can explain the metal abundances in at least 15% of the DAZ white dwarfs, a problem that has been puzzling astronomers for decades.Item Creating and measuring white dwarf photospheres in a terrestrial laboratory(2014-08) Falcon, Ross Edward; Winget, Donald Earl, 1955-As the ultimate fate of nearly all stars, including our Sun, white dwarfs (WDs) hold rich and informative histories in their observable light. To determine a fundamental parameter of WDs, mass, we perform the first measurement of the average gravitational redshift of an ensemble of WDs. We find a larger mean mass than that determined from the primary and expansive technique known as the spectroscopic method. The potential inaccuracy of this method has broad astrophysical implications, including for our understanding of Type 1a supernova progenitors and for constraining the age of the Universe. This motivates us to investigate the WD atmosphere models used with the spectroscopic method, particularly the input theoretical line profiles, by developing a new experimental platform to create plasmas at WD photospheric conditions (T_e ~ 1 eV, n_e ~ 10^17 cm^-3). Instead of observing WD spectra to infer the plasma conditions at the surface of the star, we set the conditions and measure the emergent spectra in the laboratory. X-rays from a z-pinch dynamic hohlraum generated at the Z Pulsed Power Facility at Sandia National Laboratories irradiate a gas cell to initiate formation of a large (120x20x10 mm or 24 cm^3) plasma. We observe multiple Balmer lines from our plasma in emission and in absorption simultaneously along relatively long (~120 mm) lines of sight perpendicular to the heating radiation. Using a large, radiation-driven plasma aides us to achieve homogeneity along our observed lines of sight. With time-resolved spectroscopy we measure lines at a range of electron densities that spans an order of magnitude, and we do this within one pulsed power shot experiment. Observing our plasma in absorption not only provides the signal-to-noise to measure relative line shapes, it allows us to measure relative line strengths because the lines share the same lower level population. This constrains the theoretical reduction factors used to describe ionization potential depression or the occupation probabilities associated with these Balmer lines. We compare our measured line shapes with the theoretical ones used in WD atmosphere models as part of the first fruits of this rich experimental platform.Item Ensemble characteristics of the ZZ Ceti stars(2004) Mukadam, Anjum Shagufta; Winget, Donald EarlGlobal pulsations of stars can be used to probe their interiors, similar to the method of using earthquakes to explore the Earth’s interior. This technique, called asteroseismology, is the only systematic way to study stellar interiors. White dwarf stars represent a relatively simple stellar end state for most main sequence stars like the Sun. This is because they are not expected to have any central nuclear fusion and their evolution is dominated by cooling. These stars are scientifically interesting since they contain a fossil record of their previous evolution. Their high densities and temperatures make them good cosmic laboratories to study fundamental physics under extreme conditions. Besides, white dwarfs are not as centrally condensed as some other classes of variables, and hence the observed pulsations sample their interior better. Each pulsation mode is an independent constraint on the structure of the star. We can probe stellar structure and composition by finding a single star rich in pulsation modes, and/or by finding a large number of pulsators to use the method of ensemble asteroseismology. A fraction of white dwarf pulsators are observed to be extremely stable clocks; this property allows us to look for any orbiting planets. The drift rates of these stable clocks are expected to reveal the stellar cooling rate. Including this information in evolutionary white dwarf models allows us to determine the age of the star. Since most stars evolve into white dwarfs, we can use the distribution of white dwarf ages in different parts of the Galaxy to constrain the age of the Galaxy and its evolution. Variable white dwarfs can also be used as a means to measure Galactic distances. All these reasons motivate us to search for additional white dwarf pulsators. Four out of five white dwarfs show hydrogen in their outermost layers and are classified as DAs. These are observed to pulsate in a temperature range of 11000–12000 K. I decided to search specifically for DA white dwarf variables (DAVs), also known as ZZ Ceti stars. To substantially increase the sample of ZZ Ceti stars, I was forced to search at greater distances (or fainter magnitudes). This is because various research groups around the world have already examined the relatively nearby (or bright) candidates for variability. Hence, I helped Dr. R. E. Nather in building a high speed time-series CCD photometer for the prime focus of the 2.1m telescope at McDonald Observatory. This CCD instrument allows us to obtain usable time-series data on 19th magnitude objects, as opposed to a limiting magnitude of 17 with our previous instrument. The combination of an efficient new instrument and a large amount of telescope time (' 100 nights/yr) gave me a unique opportunity to search extensively for new ZZ Ceti stars. Other members of my research group also contributed towards the 15 month long observations at McDonald Observatory, and helped me in data analyses. We pre-selected candidates by using the photometric and spectroscopic observations of the Sloan Digital Sky Survey. I present 35 new pulsating DA (hydrogen atmosphere) white dwarf stars discovered from the Sloan Digital Sky Survey (SDSS) and the Hamburg Quasar Survey (HQS). This increases the sample of 39 known ZZ Ceti stars to 74; the first ZZ Ceti star was accidentally discovered in 1968. This is the first time in the history of white dwarf variables that we have a homogeneous set of spectra acquired using the same instrument on the same telescope, and with consistent data reductions, for a statistically significant sample of ZZ Ceti stars. The homogeneity of the spectra reduces the scatter in the spectroscopic temperatures; we have essentially re-defined the ZZ Ceti instability strip. We find a narrow ZZ Ceti strip of width ' 1000 K, as opposed to the previous determination of 1500 K. We question the purity of the DAV instability strip as we find several non-variables within. We present our best fit for the red (cool) edge and our constraint for the blue (hot) edge of the instability strip, determined using a statistical approach. I also present the observed pulsation spectra of 67 ZZ Ceti stars with reliable spectroscopic temperatures. I verify the well-established relation of the increase in observed pulsation periods and amplitudes for the new ZZ Ceti stars, traversing from the blue to the red edge of the instability strip. The data on the new ZZ Ceti stars suggests that pulsation amplitude declines prior to the red edge. This means that ZZ Ceti pulsations do not shut down abruptly at the red edge of the instability strip. This is the first possible detection of such an effect.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 Probing exotic physics with pulsating white dwarfs(2007) Kim, Agnès; Winget, D. E.Item Probing exotic physics with pulsating white dwarfs(2007-05) Kim, Agnès, 1975-; Winget, Donald Earl, 1955-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.Item Substellar companions to white dwarves(2007) Mullally, Fergal Robert, 1979-; Winget, Donald Earl, 1955-We search for planets and brown dwarves around white dwarves (WDs). Finding extra-solar planets is the first step toward establishing the existence and abundance of life in the Universe. The low mass and luminosity of WDs make them ideal stars to search for low mass companion objects. Theoretical predictions generally agree that a star will consume and destroy close-in, low mass planets as it ascends the red giant and asymptotic giant branch evolutionary tracks, but larger mass objects and those further out will survive. The matter ejected from the star as it evolves into a white dwarf may also be accreted onto daughter planets, or may coalesce into a disk from which planets can form. We employ two techniques to search for planets and brown dwarves (BDs) around WDs. A subset of pulsating white dwarf stars have a pulsational stability that rivals pulsars and atomic clocks. When a planet is in orbit around a such a star the orbital motion of the star around the centre of mass is detectable as a change in arrival times of the otherwise stable pulsations. We search for, and find, a sample of suitable pulsators, monitor them for between three and four years, and place limits on companions by constraining the variation consistent with a 2.4M[subscript J] planet in a 4.6 year orbit. We also observe a large sample of WDs to search for a mid-infrared excess caused by the presence of sub-stellar companions. We present evidence for a potential binary system consisting of a WD and a BD on the basis of an observed excess flux at near and mind-infrared wavelengths. We also place limits on the presence of planetary mass companions around those stars and compare our results to predictions of planetary survival theories. Our findings do not support suggestions of planet formation or accretion of extra mass during stellar death.