Dusty Exoplanetary Debris Disks in the Single-Temperature Blackbody Plane

The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018We present a bulk sample analysis of the metal polluted white dwarfs which also host infrared bright dusty debris disks, known to be direct signatures of an active exoplanetary accretion source. We explore the relative positions of these systems in a “single-temperature blackbody plane”, defined as the temperature and radius of a single temperature blackbody as fitted to the infrared excess. We find that the handful of dust systems which also host gaseous debris in emission congregate along the high temperature boundary of the dust disk region in the single-temperature blackbody plane. We discuss interpretations of this boundary and propose the single-temperature blackbody plane selection technique for use in future targeted searches of gaseous emission.Astronom

Battles with Words: Literate and Linguistic Resistance in Multi-Ethnic U.S. Literature and Everyday Life

Battles with Words analyzes the role of multi-ethnic U.S. literature as an alternative form of cultural production which critiques and challenges U.S. linguistic and literate hegemony and homogeneity. The texts comprising this field continually emphasize the ways in which words, through language and literacy, become tools of power and action used by the ethnically marginalized to negotiate everyday advantages for themselves and challenge the linguistic and cultural domination of Anglo America. Through their critiques of the culture of English-only monolingualism that has continued to dominate the national landscape of the U.S. throughout the twentieth and twenty-first centuries, these authors indicate their concern with the ways language intersects with and impacts literature, as well as their interest in using literature to explore and critique the relationship between language, literacy, race, ethnicity, and citizenship in the U.S. Using seven contemporary multi-ethnic U.S. novels, I examine how these novels portray language and literacy as weapons of the dominant which maintain and reproduce racist, classist systems of power and bureaucracy and as tools for those who are positioned as ethnically, linguistically, and nationally unauthorized, subjugated, and illegitimate to resist their subordination and disenfranchisement. By examining these works through a rhetorical lens, my analyses attempt to elucidate what is (un)said, (un)speakable, and (un)recorded when subordinates confront authorities in various public and private contexts including classrooms, social services offices, immigration stations, neighborhoods, and homes. The high-stakes literate and linguistic exchanges these works portray offer a multitude of perspectives from which to consider the seemingly mundane, ordinary ways in which language and literacy are used by the marginalized and the powerful as they negotiate various everyday contexts and encounters. While these novels reveal the many problematic uses of literacy and language in power struggles in the U.S., especially as they relate to race, ethnicity, and citizenship, they also suggest alternative ways that language and literacy might be used less hierarchically and more democratically in everyday life, offering models for transforming bureaucratic, institutional, and social encounters. These alternative models should interest not only literary scholars, but also those in the fields of composition, pedagogy, language, literacy and education

Explorations of the Remnant Exoplanetary Debris Disks around White Dwarf Stars

Always amenable to modern astrophysical needs, the compact degenerate remnants of stars like our Sun known as white dwarf stars have recently been put into service as exoplanetary laboratories. Their nominally pure hydrogen or helium atmospheres reveal pollution from the debris of crushed up exoplanets, and our understanding of the physics of their atmospheres provides a means to explore the chemical makeup of the rocky bodies they accrete. This thesis work will build on observations of the exoplanetary systems around white dwarf stars by providing new discoveries of such systems, including one that appears to have accreted the crust of an exo-earth analogue. We also pioneer a novel discovery technique which is capable of both revealing new systems and isolating the most interesting among them. Some of our new observations required a major upgrade to a facility class scientific instrument at the Southern Astrophysical Research Telescope in Chile, and we detail the design and implementation of this new upgrade which is now available to the entire US astronomical community. With this upgrade, we discovered a rapidly evolving white exoplanetary system which both challenges the existing theories of white dwarf exoplanetary system evolution and provides the only path forward to test such theories on a reasonable timeline. We conclude with a discussion of the impact these new observations and discovery tools have on the field, and the future directions they enable.Doctor of Philosoph

When flux standards go wild: white dwarfs in the age of Kepler

White dwarf stars have been used as flux standards for decades, thanks to their staid simplicity. We have empirically tested their photometric stability by analyzing the light curves of 398 high-probability candidates and spectroscopically confirmed white dwarfs observed during the original Kepler mission and later with K2 Campaigns 0-8. We find that the vast majority (>97 per cent) of non-pulsating and apparently isolated white dwarfs are stable to better than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales, confirming that these stellar remnants are useful flux standards. From the cases that do exhibit significant variability, we caution that binarity, magnetism, and pulsations are three important attributes to rule out when establishing white dwarfs as flux standards, especially those hotter than 30,000 K.Comment: Accepted for publication in MNRAS; 7 pages, 4 figures, 2 table

The magnetic cataclysmic variable LSQ1725-64

We present new photometry and spectroscopy of the 94 m eclipsing binary LSQ1725-64 that provide insight into the fundamental parameters and evolutionary state of this system. We confirm that LSQ1725-64 is a magnetic cataclysmic variable whose white dwarf has a surface-averaged magnetic field strength of 12.5 ± 0.5 MG measured from Zeeman splitting. The spectral type and colour of the secondary, as well as the eclipse length, are consistent with other secondaries that have not yet evolved through the period minimum expected for cataclysmic variables. We observe two different states of mass transfer and measure the transition between the two to occur over about 45 orbital cycles. In the low state, we observe photometric variations that we hypothesize to arise predominantly from two previously heated magnetic poles of the white dwarf. Our precise eclipse measurements allow us to determine binary parameters of LSQ1725-64 and we find it contains a high-mass (0.966 ± 0.027 M⊙) white dwarf if we assume a typical mass–radius relationship for a CO core white dwarf. We also measure an eclipse of the accretion stream after the white dwarf eclipse, and use it to estimate an upper limit of the mass transfer rate. This derived limit is consistent with that expected from angular momentum loss via gravitational radiation alone

A SUBTLE INFRARED EXCESS ASSOCIATED WITH A YOUNG WHITE DWARF IN THE EDINBURGH-CAPE BLUE OBJECT SURVEY

We report the discovery of a subtle infrared excess associated with the young white dwarf EC 05365–4749 at 3.35 and 4.6 μ m. Follow-up spectroscopic observations are consistent with a hydrogen atmosphere white dwarf of effective temperature 22,800 K and log [ g (cm s{sup −2})] = 8.19. High-resolution spectroscopy reveals atmospheric metal pollution with logarithmic abundances of [Mg/H] = −5.36 and [Ca/H] = −5.75, confirming the white dwarf is actively accreting from a metal-rich source with an intriguing abundance pattern. We find that the infrared excess is well modeled by a flat, opaque debris disk, though disk parameters are not well constrained by the small number of infrared excess points. We further demonstrate that relaxing the assumption of a circular dusty debris disk to include elliptical disks expands the widths of acceptable disks, adding an alternative interpretation to the subtle infrared excesses commonly observed around young white dwarfs

Destroying Aliases from the Ground and Space: Super-Nyquist ZZ Cetis in K2 Long Cadence Data

With typical periods of order 10 minutes, the pulsation signatures of ZZ Ceti variables (pulsating hydrogen-atmosphere white dwarf stars) are severely undersampled by long-cadence (29.42 minutes per exposure) K2 observations. Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling precision asteroseismology. We report the discovery of two new ZZ Cetis in long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by 3-4 nights of follow-up, high-speed (<=30 s) photometry from McDonald Observatory, we recover accurate pulsation frequencies for K2 signals that reflected 4-5 times off the Nyquist with the full precision of over 70 days of monitoring (~0.01 muHz). In turn, the K2 observations enable us to select the correct peaks from the alias structure of the ground-based signals caused by gaps in the observations. We identify at least seven independent pulsation modes in the light curves of each of these stars. For EPIC 220274129, we detect three complete sets of rotationally split ell=1 (dipole mode) triplets, which we use to asteroseismically infer the stellar rotation period of 12.7+/-1.3 hr. We also detect two sub-Nyquist K2 signals that are likely combination (difference) frequencies. We attribute our inability to match some of the K2 signals to the ground-based data to changes in pulsation amplitudes between epochs of observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC 220274129 near the middle of the ZZ Ceti instability strip, with Teff = 11590+/-200 K and 11810+/-210 K, and masses 0.57+/-0.03 Msun and 0.62+/-0.03 Msun, respectively.Comment: 13 pages, 9 figures, 7 tables; accepted for publication in Ap

A Word to the WISE: Confusion is Unavoidable for WISE-selected Infrared Excesses

Stars with excess infrared radiation from circumstellar dust are invaluable for studies of exoplanetary systems, informing our understanding on processes of planet formation and destruction alike. All-sky photometric surveys have made the identification of dusty infrared excess candidates trivial, however, samples that rely on data from WISE are plagued with source confusion, leading to high false positive rates. Techniques to limit its contribution to WISE-selected samples have been developed, and their effectiveness is even more important as we near the end-of-life of Spitzer, the only facility capable of confirming the excess. Here, we present a Spitzer follow-up of a sample of 22 WISE-selected infrared excess candidates near the faint-end of the WISE detection limits. Eight of the 22 excesses are deemed the result of source confusion, with the remaining candidates all confirmed by the Spitzer data. We consider the efficacy of ground-based near-infrared imaging and astrometric filtering of samples to limit confusion among the sample. We find that both techniques are worthwhile for vetting candidates, but fail to identify all of the confused excesses, indicating that they cannot be used to confirm WISE-selected infrared excess candidates, but only to rule them out. This result confirms the expectation that WISE-selected infrared excess samples will always suffer from appreciable levels of contamination, and that care should be taken in their interpretation regardless of the filters applied.Comment: 13 pages, 4 Figures; Accepted for publication in Ap