Pulsational oddities at the extremes of the DA white dwarf instability strip

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.Astronom

A Dark Spot on a Massive White Dwarf

We present the serendipitous discovery of eclipse-like events around the massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We selected J1529+2928 for time-series photometry based on its spectroscopic temperature and surface gravity, which place it near the ZZ Ceti instability strip. Instead of pulsations, we detect photometric dips from this white dwarf every 38 minutes. Follow-up optical spectroscopy observations with Gemini reveal no significant radial velocity variations, ruling out stellar and brown dwarf companions. A disintegrating planet around this white dwarf cannot explain the observed light curves in different filters. Given the short period, the source of the photometric dips must be a dark spot that comes into view every 38 min due to the rotation of the white dwarf. Our optical spectroscopy does not show any evidence of Zeeman splitting of the Balmer lines, limiting the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display kG magnetic fields, such eclipse-like events should be common around white dwarfs. We discuss the potential implications of this discovery on transient surveys targeting white dwarfs, like the K2 mission and the Large Synoptic Survey Telescope.Comment: ApJ Letters, in pres

The composition of the zebrafish intestinal microbial community varies across development

The assembly of resident microbial communities is an important event in animal development; however, the extent to which this process mirrors the developmental programs of host tissues is unknown. Here we surveyed the intestinal bacteria at key developmental time points in a sibling group of 135 individuals of a model vertebrate, the zebrafish (Danio rerio). Our survey revealed stage-specific signatures in the intestinal microbiota and extensive interindividual variation, even within the same developmental stage. Microbial community shifts were apparent during periods of constant diet and environmental conditions, as well as in concert with dietary and environmental change. Interindividual variation in the intestinal microbiota increased with age, as did the difference between the intestinal microbiota and microbes in the surrounding environment. Our results indicate that zebrafish intestinal microbiota assemble into distinct communities throughout development, and that these communities are increasingly different from the surrounding environment and from one another

Contribution of neutral processes to the assembly of gut microbial communities in the zebrafish over host development

Despite their importance to host health and development, the communities of microorganisms associated with humans and other animals are characterized by a large degree of unexplained variation across individual hosts. The processes that drive such inter-individual variation are not well understood. To address this, we surveyed the microbial communities associated with the intestine of the zebrafish, Danio rerio, over developmental time. We compared our observations of community composition and distribution across hosts with that predicted by a neutral assembly model, which assumes that community assembly is driven solely by chance and dispersal. We found that as hosts develop from larvae to adults, the fit of the model to observed microbial distributions decreases, suggesting that the relative importance of non-neutral processes, such as microbe-microbe interactions, active dispersal, or selection by the host, increases as hosts mature. We also observed that taxa which depart in their distributions from the neutral prediction form ecologically distinct sub-groups, which are phylogenetically clustered with respect to the full metacommunity. These results demonstrate that neutral processes are sufficient to generate substantial variation in microbiota composition across individual hosts, and suggest that potentially unique or important taxa may be identified by their divergence from neutral distributions

The first IEEE workshop on the Future of Research Curation and Research Reproducibility

This report describes perspectives from the Workshop on the Future of Research Curation and Research Reproducibility that was collaboratively sponsored by the U.S. National Science Foundation (NSF) and IEEE (Institute of Electrical and Electronics Engineers) in November 2016. The workshop brought together stakeholders including researchers, funders, and notably, leading science, technology, engineering, and mathematics (STEM) publishers. The overarching objective was a deep dive into new kinds of research products and how the costs of creation and curation of these products can be sustainably borne by the agencies, publishers, and researcher communities that were represented by workshop participants.National Science Foundation Award #164101

CATKE: a turbulent-kinetic-energy-based parameterization for ocean microturbulence with dynamic convective adjustment

We describe CATKE, a parameterization for ocean microturbulence with scales between 1 and 100 meters. CATKE is a one-equation model that predicts diffusive turbulent vertical fluxes a prognostic turbulent kinetic energy (TKE) and a diagnostic mixing length that features a dynamic model for convective adjustment (CA). With its convective mixing length, CATKE predicts not just the depth range where microturbulence acts but also the timescale over which mixing occurs, an important aspect of turbulent convection not captured by convective adjustment schemes. As a result, CATKE can describe the competition between convection and other processes such as baroclinic restractification or biogeochemical production-destruction. We estimate CATKE's free parameters with a posteriori calibration to eighteen large eddy simulations of the ocean surface boundary layer, and validate CATKE against twelve additional large eddy simulations with stronger and weaker forcing than used during calibration. We find that a CATKE-parameterized single column model accurately predicts the depth structure of buoyancy and momentum at vertical resolutions between 2 and 16 meters and with time steps of 10-20 minutes. We propose directions for future model development, and future efforts to recalibrate CATKE's parameters against more comprehensive and realistic datasets.Comment: submitted to J. Adv. Model. Earth Sy., 24 pages, 8 figure

Ontogenetic Differences in Dietary Fat Influence Microbiota Assembly in the Zebrafish Gut

ABSTRACT Gut microbiota influence the development and physiology of their animal hosts, and these effects are determined in part by the composition of these microbial communities. Gut microbiota composition can be affected by introduction of microbes from the environment, changes in the gut habitat during development, and acute dietary alterations. However, little is known about the relationship between gut and environmental microbiotas or about how host development and dietary differences during development impact the assembly of gut microbiota. We sought to explore these relationships using zebrafish, an ideal model because they are constantly immersed in a defined environment and can be fed the same diet for their entire lives. We conducted a cross-sectional study in zebrafish raised on a high-fat, control, or low-fat diet and used bacterial 16S rRNA gene sequencing to survey microbial communities in the gut and external environment at different developmental ages. Gut and environmental microbiota compositions rapidly diverged following the initiation of feeding and became increasingly different as zebrafish grew under conditions of a constant diet. Different dietary fat levels were associated with distinct gut microbiota compositions at different ages. In addition to alterations in individual bacterial taxa, we identified putative assemblages of bacterial lineages that covaried in abundance as a function of age, diet, and location. These results reveal dynamic relationships between dietary fat levels and the microbial communities residing in the intestine and the surrounding environment during ontogenesis. IMPORTANCE The ability of gut microbiota to influence host health is determined in part by their composition. However, little is known about the relationship between gut and environmental microbiotas or about how ontogenetic differences in dietary fat impact gut microbiota composition. We addressed these gaps in knowledge using zebrafish, an ideal model organism because their environment can be thoroughly sampled and they can be fed the same diet for their entire lives. We found that microbial communities in the gut changed as zebrafish aged under conditions of a constant diet and became increasingly different from microbial communities in their surrounding environment. Further, we observed that the amount of fat in the diet had distinct age-specific effects on gut community assembly. These results reveal the complex relationships between microbial communities residing in the intestine and those in the surrounding environment and show that these relationships are shaped by dietary fat throughout the life of animal hosts