UNC-CH Physics and Astronomy PhD Dissertation Defense

Jeff Ratzloff

“Designing and Building the Evryscopes, Fast Transit Search Results”

Fast astronomical events offer a window into exciting stellar, binary, and planetary astrophysics that enhance our understanding of stellar formation and evolution, binary interaction, planet characteristics, and lead to new discoveries of object types not seen before which constrain astrophysical models in new environments. Typically the events are very rare, from a combination of factors – the astrophysical causes are infrequent and the sources with these types of events are rare and the objects are observationally challenging. Our targeted research areas include hot subdwarf (HSD – small, dense stars, under-luminous for their high temperatures) transits or eclipses (from a gas planet or compact star), white dwarf (WD – final stage stellar remnants) transits, late dwarf star transits, and a host of sought after fast transient events including high-amplitude flares and supernovae.

Our solution to the challenges of detecting fast transits, eclipses, and transients is the Evryscope – a new type of telescope that monitors the entire sky, continuously, and at high cadence, with good angular resolution (13 arcsec/pixel). The first part of this work describes the Evryscope concept and how the wide field-of-view, observation strategy, and 2-minute images provide sensitivity to the sought-after fast events while covering the millions of targets necessary to find the rarest phenomena. For example, to catch a white dwarf cooling towards a degenerate object in an eclipsing binary, where the system properties can be precisely measured, several million targets need to be searched to have a chance at finding just one of these very short timescale (in evolutionary terms) objects.

We describe the design process, the 3-D models created to test and finalize the design, the construction and deployment. We also describe the innovative camera and optics automated alignment system (the Robotilters), critical in reaching the level of image quality necessary to support our science goals.

The second part of this dissertation work presents three fast transit surveys (Polar, WD and HSD) conducted with Evryscope light curves. We discovered numerous HSD reflection effect and eclipsing binaries, and peculiar variability. Two of the discoveries (both compact binaries with remnant stellar cores) are new classes of objects discovered by our survey. EVR-CB-001 is expected to merge and is a likely single HSD progenitor system (single HSDs are observed and predicted but single HSD progenitor systems have been elusive). The primary of EVR-CB-004 is likely a HSD that has exhausted its core fuel and is in the short-lived final shell burning stage predicted by stellar evolution models but not seen before in a compact WD + HSD system.

The eclipsing and reflection effect discoveries allowed us to precisely measure the primary HSD and M-dwarf properties of several of the systems, helping to constrain parameter spaces and stellar models for these objects. Several of our binaries contain higher mass HSDs or M-dwarfs, suggestive that the prediction of a small fraction of high-mass outliers from binary modeling simulations is correct. Our compact binary discoveries offer a window into short-lived stellar evolution stages and precursor systems to eventual mergers.