Pathways to Probing the Circumstellar Environments: A Search for the Rarest Eclipses Across Stellar Populations

Abstract

When circumstellar dust orbiting a star crosses our line of sight, it leaves a fingerprint in the light curve: a complex dimming event that carries information about stellar evolution, planet formation, and the dynamics of circumstellar material that no other observational technique can access at scale. These dimming phenomena are rare, diverse, and until recently, were largely hidden by the lack of a digital footprint across the dynamic night sky. Their discovery required photometric baselines spanning years, sky coverage spanning billions of stars, and the computational infrastructure to flag anomalies in large survey catalogs. The following dissertation focuses on a subclass of three types of rare circumstellar eclipses that probe stellar and planetary formation. We begin with the first large-scale systematic search for main-sequence dipper stars, conducted across 63 million FGK stars using the Zwicky Transient Facility (ZTF) survey and Gaia EDR3. Using a novel light-curve scoring algorithm, we identify 81 new dipper candidates exhibiting diverse dimming morphologies, from symmetric to highly asymmetric, spanning timescales of days to years, with no detectable infrared excess or periodicity in many cases. We investigate multiple classification scenarios and compare them against typical Young Stellar Object (YSO) variability. We conclude that the dimming events are driven either by transiting circumstellar clumps or occultations by substellar companions with disks, establishing the first comprehensive census of main-sequence dippers and placing new constraints on the occurrence rates of circumstellar material around solar analogs. We then turn to the discovery of Gaia17bpp, a cool M0-III giant star that underwent a single deep dimming event of ~4.5 magnitudes lasting over 6.5 years. Gaia17bpp represents one of the longest and deepest stellar occultations recorded in the modern photometric era to date. Using optical-to-infrared spectral energy distribution fitting and a monochromatic transit model, we constrain the occulting body to be a large, optically thick disk with a radius of ~1.4 au, moving at a slow transverse velocity of ~0.005 km/s. We propose that Gaia17bpp belongs to a rare class of binary systems analogous to ε-Aurigae, in which a secondary star enshrouded by a circumstellar disk periodically eclipses an evolved giant primary. The discovery of Gaia17bpp demonstrates that the disk-eclipse phenomenon is not limited to young stellar systems, and that long photometric baselines are essential for recovering such events. Finally, we report the discovery of Gaia20ehk (Gaia-GIC-1), the first giant impact candidate identified by Gaia. This system exhibits a striking anticorrelation between its optical and mid-infrared light curves: large-amplitude irregular optical dimming accompanied by sustained infrared brightening over more than four years. We identify the host as a likely young F-type star, detect a significant ~380.5-day periodic modulation in the pre-outburst light curve consistent with a ~1.1 au orbit, and measure a dust temperature of ~900 K and a minimum dust mass of 4E20 kg from WISE photometry. We interpret Gaia-GIC-1 as the afterglow of a recent catastrophic collision between two planetesimals, producing a clumpy debris cloud on a bound orbit, one of only a handful of such systems known, and the first discovered through Gaia's Photometric Science Alerts. In summary, these three results trace circumstellar dust across the full arc of stellar evolution: from the planet-forming environments of young stars, to the mature main sequence, to evolved giant systems. The following thesis demonstrates that rare dust-driven occultations are discoverable at scale when time-domain survey infrastructure is paired with targeted algorithmic search strategies, and that the coming era of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will vastly expand the known population of such systems. This thesis establishes both the observational framework and the scientific context needed to interpret that forthcoming census.