Astrometry from the Gaia mission has revealed a large population of white dwarf (WD) + main sequence (MS) binaries with periods of $100 - 1000\,$d. Such systems have separations intermediate to predictions from standard binary evolution scenarios, challenging models of binary interaction and mass transfer. Because the selection function of Gaia astrometric catalogs is complex, the underlying population demographics of WD+MS binaries remain imperfectly understood. We present a forward-model of the au-scale WD+MS binary population probed by Gaia that begins with a realistic binary population and incorporates a full model of Gaia mock observations and astrometric model fitting, as well as cuts employed in producing the Gaia astrometric catalog and selecting WD+MS binary candidates. Our model allows us to constrain the intrinsic population demographics of intermediate-separation WD+MS binaries. (...)

Five self-lensing binaries (SLBs) have been discovered with Kepler light curves. They contain white dwarfs (WDs) in AU-scale orbits that gravitationally lens solar-type companions. Forming SLBs likely requires common envelope evolution when the WD progenitor is an AGB star and has a weakly bound envelope. No SLBs have yet been discovered with data from the Transiting Exoplanet Survey Satellite (TESS), which observes far more stars than Kepler did. Identifying self-lensing in TESS data is made challenging by the fact that TESS only observes most stars for ∼25 days at a time, so only a single lensing event will be observed for typical SLBs. TESS's smaller aperture also makes it sensitive only to SLBs a factor of ∼100 brighter than those to which Kepler is sensitive. We demonstrate that TESS has nevertheless likely already observed ∼4 times more detectable SLBs than Kepler. We describe a search for non-repeating self-lensing signals in TESS light curves and present preliminary candidates for which spectroscopic follow-up is ongoing. (...)

Astrometry from Gaia DR3 has enabled the discovery of a sample of 3000+ binaries containing white dwarfs (WD) and main-sequence (MS) stars in relatively wide orbits, with orbital periods Porb = (100−1000) d. This population was not predicted by binary population synthesis models before Gaia and -- if the Gaia orbits are robust -- likely requires very efficient envelope ejection during common envelope evolution (CEE). To assess the reliability of the Gaia solutions, we measured multi-epoch radial velocities (RVs) of 31 WD+MS binary candidates with Porb = (40−300) d and AstroSpectroSB1 orbital solutions. We jointly fit the RVs and astrometry, allowing us to validate the Gaia solutions and tighten constraints on component masses. (...)

Five self-lensing binaries (SLBs) have been discovered with data from the Kepler mission. One of these systems is KIC 8145411, which was reported to host an extremely low mass (ELM; 0.2 M ⊙) white dwarf (WD) in a 456 days orbit with a solar-type companion. The system has been dubbed "impossible," because evolutionary models predict that ∼0.2 M ⊙ WDs should only be found in tight orbits (P orb ≲ days). In this work, we show that KIC 8145411 is in fact a hierarchical triple system: it contains a WD orbiting a solar-type star, with another solar-type star ∼700 au away. (...)

Post-common envelope binaries (PCEBs) containing a white dwarf (WD) and a main-sequence (MS) star can constrain the physics of common envelope evolution and calibrate binary evolution models. Most PCEBs studied to date have short orbital periods (Porb ≲ 1 d), implying relatively inefficient harnessing of binaries' orbital energy for envelope expulsion. Here, we present follow-up observations of five binaries from 3rd data release of Gaia mission containing solar-type MS stars and probable ultramassive WDs (M≳1.2 M⊙) with significantly wider orbits than previously known PCEBs, Porb = 18-49 d. The WD masses are much higher than expected for systems formed via stable mass transfer at these periods, and their near-circular orbits suggest partial tidal circularization when the WD progenitors were giants. These properties strongly suggest that the binaries are PCEBs. (...)​

We present follow-up spectroscopy of 21 cataclysmic variables (CVs) with evolved secondaries and ongoing or recently terminated mass transfer. Evolutionary models predict that the secondaries should have anomalous surface abundances owing to nuclear burning in their cores during their main-sequence evolution and subsequent envelope stripping by their companion white dwarfs. To test these models, we measure sodium (Na) abundances of the donors from the Fraunhofer 'D' doublet. Accounting for interstellar absorption, we find that all objects in our sample have enhanced Na abundances. We measure 0.3 dex ≲ [Na/H] ≲ 1.5 dex across the sample, with a median [Na/H] = 0.956 dex, i.e. about an order of magnitude enhancement over solar values. To interpret these values, we run Modules for Experiments in Stellar Astrophysics binary evolution models of CVs in which mass transfer begins just as the donor leaves the main sequence. (...)

One of the key processes driving galaxy evolution during the Cosmic Dawn is supernova feedback. This likely helps regulate star formation inside of galaxies, but it can also drive winds that influence the large-scale intergalactic medium. Here, we present a simple semi-analytic model of supernova-driven galactic winds and explore the contributions of different phases of galaxy evolution to cosmic metal enrichment in the high-redshift (z ≳ 6) Universe. We show that models calibrated to the observed galaxy luminosity function at z ~ 6-8 have filling factors ∼1% at z ~ 6 and ∼0.1% at z ~ 12, with different star formation prescriptions providing about an order of magnitude uncertainty. Despite the small fraction of space filled by winds, these scenarios predict an upper limit to the abundance of metal-line absorbers in quasar spectra at z≳5 which is comfortably above that currently observed. (...)