Astrophysics and Relativity Seminars

A stellar explosion launches a radiation-mediated shock that propagates through the star and breaks out when the optical depth ahead of it becomes too low to sustain the shock. The photons released during shock breakout are the first electromagnetic signal from a cosmic explosion and carry unique information about the medium the shock emerges from, including its geometry, radius, composition, and density profile. In this talk, I will discuss the distinctive properties of radiation-mediated shocks and describe the resulting breakout emission in a variety of cosmic explosions. I will highlight some of the things that observations of shock breakout and early emission have taught us so far, and some of what we expect to learn over the coming decade.

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Massive black hole binaries (MBHBs) are a natural outcome of galaxy mergers and are expected to be among the strongest sources of low-frequency gravitational waves. At sub-parsec separations, however, these binaries are extremely difficult to resolve directly. One promising approach is to search for their signatures in the time-domain variability of AGN lightcurves. Orbital motion can imprint periodic or evolving (“chirping”) signals in quasar light curves through mechanisms such as relativistic Doppler boosting. In this talk, I present a series of studies exploring methods for detecting compact SMBHBs in large optical surveys, building towards a systematic search. Using mock data motivated by the new time-domain survey LSST, we evaluate the performance of these methods. I will demonstrate that both periodic and chirping signals, consistent with GW-driven inspiral, can be identified in large optical surveys alone, and that these detections can be confirmed with even higher confidence when combined with GW observations. Finally, I will discuss a potential multi-wavelengths signature of MBHBs due to multiple disk- and jet- components.

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" Planet formation begins with the coagulation of micrometer-sized dust grains in a protoplanetary disk. This particle growth not only provides the building material for planetesimals but also shapes the environments they form in. Aerodynamic drag forces between gas and dust determine how fast the grains drift and how efficiently they can be trapped in the frequently observed rings and vortices in protoplanetary disks. The size of the particles thus affects the available dust mass and composition throughout the disk. Dust also dominates the opacities and therefore regulates the thermal structure and the onset of hydrodynamic instabilities that depend on thermal relaxation. In my talk, I will present two recent applications of coupled multifluid simulations and dust coagulation models that capture the interplay of microphysics and large-scale dynamics in protoplanetary disks: Our simulations of planet-disk systems with dust coagulation quantify the amount of small dust grains that diffuse and drift through planet-induced gaps. These calculations constrain the composition and available mass of solids in the inner disk regions. We furthermore investigate the effects of dust evolution on protoplanetary disk turbulence, induced by the Vertical Shear Instability. Our simulations show that efficient dust coagulation can suppress turbulence in protoplanetary disks, due to the reduced collisional cooling in the dust-depleted upper layers. Understanding how dust coagulation influences the dynamics of protoplanetary disks is thus key to understanding the initial conditions for planet formation and to interpreting multi-wavelength observations."

Binary black holes (BHBs) embedded in dense gas hold the promise of "multi-messenger astrophysics": when they are detected both through gravitational waves (GWs) and electromagnetic (EM) observations, they will enable novel science. This is true both for massive BHBs, whose GWs will be detectable by the future LISA satellite and by on-going pulsar timing arrays (PTAs), as well as for stellar-mass BHBs detected through ground-based GW detectors. In both cases, identifying coalescing binaries through their EM signatures will help clarify their astrophysical origin and yield novel probes of cosmology, fundamental physics, and accretion physics. In this talk, I will describe how circumbinary gas may impact the orbital evolution of binaries, while producing characteristic EM signatures for both massive and stellar-mass BHBs, based on hydrodynamical simulations and analytic models. In both cases, several binary candidates have been identified in optical surveys, and LSST should soon be able to uncover robust binaries based on their high-quality, multi-band light-curves.

The tidal disruption of a star that wanders too close to a central black hole is a unique probe of quiescent supermassive black holes lurking in the nuclei of galaxies. We have demonstrated the ability to discover and spectroscopically classify large samples of tidal disruption events (TDEs) in the optical from the Zwicky Transient Facility (ZTF) Northern Sky Survey, with systematic high-energy follow-up with Swift and XMM-Newton. From these efforts, we now know the spectroscopic signatures, multi-wavelength properties, and host galaxy preferences of TDEs, and can begin to use TDEs as powerful probes of massive black hole demographics in quiescent galaxies. One of the most exciting applications of TDEs is the ability to probe intermediate mass black holes (IMBHs) that are expected to reside in dwarf galaxies, with demographics that encode the seeding mechanism of massive black holes in the early Universe. I will outline our strategy in the next decade for searching for TDEs across the black hole mass function, and over cosmic time, by extending our sensitivity to TDEs to lower central black hole masses and higher redshifts, by exploiting the discovery power of the next generation of wide-field time domain surveys, including Rubin, Roman, ULTRASAT, and UVEX.

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