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Eonho Chang, Applied Math

“Halfway to Rayleigh” and Other Insights into the Rossby Wave Instability

 

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Abstract:  The Rossby wave instability (RWI) is the fundamental nonaxisymmetric radial shear instability in disks. The RWI can facilitate disk accretion, set the shape of planetary gaps, and produce large vortices. It arises from density and/or temperature features, such as radial gaps, bumps, or steps. A general, sufficient condition to trigger the RWI is lacking, which we address by studying the linear RWI in a suite of simplified models, including incompressible and compressible shearing sheets and global, cylindrical disks. We focus on enthalpy amplitude and width as the fundamental properties of disk features with various shapes. We find analytic results for the RWI boundary and growth rates across a wide parameter space, in some cases with exact derivations and in others as a description of numerical results. Features wider than a scale height generally become unstable about halfway to Rayleigh instability, i.e., when the squared epicyclic frequency is about half the Keplerian value, reinforcing our previous finding. RWI growth rates approximately scale as enthalpy amplitude to the 1/3 power, with a weak dependence on width, across much of the parameter space. Global disk curvature affects wide planetary gaps, making the outer gap edge more susceptible to the RWI. Our simplified models are barotropic and height integrated, but the main results should carry over to more complex and realistic scenarios.

The Astrophysical Journal, Volume 976, Number 1

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Yangyang Cai, Physics

Dynamics of Ultrarelativistic Charged Particles with Strong Radiation Reaction in Aristotelian Equilibrium

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Abstract:  As first proposed by Gruzinov, a charged particle moving in strong electromagnetic fields can enter an equilibrium state where the power input from the electric field is balanced by radiative losses. When this occurs, the particle moves at nearly light speed along special directions called the principal null directions (PNDs) of the electromagnetic field. This equilibrium is “Aristotelian” in that the particle velocity, rather than acceleration, is determined by the local electromagnetic field. In Paper I of this series, we analytically derived the complete formula for the particle velocity at leading order in its deviation from the PND, starting from the fundamental Landau-Lifshitz (LL) equation governing charged particle motion, and demonstrated agreement with numerical solutions of the LL equation. We also identified five necessary conditions on the field configuration for the equilibrium to occur. In this paper we study the entry into equilibrium using a similar combination of analytical and numerical techniques. We simplify the necessary conditions and provide strong numerical evidence that they are also sufficient for equilibrium to occur. Based on exact and approximate solutions to the LL equation, we identify key timescales and properties of entry into equilibrium and show quantitative agreement with numerical simulations. Part of this analysis shows analytically that the equilibrium is linearly stable and identifies the presence of oscillations during entry, which may have distinctive radiative signatures. Our results provide a solid foundation for using the Aristotelian approximation when modeling relativistic plasmas with strong electromagnetic fields.

Physical Review D, Volume 108, Number 063018

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Haowen Zhang, Physics

Trinity I: Self-consistently Modelling the Dark Matter Halo–Galaxy–Supermassive Black Hole Connection from z = 0–10

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Abstract:  We present Trinity, a flexible empirical model that self-consistently infers the statistical connection between dark matter haloes, galaxies, and supermassive black holes (SMBHs). Trinity is constrained by galaxy observables from 0 < z < 10 [galaxies’ stellar mass functions, specific and cosmic star formation rates (SFRs), quenched fractions, and UV luminosity functions] and SMBH observables from 0 < z < 6.5 (quasar luminosity functions, quasar probability distribution functions, active black hole mass functions, local SMBH mass–bulge mass relations, and the observed SMBH mass distributions of high-redshift bright quasars). The model includes full treatment of observational systematics [e.g. active galactic nucleus (AGN) obscuration and errors in stellar masses]. From these data, Trinity infers the average SMBH mass, SMBH accretion rate, merger rate, and Eddington ratio distribution as functions of halo mass, galaxy stellar mass, and redshift. Key findings include: (1) the normalization and the slope of the SMBH mass–bulge mass relation increases mildly from z = 0 to z = 10; (2) The best-fitting AGN radiative+kinetic efficiency is ∼0.05–0.06, but can be in the range ∼0.035–0.07 with alternative input assumptions; (3) AGNs show downsizing, i.e. the Eddington ratios of more massive SMBHs start to decrease earlier than those of lower mass objects; (4) The average ratio between average SMBH accretion rate and SFR is ∼10⁻³ for low-mass galaxies, which are primarily star-forming. This ratio increases to ∼10⁻¹ for the most massive haloes below z ∼ 1, where star formation is quenched but SMBHs continue to accrete.

The Royal Astronomical Society, Volume 522, Issue 3

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Gabriele Bozzola, Astronomy

Does Charge Matter in High-energy Collisions of Black Holes?

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We perform numerical-relativity simulations of high-energy head-on collisions of charged black holes with the same charge-to-mass ratio λ. We find that electromagnetic interactions have subdominant effects already at low Lorentz factors γ, supporting the conjecture that the details of the properties of black holes (e.g., their spin or charge) play a secondary role in these phenomena. Using this result and conservation of energy, we argue these events cannot violate cosmic censorship.

Physical Review Letters 128, 071101

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Nicolas Garavito-Camargo, Astronomy

Hunting for the Dark Matter Wake Induced by the Large Magellanic Cloud

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Abstract:  Satellite galaxies are predicted to generate gravitational density wakes as they orbit within the dark matter (DM) halos of their hosts, causing their orbits to decay over time. The recent infall of the Milky Way’s (MW) most massive satellite galaxy, the Large Magellanic Cloud (LMC), affords us the unique opportunity to study this process in action. In this work, we present high-resolution (m_dm = 4 × 10⁴M_⊙) N-body simulations of the MW–LMC interaction over the past 2 Gyr. We quantify the impact of the LMC’s passage on the density and kinematics of the MW’s DM halo and the observability of these structures in the MW’s stellar halo. The LMC is found to generate a pronounced wake, which we decompose in Transient and Collective responses, in both the DM and stellar halos. The wake leads to overdensities and distinct kinematic patterns that should be observable with ongoing and future surveys. Specifically, the Collective response will result in redshifted radial velocities of stars in the north and blueshifts in the south, at distances >45 kpc. The Transient response traces the orbital path of the LMC through the halo (50–200 kpc), resulting in a stellar overdensity with a distinct, tangential kinematic pattern that persists to the present day. The detection of the MW’s halo response will constrain the infall mass of the LMC, its orbital trajectory, and the mass of the MW, and it may inform us about the nature of the DM particle itself.

The Astrophysical Journal, Volume 884, Number 1

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Joshua Lothringer, Planetary Sciences

Extremely Irradiated Hot Jupiters: Non-oxide Inversions, H⁻ Opacity, and Thermal Dissociation of Molecules

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Youngmin JeongAhn, Planetary Sciences

On the Non-uniform Distribution of the Angular Elements of Near-Earth Objects

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Abstract:  We examine the angular distributions of near-Earth objects (NEOs) which are often regarded as uniform. The apparent distribution of the longitude of ascending node, Ω, is strongly affected by well-known seasonal effects in the discovery rate of NEOs. The deviation from the expected π-periodicity in the apparent distribution of Ω indicates that its intrinsic distribution is slightly enhanced along a mean direction, Ω‾=111°; approximately 53% of NEOs have Ω values within ±90° of Ω‾. We also find that each subgroup of NEOs (Amors, Apollos and Atens) has different observational selection effects which cause different non-uniformities in the apparent distributions of their arguments of perihelion ω, and longitudes of perihelion ϖ. For their intrinsic distributions, our analysis reveals that the Apollo asteroids have non-uniform ω due to secular dynamics associated with inclination-eccentricity-ω coupling, and the Amors’ ϖ distribution is peaked towards the secularly forced eccentricity vector. The Apollos’ ω distribution is axial, favoring values near 0° and 180°; the two quadrants centered at 0° and 180° account for 55% of the Apollos’ ω values. The Amors’ ϖ distribution peaks near ϖ‾=4°; 61% of Amors have ϖ within ±90° of this peak. We show that these modest but statistically significant deviations from uniform random distributions of angular elements are owed to planetary perturbations, primarily Jupiter’s. It is remarkable that this strongly chaotic population of minor planets reveals the presence of Jupiter in its angular distributions.

Icarus, Volume 229, p. 236-246

Evan Schneider, Astronomy

Cholla: A New Massively Parallel Hydrodynamics Code for Astrophysical Simulation

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Abstract:  We present Computational Hydrodynamics On ParaLLel Architectures (Cholla ), a new three-dimensional hydrodynamics code that harnesses the power of graphics processing units (GPUs) to accelerate astrophysical simulations. Cholla models the Euler equations on a static mesh using state-of-the-art techniques, including the unsplit Corner Transport Upwind algorithm, a variety of exact and approximate Riemann solvers, and multiple spatial reconstruction techniques including the piecewise parabolic method (PPM). Using GPUs, Cholla evolves the fluid properties of thousands of cells simultaneously and can update over 10 million cells per GPU-second while using an exact Riemann solver and PPM reconstruction. Owing to the massively parallel architecture of GPUs and the design of the Cholla code, astrophysical simulations with physically interesting grid resolutions (≳256³) can easily be computed on a single device. We use the Message Passing Interface library to extend calculations onto multiple devices and demonstrate nearly ideal scaling beyond 64 GPUs. A suite of test problems highlights the physical accuracy of our modeling and provides a useful comparison to other codes. We then use Cholla to simulate the interaction of a shock wave with a gas cloud in the interstellar medium, showing that the evolution of the cloud is highly dependent on its density structure. We reconcile the computed mixing time of a turbulent cloud with a realistic density distribution destroyed by a strong shock with the existing analytic theory for spherical cloud destruction by describing the system in terms of its median gas density.

The Astrophysical Journal, Volume 217, Number 2

Tim Johannsen, Physics

Testing the No-Hair Theorem with Observations in the Electromagnetic Spectrum. 1. Properties of a Quasi-Kerr Spacetime

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Abstract:  According to the no-hair theorem, an astrophysical black hole is uniquely described by only two quantities, the mass and the spin. In this series of papers, we investigate a framework for testing the no-hair theorem with observations of black holes in the electromagnetic spectrum. We formulate our approach in terms of a parametric spacetime which contains a quadrupole moment that is independent of both mass and spin. If the no-hair theorem is correct, then any deviation of the black hole quadrupole moment from its Kerr value has to be zero. We analyze in detail the properties of this quasi-Kerr spacetime that are critical to interpreting observations of black holes and demonstrate their dependence on the spin and quadrupole moment. In particular, we show that the location of the innermost stable circular orbit and the gravitational lensing experienced by photons are affected significantly at even modest deviations of the quadrupole moment from the value predicted by the no-hair theorem. We argue that observations of black hole images, of relativistically broadened iron lines, as well as of thermal X-ray spectra from accreting black holes will lead in the near future to an experimental test of the no-hair theorem.

The Astrophysical Journal, Volume 716, Number 1

Curtis Cooper, Planetary Sciences

Dynamic Meteorology at the Photosphere of HD 209458b

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Abstract:  We calculate the meteorology of the close-in transiting extrasolar planet HD 209458b using a global, three-dimensional atmospheric circulation model. Dynamics are driven by perpetual irradiation of one hemisphere of this tidally locked planet. The simulation predicts global temperature contrasts of ~500 K at the photosphere and the development of a steady superrotating jet. The jet extends from the equator to midlatitudes and from the top model layer at 1 mbar down to 10 bar at the base of the heated region. Wind velocities near the equator exceed 4 km s^-1 at 300 mbar. The hottest regions of the atmosphere are blown downstream from the substellar point by ~60° of longitude. We predict from these results a factor of ~2 ratio between the maximum and minimum observed radiation from the planet over a full orbital period, with peak infrared emission preceding the time of the secondary eclipse by ~14 hr.

The Astrophysical Journal, Volume 629, Number 1

Martin Pessah, Astronomy

The Stability of Magnetized Rotating Plasmas with Superthermal Fields

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Abstract:  During the last decade it has become evident that the magnetorotational instability is at the heart of the enhanced angular momentum transport in weakly magnetized accretion disks around neutron stars and black holes. In this paper we investigate the local linear stability of differentially rotating, magnetized flows and the evolution of the magnetorotational instability beyond the weak-field limit. We show that, when superthermal toroidal fields are considered, the effects of both compressibility and magnetic tension forces, which are related to the curvature of toroidal field lines, should be taken fully into account. We demonstrate that the presence of a strong toroidal component in the magnetic field plays a nontrivial role. When strong fields are considered, the strength of the toroidal magnetic field not only modifies the growth rates of the unstable modes but also determines which modes are subject to instabilities. We find that, for rotating configurations with Keplerian laws, the magnetorotational instability is stabilized at low wavenumbers for toroidal Alfvén speeds exceeding the geometric mean of the sound speed and the rotational speed. For a broad range of magnetic field strengths, we also find that two additional distinct instabilities are present; they both appear as the result of coupling between the modes that become the Alfvén and the slow modes in the limit of no rotation. We discuss the significance of our findings for the stability of cold, magnetically dominated, rotating fluids and argue that, for these systems, the curvature of toroidal field lines cannot be neglected even when short-wavelength perturbations are considered. We also comment on the implications of our results for the validity of shearing box simulations in which superthermal toroidal fields are generated.

The Astrophysical Journal, Volume 628, Number 2

Todd Thompson, Physics

Supernova Neutrino Thermalization:  Interactions and Timescales

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Abstract:  We solve the Boltzmann equation for the evolution of mu and tau-type neutrino distribution functions including contributions from electron scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and nucleon scattering at temperatures and densities relevant to supernova and protoneutron star calculations, but in an idealized system with no spatial or angular gradients. We incorporate the structure function formalism of Reddy et al.(1998) and Burrows and Sawyer (1998) in electron scattering and nucleon scattering, respectively, in order to include the full scattering kinematics at arbitrary degeneracy. Particularly, we examine the timescales for thermalization with the ambient nuclear medium and the approach to equilibrium.

APS Four Corners Section Meeting Abstracts

Sharada Iyer, Physics

Searching for νμ→ντ Oscillations with Extragalactic Neutrinos

Abstract:  We propose a novel approach for studying νμ→ντ oscillations with extragalactic neutrinos. Active Galactic Nuclei and Gamma Ray Bursts are believed to be sources of ultrahigh energy muon neutrinos. With distances of 100 Mpc or more, they provide an unusually long baseline for possible detection of νμ→ντ with mixing parameters Δm2 down to 10−17eV2, many orders of magnitude below the current accelerator experiments. By solving the coupled transport equations, we show that high-energy ντ's, as they propagate through the earth, cascade down in energy, producing the enhancement of the incoming ντ flux in the low energy region, in contrast to the high-energy νμ's, which get absorbed. For an AGN quasar model we find the ντ flux to be a factor of 2 to 2.5 larger than the incoming flux in the energy range between 102 GeV and 104 GeV, while for a GRB fireball model, the enhancement is 10%-27% in the same energy range and for zero nadir angle. This enhancement decreases with larger nadir angle, thus providing a novel way to search for ντ appearance by measuring the angular dependence of the muons. To illustrate how the cascade effect and the ντ final flux depend on the steepness of the incoming ντ, we show the energy and angular distributions for several generic cases of the incoming tau neutrino flux, F0ν∼E−n for n=1,2 and 3.6. We show that for the incoming flux that is not too steep, the signal for the appearance of high-energy ντ is the enhanced production of lower energy μ and their distinctive angular dependence, due to the contribution from the τ decay into μ just below the detector.

Physical Review D, 61, 053003

Chris L. Fryer, Astronomy

The Dynamics and Outcomes of Rapid Infall onto Neutron Stars

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We present an extensive study of accretion onto neutron stars in which the velocity of the neutron star and structure of the surrounding medium is such that the Bondi-Hoyle accretion exceeds .001 Msun/y. For most cases, hypercritical accretion due to rapid neutrino cooling allows the neutron star to accrete above the Bondi-Hoyle rate as previously pointed out by Chevalier. However, for a subset of simulations which corresponds to evolutionarily common events, convection driven by neutrino heating can lead to explosions by a mechanism similar to that found in core-collapse supernovae.
Armed with the results from our calculations, we are in a position to predict the fate of a range of rapid-infall neutron star accretors present in certain low-mass X-ray binaries, common envelope systems, supernova fallbacks and Thorne-Zytkow objects (TZOs). A majority of the common envelope systems that we considered led to explosions expelling the envelope, halting the neutron star's inward spiral, and allowing the formation of close binary systems. Smothered neutron stars produced in collisions also lead to explosions, preventing them from forming millisecond pulsars. For supernovae in which the fallback of material towards the neutron star is large, we find that a black hole is formed within a few seconds. Finally, we argue that the current set of TZO formation scenarios is inadequate and leads instead to hypercritical accretion and black hole formation. Moreover, it appears that many of the current TZ models have structures ill-suited for modeling by mixing length convection. This has prompted us to develop a simple test to determine the viability of this approximation for a variety of convective systems.

The Astrophysical Journal, Volume 460:801, 1996

William D. Sears, Planetary Sciences

Diffusive Redistribution of Water Vapor in the Solar Nebula Revisited

Stevenson and Lunine presented a model for enhancing the abundance of solid material in the region of the solar nebula at the water condensation point. This was used to provide a means to produce a much more rapid formation of Jupiter than the standard solar nebula models. However, they underestimated the drag induced sun-ward radial drift of the planetesimals of interest. Reanalysis reveals that these particles would spread over the inner solar system and might influence the formation of the asteroids.

24th Lunar and Planetary Science Conference, 1993, Abstract #1637

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Aurora Wilde presented research at the 247th American Astronomical Society (AAS) Winter Meeting in Phoenix, AZ, January 2026.

Elaheh Hayati presented research at the 247th American Astronomical Society (AAS) Winter Meeting in Phoenix, AZ, January 2026.

Hayden Foote presented research at the 247th American Astronomical Society (AAS) Winter Meeting in Phoenix, AZ, January 2026.

Himansh Rathore presented research at the Cosmic Collisions Conference in Abu Dhabi, UAE, in January 2026.

A large group of TAP Graduate Students represented the University of Arizona and presented research at the 247th American Astronomical Society (AAS) Winter Meeting in Phoenix, AZ, January 2026.

Mahdi Naseri

Rafael Bertolotto presented research at the 247th American Astronomical Society (AAS) Winter Meeting in Phoenix, AZ, January 2026.

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2026 Travel Grant Recipients

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Mahdi Naseri attended the American Physical Society (APS) Global Summit in Anaheim, CA, March 2025.

Haley Bowden attended the Simulation Based Inference (SBI) for Galaxy Evolution in Bristol, UK, May 2025

Vikram Manikantan attended the American Physical Society (APS) Global Summit in Anaheim, CA, March 2025.

Maria Mutz attended the American Physical Society (APS) Global Summit in Anaheim, CA, March 2025.

M Smith attended the 24th International Conference on General Relativity and Gravitation & 16th Edoardo Amaldi Conference on Gravitational Waves & Early Career Workshop in Glasgow, UK, July 2025.

Neev Shah attended the Modules for Experiments in Stellar Astrophysics (MESA) Summer School in KU Leuven, Belgium, July 2025.

Himansh Rathore attended XMCII: Clouds over Yellowstone at Montana State University in Bozeman, MT, May 2025.

Himansh Rathore attended XMCII: Clouds over Yellowstone at Montana State University in Bozeman, MT, May 2025.

Connor Sweeney attended Cosmological Surveys and Synergies (COLOURS) program, a 2-week summer school and workshop intended for Early Career Researchers, in Paris, France, June 2025.

Connor Sweeney attended Cosmological Surveys and Synergies (COLOURS) program, a 2-week summer school and workshop intended for Early Career Researchers, in Paris, France, June 2025.

Erik Wessel attended the American Physical Society (APS) Global Summit in Anaheim, CA, March 2025.

Jackson Zariski attended the Research collaboration with the MIT Kavli Institute to apply AI-based telescope pointing techniques in Boston, MA, January 2025.

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2025 Travel Grant Recipients