Athena++ 2019

Time	Speaker	Topic (click titles to see abstracts)
8:30 - 9:00		Registration (Hospitality Hall, first floor)

9:00 - 9:05	Eric Chronister	Dean's welcome

9:05 - 9:20	Jim Stone	Summary

9:20 - 9:40	Omer Blaes	Athena++ simulations of AM CVn disks
		I will give a brief overview of some of the challenges we have faced in trying to do global 3D simulations of accretion disks in compact binary systems.
9:40 - 10:00	Bryance Oyang	AMCVn Disk Simulations
		We've been running AMCVn disk simulations with ideal MHD and radiation. The goal is to understand the physics that leads to the observed lightcurve behavior, including the rms-flux relation, superhumps, and dwarf nova outbursts. Difficulties with angular momentum conservation will be mentioned.
10:00 - 10:20	Morgan MacLeod	Resonant Tides in Close Binaries
		This talk discusses the methodology and results simulations of close binary systems being driven toward coalescence under the influence of unstable mass transfer. We find that the decaying orbit traverses resonances with oscillatory modes of the donor star's envelope, exciting large-amplitude tidal waves and distorting the star.
10:20 - 10:40	Mackenzie Moody	Hydrodynamic Torques in Circumbinary Accretion Disks
		Gaseous disks have been proposed as a mechanism for facilitating mergers of binary black holes. We explore circumbinary disk systems to determine the evolution of the central binary. To do so, we perform 3D, hydrodynamic, locally isothermal simulations of circumbinary disks on a Cartesian grid. We focus on binaries of equal mass ratios on fixed circular orbits. To investigate the orbital evolution of the binary, we examine the various torques exerted on the system. We study the case where the binary and disk are aligned, and also study the case when the two are misaligned.
10:40 - 11:00		Coffee Break

11:00 - 11:20	Patryk Pjanka	Stratified global simulations of accretion disks with Roche-lobe overflow
		Observational efforts have brought forth large quantities of photometric and spectral data concerning accretion disks in cataclysmic variables and low-mass X-ray binaries, making these systems an exciting target for research in the field of accretion physics. Unfortunately, few detailed global MHD and radiation-MHD models of these systems exist, causing the observed spectral and variability features to lack a detailed numerical prediction and often only allowing interpretation in a qualitative manner. I will present preliminary results of global 3D stratified MHD simulations of accretion disks, an extension of the work of Ju, Stone & Zhu (2016, 2017), who investigated unstratified accretion disks formed self-consistently through Roche-lobe overflow. I will focus on the properties of angular momentum transport via MHD turbulence as compared to spiral density waves in the disk, as well as the global properties of the flow.
11:20 - 11:40	Bhupendra Mishra	Strongly magnetized accretion disks around black holes slides
		I will talk about physics of strongly magnetized accretion flows using fully global three dimensional MHD simulations. The models use cutting edge numerical techniques in Athena++ to resolve a geometrically thin disk with H/r ~ 0.05. I used static mesh refinement to substantially reduce the computational cost and performed fully global simulation including poles and entire azimuthal domain. In a set of simulations with different initial non-zero field strength, I find a qualitative agreement with previous local shearing box simulations of strongly magnetized disks.
11:40 - 12:00	Lizhong Zhang	Implementation of magnetic opacity and radiative transfer in relativistic hydrodynamics in Athena++
		With a view toward numerical simulations of neutron star accretion columns, we need to couple special relativistic MHD with the radiation transfer equation. The Athena++ module developed by Jiang (Jiang et al. 2014; Jiang er al. 2017b) is capable of solving the frequency-integrated, time-dependent radiative transfer equation. However, we still need to couple the photon-matter momentum and energy exchange source terms in special relativistic MHD. Also, implementation of opacities that depend explicitly on magnetic field strength and direction is also important for neutron star applications.
12:00 - 12:30		Discussion (leader: Omer Blaes)

12:30 - 2:00		Lunch

Session 2

Accretion Disks and Winds (Chair: Omer Blaes)

Time	Speaker	Topic (click titles to see abstracts)
2:00 - 2:20	Sergei Dyda	Opacity Effects on AGN Cloud Dynamics
		We study the effects of opacity on the dynamical evolution of AGN clouds using time-dependent, radiation-hydrodynamics simulations. When clouds are optically thick, the bright side heats up and expands, accelerating the cloud via the rocket effect. This acceleration can be enhanced by ˜20% when cloud opacity is a non-monotonic function of temperature, as expected from the contribution to opacity from certain chemical species. We find that ˜1% of incident radiation is re-emitted by accelerating clouds, which we estimate as the contribution of a single accelerating cloud to an emission or absorption line. Re-emission is suppressed by features in the opacity profile since these decrease the opacity of the hot, evaporating gas, primarily responsible for the re-radiation. If clouds are optically thin, they heat nearly uniformally, expand and form shocks. This triggers the Richtmyer-Meshkov instability, leading to cloud dissipation.
2:20 - 2:40	Tim Waters	Cloud coalescence and condensation splatter
		We recently utilized Athena++ to uncover new phenomena associated with multiphase gas dynamics. We present two topics in some detail: (i) cloud coalescence, a dynamical instability causing clouds to merge even in the absence of gravity; (ii) 'splattering', a nonlinear effect of the non-isobaric regime of thermal instability by which initially stationary gas can possibly give rise to supersonic condensations. We discuss the astrophysical implications of these findings and the numerical requirements to accurately simulate these effects.
2:40 - 3:00	Randall Dannen	On Modeling Line Driven AGN Winds in the Presence of Heating slides
		We will discuss our latest efforts modeling line driven AGN winds using the most up-to-date atomic data available coupled with SED specific heating and cooling rates. We present our current findings that 1) heating dominates driving unless the gas is nearly isothermal and 2) adopting non-Sobolev prescriptions for the optical depth may help alleviate this problem.
3:00 - 3:20	Lia Hankla	Global Three-dimensional MHD Simulations of Accretion Flows and Dynamo Effects slides
		Accretion disks around black holes can lead to the formation of astrophysical jets under certain conditions, the most important being that large-scale poloidal magnetic fields exist close to the black hole. However, that these poloidal magnetic fields arise naturally is not at all obvious; a typical object shredded by the black hole most likely results in a toroidal field, as in a tidal disruption event. Recent work with very specific initial conditions has shown that toroidal flux can, likely through a dynamo mechanism, create large-scale poloidal flux. Now, the question is whether generic initial conditions are also subject to such a dynamo, or if these results were due to a fine-tuning of certain parameters. This project aims to study various magnetic field configurations to further investigate the dynamo mechanism and examine when large-scale poloidal fields can develop, with the ultimate goal of understanding the circumstances under which jets form.
3:20 - 3:50		Coffee break

3:50 - 4:10	Matthew Coleman	Accretion Disk Boundary Layers slides
		The region where an accretion disk meets a star is referred to as the boundary layer. In this region the MRI is unable to operate, so some other angular momentum transport mechanism is required. The supersonic shear here is hydrodynamically unstable, leading to the generation acoustic waves which provide this transport. I discuss recent Athena++ simulations designed to gain physical insight into this transport mechanism.
4:10 - 4:30	Bri Mills	Athena++ Monte Carlo Radiation Transfer Calculations of Accretion Disk: Spectra from Radiation Hydrodynamic Simulations
		Radiation transfer in optically thick regions can be computationally expensive and time-consuming. We have implemented a modified random walk method to accelerate the Monte Carlo radiation transfer solver in Athena++ to more quickly post-process optically thick accretion disk simulation snapshots. We report on efforts to use the Monte Carlo transfer solver to model accretion onto 5 x 10^8 solar mass black holes and discuss implications for super-Eddington accretion in ultra-luminous X-ray sources (ULXs) and active galactic nuclei (AGN).
4:50 - 4:50	Shengtai Li	Coupling Hydrodynamics with Dust Coagulation for Simulating Dusty Disks
		We have developed a new capability to combine the hydrodynamics of disks with a dust coagulation model (Birnstiel et al 2010) in our code LA-COMPASS (Los Alamos Computational Astrophysics Suite). LA-COMPASS is a code package for simulations of interaction between gas and dust in proto-planetary disks, and between disks and their embedded proto-planets. It uses a new one-fluid method for simulating the dynamics of dust grains in a dusty gaseous disk. The dust grains are treated as pressure-less fluid and their coupling with the gas is through stiff drag terms. It handles dust species with small stopping time as well as large stopping time. The coupling of hydrodynamics with dust coagulation model is done in an operator-split manner. Since one-step coagulation process is much more expensive than one-step hydro solver, we perform a number of hydro step for one coagulation step. We have also developed several approaches to speed up the coupled computing between the hydro and coagulation process. A new coagulation model with conserved total momentum is also developed and tested. Numerical examples are provided to demonstrate the effectiveness and efficiency of our code.
4:50 - 5:20		Discussion (leader: Daniel Proga)

Thursday, March 21st

(Day 2 - User Meeting)

Session 3

The ISM and Gas Flows in Galaxies (Chair: Munan)

Time	Speaker	Topic (click titles to see abstracts)
9:00 - 9:20	Kris Beckwith	Correlations and Energy Cascades in Magnetized Turbulence
		Many terrestrial and astrophysical plasmas encompass very large dynamical ranges in space and time, which are not accessible by direct numerical simulations. Thus, idealized sub-volumes are often used to study small-scale effects including the dynamics of magnetized turbulence. One significant aspect of the dynamics of the turbulent cascade is the transfer of energy from the large to small scales. In this work, we present a new shell-to-shell energy transfer analysis framework for understanding cascades of energy within compressible magnetized turbulence. We demonstrate the viability of this framework through application to a series of isothermal subsonic and supersonic magnetized turbulence simulations and utilize results from this analysis to establish benchmarks in the non-linear regime for cross-code comparison. We further utilize these simulations to study how the autocorrelation time and normalization of the large-scale driving systematically change properties of the turbulence. In general, we show that shorter autocorrelation times require more power in the acceleration field. More power in the acceleration field results in more power in compressive modes that weaken the anti-correlation between density and magnetic field strength. We examine how these results can impact a range of diagnostics relevant for a range of terrestrial and astrophysical applications. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. SAND No: SAND2018-1999 A
9:20 - 9:40	Sean Michael Ressler	Accretion In The Galactic Center Via Magnetized Stellar Winds slides
		I will present the results of using Athena++ to simulate the accretion onto Sagittarius A* (Sgr A), the supermassive black hole in the galactic center, as it is fed by the stellar winds of the ˜30 Wolf Rayet (WR) Stars orbiting at ˜parsec scales. The winds in our calculation are treated point sources of mass, momentum, energy, and magnetic fields with properties based on spectral observations of the WR stars. Using 3D MHD simulations with nested static mesh refinement, we are able to track the gas down to a few hundred gravitational radii of the black hole, roughly 4 orders of magnitude in radius. This allows us to calculate, from first principles, e.g., the rotation measure of Sgr A and the net vertical flux ultimately threading the black hole, a key parameter for horizon scale simulations, as well as to study the dynamics of the accretion flow. I will present a summary of these results and discuss what they mean in the context of emission modeling of Sgr A*.
9:40 - 10:00	Miao Li	Effect of Streaming Cosmic Rays on the Evolution of Supernova Remnant
		"Feedback from supernovae (SNe) plays a critical role in regulating the interstellar medium (ISM), which has important implications for star formation and galaxy evolution. Cosmic rays (CRs) are an important product of SNe, and permeate throughout the ISM. But it is not clear how CRs affect SN feedback. I will talk about our experiments on the evolution of a supernova remnant with CRs, utilizing the recent development of CR streaming and diffusion module in Athena++. We evaluate the impact of CRs on the SN feedback in different environments. I will discuss its implications for the formation of a multiphase ISM and galactic outflows.
10:00 - 10:20	Dong Zhang	Supernova Remnants in Turbulent Medium slides
		Core-collapse supernova explosions may occur in the highly inhomogeneous molecular clouds in which their progenitors were born. We perform a series of 3-dimensional hydrodynamic simulations to model the interaction between an individual supernova remnant (SNR) and a turbulent molecular medium, in order to investigate possible observational evidence for the turbulent structure of MCs. We find that the properties of SNRs are mainly controlled by the mean density of the surrounding medium, while a SNR in a more turbulent medium with higher supersonic turbulent Mach number shows lower interior temperature, lower radial momentum, and dimmer X-ray emission compared to one in a less turbulent medium with the same mean density. We compare our simulations to observed SNRs, in particular, to W44, W28 and IC 443. We also explore the impact of cosmic rays on the dynamics of remnants and momentum feedback in molecular clouds.
10:20 - 10:50		Coffee break

10:50 - 11:10	Drummond Fielding	Clustered supernovae drive powerful galactic winds after superbubble breakout slides
		Using 3D hydrodynamic simulations of vertically stratified patches of turbulent galactic discs to I will show how the spatio-temporal clustering of supernovae (SNe) enhances the power of galactic winds. Randomly distributed SNe drive inefficient galactic winds because most supernova remnants lose their energy radiatively before breaking out of the disc. Accounting for the fact that most star formation is clustered alleviates this problem. Superbubbles driven by the combined effects of clustered SNe propagate rapidly enough to break out of galactic discs well before the clusters' SNe stop going off. The radiative losses post-breakout are reduced dramatically and a large fraction (greater than 0.2) of the energy released by SNe vents into the halo powering a strong galactic wind. These energetic winds are capable of providing strong preventative feedback and eject substantial mass from the galaxy with outflow rates of the order of the star formation rate. The momentum flux in the wind is only of order that injected by the SNe, because the hot gas vents before doing significant work on the surroundings. I will demonstrate that our conclusions hold for a range of galaxy properties, both in the local Universe (e.g. M82) and at high redshift (e.g. z ˜ 2 star-forming galaxies). Building upon these results I will show that if the efficiency of forming star clusters increases with increasing gas surface density, as suggested by theoretical arguments, the condition for star cluster-driven superbubbles to break out of galactic discs corresponds to a threshold star formation rate surface density for the onset of galactic winds of order that which is observed.
11:10 - 11:30	Xiaoshan Huang	Radiation's Role in Accelerating Galactic Outflow
		Cold galactic outflows are widely observed with a range of outflow speeds, but questions remain about how accelerate cold gas clouds to observed speeds without overheating them. We preform 2D and 3D radiation hydrodynamics simulations using Athena++ to test if radiation pressure on dust embedded in the gas a promising mechanism to accelerate the outflow. We studied relative impact of infrared (IR) and ultraviolet (UV) radiation on cloud dynamical evolution. We found that, in constrast to IR radiation, a pure UV radiation field tends to overheat gas, sublimating the dust and destroying the cloud quickly. We are now exploring the range of UV to IR radiation where acceleration is most promising.
11:30 - 11:50	Thomas Gardiner	Momentum and Energy Conserving Formulations for Multi-fluid Plasmas
		In recent years, there has been considerable effort devoted to developing numerical methods for the solution of two- and multi-fluid plasma equations. The interest in such methods stems from the fact that they are widely applicable to both laboratory and astrophysical systems. The dynamics in many of these systems are such that one typically finds smooth waves, instabilities and shock waves present and ideally one would like a numerical method capable of capturing these phenomena faithfully. In the case of shock waves, it is well known that failure to conserve momentum and energy may lead to incorrect shock speeds. In this talk I will present a new approach to formulating the multi-fluid plasma system of equations such that when discretized they exactly conserve momentum and energy. This research is part of a 3-year, Grand Challenge Laboratory Directed Research and Development project at Sandia National Labs with the goal of developing integrated, multi-scale plasma simulation methods that span the range of scales from kinetic to single-fluid MHD regimes. Sandia National Labs is managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a subsidiary of Honeywell International, Inc., for the U.S Dept. of Energy's National Nuclear Security Administration under contract DE-NA0003525.
11:50 - 12:20		Discussion (leader: Chang-Goo Kim)

12:20 - 2:00		Lunch

Session 4

ISM and Star Formation (Chair: Chang-Goo Kim)

Time	Speaker	Topic (click titles to see abstracts)
2:00 - 2:20	Munan Gong	Post-processing chemistry and its applications to the X_CO conversion factor slides
		Chemistry is important in many astrophysical environments, both for heating and cooling of the gas and comparison with observations. I will introduce the post-processing chemistry that has been implemented in a branch of Athena++. The current implementation has been used to study the X_CO conversion factor of molecular clouds in different environments. I will also discuss the future extension of the implementation to time-dependent chemistry and possible methods for improving the code performance.
2:20 - 2:40	Yuan Li	Simulating thermal instabilities
		Multiphase gas exists in many environments in astrophysics, from the ISM to the ICM and CGM. I will first talk about the importance of understanding multiphase gas in simulations of AGN feedback in massive galaxies and clusters, and the technical challenges we face if we try to resolve it. I will then outline a few project plans that use small-scale simulations to study the properties of the multiphase gas in the ISM, CGM, and in the context of SMBH accretion. All of these will be done using Athena++. Some of them are still in the design phase, so comments and suggestions are welcome.
2:40 - 3:00	Alwin Mao	Star Formation and Gas. slides
		I study how gas becomes stars on kiloparsec and parsec scales. On kiloparsec scales I experiment with ways to predict whether regions of gas will form stars, inferring a star formation efficiency. I test different methods using snapshots from the TIGRESS simulation. On parsec scales I plan to use tracer particles to understand gas motions in gravitoturbulent collapse.
3:00 - 3:20	Aleksandra Kuznetsova	Numerical Experiments on (Proto)star Formation slides
		Top-down simulations of star formation, from cluster to core scales can help us characterize protostellar populations and understand the range of initial conditions for protostellar disks. I will present on a series of (hydro and MHD) Athena simulations in which we used a sink-patch algorithm to track how protostars in the simulation accrued mass and angular momentum. We find that protostellar angular momentum is imparted locally, that is, protostars do not inherit angular momentum from the parent cloud. We report on the distribution of initial conditions to be used for more high resolution studies and comment on the implications for disk formation and evolution.
3:20 - 3:40		Coffee break

3:40 - 4:00	Ka Ho (Andy) Lam	Disk Formation in Magnetized Dense Cores with Turbulence and Ambipolar Diffusion slides
		In the presence of a moderate level of magnetization, disks are easily suppressed in the simplest case of a laminar core with aligned magnetic field and rotation in the ideal MHD limit. Although there are already numerous studies showing the effects of turbulence and non-ideal MHD independently, there has not been any study that includes both turbulence and all of the three non-ideal MHD effects simultaneously. As the first step, we study magnetized core collapse and disk formation including different levels of turbulence and ambipolar diffusion (AD) using Athena and sink particle treatment, with focus on the protostellar mass accretion phase. We find that turbulence affects the accretion flow by modifying the magnetically flattened pseudodisk, facilitates the escape of dragged-in magnetic flux, and reduces the angular momentum redistribution between flows with different specific angular momenta. In our models, turbulence is able to enable the formation of transient disks. However, a moderately strong AD is needed to enable the formation of persistent disks. During the collapse of the laminar cores with AD, we find a novel structure produced by the magnetic flux decoupled from the stellar mass, which is related to the AD shock found in previous analytic models and 2D simulations. We show that turbulence and AD work together in a complementary way, enabling more robust disk formation. The future direction of our project is to include both turbulence and all non-ideal MHD effects, resolving structures in higher resolution using AMR or SMR, and study the long-term behavior using Athena++ with a sink particle treatment, which I hope to implement into ATHENA++.
4:00 - 4:20	Kei E. I. Tanaka	Synthetic Observations of Molecular Cloud Formation
		We are developing a post-processing radiative transfer code for synthetic observations. As a first test case, we carry out the radiative transfer of dust continuum, polarization, and CO line based on the simulations of molecular cloud formation by Iwasaki et al. (2019).
4:20 - 4:40		Discussion (leader: Kengo Tomida)

5:00		Conference Dinner (Bus pickup - library at 5:00 pm)

Friday, March 22nd

(Day 3 - User Meeting)

Session 5

Protoplanetary Disks (Chair: Jake Simon)

Time	Speaker	Topic (click titles to see abstracts)
9:00 - 9:10	Zhaohuan Zhu	Protoplanetary Disks

9:10 - 9:30	Lile Wang	Athena++ simulations with consistent thermochemistry and beyond
		Thermochemical calculations become increasingly desired by modern astrophysical simulations for model consistency. In this talk, I will introduce and elaborate simulations that involve consistent thermochemistry based on Athena++ and their applications, especially in the research of protoplanetary disks (PPDs) and exoplanetary atmospheres. Current attempts in bridging these simulations to observations will also be discussed.
9:30 - 9:50	Xiao Hu	Non-ideal MHD simulation of HL Tau disk: formation of rings
		Recent high resolution observations unveil various ring structures in circumstellar disks. The origin of these rings has been widely investigated through different theoretical assumptions. In this work we perform global 3D non-ideal MHD simulations including effects from both Ohmic dissipation and ambipolar diffusion (AD) to model the HL Tau disk.. The non-ideal MHD diffusion profiles are calculated self-consistently based on the global dust evolution calculation including sintering effects. We find that accretion is mainly driven by disk wind. Gaps and rings can be quickly produced close to snowlines of major volatiles. Strong Ambipolar diffusion (AD) leads to highly preferential accretion at midplane, followed by magnetic reconnection. The draining of mass in the field reconnection area makes gaps and adjacent rings from mass accumulation
9:50 - 10:10	Shinsuke Takasao	3D MHD Simulations of Accreting Young Stars
		Youg stars such as protostars and pre-main-sequence stars grow through the interaction with the surrounding disks. Since their evolution history and final state depends on the interaction process, we have been developing a 3D MHD star-disk model to reveal the accretion process in the vicinity of a central star. In this paper, we will describe how functions of Athena++ are utilized for the model. Then, we will briefly introduce our results and discuss the future work.
10:10 - 10:30	Haifeng Yang	Global simulations of truncated protoplanetary disks
		We present preliminary results from a global 2D simulation of a magnetized protoplanetary disk with truncated outer disk aiming to understand the interaction of the disk with the ISM environment, as well as magnetic flux evolution with realistic boundary conditions. We find that as the system relaxes, poloidal magnetic field threading the disk beyond the truncation radius collapses towards the midplane, leading to rapid reconnection. This process removes a substantial amount of magnetic flux from the system, and forms closed poloidal magnetic flux loops encircling the outer disk in quasi-steady-state. Implications from such field configurations on disk structure and evolution will be briefly discussed.
10:30 - 10:50		Coffee break

10:50 - 11:10	Fulvia Pucci	Evolution of MRI turbulence in protoplanetary disks: a local simulation including dead-zone boundaries
		Understanding the complex dynamical evolution of protoplanetary disks is of key interest both for building a comprehensive theory of planet formation, as well as a means of explaining the observationally inferred properties of these objects. Gammie (1996) proposed what has now become the traditional dead-zone model in which disk surface layers accrete by sustaining MRI turbulence, with the shielded interior maintaining an inert and magnetically decoupled dead zone, where MRI turbulence is quenched by the ohmic resistivity. In this region dust is believed to accumulate, initiating the planet formation. I will show local high resolution shearing box simulations to study the evolution of the dead zone boundaries between active and non active zone. I will discuss the classic turbulence diagnostics such as the Reynolds and Maxwell tensors' evolution, 1D and 2D spectra, to characterize the penetration of turbulence in the inactive region.
11:10 - 11:30	Kazunari Iwasaki	Global Non-ideal MHD Simulations of Protoplanetary Disks: Inner Dead Zone Boundaries
		We perform global three-dimensional non-ideal magnetohydrodynamic simulations of protoplanetary disks with Ohmic resistivity and Ambipolar diffusion using Athena++. We take into account realistic Ohmic and Ambipolar diffusion coefficients in the assumed disk profile. By simulating the disk with a wide range of radii from 0.1 AU to 10 AU, our simulation box contains the inner dead zone boundaries both for Ohmic and Ambipolar diffusion. In the active zone, magnetorotational instability is developed and alpha reaches 10&Hat-2 level in the case where the initial plasma beta is 10&Hat4. By contrast, in the dead zone due to Ohmic diffusion and Ambipolar diffusion, magneto-centrifugal wind is driven and global magnetic fields extract angular momentum from the disk. In this talk, we will discuss the gas dynamics around the inner dead zone boundaries.
11:30 - 11:50	Alexei Poludnenko	Modeling of Terrestrial and Astrophysical Reacting Flows with Athena and Athena++ slides
		Over the past ten years, Athena has been extensively used for DNS modeling of a wide range of chemical reacting flows in various combustion systems on Earth and of thermonuclear reacting flows in astrophysical contexts, in particular in Type Ia supernovae. In this talk, I briefly discuss the reacting flow extensions implemented in Athena 3.1 (referred to as Athena-RFX) to represent combustion physics. I also provide a brief summary of various contexts, in which this code has been applied, including a broad survey of turbulent reacting flows in aerospace propulsion and energy conversion systems, and the problem of the deflagration-to-detonation transition in Type Ia supernovae. Furthermore, various associated algorithmic extensions to Athena-RFX will be presented including the implementation of the load-balancing strategy. Finally, I discuss the planned transition to Athena++ for combustion modeling.
11:50 - 12:10	Yoram Kozak	Extensions of the Athena Particle Solver for Highly-Compressible Flow Regimes
		A wide range of multi-phase flows involve motion of massive particles through the embedding fluid. On Earth, these include various propulsion and energy conversion systems, e.g., sprays in engines, industrial explosions, e.g., in mines or in storage silos, various military applications, etc. In many of these situations, particles can move supersonically, or the host flow can be highly compressible with shocks or other structures associated with sharp property gradients, e.g., unresolved flames. Here we discuss an extension of the particle integration algorithm implemented in Athena 4.2 intended to provide high solution accuracy for compressible flows. We also discuss the implementation of the massless Lagrangian tracer particles allowing Lagrangian analysis of the flow. Once strong shocks are introduced into the flow, various central interpolation schemes tend to oscillate and produce non-physical values. In order to address this issue, two Weighted-Essentially-Non-Oscillatory (WENO) interpolation schemes are introduced. It is shown that WENO schemes effectively do not oscillate and capture shocks in a sharper manner than centered schemes. Furthermore, for massive particles, the drag law is extended for flows with high compressibility to depend also on the Mach number. Proper modifications of the numerical schemes are made in order to maintain the integrator properties.
12:10 - 12:40		Discussion (leader: Chao-Chin Yang) slides

12:40 - 2:00		Lunch

Session 6

Protoplanetary Disks and Planetary Sciences (Chair: Chao-Chin Yang)

Time	Speaker	Topic (click titles to see abstracts)
2:00 - 2:20	Jake Simon	Planetesimal Formation and the Influence of the Radial Pressure Gradient
		The streaming instability leads to significant enhancement of solid particles in protoplanetary disks such that these particles eventually collapse due to their own gravity and form planetesimals. The source of energy driving this instability is the radial gas pressure gradient in the disk. In this talk, I will present a series of high resolution simulations of the streaming instability that include particle self-gravity to study how the initial mass function of planetesimals depends on the radial pressure gradient. Consistent with our previous results that examined other parameters, we find that the mass distribution is at best weakly dependent on the pressure gradient. Furthermore, the largest mass planetesimals are also weakly dependent on the pressure gradient; our statistical errors are consistent with either a constant mass or linear dependence of the mass on the pressure gradient. Furthermore, we can safely rule out a cubic dependence, which is what would be expected from linear considerations. I will conclude with a brief comparison between the theoretically predicted mass function with observations of the Kuiper Belt.
2:20 - 2:40	Tomohiro Ono	Studies of the Rossby Wave Instability Using the Athena++ Code slides
		Recent observations have revealed protoplanetary disks with crescent structures. A formation theory of those structures suggests the presence of gas vortices, which can trap dust particles due to gas drag. The Rossby wave instability (RWI) is a hydrodynamic instability and forms gas vortices when a disk has a rapid radial variation. As far as the observations show, all the crescent structures are accompanied by ring-like structures or inner holes. Therefore, the RWI is one of the promising mechanisms for explaining the crescent structures. We investigate the RWI and vortices produced by the RWI with numerical simulations using the Athena++ code. We obtain some empirical relations to estimate the properties of the vortices from the initial conditions. We also talk about results of tracer particle analyses. Our studies provide solid theoretical ground for quantitative interpretation of the observed crescent structures in protoplanetary disks.
2:40 - 3:00	Avery Bailey	Simulations of Accretion onto Planetary Cores
		I will discuss our current understanding of accretion onto planetary cores and what extensions can be made using Athena++.
3:00 - 3:20	Patrick Mullen	Giant Impacts with Athena++ slides
		The giant impact hypothesis suggests that ˜4.5 billion years ago, a Mars-sized impactor struck the proto-Earth in an off-centered collision. This collision sends the proto-Earth spinning with a period of ˜5 hours, while the impact ejecta, comprised of melted and vaporized silicates, wraps around the proto-Earth forming a Keplerian disk. It is from this "protolunar disk", that the Moon is thought to have formed. This problem has been studied via numerical simulations, however, most have utilized smoothed particle hydrodynamics (SPH) codes. Here, we apply Athena++ to study this Moon-forming giant impact, and for the first time, we consider the potential role of magnetic fields. This talk will address (1) recent results from magnetized moon-forming giant impact models with Athena++, (2) past and ongoing development in Athena++ to include (a) super-time-stepping algorithms for diffusive physics and (b) multiple material flow evolution, and (3) future directions for adapting Athena++ for planetary science applications.
3:20 - 3:50		Coffee break

3:50 - 4:10	Cheng Li	Simulating Non-hydrostatic atmospheres on Planets (SNAP): formulation, validation and application to the Jovian atmosphere
		A new non-hydrostatic and cloud-resolving atmospheric model is developed for studying moist convection and cloud formation in planetary atmospheres. It is built on top of the Athena++ framework, utilizing its static/adaptive mesh-refinement, parallelization, curvilinear geometry, and dynamic task scheduling. We extend the original hydrodynamic solver to vapors, clouds, and precipitation. Microphysics is formulated generically so that it can be applied to both Earth and Jovian planets. We implemented the Low Mach number Approximate Riemann Solver (LMARS) for simulating low speed atmospheric flows in addition to the usual Roe and HLLC Riemann solvers. Coupled with a fifth-order Weighted Essentially Nonoscillatory (WENO) subgrid-reconstruction method, the sharpness of critical fields such as clouds is well-preserved, and no extra hyperviscosity or spatial filter is needed to stabilize the model. Unlike many atmospheric models, total energy is used as the prognostic variable of the thermodynamic equation. One significant advantage of using total energy as a prognostic variable is that the entropy production due to irreversible mixing process can be properly captured. The model is designed to provide a unified framework for exploring planetary atmospheres across various conditions, both terrestrial and Jovian. First, a series of standard numerical tests for Earth's atmosphere is carried out to demonstrate the performance and robustness of the new model. Second, simulation of an idealized Jovian atmosphere in radiative-convective equilibrium shows that 1) the temperature gradient is superadiabatic near the water condensation level because of the changing of the mean molecular weight, and 2) the mean profile of ammonia gas shows a depletion in the subcloud layer down to nearly 10 bars. Relevance to the recent Juno observations is discussed.
4:10 - 4:30	Huazhi Ge	A Horizontally Explicit and Vertically Implicit Time (HEVI) Integration Scheme for Athena++ to Study Non-Hydrostatic Planetary Tropospheric Dynamics
		Recent observations from Juno spacecraft and ground-based telescopes (e.g., VLA, VLT) reviewed puzzling distributions of tracers in Jupiter's weather layer. The observed distributions of NH3, GeH4 and AsH3, which are different from the theoretical predictions, suggest that the equator-to-pole transports and the moist convection induced by the water condensation might play important roles in Jupiter's tropospheric dynamics. In order to study the complex non-hydrostatic atmospheric dynamics with cloud physics, we adapt Athena++ to a non-hydrostatic atmospheric general circulation model to study the above physical processes in Jupiter's troposphere. We use Horizontally Explicit and Vertically Implicit (HEVI) time integration scheme to enlarge the time step in the simulation. We also use energy as the prognostic value to solve the 3D Euler equation. For most conventional atmospheric models, the prognostic value of the energy equation is chosen to be the potential temperature due to the assumption that the atmosphere essentially follows adiabatic processes. By applying our HEVI scheme on a simple 2D Radiative-Convective Equilibrium (RCE) problem, we show that the model using energy as the prognostic value has the advantage of ensuring the conservation of energy in a closed system. This project is funded by the NASA Earth and Space Science Fellowship.
4:50 - 4:50	Roseanne Cheng	Multi-material Athena++ with Mie-Gruneisen EOS for Planetary Science and Shock Physics Applications slides
		We implement a multi-material capability into Athena++ where the evolution of several materials (gas and/or solids) is modeled in a fluid approximation with a set of conservative equations coupled to the basic hydrodynamic equations. Each material is governed by a separate equation of state (EOS). The current implementation includes analytic EOS, but an extension to tabular form is straightforward. We consider ideal gases as well as Mie-Gruneisen EOS based on a Murnaghan reference isentrope. In this talk, we describe the numerical method, Mie-Gruneisen EOS parameters fitted to experimental data, and numerical tests which verify this new capability.
4:50 - 5:30		Discussion & Conference Summary (leader: Jim Stone)