I am a theoretical astrophysicist who often works directly with observational data. My work frequently involves some combination of "pen and paper" calculations, numerical simulations, code development, and data analysis. I have benefited greatly from my involvement in numerous collaborations and enjoy working with both observers and theorists.

I have organized my astrophysics research into several categories. Click on the links below for a summary of previous and (in many cases) on-going work along with links to the relevant publications. The organization is by topic with little attempt at chronological ordering. The easiest way to get a sense of what I have been doing as a function of time is to examine my publication list.

Radiation Hydrodynamics
Magnetorotational Turbulence in Stratified Disks
Accretion Disks: Theory and Models
X-ray Binaries and Ultraluminous X-ray Sources
Active Galactic Nuclei
Magnetar Polarization
Star Formation Feedback

RADIATION HYDRODYNAMICS

Over the last two years I have focused a large fraction of my efforts on developing and implementing numerical methods for solving the the equations of radiative transfer and radiation magnetohydrodynamics.  Along with Jim Stone and Yan-Fei Jiang (Princeton U.), I have written routines for solving the equations of radiation transfer in the Athena astrophysical fluid dynamics code. These modules allow us to solve the equations of radiation hydrodynamics using the variable Eddington tensor method, generate spectra and image diagnostics in real time as the simulation runs, or directly compute the heating and cooling rates due to radiative transfer.  We have extensively tested the code and published papers describing our implementations and code tests in ApJ supplements (Jiang, Stone, & Davis 2012; Davis, Stone, & Jiang, 2012).  We demonstrate that for some problems the computational cost of full radiative transfer is comparable to alternative methods such as flux-limited diffusion while still giving a superior representation of the radiation field.

We anticipate a wide variety of astrophysical applications but we have initially focused on simulations of accretion flows and Yan-Fei Jiang has already lead an extensive study of shearing box simulations with radiation.  The first phase of this program looked at unstratifed shearing box simulations and is largely completed. More recent work is underway focused on assessing the thermal stability of radiation dominated accretion disks with stratified shearing box simulations. In the longer term, we intend to consider the question the viscous stability of radiation dominated disks, but also take a closer look at how both gas and radiation pressure dominated disks are affected by radiation in environments with different opacities, temperature, densities, and explore the impact of irradiation by an external source.

Exploring the feedback of star formation on surrounding gas is a second topic of interest.  We would like to consider whether the momentum input directly by absorbed and scattered photons can accelerate cold gas surrounding stars or star clusters without disruption or over heating.  "Test problems" have already extensively examined the Rayleigh-Taylor instability in a radiation dominated background and the ablation of a cold gas cloud by external irradiation.  We have also reexamined the linear growth rates and non-linear evolution of the photon bubble instability, confirming previous results from Turner et al. (2005).

We anticipate a large number of other applications with a variety of collaborators.  In addition to the projects mentioned above, we have plans to examine the effects of radiation in boundary layers where accretion settles on to stellar surfaces, thermal stability in irradiated clouds, outflows from accretion disks, evolution of HII regions, and massive star formation.

Relevant Publications:

Non-linear Evolution of Rayleigh-Taylor Instability in a Radiation Supported Atmosphere
Jiang, Y.-F., Davis, S. W. & Stone, J. M., ApJ, submitted

A Radiation Transfer Solver for Athena using Short Characteristics
Davis, S. W., Stone, J. M., and Jiang, Y.-F., ApJS, 199, 9

A Godunov Method for Multidimensional Radiation Magnetohydrodynamics based on a variable Eddington tensor
Jiang, Y.-F., Stone, J. M., and Davis, S. W., ApJS, 199, 14

The Effects of Photon Bubble Instability in Radiation-Dominated Accretion Disks
Turner, N. J., Blaes, O. M., Socrates, A., Begelman, M. C., & Davis, S. W. 2005, ApJ, 624, 267
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