Brief summaries of various research areas can be found below. More detailed descriptions can be found by clicking on the links. The easiest way to get a chronological ordering of what I have been doing as a function of time is to examine my publication list.
Magnetorotational Turbulence in Stratified Disks
Accretion Disks: Theory and Models
X-ray Binaries and Ultraluminous X-ray Sources
Active Galactic Nuclei
Star Formation Feedback
RADIATION HYDRODYNAMICSOver the last several 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 developed methods for solving the equations of radiation transfer in the Athena. 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.
To date, we have pursued a number of scientific applications. We studied the effects of radiation on the Rayleigh-Taylor instability (Jiang et al. 2013a) and the magnetorotational instability (Jiang et al. 2013b). We find that at high radiation to gas pressure ratios, radiation forces (particularly radiation drag and possibly radiation viscosity) can play an important role in the non-linear development and (for the MRI) saturation of these instabilities. We have also studied the evolution of stratified shearing box simulations to examine the possibility of thermal instability in radiation dominated accretion flows (Jiang et al. submitted to ApJ). Contrary to previous numerical simulation results where radiation was modeled with flux-limited diffusion, we find that radiation dominated simulations always show thermal runaways, consistent with some form of thermal instability. Ongoing work aims to better understand the nature of these thermal runaways and lay the groundwork for future global simulations.
Exploring the feedback of star formation on surrounding gas is a second topic of interest. In collaboration with Norm Murray, we have been examining the role of Rayleigh-Taylor instabilities in limiting the coupling between radiation and the dusty gas in rapidly star-forming environments. Our work to date shows qualitative similarity to previous work that used the Orion code with flux-limited diffusion (Krumholz & Thompson, 2012), but the coupling between the radiation and dusty gas seems stronger than was previously found. The implications of this finding for astrophysical systems is the focus of current study. Another study, in collaboration with Daniel Proga is studying the effect of external radiation on clouds with different types and amounts of opacity to the external radiation field.
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.
On the Thermal Stability of Radiation Dominated Disks
Jiang, Y.-F., Stone, J. M. & Davis, S. W., ApJ, submitted
Saturation of the Magneto-rotational Instability in Strongly Radiation-dominated Accretion Disks
Jiang, Y.-F., Stone, J. M. & Davis, S. W., ApJ, 2013, 767, 148
Non-linear Evolution of Rayleigh-Taylor Instability in a Radiation Supported Atmosphere
Jiang, Y.-F., Davis, S. W.& Stone, J. M., ApJ, 2013, 763, 102
A Radiation Transfer Solver for Athena using Short Characteristics
Davis, S. W., Stone, J. M., and Jiang, Y.-F., ApJS, 2012, 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, 2012, 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