David P. Fleming

About Me

As a native of St. Louis, MO, I love all things St. Louis, especially the beer, the Imo's Pizza, and the sports teams. The St. Louis Blues hold a special place in my heart, and I eagerly await their first Stanley Cup. Being a Notre Dame football fan has made me almost too good at waiting for a championship, but there’s always next year, right? While not working on my simulations, you can find me either watching sports or playing foosball.

Education:

I graduated from the University of Notre Dame in 2014 with a Bachelor of Science with a major in Physics and concentrations in Astrophysics and Advanced Physics. I earned my Master's in Science from UW in 2016 and became a PhD Candidate in the Fall of 2018. I expect to graduate with my PhD in Astronomy in the summer of 2020.

Research

On the Lack of Circumbinary Planets Orbiting Isolated Binary Stars

In this paper, we outlined a mechanism that explains the observed lack of circumbinary planets (CBPs) via coupled stellar-tidal evolution of isolated binary stars. Tidal forces between low-mass, short-period binary stars on the pre-main sequence slow the stellar rotations transferring rotational angular momentum to the orbit as the stars approach the tidally locked state. This transfer increases the binary orbital period, expanding the region of dynamical instability around the binary, and destabilizing CBPs that tend to preferentially orbit just beyond the initial dynamical stability limit. After the stars tidally lock, we find that angular momentum loss due to magnetic braking can significantly shrink the binary orbit, and hence the region of dynamical stability, over time, impacting where surviving CBPs are observed relative to the boundary. We perform simulations over a wide range of parameter space and find that the expansion of the instability region occurs for most plausible initial conditions and that, in some cases, the stability semimajor axis doubles from its initial value. We examine the dynamical and observable consequences of a CBP falling within the dynamical instability limit by running N-body simulations of circumbinary planetary systems and find that, typically, at least one planet is ejected from the system. We apply our theory to the shortest-period Kepler binary that possesses a CBP, Kepler-47, and find that its existence is consistent with our model. Under conservative assumptions, we find that coupled stellar-tidal evolution of pre-main sequence binary stars removes at least one close-in CBP in 87% of multi-planet circumbinary systems.

Check out the ApJ paper, Fleming et al. (2018), here!

Coevolution of Binaries and Gaseous Discs

The recent discoveries of circumbinary planets by Kepler raise questions for contemporary planet formation models. Understanding how these planets form requires characterizing their formation environment, the circumbinary protoplanetary disc, and how the disc and binary interact and change as a result. The central binary excites resonances in the surrounding protoplanetary disc that drive evolution in both the binary orbital elements and in the disc. To probe how these interactions impact binary eccentricity and disc structure evolution, N-body smooth particle hydrodynamics (SPH) simulations of gaseous protoplanetary discs surrounding binaries based on Kepler 38 were run for 104 binary periods for several initial binary eccentricities. We find that nearly circular binaries weakly couple to the disc via a parametric instability and excite disc eccentricity growth. Eccentric binaries strongly couple to the disc causing eccentricity growth for both the disc and binary. Discs around sufficiently eccentric binaries that strongly couple to the disc develop an m=1 spiral wave launched from the 1:3 eccentric outer Lindblad resonance (EOLR) that corresponds to an alignment of gas particle longitude of periastrons. All systems display binary semimajor axis decay due to dissipation from the viscous disc.

Check out the MNRAS paper, Fleming & Quinn (2017), here!

approxposterior: machine learning stellar and exoplanet dynamics

The code is open-source and publicly available on GitHub!

I am the lead developer of approxposterior, a Python implementation of Bayesian Active Learning for Posterior Estimation by Kandasamy et al. (2015) and Adaptive Gaussian process approximation for Bayesian inference with expensive likelihood functions by Wang & Li (2017). These algorithms allows the user to compute approximate posterior probability distributions using computationally expensive forward models by training a Gaussian Process (GP) surrogate for the likelihood evaluation. The algorithms leverage the inherent uncertainty in the GP's predictions to identify high-likelihood regions in parameter space where the GP is uncertain. The algorithms then run the forward model at these points to compute their likelihood and re-trains the GP to maximize the GP's predictive ability while minimizing the number of forward model evaluations. Check out Bayesian Active Learning for Posterior Estimation by Kandasamy et al. (2015) and Adaptive Gaussian process approximation for Bayesian inference with expensive likelihood functions by Wang & Li (2017) for in-depth descriptions of the respective algorithms.

Publications: Click here for all of David's publications on ADS

Contact

Email: dflemin3 (at) uw (dot) edu

Publications: Click here for all of David's publications on ADS

• GitHub GitHub

• Twitter @astrofleming1

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Elements

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