J. Rafael Fuentes

About me

I was an undergraduate and MSc student at the Pontificia Universidad Católica de Chile, and received my PhD from McGill University in February 2023 under the supervision of Andrew Cumming. You can read my PhD thesis in 400 words here, or the full thesis. These days, I'm a Postdoctoral Researcher at CU Boulder.

In my free time, I take advantage of the many mountains around Boulder Colorado, for daily hikes and runs. I love trail running and in the past, I used to run long-distance (50km - 100 km) races in the Andes Mountains.

The best way to contact me is at jofu5477@colorado.edu.
My office is ECOT 213 in the Engineering building.
My mailing address is
Department of Applied Mathematics,
University of Colorado Boulder,
1111 Engineering Center, ECOT 225
Boulder, CO 80309-0526

Research

My research combines numerical simulations and analytical methods to investigate the interiors of stars and planets—anything round catches my interest! Recently, I’ve focused on how convection mixes the core of gas giant planets, crystallization-driven dynamos in white dwarfs, and superfluidity in neutron stars. My research has the goal of using observations of these objects to learn about the fundamental physics processes at work in their interiors.

Currently, I'm performing hydrodynamic simulations of rotationally-constrained turbulence in ice giants and thermohaline convection in polluted white dwarfs.

In this study, published in Fuentes 2025, I present the first 3D simulations of semiconvection in the full sphere (i.e. including r=0), and accounting for rotation. These simulations explore how double-diffusive convection — relevant in stars and giant planets — behaves in more realistic geometries and under rotational constraints. I show that layer formation in non-rotating spheres closely resembles that in Cartesian domains, with similar critical density ratios and transport properties. When rotation is introduced, mixing becomes less efficient and more anisotropic, with transport proceeding in cylindrical columns aligned with the rotation axis. The presence of rotation significantly prolongs the time it takes for the fluid to become fully mixed and alters the spatial structure of convective layers. The results align better with theoretical predictions by Henk Spruit than other models, and underscore the stabilizing influence of rotation on heat and compositional transport. These findings have important implications for understanding the internal dynamics of stars and gas giants, including how layered interiors evolve under realistic astrophysical conditions. In the videos below you can watch a movie of the evolution of semiconvection in rapidly rotating flows (left panel). The video on the right is an informal interview with Frank Timmes discussing the paper.

When a multicomponent plasma freezes, the composition of the solid is typically different from the composition of the liquid. If the solid preferentially retains heavy elements, the liquid left behind is lighter and buoyant, driving convection. I have combined analytic theory and 3D simulations to investigate in great detail the transport properties of convection during crystallization in cooling white dwarfs. My results have shed light on whether convection is efficient enough to drive a magnetic dynamo (see Fuentes et al. 2023, and Fuentes et al. 2024) . Below you can watch two videos of simulations of compositionally-driven convection in cooling white dwarfs (non-rotating case is on the left, and rotating case is on the right). There is also a video of an informal interview with Frank Timmes discussing our ApJ Letter on crystallization-driven dynamos.

The interiors of giant planets are a window into the history of the solar system. While these planets have traditionally been viewed as well-mixed objects that have been convectively cooling for billions of years, recent data from NASA's Juno and Cassini Missions indicate that large regions in the deep interiors of these planets may be “stably stratified”, meaning they are not convective and consequently have primordial composition gradients. This discovery suggests that giant planets have “dilute cores”. Answering the question why gas giants are not fully mixed is essential for understanding their internal structure and evolution, since the strength and spatial extent of the compositional gradients determine the nature of the heat transport, long term cooling processes, and magnetism.
Composition gradients are known to inhibit convection entirely or partially within planets and stars, and under certain circumstances, can lead to the formation of a convective-staircase, i.e., series of turbulent convective layers separated by sharp interfaces across which transport of heat and chemical species is achieved by molecular diffusion. This is known as double-diffusive convection, similar to what is seen in the Earth's Arctic Sea. This mechanism has been proposed to prevent mixing in the deep interior of gas giants. However, we have shown using numerical simulations that staircases do not survive over long timescales (see Fuentes et al. 2022, and the movie below on the right panel).
Another potential mechanism to prevent mixing, rotation, has received less attention despite being a well-known factor that hampers convection in fluid dynamics. Recently, I have shown that rotation decreases the convective velocities and the kinetic energy flux available for the mixing of primordial composition gradients. When plugging in Jovian values, the results suggest that rotation can delay the mixing time of the core by about three orders of magnitude when compared to the mixing time without rotation (see Fuentes et al. 2023, and the movie below on the left panel). Further, if multiple convective layers form, rotation prolongs their survival in time (see Fuentes et al. 2024).

Neutron stars are the densest objects in the universe that can be observed directly. Their mean density is even higher than that of atomic nuclei, providing a unique environment to study the physics of extremely dense matter. The rotation of these objects is exceptionally stable and shows a regular deceleration trend. This trend is sometimes interrupted by spin-up events called ''glitches'', which are believed to be caused by an exchange of angular momentum between a superfluid of neutrons inside the star and the other components of the star. The observations of glitches can lead to constraints on the mass of the star and on the equation of state of dense matter. Further, they are a direct way to investigate the interiors of neutron stars.

During my early years as a graduate student, I collaborated with the pulsar group at the Jodrell Bank Centre for Astrophysics of the University of Manchester. I led two studies (see Fuentes et al. 2017, and Fuentes et al. 2019) to infer the properties of pulsar glitches. One of the main results is that the glitch activity (spin-up rate due to glitches) of all rotation-powered pulsars is strongly correlated with the spin-down rate, and that no other pulsar parameter offers the same level of correlation. My results have shown that at least 1% of a neutron star's moment of inertia is needed in order for there to be sufficient angular momentum to be transferred during glitches. Another important conclusion from these studies is that there are two classes of glitches: small glitches, which seem to occur randomly in time, and large glitches, which are more regular and occur quasi-periodically. The results from both studies have given insights into the glitch mechanisms. Further, they have been used to constrain models of fast radio bursts and to search for gravitational waves.

Teaching resources

These are the slides for three lectures I gave on 2024 at CU Boulder as part of the courses ASTR 5700 - Stellar Astrophysics and ASTR 5400 -Introduction to Fluid Dynamics. You will need the app keynote to access them.

During my undergraduate years in Chile, I worked as a teaching assistant for many courses, including: Electricity and Magnetism, Statistical Mechanics, Stellar Astrophysics, Classical Mechanics, and General Astrophysics. Before leaving, I put together a collection of notes that included problem sets and solutions for some of those classes. I know that these notes are still being used, so I've made them available for download below. Just keep in mind that they're written in Spanish!

Statistical Mechanics
Electricity and Magnetism
General Astrophysics

Slides

The effects of rotation on entrainment and layered convection in gas giants, from the IPAM workshop Rotating Turbulence: Interplay and Separability of Bulk and Boundary Dynamics, UCLA, January 2025.

Crystallization-driven dynamo in cooling White Dwarfs, ICE-CSIC, Barcelona, February 2024.

Solar meridional circulation through the lens of helioseismology, COFFIES NSSL Meeeting, January 2024

The glitch activity of neutron stars, from IAU Symposium 337: Pulsar Astrophysics The Next Fifty Years , Jodrell Bank, October 2017

Elements

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