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Dhruv Muley
Dhruv Muley
PhD student, Max-Planck-Institut für Astronomie (MPIA)
Hydrodynamical simulations of disk/planet interaction
Intro
I am a final-year PhD student at the Max Planck Institute for Astronomy (MPA), in Heidelberg, Germany. As a member of Hubert Klahr's group, I develop and employ numerical hydrodynamical simulations in the context of protoplanetary disks, with a particular interest in radiation hydrodynamics. Previously, I was a research assistant with Ruobing Dong at the University of Victoria from 2020-21, and studied at UC Berkeley from 2016-2020 where I worked with Jeffrey Fung. In Fall 2025, I will take up a postdoctoral fellowship at the Max Planck Institute for Astrophysics (MPA) in Garching bei München.
Exchange of energy between gas, dust, and radiation; after some time, the gas and dust come into equilibrium with one another before relaxing simultaneously to the radiation temperature.
In a protoplanetary disk system, stellar irradiation strikes the disk at the τ=1 optical surface, where it is absorbed, thermalized, and propagated throughout the bulk of the disk; the balance between the incident stellar radiation and reprocessed thermal radiation determines its temperature structure. Although most of the disk's mass and heat capacity is contained within the gas component, most of its opacity—and thus, most of the interaction with these radiation fields—is associated with the dust component, typically held to constitute just 1% of the total mass. Thus, heating and cooling of the disk must be mediated by gas-grain collisions, a process not typically included in hydrodynamical situations. To meet this need and better model the thermodynamics of protoplanetary disks, we have developed a "three-temperature scheme" (3T) for the PLUTO hydrodynamics code, incorporating thermal coupling between gas, dust, and radiation. Transport of radiation takes place using the explicit M1 scheme already present in PLUTO, while the local energy-exchange source terms are treated implicitly using a Newton-Raphson scheme. This work forms the foundation of my doctoral work, and serves as a starting point for future work including the dynamics and thermodynamics of multiple dust species (small micron/sub-micron grains well-coupled to the gas, as well as larger millimeter/submillimeter grains that settle to the midplane).
As a research assistant with Ruobing Dong at the University of Victoria, I simulated the strength and observability of the spiral temperature perturbations created by planet-driven spirals in protoplanetary disks, for a range of cooling prescriptions (effectively isothermal, intermediate, and effectively adiabatic). We found that the spirals generated by Saturn- and Jupiter-mass planets in adiabatic disks should be clearly detectable in the 12-CO brightness temperature—creating signatures reminiscent of those found in the TW Hya system by Richard Teague et al.. Buoyancy spirals were found to be thin but prominent close to the planet, while Lindblad spirals extended further away.
The PDS 70 system, imaged in scattered-light by SPHERE. The super-Jupiter planet candidate, PDS 70b, is visible as a bright blob to the right of the coronagraph. Credit: ESO/A. Müller et al.
Planets have long been believed to have formed in circumstellar disks, but how exactly they influence disk evolution and dispersal has remained a mystery. The PDS 70 system—one of a handful in which a planet candidate has been directly imaged within its natal disk—provides a valuable window onto this process. Traditional theory states that the super-Jupiter companion, located at 22 au, should exchange angular momentum with disk material and carve out a gap of approximately equal width. Indeed, PDS 70 does have a gap—but it extends to ~65 AU, making it twice as wide as the theory predicts! To make matters more puzzling, this wide gap is not an anomaly, but rather represents many late-stage "transition disks" in the process of dispersing to give way to planetary systems.
Using the PEnGUIn hydrodynamical code, we simulated the the accretion and orbital evolution of the companion, PDS 70b, in its disk over the system's ~5 Myr lifetime. We find that when the planet reaches super-Jupiter mass, it spontaneously becomes eccentric and accretes from a broader region of the disk. This "fiducial" model accurately reproduces the gap profile observed in PDS 70, which our 100+ alternative models (varying disk temperature, density, fixed vs. moving planets, etc.) failed to do. Moreover, the fiducial setup is simple and requires little fine-tuning for its predictive power. Our results show that single, super-Jupiter planets can sculpt the characteristic wide cavities of transition disks, and that the high eccentricities measured for such planets may in part arise from disk-planet interaction.
Dwarf galaxies
Muley, Dhruv; Wheeler, Coral R.; Hopkins, Philip F.; Wetzel, Andrew; Emerick, Andrew; Kereš, Dušan. "Progenitor-mass-dependent yields amplify intrinsic scatter in dwarf-galaxy elemental abundance ratios," Monthly Notices of the Royal Astronomical Society, (2021; accepted, ArXiv: 1909.02006)
The FIRE project has, over the past decade, produced increasingly accurate simulations of galaxy evolution, addressing problems from missing-satellites to the Kennicutt-Schmidt relation. Among the remaining discrepancies is the fact that in observed stars, the ratio of α-elements (e.g., Mg, Si, Ca, Ti) to iron ([α/Fe]) exhibits a wide intrinsic scatter at any given metallicity [Fe/H]; in simulations, however, stars tend to lie on a tight [α/Fe]-[Fe/H] relation. One potential cause is that in typical simulations, all Type II supernovae (which produce most α-elements, particularly at early times) are identical, with uniform IMF-averaged yields. Thus, when they enrich the surrounding gas (which subsequently forms future generations of stars), they do so with a constant ratio of [α/Fe].
In reality, however, the yields of Type II supernovae, and thus the production of α-elements and Fe, can vary substantially for different progenitor masses. To account for this in the FIRE simulations, I spent my summer 2019 as a SURF fellow at Caltech, implementing time- (and thus progenitor-mass-) dependent yields from the NuGrid stellar evolution suite. Our simulations of dwarf galaxies show a clearly wider scatter in [α/Fe] for each [Fe/H], and we have a paper in preparation. In the future, our prescription could be used to perform mock galactic archaeology on individual stellar populations within simulated Milky Way-type galaxies.
Teaching
The Mice Galaxies, as seen by Hubble. Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, ESA
In Fall 2018 I was an "undergraduate student instructor" (teaching assistant) for Alex Filippenko's Astronomy C10, a general astronomy class for non-science majors at UC Berkeley. In this capacity, I led weekly discussion sections and held office hours for ~60 students in a class of 900. Selected course materials I wrote are linked below; quizzes and practice exams available upon request.
Evolution of the Kelvin-Helmholtz instability. Credit:Jim Stone
I am a fourth-year PhD student at the Max-Planck-Institut for Astronomie, having previously worked at the University of Victoria (2020-21) and studied at UC Berkeley (2016-20). My full curriculum vitae may be found here; also find me on LinkedIn here.
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I am a final-year PhD student at the Max Planck Institute for Astronomy (MPA), in Heidelberg, Germany. As a member of Hubert Klahr's group, I develop and employ numerical hydrodynamical simulations in the context of protoplanetary disks, with a particular interest in radiation hydrodynamics. Previously, I was a research assistant with Ruobing Dong at the University of Victoria from 2020-21, and studied at UC Berkeley from 2016-2020 where I worked with Jeffrey Fung. In Fall 2025, I will take up a postdoctoral fellowship at the Max Planck Institute for Astrophysics (MPA) in Garching bei München. 
