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Astro fluids / Star and planet formation

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Protoplanetary discs and Planet Formation

Star Formation

Turbulence

Fundamental SPH

The Astro Fluids / Star and Planet formation group studies the processes involved in star and planet formation, as well as more general accretion phenomena (e.g. onto black holes). The primary numerical method we utilise is Smoothed Particle Hydrodynamics (SPH), originally developed by Joe Monaghan here at Monash. We maintain and develop a public SPH code, PHANTOM, and also actively maintain a Monte Carlo radiative transfer code (MCFOST). We are increasingly involved in observations of star and planet formation using telescopes (and collaborators) around the world.

Our team pioneered a technique to detect newborn planets in discs of gas and dust around newborn stars, with detections of baby planets in 2018 and 2019, and first observational evidence for a circumplanetary disc around PDS70 b.

We are always on the lookout for interested and motivated students to join us in pursuing research towards a PhD or as part of our honours or masters degrees (contact either Daniel Price or Christophe Pinte if you are interested). The group often hosts long term visitors interested in applying our codes to their own research fields. We utilise a range of national, state-level and university-level supercomputing facilities.

Members

Former group members: Mark Hutchison, James Wurster, Christoph Federrath (ARC Fellow), Guillaume Laibe, Ben Ayliffe, Jules Kajtar, Alireza Valizadeh (postdocs); Rebecca Nealon, Hauke Worpel, Terrence Tricco (PhD students)


Protoplanetary discs and planet formation

Second newborn planet discovered, seen as a `kink' in the velocity map
That's no moon - it's a circumplanetary disc in PDS70b

Circumbinary mayhem in the HD142527 protoplanetary disc

Researchers: Daniel Price, Christophe Pinte, Valentin Christiaens

We use computer simulations, radiative transfer and observations to understand the swirling discs of gas and dust around young stars, known as protoplanetary discs. Using these techniques, we discovered two baby planets in 2018 and 2019, along with the first evidence for a circumplanetary disc around a newborn planet. We use theoretical models in close comparison with observations to understand images produced by telescopes such as the Atacama Large Millimetre/submillimetre Array (ALMA) and the Very Large Telescope (VLT), both in Chile.

We also develop computer algorithms with which to simulate and observe planet formation. Using complex numerical simulations, we are striving to model protoplanetary discs in an increasingly realistic way, in order to better understand what we are observing. To this end we must include the relevant physics, such as the transport of energy by radiation, the gravitational pull of the gas and dust media upon themselves, known as self-gravity, and the interactions of the solid and gaseous states of matter. Smoothed Particle Hydrodynamics (SPH) is an ideal tool for tackling these problems, allowing us to simultaneously model regions of material with huge contrasts in density and temperature, with a natural approach to modelling self-gravity, and the ability to readily trace the flow of matter. In concert with these numerical simulations, we also produce analytical models that are simpler, but can be used to pin down the essential physics involved in the different processes.


Star Formation and Accretion discs


Snapshot from a calculation of star cluster formation (see the movies)

Calculation of warp propagation in an astrophysical accretion disc.
Accretion of material onto a spinning black hole

Researchers: Daniel Price

The formation of stars is one of the most fundamental processes in the universe without which galaxies and indeed, ourselves would not exist. However our theoretical understanding of star formation is relatively poor, primarily because of the difficulty in modelling the physical process which involve gravity, highly turbulent gas dynamics (at supersonic velocities), magnetic fields, radiation, molecular and dust chemistry. Star formation also involves a truly enormous range in length and time scales which makes simulating the process difficult even with the fastest computers. One of the key aspects of our work here at Monash is to understand the role of magnetic fields in the star formation process, using simulations that form whole star clusters rather than just isolated stars, which gives a much deeper, statistical, picture of star formation that can be compared to real molecular clouds. Much of this work involves developing accurate numerical algorithms for simulating self-gravitating radiation-magnetohydrodynamics over the wide range of length and time scales involved in star formation (which we achieve using the Smoothed Particle Hydrodynamics method). The simulation methods thus develope are readily applicable to other areas of astronomy (and also to many earth-bound problems for which SPH is increasingly being used), amongst which we have or are working on application to the merger of binary neutron star systems; to the nature and statistics of turbulence in molecular clouds; to the role of magnetic fields in galaxies and to warp propagation in accretion discs.


Turbulence


Drawing by Leonardo Da Vinci of pouring water creating turbulent flow

Snapshot from a calculation of supersonic turbulence in star forming molecular clouds

Researchers: Daniel Price, Joe Monaghan

Leonardo da Vinci was one of the first to be fascinated by turbulent fluid flows (see his drawings of water flowing into a tank and creating vortices). Since then generations of scientists have tried to explain and to model turbulence. The most successful phenomenological model for incompressible turbulence was presented by Kolmogorov in 1941. However, turbulence in the interstellar medium or primordial gas halos in the early Universe is highly compressible and characterized by supersonic random motions. To gain insight into this multi-scale, highly non-linear regime of compressible turbulence, we use large-scale computer simulations here at MoCA. Such simulations require massively parallelized supercomputers with thousands of processors working together in parallel to solve the gas and plasma equations governing turbulent gas flows. To enable simulations of highly compressible turbulence, learn about its statistics and role for astrophysical systems, we develop efficient parallel codes and state-of-the-art supercomputing techniques here at MoCA.

One of the most important applications of our simulations is to study the role of turbulence for the formation of stars in magnetized gas clouds. We also aim to understand fundamental properties of turbulence and its role for star formation in the present-day and early Universe.


Numerical methods, fundamentals of SPH

Researchers: Daniel Price, Joe Monaghan

Our group is one of the leading centres in the world developing the SPH method, a numerical method for simulating hydrodynamic fluid flow which is widely used in astrophysics. We have pioneered methods for simulating magnetic fields and dust-gas mixtures with SPH, but are also considered with other fundamentals such as interpolation methods. We have performed comparison studies of SPH and grid based methods, which highlight the differing strengths or agreements between these methods.