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Pinte, Christophe; Van der Plas, G.; Menard, F.; Price, D. J.; Christiaens, V.; Hill, T.; Mentiplay, D.; Ginski, C; Choquet, E; Boehler, Y.; Duchene, G.; Perez S.; Casassus, S., 2019, Kinematic detection of a planet carving a gap in a protoplanetary disc, Nature Astronomy (arXiv:1907.02538) | CNET article (public level) | Monash press release |

Christiaens, V., et al. 2019, Evidence for a circumplanetary disc around protoplanet PDS 70 b, ApJL 877, L33 (arXiv:1905.06370, ADS) | Monash press release |

Pinte, Christophe; Price, D. J.; Menard, F.; Duchene, G.; Dent, W.; Hill, T.; de Gregorio-Monsalvo, I.; Hales, A; Mentiplay, D.; 2018, Kinematic evidence for an embedded protoplanet in a circumstellar disc, accepted to ApJL (arXiv:1805.10293)

Hayley Macpherson, Paul D. Lasky and Daniel J. Price, 2018, The trouble with Hubble: Local versus global expansion rates in inhomogeneous cosmological simulations with numerical relativity, ApJL 865, L4 (arXiv:1807.01714, ADS) | AAS NOVA article (public level) | Monash press release |

Software

Publications

My research publications are available as an ADS library, via my ORCID page, or on google scholar.

Research

See the publications page for links to the full range of movies and images.

The role of magnetic fields in star birth — Protoplanetary discs — Turbulence in star forming clouds — Warped discs: How black holes eat — Simulation techniques for astrophysical fluid dynamics

Please contact me if you are interested in PhD research in any of these areas.

Star formation

On a dark, dark night, in a dark, dark place, look up at the Milky Way. Hopefully you might see something like this:

What is immediately striking is that, for a galaxy containing 100 billion stars, much of the Milky Way is obscured by dark, dusty blotches. These dark "molecular clouds" are the stellar nurseries of our galaxy, the birthplace of new stars. Here are two of our nearest star forming regions up close:

Rho Ophiuchus molecular cloud, one of the closest star forming regions to us, at a distance of ~140 parsecs (450 light-years).	Hubble Space Telescope mosaic of the Orion Nebula, our most spectacular nearby star forming region (~350 parsecs/1000 light-years away)	Baby stars in the heart of the Orion Nebula, complete with planet-forming discs silhouetted against the nebula.

The problem for us as humans is that star formation is a relatively long process, taking of order 1-10 million years, simply because it involves gathering material over huge distances by the gentle but steady pull of gravity. Thus when we observe the molecular clouds of the Milky Way, all we get is a momentary snapshot of what is really a highly dynamic process. We therefore use computer simulations to speed up the process to a few months inside a supercomputer, by solving the equations which govern the basic physical processes, i.e. gravity, gas dynamics and, through my research, the effects of magnetic fields. The results go something like this:

In our calculations we have found that magnetic fields can substantially change the picture of star cluster formation — they strongly reduce the rate at which stars form by stopping the gas from collapsing under its own gravity, producing star formation that is less vigourous, in turn leading to fewer low mass objects known as "brown dwarfs" (failed stars). This brought the predicted number of stars as a function of mass from the calculations into better agreement with the observed mass distribution of young stars. Predicting the mass distribution of stars (i.e., how stars get their mass) is a key aim of our work since it is very important for galaxy formation models.

Jets from young stars

Magnetic fields can also produce some of the most spectacular phenomena in astrophysics, such as the launch of jets from newborn stars:

In work with Matthew Bate (Exeter, UK), and Terry Tricco (Monash/CITA), we were able to reproduce this phenomena, capturing the magnetic launching of jets during a short (10,000-year) stall phase (known as the "first core") during the collapse of gas under gravity to form a protostar:

Previously it was not thought that collimated (needle-like) jets could be launched during this phase, but in November 2011 there was a spectacular detection (by Mike Dunham and colleagues) of a candidate first-core jet, with properties matching those predicted from the simulations. These observations and theoretical predictions are currently being followed up with the new ALMA telescope high in the mountains of Chile.

Protoplanetary discs and planet formation

Newborn stars are observed surrounded by swirling discs of gas and dust known as protoplanetary discs. These are thought to be the birthplace of planets like those in the solar system, but many details of the planet formation process are not well understood.

New telescopes have brought amazing new images of nearby protoplanetary discs in the Milky Way. Since the first light observations from the ALMA telescope in Chile, our group has undertaken a major modelling effort to try to understand the observed features, including gaps, rings, spirals and warps. Our first project modelling HL Tau already suggested the presence of fully-formed planets less than 1 million years after the formation of the star:

Since then we have been involved in many subsequent modelling projects, trying to understand how planets are formed.

Detection of baby planets using disc kinematics

One of our most spectacular findings, led by my colleague Christophe Pinte (Monash), has been the direct detection of planets caught in the process of formation from the disturbance they create in the gas flow in protoplanetary discs. 3D modelling of our first detection in 2018 suggested a newborn planet with a mass 2-3 times heavier than Jupiter.

In 2019 our team made a spectacular second detection using a similar technique, this time with the planet location coincident with a `gap` in the dust disc, similar to those observed in HL Tau, confirming that in this case the planet is the likely culprit for carving this gap:

In 2020 we followed up with 8 more tentative detections of planets embedded in several other discs. Work is ongoing to confirm and follow-up these detections with higher resolution observations and improved models to better constrain the planet properties.

Turbulence in molecular clouds

With colleagues Christoph Federrath (Monash) and Chris Brunt (Exeter, UK) we investigated various aspects of the turbulence observed in molecular clouds. While turbulence in ordinary fluids like water is only partially understood, the motions observed in star-forming clouds are much faster than the speed of sound, i.e. highly supersonic, with Mach numbers ranging from around 5 up to 20, making the properties quite different to turbulence in water. Understanding turbulence in this regime can help us predict the process of star formation, as well as being of more fundamental importance. Below is an example of a calculation from a comparison between different simulation codes, showing gas driven to supersonic speeds in a box with periodic boundary conditions:

Of particular relevance to star formation is how turbulence distributes the gas as a function of density — i.e., the probability of a given parcel of gas being at a particular density, measured by the "Probability Density Function" or PDF. In particular the PDF in supersonic turbulence shows a characteristic "log-normal" shape (see right panel, below) that is very similar to the mass distribution of young stars, suggesting a link between the two.

One of the more fundamental aspects that we have investigated is how the distribution of gas as a function of density changes with the Mach number of the turbulence. Knowing this relationship means that we can make theoretical predictions for the mass distribution of stars and the star formation rate that can be compared against observational measurements.

Warped discs: How black holes eat

With colleagues in Italy and the UK I have a long-term project to understand the dynamics of accretion discs — the swirling of gas that form when material tries to land on a star or black hole, observed around many different kinds of objects including young stars, white dwarfs, neutron stars, black holes and supermassive black holes — essentially whereever there is gas falling onto a gravitating object. In particular we are trying to understand what happens when the disc is warped, tilted, buckled, bent or broken, and how these disturbances travel in the disc:

In one particular study we looked at warps driven in a misaligned accretion disc by a spinning black hole. The results were completely unexpected, that the black hole can "tear up" the disc, rapidly accelerating it's ability to feed itself:

Computer simulation techniques for astrophysical fluid dynamics

What enables all of the research above is the development of ever improving algorithms for solving the equations of astrophysical gas dynamics on a computer. Although the least "glamourous" part of my research in terms of public appeal, it is what drives our ability to perform ever more realistic simulations of astrophysical phenomena. A brief scan of my list of publications shows the kind of highly technical nature of the work that underpins our applications to astrophysics.

The method we use and develop, Smoothed Particle Hydrodynamics, uses moving particles to represent the fluid, instead of the standard approach which uses a grid. This has advantages for many problems, not only in astrophysics but also for simulation of water and other fluids or gases on Earth.

Teaching

Contact

School of Physics and Astronomy,
Monash University,
Vic 3800
Australia
tel: +61 3 9905 1760
daniel.price [ @ ] monash.edu
ORCID: 0000-0002-4716-4235