Research

The SIE3D group studies the physics of shocks, winds, stellar pulsation, radiative transport, mixing and instabilities, dust and nuclear fusion applied to interacting stars and their explosions. Current projects include investigating how stars exchange mass, energy and momentum with each other and with their surroundings, both during their lives and their deaths; the formation and evolution of aspherical outflows (e.g., bow shocks, bipolar and spiral outflows), interacting low- and high-mass binary systems (e.g., symbiotic and X-ray binaries), stellar collisions, and nova and supernova explosions (see brief highlights below).

We utilize a variety of numerical approaches including smoothed particle hydrodynamics (SPH), moving-mesh (magneto) hydrodynamics and Monte Carlo radiative transfer. The models include detailed microphysics, e.g., radiative cooling, thermal conduction and dust formation. Ultimately we aim to compare our simulations with current observations, and to a make predictions for upcoming facilities.

BINARY INTERACTIONS

The majority of stars have at least one (sub-)stellar companion. In many cases the components are close enough that they will exchange mass and angular momentum - a process that can fundamentally change the structure and subsequent evolution of both stars. Indeed, these systems produce a wide range of weird and wonderful astrophysical phenomena; selected topics of interest include:

The amount of mass and angular momentum transfer in binary star systems and the implications for e.g., symbiotic and X-ray binaries, chemically peculiar stars (CH and Ba stars), blue stragglers, the progenitors of Type Ia supernovae and gravitational wave sources.
The shaping of cirumstellar environments in low- and high-mass systems, and the connection to aspherical planetary and symbiotic nebulae, luminious blue variables, and the progenitors of interaction supernovae.
The formation of accretion disks, circumbinary disks, equatorial outflows, torii, jets and collimated fast outflows.
Going beyond two stars, multi-body interactions and studies of von-Zeipel-Kozai-Lidov oscillations and the outflow geometries and fates of triple systems.

Mass transfer via Wind Roche-Lobe Overflow (WRLOF) in an eccentric, symbiotic-type binary with a red giant donor losing mass via a stellar wind, and a nearby accreting white dwarf companion. The slow, dense wind of the giant is transferred to the companion via the inner Lagrangian point (L1), similar to standard RLOF except here the stellar wind and not the star fills the Roche lobe.

The first 3D hydrodynamic simulations of the formation and evolution of Betelgeuse’s bow shock, viewed at different inclination angles and covering a wide range of plausible ISM densities and stellar velocities – assuming the star is moving through the ISM at 32 km/s (top) and 73 km/s (bottom). Image featured on A&A journal cover, 2012.

STELLAR OUTFLOWS

In their final stages of evolution, stars lose copious amounts of mass and momentum via powerful, dense stellar winds and eruptions. These outflows, enriched in dust and heavy elements forged in the cores of stars, not only provide the raw material for planetary systems, but also play a central role in the chemical evolution of galaxies. Selected topics of interest include:

Mass loss mechanisms in evolved stars, e.g., pulsation and dust-driven winds in red (super)giants, outbursts in luminous blue variables.
Runaway stars and the formation of bow shocks, cometary structures produced by the supersonic collision of their stellar winds and the interstellar medium (ISM), e.g., Betelgeuse, Mira AB.
Colliding wind binaries where the shock interface between the two winds is the site for dust formation, and radio, X-ray and gamma-ray emission, e.g. WR 140.
Interacting winds that lead to the formation of shells, arcs, spirals and bipolar outflows, e.g., R Scl and many planetary nebulae.

STELLAR EXPLOSIONS

The blast waves resulting from stellar explosions produce emission from radio to X-ray and gamma-ray wavelengths that can persist for hundreds of years. Thus these systems form a rich landscape to study processes like shock physics, particle acceleration, fluid instabilities, dust formation and destruction, and explosive nucleosynthesis. The explosions not only drive the chemical evolution of and feedback in galaxies, but also, due to their incredible luminosities, are important cosmic probes of the structure and evolution of the universe. Selected topics of interest include:

Exploring the wide diversity of interaction supernovae and their progenitors, including those with extended atmospheres, photoionization-confined shells and outflows produced by binary mass transfer.
Understanding the origin of the aspherical morphologies of classical and recurrent nova explosions, and the implications for pre-outburst binary mass transfer. For the recurrent systems, investigating the link with Type Ia supernovae.
Stellar collisions between white dwarfs and main sequence stars in different environments. More generally, making predictions for and characterising unusual transients that will be detected with upcoming large surveys, e.g., the Rubin Observatory (LSST) and in the future, the Square Kilometer Array (SKA).

3D model of a nova explosion. The spherical ejecta decelerates rapidly in the equatorial direction due to interaction with the dense material lost during the quiescent mass-transfer phase, and flows more freely perpendicular to the orbital plane producing bipolar lobes.

Velocity cross-section in the orbital plane showing a three-armed spiral structure produced due to an approximately 1:3 "stellar pulsation - planet orbit" resonance.

STAR-PLANET INTERACTIONS

Stars and their sub-stellar companions (planets and brown dwarfs) are not only gravitationally linked, but also interact via tides, magnetic fields, winds and irradiation. The interactions can have dramatic impacts on the star and the sub-stellar companion. Topics of interest include:

Close planets or brown dwarfs (within a few stellar radii from the star) can have a strong effect on the slow, dense winds ejected by red giants. The sub-stellar companion is not only able to shape the outflow into multiple spiral structures, but can also significantly enhance the mass-loss rate.
Using circumstellar structures, e.g., spirals, as a means to ‘detect’ sub-stellar companions around very bright stars that are otherwise difficult to find with traditional methods (e.g., radial velocities and transits).
The stellar winds and radiation fields can also impact the planetary companions; the latter may grow, 'thaw'/heat-up or even evaporate.

3D SIMULATIONS MEET OBSERVATIONS...

In addition to broad studies of the physical processes mentioned above, we aim to model stellar systems that enable us to challenge our understanding and better constrain the models. Extracting synthetic observables (via radiative transfer processing) is a major component of our work as these can be used to more directly interpret (and make predictions for) exquisitely high-resolution, high-sensitivity observational facilities, e.g., the Atacama Large Millimeter Array (ALMA) and JWST.

We also explore the parameter space to provide inputs and prescriptions for e.g., stellar population synthesis studies, and feedback processes in galaxies.

ALMA CO observation of the detached shell and spiral around carbon-rich red giant, R Scl [left], and 3D SPH simulation used to derive mass loss propertices, and binary companion parameters [right].