Astrophysics

Multi-scale physics of star formation

I study the process of star formation on scales ranging from the cloud-scale averaged across entire galaxies to the sub-pc scale concerning the accretion rates and disc lifetimes of individual protostellar systems. Most of my work is focussed on the Magellanic Clouds, two of the Milky Way’s closest and most massive satellite galaxies.

Zooming in on massive young stellar objects in the Large Magellanic Cloud

Developing a multi-tracer timeline of star formation in the Large Magellanic Cloud

While the formation of stars is one of the most fundamental processes contributing to the evolution of galaxies, the timescales on which the various phases of star formation operate remain poorly constrained. By constraining the timescales on which star formation occurs, we can break the degeneracy between star formation that is slow and efficient and star formation that is fast and inefficient.

By utilising the uncertainty principle for star formation we can measure the timescales associated with various stages of star formation to develop a multi-tracer timeline of star formation. The Large Magellanic Cloud is fully sampled at a wider range of wavelengths than any other galaxy in the universe. As such it is the ideal target for the development of the first multi-tracer of star formation. The relative timescales from overdensities of atomic gas through the condensation of molecular clouds and the formation of the the first protostars through to the ionised emission of newly formed stars are visualised in the video below.

Probing the origins of OB associations

It is often been asserted that most, if not all, stars form in gravitationally bound, compact clusters. If this were the case, then the gravitationally unbound OB associations that we see today would exhibit well-ordered expansion-like velocity fields originating from a small number of discrete sites of star formation.

Using data from the Tycho-Gaia Astrometric Solution (TGAS) I showed that 18 nearby OB associations exhibited velocity fields inconsistent with those of expanding star clusters and that therefore they must have formed in-situ as gravitationally unbound associations.

In 2019, we have expanded this work by applying a clustering algorithm to independently select over 100 OB associations from the 2nd Gaia data release. We show that these associations could not have formed as much more compact clusters. Rather, these associations must have been formed in-situ on the scale of molecular clouds and exhibit a wide range of kinematic properties inherited from the complex kinematics of the supersonic interstellar medium from which they form.

Investigating the effects of metallicity on the formation of massive stars

The Magellanic Clouds are the Milky Way’s largest satellites and our closest star-forming galactic neighbours. The Magellanic Clouds exhibit lower metallicities than the Milky Way, meaning that they have contain lower levels of elements heavier than helium than in the Milky Way. The Magellanic Clouds are therefore an ideal location in which to study star formation in more prestine environments than those in our own galaxy, giving us an insight into star formation earlier in the history of the universe.

In 2013 we performed the highest resolution integral field spectroscopy observations of young stellar objects (YSOs) in the Magellanic Clouds to date. The results, published as part of two papers in 2016 and 2017, showed that accretion rates are higher in the low metallicity Small Magellanic Cloud than those in our own galaxy, consistent with previous studies of low to intermediate mass YSOs. We also found a few exceptional examples of compact HII regions and colossal protostellar outflows, with velocities of up to 15 km per second and extending up to a parsec in length. For context, that is about the distance from our solar system to the closest star.

The compact HII region surrounding N113-YSO3-A in the LMC (shown in Brg emission, blue), along side the earlier stage YSOs B and C. Continuum emission is shown in green with molecular H2 emission shown in red (Ward+ 2016).
The protostellar outflow from YSO 22 in the SMC (Ward+ 2017) traced in molecular H2 emission.

I am also part of the team that first detected the complex organic molecules dimethyl ether and methyl formate outside of our galaxy for the first time. The presence of these complex organic molecules indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low metallicity systems in the earlier Universe. This study was led by Marta Sewilo in a detection paper in 2018 as well as a review in ACS Earth and Space Chemistry.