By Morgan Owen
For the first time ever, an accretion disk has been found feeding a young, massive star outside of our own galaxy. This groundbreaking discovery – which recently featured on the front cover of the leading multi-disciplinary journal, Nature – was completed by an international team of astronomers, led by Dr Anna McLeod from the Department of Physics at Durham University.
This breakthrough has only become possible in recent years, following increased spatial resolution and sensitivity of our astronomical observatories. The Atacama Large Millimeter/ Submillimeter Array (ALMA) in the Chilean desert has been at the forefront of this. Consisting of over 60 radio dishes, astronomers can combine these dishes such that they all focus on the same source at the same time; the full array has only been operational for a couple of years.
The star, called HH 1177, and its disk live in our neighbouring dwarf galaxy, the Large Magellanic Cloud (LMC). At a distance of around 163,000 light years away, it is the most distant disk around a massive star ever to be directly observed and resolved. The star itself, which is about 15 times the mass of the Sun was however already known to researchers.
In 2018, this young and growing massive star was first observed in the optical wavelength regime, as opposed to ALMA’s radio wavelength regime. This was using data from the MUSE instrument on the European Southern Observatory’s Very Large Telescope (ESO’s VLT). These observations showed the powerful jets emerging from the star. Such jets were a signpost for ongoing accretion’ according to Dr Anna McLeod, and this has now been confirmed.
As material is pulled towards a growing star due to gravity, because of physical laws such as the conservation of angular momentum, the material cannot fall directly onto it. Rather, it forms a flattened, disk-like structure. The ALMA data showed that this disk exhibits signs of Keplerian rotation, such that the material in the disk is feeding the star through infalling material.
Accretion disks are a phenomenon that we observe across a range of different scales as part of systems including different astrophysical objects, ranging from low-mass stars, to high-mass stars, and to the supermassive black holes at the centre of galaxies.
Across these size and mass scales, “accretion physics seems to be ubiquitous,” says McLeod. This discovery provides the perfect extragalactic testbed to investigate this physics.
The Large Magellanic Cloud is a very different environment to our own galaxy, the Milky Way. It has a lower dust and metal content (to astronomers, everything heavier than Hydrogen and Helium are referred to as metals). The combination of lower metallicity and dust content is, in fact, what enabled researchers to detect this star in the first place. Massive stars that are so young, and still in their accretion phase, are “deeply embedded in their natal molecular cloud” says McLeod.
This means that such stars, if they were in the Milky Way, would typically be hidden from the gaze of our optical telescopes. However, the lower metallicity in the LMC means that stars, of comparable mass to those in the Milky Way, are hotter. Hence, they have stronger ionising radiation. In combination with a lower dust content, this means that these stars shed their natal cocoon on much faster timescales, revealing themselves to us.
This research therefore offers new insights into extragalactic massive star formation. This will allow astronomers to “perform an empirical study of an accretion disk and a growing young star in these different environmental conditions,” says McLeod, compared to our fundamentally different galactic environment.
This object will serve as a benchmark for both theorists and those creating simulations of the formation of massive stars. These researchers want to investigate the impact of changing parameters – like the metallicity and dust content of the material a star forms from – on the evolutionary pathway of a star.
This accretion disk is feeding the growth of the star at its centre, but what about planets? Could they form in this disk? Well, according to McLeod, “it’s bad news for [material] that would want to […] form planets in that disk.” The star is outputting a huge amount of strong, ionising radiation. This is likely to evaporate the disk before planets have the opportunity to form. Furthermore, likely an early type B-star, HH 1177, is expected to live for a couple of million years; a long time for us, but a short time in astronomy, and a lot shorter than the lifetime of stars like our own Sun. The accretion disk is therefore likely not to have the time to form planets.
Despite this, we can still learn a massive amount from this system. With its low metallicity, the LMC allows astronomers to observe stars like those in the very early universe, but in our own galactic neighbourhood. Where do researchers go next? McLeod and others, in Durham and beyond, now hope to observe this star with the James Webb Space Telescope (JWST), whose near-infrared capabilities could reveal even more beautiful details.
With the JWST, astronomers can observe all three components of this system simultaneously: the star, the disk and the jets. The physics of jets in particular is still relatively poorly understood. Researchers can start to address fundamental astronomical questions like how the jet is being launched from the star and what stellar origins look like across different galaxies in our Universe. A groundbreaking discovery, but not the end; more answers to these questions are on the horizon of this active field of research.
Image: ESO/M. Kornmesser