Baby Stars And Planets Born To Napping Parents

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It has long been suspected that the build up of material onto young stars is not continuous but happens in episodic events, resulting in short outbursts of energy from these stars.

However, this has been largely ignored in models of star formation.

Now, by developing advanced computer models to simulate the behaviour of young stars, University Astrophysicists from the School of Physics and Astronomy Dr Dimitris Stamatellos and Professor Anthony Whitworth, have offered a new insight in star formation.

Figure: Three images showing a disc around a young star at several stages during its early life, with red material showing denser gas and dust, blue less dense material, and green colours intermediate densities. The disc forming around the young star (left) is unstable as indicated by the spiral structure, but it is temporarily stabilized during a burst of energy from the star (centre) that destroys the spiral structure. After the outburst, the central star “falls asleep” and the spiral structure returns. Eventually the disc breaks up to form two low-mass stars (black dots).

Image credit: D.Stamatellos, A.Whitworth (Cardiff University).

While stars are young they are surrounded by discs of gas and dust, and grow by accreting material from these discs. The discs may break-up to give birth to smaller stars, planets and brown dwarfs – objects larger than planets but not large enough to burn hydrogen like our Sun.

“We know that young stars spend most of their early lives sleeping,” said Dr Dimitris Stamatellos.

“After they have their lunch, a large chunk of dust and gas from their discs, they take a nap that lasts for a few thousand years. During this nap their brightness is very low.

“As they sleep, their discs grow in mass, but they remain relatively cool, despite the presence of stars right at their centres. Eventually, these discs become unstable and fragment to form low-mass stars and substellar objects, like brown dwarfs and planets,” he added.

To date, research has suggested that the radiation from the parent star could heat and stabilize the disc, suppressing its breaking up. However, the researchers discovered that there is ample time in between outbursts to allow the disc to break up and give birth to a new generation of low-mass stars, brown dwarfs, and planets.

The new theory provides an explanation for the formation and the properties of stars with masses below a fifth of that of our Sun, which are estimated to constitute more than 60% of all stars in our Galaxy.

“Our findings suggest that disc fragmentation is possible in nature,” says Dr Stamatellos.

“It is important now to investigate whether this is the dominant mechanism for the formation of low-mass stars and brown dwarfs,” he adds.

A star acquires much of its mass by accreting material from a disc. Accretion is probably not continuous but episodic. We have developed a method to include the effects of episodic accretion in simulations of star formation. Episodic accretion results in bursts of radiative feedback, during which a protostar is very luminous, and its surrounding disc is heated and stabilised. These bursts typically last only a few hundred years. In contrast, the lulls between bursts may last a few thousand years; during these lulls the luminosity of the protostar is very low, and its disc cools and fragments. Thus, episodic accretion enables the formation of low-mass stars, brown dwarfs and planetary-mass objects by disc fragmentation. If episodic accretion is a common phenomenon among young protostars, then the frequency and duration of accretion bursts may be critical in determining the low-mass end of the stellar initial mass function.

Credit: Dimitris Stamatellos

The research was funded by the Science and Technology Facilities Council (STFC) and the Leverhume Trust. The computer simulations can be viewed and downloaded at: www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/Movies.html

Related links
School of Physics and Astronomy
The Astrophysical Journal