Star embryo with hot flashes


Artist’s impression of a protostar that accretes and grows gas from a circumstellar disk. (Image: NASA / JPL-Caltech, R. Hurt / SSC)

Massive stars do not grow gradually, but in spurts – at least that’s what the current theory says. Astronomers have now succeeded in demonstrating such growth spurts in a protostar. Using a network of radio telescopes, they observed rapidly spreading radiation sources in the vicinity of the young star. They suggest that waves of intense radiation and heat pass through the gas in the star environment. Accordingly, the star must have taken in new food in the form of material from the surrounding gas and dust disk at this time.

According to current theory, stars form dense clouds of cold, interstellar gas. Vibrations, such as the shock waves from a nearby supernova, compress these gas clouds in some places. These lumps of gas are then further compressed by their own gravity and eventually collapse. This collapse creates a star embryo. This protostar grows by absorbing additional material from its surroundings. Above a certain mass, the gravitational pressure inside becomes so great that atomic nuclei fuse – the nuclear fusion ignites and the star begins to shine. So much for the classic scenario for sun-like stars.

Gradual accretion

But with massive stars, the whole thing runs much less smoothly. Because of the enormous pressure of gravity inside these stellar heavyweights, nuclear fusion starts while they are still growing. The problem, however, is that the outward radiation pressure pushes the surrounding gas away, making it more difficult for the protostar to absorb additional material. How such stars manage to continue growing despite this resistance has so far only been partially clarified. According to current theory, this growth could be made possible by short, strong bursts of material absorption. The star gas pulls from the circumstellar disk in phases in individual large packages and becomes brighter for a short time. “Such accretion events are very rare and difficult to observe directly,” explains Ross Burns from the National Astronomical Observatory of Japan and his colleagues. Because protostars are usually deeply embedded in dense clouds of dust and gas.

But there is an indirect indication of such stellar heat and growth spurts, as astronomers explain. This consists in an increased maser emission in the vicinity of the protostar. Masers are laser-like radiation sources that emit microwave radiation instead of visible light. This radiation arises when certain molecules in the circumstellar disks are excited by the intense radiation of the protostar and then emit microwaves. “It would be very complicated to observe the actual heat wave directly in the thermal infrared,” explains co-author Hendrik Linz from the Max Planck Institute for Astronomy in Heidelberg. “The burls as strong radiation sources in an easily accessible wavelength range are a very good observation tool to indirectly understand the passage of such a heat wave on small spatial scales, and thus on short time scales after an outbreak.”

Fast burls reveal stellar waves

This is exactly what the astronomer team has now achieved with the massive proto star G358.93-0.03-MM1. Already in 2019, observations had shown that there are strong burls caused by excited methanol in the vicinity of this star. That is why astronomers as part of the Maser Monitoring Organization (M2O) have been closely observing this star in the past few months with several radio telescopes, including the Very Long Baseline Array (VLBA). Burns and his team recorded radio interferometry data with a high spatial resolution of 0.005 arcseconds every few weeks. When evaluating this data, they found that the burls changed their position quickly – they moved outward at up to eight percent of the speed of light. “Such a rapid transformation cannot go back to the movements of the methanol gas clouds,” say the researchers. The speed was too high for that. Instead, they assume that these spreading maser emissions are triggered by an energy wave emanating from the star.

The grain sources thus provide evidence that the young protostar G358.93-0.03-MM1 periodically emits waves of heat and radiation – as is the case with current theory based on batchwise accretion. “The M2O observations are among the first to testify in detail the immediate effects of an accretion spurt in a massive protostar,” explains Burns. These observations confirm that massive young stars actually grow in spurts.

Source: Ross Burns (National Astronomical Observatory of Japan, Tokyo) et al., Nature Astronomy, doi: 10.1038 / s41550-019-0989-3

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