Astronomy - Are all stars created equal?

Artist’s impression of an accretion burst in a high-mass young stellar object like S255IR NIRS 3 some 6,000 light-years away. Image credit: Deutsches SOFIA Institut (DSI).

Astronomers using critical observations from the Gemini Observatory have found the strongest evidence yet that the formation of massive stars follows a path similar to their lower-mass brethren — but on steroids!

The new findings, that include data from Gemini, SOFIA, Calar Alto Observatory, and the European Southern Observatory, show that the episodic explosive outbursts within what are called accretion discs, known to occur during the formation of average mass stars like our Sun, also happen in the formation of much more massive stars.

“These outbursts, which are several orders of magnitude larger than their lower mass siblings, can release about as much energy as our Sun delivers in over 100,000 years,” said Dr. Alessio Caratti o Garatti of the Dublin Institute for Advanced Studies (Ireland). “Surprisingly, fireworks are observed not just at the end of the lives of massive stars, as supernovae, but also at their birth!”

The international team of astronomers (led by Caratti o Garatti) published their work in the 14 November issue of the journal Nature Physics, presenting the first clear case that massive stars can form from clumpy discs of material — in much the same way as less massive stars. Previously it was thought that the accretion discs seen around lower mass stars would not survive around stars of higher mass due to their strong radiation pressure. Therefore, some other process would be necessary to account for the existence of more massive stars — which can exceed 50-100 times the mass of our Sun.

“How accretion discs can survive around these massive stars is still a mystery, but the Gemini spectroscopic observations show the same fingerprints we see in lower mass stars,” said Caratti o Garatti. “Probably the accretion bursts reduce the radiation pressure of the central source and allow the star to form, but we still have a lot of explaining to do in order to account for these observations.”

According to team member Dr. Bringfried Stecklum of the Thüringer Landessternwarte Tautenburg (Germany), “Studying the formation of high-mass stars is challenging because they are relatively rare and deeply embedded in their natal cloud, thus not visible in optical, or visible, light. This is why we need infrared instruments like the Near-infrared Integral Field Spectrograph at Gemini North on Maunakea in Hawaii.” The outburst events are also very rapid, probably lasting only a few years or less — which, for a star, is the blink of an eye, adding to their rarity.

“The birth of truly massive stars has been a mystery that astronomers have been studying for decades. Only now, with large, infrared-optimised telescopes like Gemini, are we able to probe the details of this short-lived and, now it seems, rather explosive process,” notes Chris Davis, Program Director at the National Science Foundation which supports the operation of the Gemini Observatory and the development of its instruments. “These NIFS observations represent yet another coup for the Gemini Observatory.”

The developing star observed in this study, S255IR NIRS 3, is relatively distant, some 6,000 light-years away, with a mass estimated at about 20 times the mass of our Sun. The Gemini observations reveal that the source of the explosive outburst is a huge clump of gas, probably about twice the mass of Jupiter, accelerated to supersonic speeds and ingested by the forming star. The team estimates that the outburst began about 16 months ago and according to Caratti o Garatti it appears that the outburst is still active, but much weaker.

“While low-mass stars, and possible planetary systems, can form basically next door to our Sun, the formation of high-mass stars is a complex and relatively rapid process that tends to happen rather far away in our galaxy, thousands, or even tens of thousands of light-years away,” said Caratti o Garatti. He adds that the formation of these massive stars happens on timescales of 100,000 years, whereas it takes hundreds of times longer for lower-mass stars like our Sun to form. “When we study the formation of higher mass stars it’s like watching a time-lapse movie when compared to less massive stars, although the process for massive stars is fast and furious, it still takes tens of thousands of years!”

“While this research presents the strongest case yet for similar formation processes for low and high mass stars, there is still lots to understand,” concludes Stecklum. “Especially whether planets can form in the same way around stars at both ends of the mass spectrum.”