Fast-moving star is a really big loser

Which means the cosmic bullies that sent 30 Doradus 016 reeling must be even bigger

In stellar terms, weighing as much as 90 suns ought to get you some respect.

FAST MOVER The inset image to this portrait of the stellar nursery 30 Doradus in the Large Magellanic Cloud galaxy shows a heavyweight star (solid arrow), some 90 times as massive as the sun, that appears to have been ejected from its star-forming region (red circle) by two heftier siblings. The dashed arrow shows the direction of the star’s motion. Main: ESO; Inset: NASA, ESA, J. Walsh/ST-ECF

But the star 30 Doradus 016 was born in a particularly rough neighborhood. New research presented May 3 and in an upcoming journal article suggests it was unceremoniously kicked out of its native home by two even bigger bullies. And an unpublished study of the core of the same star-forming region, located in a satellite galaxy of the Milky Way called the Large Magellanic Cloud, concludes that there may be really huge stars, some tipping the scales at twice the mass previously thought possible in today’s universe, lurking in nearby reaches of the cosmos.

Astronomer Nolan Walborn of the Space Telescope Science Institute in Baltimore first noticed something strange about the star, a refugee from the 30 Doradus star-forming region, when a newly installed instrument on the Hubble Space Telescope measured the speed at which the star was hurling gas into space. Hubble’s Cosmic Origins Spectrograph determined that 30 Doradus 016 was spewing material at a whopping 3,450 kilometers per second, one of the fastest stellar winds ever recorded. Massive stars are known to expel fast winds, so he knew the star had to be big.

The finding inspired Chris Evans, an astronomer at the Royal Observatory Edinburgh in Scotland, to sift through observations of 30 Doradus 016 previously taken with a spectrograph on the Very Large Telescope in Paranal, Chile. He found that those measurements ruled out the existence of a closely orbiting companion to 30 Doradus 016, which suggested the entire heft could be attributed to a single star weighing the equivalent of 90 suns. Additional observations at the Anglo-Australian Observatory in Epping, Australia, revealed that the star, now about 400 light-years from the core of 30 Doradus, is speeding away from the star-forming region at more than 85 kilometers per second.

In theory, a supernova might have hurled the star from its birth site at such a high speed. But the 30 Doradus star-forming region is too young for any star there to have already ended its life in a supernova explosion, the researchers note. Instead, the most plausible explanation is that some kind of gravitational interaction with two other stars in the crowded 30 Doradus region ejected the massive body.

So even though 30 Doradus 016 is extraordinarily heavy, it nonetheless must have been the wimpiest member of a stellar trio in 30 Doradus. The two heavier stars in the trio gravitationally ganged up on 30 Doradus 016, kicking it out, Walborn and Evans explained in a May 3 presentation at a symposium on star formation and evolution at the Space Telescope Science Institute. Their team, which also includes Paul Crowther of the University of Sheffield in England, details its study of the runaway star in the June 1 Astrophysical Journal Letters.

Numerical simulations support such an ejection scenario, Crowther notes, and suggest “the presence of significantly higher-mass stars within the core” of 30 Doradus. The findings challenge previous studies that indicated stars in the region don’t exceed 100 solar masses.

Moreover, new independent work by Crowther, which Walborn briefly mentioned at the meeting, indicates that the region may contain stars as massive as 300 suns. Although such heavyweights were thought to be common in the early universe, theorists have previously calculated that stars in the cosmos today cannot exceed about 150 solar masses. Crowther declined to provide details of his study until it is accepted by a journal.

If stars in the present-day universe can indeed pack on much more mass than has been believed, 30 Doradus could provide astronomers with a close-up glimpse of heavyweights that were common in the first galaxies, billions of years ago. It would also have profound consequences for understanding the birth and death of stars, galaxy formation and the frequency and type of supernova explosions, commented Selma de Mink of Utrecht University in the Netherlands.

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