A giant star is forming in the same way as low and medium mass stars, astronomers report. The finding goes a long way towards settling the question of whether giant stars are born differently from those of more ordinary mass.
Many of the most prominent stars are giants with masses twenty times that of the Sun. Such stars have a disproportionate impact on galactic evolution, particularly when they end their lives as supernovae. However, their brightness hides the fact that such stars are rare. They also form particularly quickly, so we get few opportunities to study crucial phases of their evolution.
Suspicion that these stars form differently has been aroused by the failure to find equivalents with masses more than 18 times that of the Sun of the disks observed as small or medium-sized stars form. Instead we see toroidal structures, like stellar doughnuts. There has been speculation that these arise from spheres whose inner areas collapse symmetrically with outflows that clear the area above and below the star.
AFGL 4176 is an exception. A monstrous disk surrounds an O-type star 25 times the mass of the Sun that is still growing as it collects material from the disk. Dr Katharine Johnston of the University of Leeds used the Atacama Large Millimeter/submillimeter Array (ALMA) to study AFGL 4176 to see if this really was the sought after example of a giant showing a normal birth.
In the Astrophysical Journal Letters Johnston and her co-authors report that the disk can be seen to occupy 870 Astronomical Units (AU) by 330 AU. For comparison Pluto's average distance from the Sun is 40 AU. The mass of this visible disk is eight times that of the Sun, but Johnston concludes even this understates the disk's true size.
The temperature falls further from the star, making the disk harder to see, even for ALMA's mighty radio telescopes. The full size, the paper argues, stretches for 2000 AU and contains 12 solar masses. Moreover the disk is spinning in a manner similar to those around low mass stars, feeding material to the center in the process.
"We started to think that real disks may not actually exist around the most massive forming stars, and that those stars might have to form in a different way," said Johnston in a statement. "Maybe the accretion into an O-type star was much more chaotic and dynamic than for the birth of our Sun.”
However, Johnston added, “We found a disk around an O-type star, which looks very similar to the disk that we think went on to form our Sun and the rest of the Solar System, except a gigantic scaled-up version of it. The disk we have found is at least 10 times larger and 100 times more massive than the disks that we usually see around young stars."
Co-author Professor Melvin Hoare added that with more time on ALMA it may be possible to find discontinuities indicating planetary formation within this mighty disk.
