Star Clusters Host Stellar Drop Outs

Two young star clusters have been found to be littered with objects that have failed the stellar entrance exam. Research has uncovered over two dozen new, free-floating, ‘failed stars’ called brown dwarfs. 

Brown dwarfs in the young star cluster NGC 1333. (Credit: SONYC Team/Subaru Telescope)

The findings come as a surprise to an international team of astronomers at the University of Toronto, Dublin Institute for Advanced Studies and the National Astronomical Observatory of Japan as it appears that the duo of clusters harbour as many failed stars as brightly glowing normal stars.

Brown dwarfs are very low mass objects, between 75 to 80 Jupiter masses, which serve as an intermediate between a fully-fledged star and planet. Due to their size, they are not large enough to start hydrogen fusion in their cores, and so, because of this, they cool down giving a planet-like atmosphere that does not burn as brightly as the stars that we see in the night sky. With a name coined by Jill Tarter in the 1970s, these ruddy coloured dwarfs are often referred to as the link between gaseous giants like Jupiter, and red dwarfs, which despite having low masses, are true to their stellar family.

The clusters – NGC 1333 and Rho Ophiuchi – have been found to contain a surplus of these mysterious objects, some with masses less than 20 times the heftiness of Jupiter. They were scrutinized by two of the world’s largest optical-infrared instruments; the Subaru Telescope in Hawaii and the Very Large Telescope (VLT) in Chile presenting a combination of deep survey and extensive follow-up observations in the optical and infrared wavebands. During the course of the Substellar Objects in Nearby Young Clusters (SOYNC) Survey, the team found that their brown dwarf candidates were very red in colour.

“A brown dwarf appears red because it is cooler than stars [colour indicates temperature],” says the University of Toronto’s Ray Jayawardhana, who is the principal Investigator of the SONYC Survey. “Therefore it emits most of its radiation in the infrared, which is the reason why we try to observe it in this regime.” He adds: “Once we take spectra, we can find telltale signs that confirm the identity of a brown dwarf. For example, spectra of cooler brown dwarfs show prominent absorption features due to steam [such as water vapour] in their atmospheres.”

Finding a surprisingly large amount of these astronomical oddballs in these star clusters, was one thing, but the collaboration of astronomers are left puzzled by the fact that they seem to have found a lightweight youngster in reflection nebula NGC 1333, with a mass of around six times the mass of Jupiter, making it the tiniest free-floating object currently known. What’s more is that while its mass is comparable to that of a gas giant, it does not seem to be orbiting a star, leaving the collaboration of scientists scratching their heads as to how it formed.

But what is the likely explanation? “It might have formed like a giant planet but then been ejected from the orbit around a star,” says Aleks Scholz of the Dublin Institute for Advanced Studies in Ireland, lead author of one of the two upcoming papers that details the team’s findings and are set to appear in the Astrophysical Journal. “Or, it could have formed in a way similar to stars, that means from the collapse of a cloud, but in this case, it would require additional physics, for example, turbulence.”

Comparing the two star clusters, the team found that NGC 1333, which can be located in the constellation of Perseus, was more abundant in brown dwarfs than Rho Ophiuchi, a dark nebula of gas and dust and a cradle for young stars. “That difference may be hinting at how different environmental conditions affect their formation,” says Koraljka Muzic of the University of Toronto in Canada, lead author of the second paper.

But what can this team’s results tell us? “Our findings suggest once again that objects not much bigger than Jupiter could form the same way as stars do,” concludes Jayawardhana, who is also the author of Strange New Worlds: The Search for Alien Planets and Life Beyond our Solar System. “In other words, nature appears to have more than one trick up its sleeve for producing planetary mass objects.”