Astronomers already knew there were three stars in this system, but they had a totally different mental picture of what was going on. “For a long time (going back almost 20 years), we thought that the tertiary was the center of the system because it was the brightest,” Tobin says.
But ALMA’s images reveal that that’s not the case. The array’s radio “eyes” are two times sharper and ten times more sensitive than those used in previous studies. ALMA detects emission from the carbon monoxide molecule, which acts as a tracer for the molecular hydrogen reservoir that forms stars. (Molecular hydrogen by itself is nearly impossible to detect in star-forming clouds.) The motion of carbon monoxide revealed a disk centered, surprisingly, on the other two stars.
ALMA’s image isn’t even the neatest part of the result. “The strength of this study is the attention to detail in Tobin and colleagues’ analysis,” writes Adele Plunkett (ESO, Chile), author of an accompanying perspective piece. The team modeled how stable the disk is, gravitationally speaking, and showed that it’s in fact unstable right around where the tertiary star is forming. The result confirms that the disk is collapsing in on itself in this region to make the third star.
Astronomers can calculate the central duo’s mass by measuring how fast the disk rotates around them. Right now, the central two protostars have a combined mass equal to the Sun’s. They’ll still grow, but probably not too much more — most of the gas ALMA detects around them already effectively belongs to the stars.
The third protostar in the disk is more difficult to “weigh”: all we know at this point is that it’s in the process of gobbling down a chunk of gas worth 8.5% of the Sun’s mass. That explains why it’s the brightest object in the image: ALMA isn’t seeing the star itself, but rather the glow of the cool dust and gas around it.
“I expect that the tertiary in the outer disk is likely to grow the most,” Tobin adds. Out there, the protostar has access to more raw materials than the primary pair in the central cavity. (The central two stars are mostly done growing at this point, having cleared out a cavity around them.) So while its mass may have the third protostar looking more like a brown dwarf at this point, Tobin predicts it will become a more massive, red dwarf star by the time all is said and done.
With this discovery, the question remains: how common is gravitational instability? How often do stars form in the shadow of their stellar siblings? “Fragmenting disks like the one observed by Tobin and colleagues are probably not rare,” Plunkett writes. “Rather, they are waiting to be studied in more detail using the powerful (sub-)millimeter-wavelength telescopes that are now available.”