If astronomers had a few hundred thousand years to wait, they could look toward what is now just a giant cloud of gas and dust they have recently analyzed and see instead a massive newborn star—one that might rank among the heftiest stars known. No one has that much time, of course, but even now astronomers can study the cloud to obtain a detailed snapshot of a colossal star, as much as 100 times the mass of the sun, in the process of forming. Such a view will help researchers learn just how those celestial behemoths take shape.
Astronomers used the new Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to peer into a so-called molecular cloud—a cold interstellar assemblage of gas, faintly adulterated with dust—some 10,600 light-years away in our galaxy. Molecular clouds contain the raw materials for stellar formation, and when gravity causes the gas and dust within a cloud to collapse inward, a star is born. The cloud SDC 335 appears to contain at least two dense lumps, or cores, that will eventually form massive stars. One of those protostellar cores is among the most massive ever seen in the Milky Way, the researchers report in a study (pdf) that will appear in Astronomy & Astrophysics.
The researchers had access only to a small subset of the 66 telescope dishes in the ALMA telescope array, which was formally inaugurated just two months ago, but even the beta version of ALMA provided superior resolution for examining the inner structure of SDC 335. “We saw two cores sitting right in the center of the cloud, one of which was very massive,” says astronomer Nicolas Peretto of Cardiff University in Wales, lead author of the new study. “A core is the direct progenitor of a single star or a small stellar system, two or three stars. It’s basically a pocket of very cold dense gas that will feed the central star that is forming.”
The heftier of the cores, which the researchers dubbed MM1, contains some 545 times the mass of the sun, whereas the smaller core, MM2, has 65 solar masses of material. But star formation is a messy, violent process, and much if not most of the material in the cores will be ejected back into interstellar space rather than condensing into a star. Peretto and his colleagues expect that MM1 will form a star 50 to 100 times more massive than the sun.
By studying objects like MM1 and the cloud that contains it, astrophysicists hope to gain new insights into the formation of the most massive stars. At least two general ideas for how molecular clouds produce giant stars have emerged: one holds that molecular clouds remain relatively stable overall but partly fragment into cores, which then collapse and draw in nearby material to build a star. The researchers, however, believe that SDC 335 provides evidence for the alternative process, called global collapse, in which “molecular clouds are dynamic environments, full of turbulence, and are actually collapsing on very large scales,” Peretto explains. The cloud contains an interlocking network of large-scale filaments, which appear to be funneling gas toward the forming stars at the center.
But the interpretation of what SDC 335 reveals about star formation in general may prove controversial. “It’s beautiful data, it’s very, very nice, but it just does not bear the weight that they’re putting on it,” says Mark Krumholz of the University of California, Santa Cruz, who did not contribute to the study.
“I think that they’ve quite convincingly shown that there are these cores and they’re gaining mass from their environment fairly rapidly,” Krumholz adds. “I think it’s in fact quite plausible that these will form massive stars. The part that I have doubts about is the part where they say that this shows that the cloud is in a state of global collapse. The reason is that the numbers just don’t add up.” The measured rate of mass inflow to the cores at the center of the cloud is “much lower than you’d expect if the story they’re telling is correct,” Krumholz says. “If this cloud is in global collapse, why is the accretion rate not even close to what you would expect?”
Krumholz believes that the new observations best support an intermediate route to forming massive stars, in which growing protostellar cores draw in material from far and wide without the cloud collapsing everywhere at once. A 2010 study by researchers at Stanford University, the University of Virginia and Niigata University in Japan, which simulated the formation of massive stars from molecular clouds, came to a similar conclusion. “It used to be that there were these two very well-defined camps: collapsing blobs within marginally stable clouds or global collapse,” Krumholz says. “The reality is certainly in between those two.”