What we think might have been a switch for star formation doesn’t seem to work that way after all.
New observations from the Hubble Space Telescope show that the powerful astrophysical jets and stellar winds flowing from baby stars do not have the expected effect of shutting down the stellar growth process. This poses a rather significant conundrum for our star formation models.
The birth of a star is quite a long process on human timescales. It’s not like we can just sit back and watch a baby star form from start to finish. What we can do is find a bunch of stars at different stages of the formation process and put the pieces together like a puzzle.
The most commonly accepted model is this: First, you have to start with a really dense clump of material in a cloud of cool interstellar molecular gas.
With sufficient density, the group collapses under its own gravity to form a protostar, which begins to spin. This spin causes the material in the cloud around it to form a disk, which coils around the growing star like water down a drain, inexorably drawn by its gravitational force.
But only 30 percent of the initial cloud’s mass ends up in the star. Until now, we had a pretty good explanation for why: As the star grows larger, it begins to produce a powerful stellar wind. In addition, the material that falls on the star begins to interact with the star’s magnetic fields, flowing along the magnetic field lines toward the poles, where it is launched into space in the form of powerful jets of plasma.
The combined outward push of these two forces, known as stellar feedback, sculpts an ever-larger cavity in the molecular cloud around the star, eventually starving it of material for further growth and determining the star’s ultimate mass.
Or so we think.
In a study of 304 protostars in the star-forming region of the Orion Complex, highlighted in yellow in the image above, astronomers have found no evidence that exit cavities grow steadily as the star grows rapidly.
“In a star formation model, if you start with a small cavity, as the protostar evolves more rapidly, its outflow creates a larger and larger cavity until the surrounding gas eventually disappears, leaving an isolated star,” he said. the astronomer Nolan. Habel from the University of Toledo.
“Our observations indicate that there is no progressive growth that we can find, so the cavities are not growing until they expel all the mass of the cloud. Therefore, there must be some other process that removes the gas that will not end in the star “.
The study required data from several space telescopes. The Herschel Space Observatory and the Spitzer Space Telescope had conducted surveys of the Orion Complex to build a catalog of hundreds of protostars. Based on the light of these stars in the polls, Habel and his team classified the protostars by age.
Next, they took observations of the surrounding cloud region in the near infrared using Hubble; some of them are shown below. Although optical light cannot penetrate a protostellar cloud, infrared wavelengths can, and infrared observations are an excellent tool for exploring densely cloudy regions.
In this case, light from the forming star is reflected off the cavity’s boundaries, allowing astronomers to map its size.
This painstaking work resulted in a catalog of protostars and their cavities, sorted by age … and the oldest protostars did not appear to have larger cavities.
“We found that at the end of the protostellar phase, where most of the gas has fallen from the cloud surrounding the star, several young stars still have quite narrow cavities,” said astronomer Tom Megeath of the University of Toledo.
“So this still commonly held picture of what determines the mass of a star and what stops the gas fall is that this growing outlet cavity collects all of the gas. This has been quite fundamental to our idea of how the Star formation continues, but it just doesn’t seem to fit the data here. “
Although it is still possible that winds and jets play some role in star formation, that role does not appear to be as important as we thought, the researchers said. It is possible that slower and higher density outflows may be responsible, a similar mechanism, but which takes longer to clear the cavity, but without more detailed observations, it is impossible to know.
So that will be one of the next steps. Astronomers will no doubt also seek to model and simulate star formation, to try to identify other mechanisms that could halt growth with a much smaller contribution from stellar feedback. Look at this space.
The team’s research should appear in The Astrophysical Journaland it is available in arXiv.