What we thought was a star formation disable switch doesn’t seem to work that way anyway.
New observations from the Hubble Space Telescope show that the powerful astrophysical jets and stellar winds pouring out of baby stars are not having the expected effect of suppressing the stellar growth process. This is quite an important conundrum for our models of star formation.
Star birth is quite a long process on human timescales. It’s not like we can sit and look at a baby star shape from start to finish. What we can do is find a number of stars at different stages of the formation process and put the pieces together like a puzzle.
The most widely accepted model is as follows: First, you have to start with a very dense lump of material in a cloud of cool, interstellar molecular gas.
With sufficient density, the clump 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 washes like water through a drain in the growing star, drawn inexorably by its strengthening gravity.
But only 30 percent of the mass of the original cloud ends up in the star. Until now, we actually had a pretty good explanation why: As the star grows, it starts to produce a powerful stellar wind. In addition, material falling into the star begins to interact with the star’s magnetic fields, flowing along magnetic field lines to the poles, where it is blown into space in the form of powerful plasma beams.
The combined outward pressure of these two forces, known as stellar feedback, cuts an ever-expanding cavity in the molecular cloud around the star, ultimately withholding material for further growth and determining the star’s ultimate mass.
Or so we thought.
(RB Andreo / DeepSkyColors.com, NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
In a study of 304 protostars in the star-forming region Orion Complex, highlighted in yellow in the image above, astronomers found no evidence that the outflow cavities grow steadily as the star grows rapidly.
“In a stellar formation model, if you start with a small cavity, while the protostar evolves rapidly, the outflow creates an ever-larger cavity until the surrounding gas is eventually blown away, leaving an isolated star,” said astronomer Nolan. Habel from the University of Toledo.
“Our observations indicate that there is no progressive growth that we can find, so the cavities do not grow until they push out all the mass in the cloud. So there must be another process going on that removes the gas that is not in the star. end up. “
The study required data from a number of space telescopes. The Herschel Space Observatory and the Spitzer Space Telescope had conducted surveys of the Orion Complex to compile a catalog of hundreds of protostars. Based on the light from these stars in the studies, Habel and his team sorted the protostars by age.
Then they took observations of the surrounding cloud region in near infrared using Hubble; some of these are pictured below. While optical light cannot penetrate a protostellar cloud, infrared wavelengths can, and infrared observations are an excellent tool for examining dense areas.
In this case, the light from the forming star is reflected off the boundaries of the cavity, allowing astronomers to map its size.
(NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
This painstaking work resulted in a catalog of protostars and their cavities, sorted by age… and older protostars did not appear to have larger cavities.
“We see that at the end of the protostellar phase, where most of the gas from the surrounding cloud has fallen on the star, some of the young stars still have fairly narrow cavities,” said astronomer Tom Megeath of the University of Toledo.
So, this picture that is still widely held of what determines a star’s mass and what stops the gas attack is that this growing outflow cavity sucks up all the gas. it just doesn’t seem to fit the data here. “
While it’s still possible that the winds and jets play a role in star formation, that role doesn’t appear to be nearly as important as we thought, the researchers said. It’s possible that slower, higher density outflows could be responsible – a similar mechanism, but one that takes longer to empty the cavity – but without more detailed observations, it’s impossible to say.
So that’s going to be one of the next steps. Undoubtedly, astronomers will also try to model and simulate star formation – to try to identify other mechanisms that could halt growth with a much smaller contribution from stellar feedback. Watch this space.
The team’s investigation will appear in The Astrophysical Journal, and is available on arXiv.