What we thought might have been a deactivation switch for star formation doesn’t seem to work that way after all.
New observations from the Hubble Space Telescope show that powerful astrophysical jets and stellar winds flowing from baby stars do not have the expected effect of stopping the stellar growth process. This poses a fairly significant enigma to our star formation models.
The birth of a star is a fairly long process on a human scale. It’s not like we can sit and watch a baby star form from going to whoa. 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 accepted model is this: first, you need to start with a very dense material thickness in a cloud of fresh, interstellar molecular gas.
With sufficient density, the cluster collapses under its own gravity to form a protostar, which begins to rotate. This twist causes the surrounding cloud material to form a disk, which sinks toward the rising star like water from a drain, inexorably dragged by its strengthening of gravity.
But only 30 percent of the initial cloud mass ends up in the star. So far, we actually had a good explanation of why: as the star grows, it begins to produce a strong stellar wind. In addition, the material falling on the star begins to interact with the magnetic fields of the star, which flows along the lines of the magnetic field to the poles, where it is thrown into space in the form of powerful jets of plasma.
The combined external thrust of these two forces, known as stellar feedback, sculpts a growing cavity in the molecular cloud around the star, eventually depriving it of material for later growth and determining the final mass of the star.
Or so we thought.
(RB Andres / DeepSkyColors.com, NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
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 the output cavities grow steadily as the star grows rapidly.
“In a star-forming model, if you start with a small cavity, as the protostar evolves rapidly, its output creates an ever-larger cavity until the surrounding gas ends up flying and leaves a isolated star, ”said astronomer Nolan Habel of 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 expel all the mass from the cloud. Therefore, there must be some other process going on that removes the gas. that doesn’t work. I won’t end up 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 survey stars, Habel and her team classified the protostars by age.
They then made observations of the surrounding cloud region in near infrared using Hubble; some of these are shown below. While optical light cannot penetrate a protostellar cloud, infrared wavelengths can do so, and infrared observations are an excellent tool for exploring densely cloudy regions.
In this case, the light from the forming star is reflected at the boundaries of the cavity, allowing astronomers to map its size.
(NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
This meticulous work resulted in a catalog of protostars and their cavities, sorted by age … and the older 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 surrounding cloud over the star, several young stars still have fairly narrow cavities,” said astronomer Tom Megeath of the University of Toledo. .
“Therefore, this image that is still commonly maintained about what determines the mass of a star and what stops the gas from falling is that this growing outlet cavity captures all the gas. This has been quite fundamental for our idea of how star formation is formed continues, but it doesn’t seem to fit the data here. “
While 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. They may be responsible for slower, denser outflow flows (a similar mechanism, but one that takes longer to clear the cavity), but without more detailed observations, it is impossible to say.
Therefore, this will be one of the next steps. No doubt astronomers will also try to model and simulate star formation, to try to identify other mechanisms that could stop growth with a much smaller contribution from stellar feedback. Take a look at this space.
Team investigations should appear in The Astrophysical Journal, and is available at arXiv.