What I thought might have been a stop to star formation doesn’t seem to work that way after all.
New observations from the Hubble Space Telescope show that strong astrophysical jets and stellar winds flowing from baby stars do not have the expected effect of extinguishing the stellar growth process. This is quite a significant enigma for our star formation models.
The birth of a star is a rather long process on the scale of human time. It’s not like we can sit and watch a baby star form from go to whoa. What we can do is find a bunch of stars in different stages of the formation process and put the pieces together like a puzzle.
The most commonly accepted model goes like this: First, you need to start with a pile of really dense material in a cloud of cold, interstellar molecular gas.
With sufficient density, the agglomeration collapses under its own gravity to form a protostar, which begins to rotate. This rotation causes the material in the cloud around it to form a disk, which wraps itself in the rising star like water on a drainage channel, inexorably attracted by the gravitational force.
But only 30 percent of the initial mass of the cloud reaches the star. So far, I’ve actually had a pretty good explanation as to why: As the star grows, it begins to produce a strong stellar wind. In addition, the falling material in the star begins to interact with the star’s magnetic fields, flowing along the lines of the magnetic field to the poles, where it is thrown into space in the form of strong plasma jets.
The combined external thrust of these two forces, known as stellar feedback, sculpts a growing cavity into the molecular cloud around the star, eventually depriving it of material for further growth and determining the final mass of the star. .
Or so we thought.
(RB Andreo / DeepSkyColors.com, NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
In a study of 304 protosts in the star-forming region of the Orion complex, highlighted in yellow in the image above, astronomers found no evidence that the output cavities grow steadily as the star grows rapidly.
“In a star formation model, if you start with a small cavity, as the protostar becomes more rapidly evolved, its leakage creates an ever-larger cavity until the surrounding gas is finally blown, leaving 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 push the whole mass out of the cloud. So there has to be another process to get rid of the gas that won’t get to the star. “
The study required data from a series of space telescopes. The Herschel Space Observatory and the Spitzer Space Telescope conducted surveys of the Orion complex to build a catalog of hundreds of protostars. Based on the light from these survey stars, Habel and her team sorted the protostars by age.
Then they made observations of the region of the surrounding infrared clouds almost using Hubble; some of these are illustrated below. Although optical light cannot penetrate a protostellar cloud, infrared wavelengths can, and infrared observations are an excellent tool for sounding in densely cloudy regions.
In this case, the light of the forming star is reflected within the limits of the cavity, which allows astronomers to map its size.
(NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
This meticulous work led to a catalog of their protostes and cavities, sorted by age … and older protostars did not appear to have larger cavities.
“We find that at the end of the protostellar phase, where most of the gas fell from the surrounding cloud on the star, a number of young stars still have fairly narrow cavities,” said astronomer Tom Megeath of the University of Toledo.
“So this image that is 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 was pretty fundamental to our idea of how stars are formed, but it doesn’t seem to match the data here. “
Although it is still possible that winds and planes play a role in star formation, this role does not seem to be nearly as important as I thought, the researchers said. Slower and higher density flows may be responsible – a similar mechanism, but one that takes longer to clean the cavity – but without more detailed observations, it is impossible to say.
So, this will be one of the next steps. No doubt, astronomers will also seek to model and simulate star formation – to try to identify other mechanisms that could stop growth, with a much smaller contribution from stellar feedback. Look at this space.
The team’s research is to appear in The astrophysics journaland is available on arXiv.