What we thought was a switch for star formation does not seem to work that way.
New observations from the Hubble Space Telescope show that the powerful astrophysical jets and star winds flowing from baby stars do not have the expected effect of extinguishing the star-growth process. This is a very important mystery for our models of star formation.
The birth of a star is quite a long process on human time scales. It’s not like we can sit and watch a baby star form up to whoa. What we can do is find a bunch of stars at different stages of the shaping process and put the pieces together like a puzzle.
The most commonly accepted model is this: First, you need to start with a very dense amount of material in a cloud of cool, interstellar molecular gas.
With enough density, the mass collapses under its own gravity to form a protostar that begins to rotate. This twist causes the material in the cloud around it to form a disk that washes into the growing star like water in a drain, relentlessly attracted by its amplifying gravity.
But only 30 percent of the mass of the initial cloud ends up in the star. So far, we’ve had a pretty good explanation for why: as the star grows, it begins to produce a powerful stellar wind. In addition, material falling into the star begins to interact with the star’s magnetic fields and flows along magnetic field lines to the poles, where it is blown into space in the form of powerful plasma radiation.
The combined outward pressure of these two forces, known as star feedback, carves a larger and larger cavity in 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 protostars in the Orion Complex star-forming region, yellow in the picture above, astronomers found no evidence that the outflow cavities gradually grow as the star grows rapidly.
“In one star-forming model, if you start with a small cavity, as the protostar develops more rapidly, its outflow creates an ever-larger cavity until the surrounding gas finally blows away, leaving an isolated star behind,” said astronomer Nolan. Habel of the University of Toledo.
“According to our observations, there is no progressive growth that we can find, so the cavities do not grow until they expel all the mass in the cloud. So there has to be another process going on that gets rid of the gas that does not does not end up in the star. ‘
The study requires data from a number of space telescopes. The Herschel Space Observatory and Spitzer Space Telescope investigated the Orion complex to compile a catalog of hundreds of protostars. Based on the light of these stars in the surveys, Habel and his team sorted the stars by age.
Then they took observations of the surrounding cloud region in near-infrared using Hubble; some of these are shown below. Although optical light cannot penetrate a protostellar cloud, infrared wavelengths and infrared observations can be an excellent tool for exploration in dense cloudy regions.
In this case, the light of the forming star reflects the boundaries of the cavity, allowing astronomers to map its size.
(NASA, ESA, STScI, N. Habel and ST Megeath / University of Toledo)
This careful work has resulted in a catalog of protostars and their cavities, sorted by age … and it looks like older protostars do not have larger cavities.
“We find that a number of young stars at the end of the protostellar phase, where most of the gas fell from the surrounding cloud onto the star, have rather narrow cavities,” said astronomer Tom Megeath of the University of Toledo.
‘This picture, which is still widely held, of what determines the mass of a star and what stops the entry of gas, is that this growing outflow cavity traps all the gas. It was pretty fundamental to our idea of how star formation was going on, but it just doesn’t seem like the information fits here. ‘
Although it is still possible that the winds and rays play a role in star formation, it seems that the role is not nearly as important as we thought. It is possible that slower outflows with higher density may be responsible – a similar mechanism, but one that takes longer to clear the cavity – but without more detailed observations it is impossible to see.
It will therefore be one of the next steps. Astronomers will no doubt also want to model and simulate star formation – to try to identify other mechanisms that can stop growth with a much smaller contribution of star feedback. Keep an eye on this space.
The team’s research should appear in The Astrophysical Journal, and is available on arXiv.