Hubble shows torrential outflows from infant stars may not stop them from growing
NASA/GODDARD SPACE FLIGHT CENTER
Though our galaxy is an immense city of at least 200 billion stars, the details of how they formed remain largely cloaked in mystery.
Scientists know that stars form from the collapse of huge hydrogen clouds that are squeezed under gravity to the point where nuclear fusion ignites. But only about 30 percent of the cloud's initial mass winds up as a newborn star. Where does the rest of the hydrogen go during such a terribly inefficient process?
It has been assumed that a newly forming star blows off a lot of hot gas through lightsaber-shaped outflowing jets and hurricane-like winds launched from the encircling disk by powerful magnetic fields. These fireworks should squelch further growth of the central star. But a new, comprehensive Hubble survey shows that this most common explanation doesn't seem to work, leaving astronomers puzzled.
Researchers used data previously collected from NASA's Hubble and Spitzer space telescopes and the European Space Agency's Herschel Space Telescope to analyze 304 developing stars, called protostars, in the Orion Complex, the nearest major star-forming region to Earth. (Spitzer and Herschel are no longer operational).
In this largest-ever survey of nascent stars to date, researchers are finding that gas-clearing by a star's outflow may not be as important in determining its final mass as conventional theories suggest. The researchers' goal was to determine whether stellar outflows halt the infall of gas onto a star and stop it from growing.
Instead, they found that the cavities in the surrounding gas cloud sculpted by a forming star's outflow did not grow regularly as they matured, as theories propose.
"In one stellar formation model, if you start out with a small cavity, as the protostar rapidly becomes more evolved, its outflow creates an ever-larger cavity until the surrounding gas is eventually blown away, leaving an isolated star," explained lead researcher Nolan Habel of the University of Toledo in Ohio.
"Our observations indicate there is no progressive growth that we can find, so the cavities are not growing until they push out all of the mass in the cloud. So, there must be some other process going on that gets rid of the gas that doesn't end up in the star."
The team's results will appear in an upcoming issue of The Astrophysical Journal.
A Star is Born
During a star's relatively brief birthing stage, lasting only about 500,000 years, the star quickly bulks up on mass. What gets messy is that, as the star grows, it launches a wind, as well as a pair of spinning, lawn-sprinkler-style jets shooting off in opposite directions. These outflows begin to eat away at the surrounding cloud, creating cavities in the gas.
Popular theories predict that as the young star evolves and the outflows continue, the cavities grow wider until the entire gas cloud around the star is completely pushed away. With its gas tank empty, the star stops accreting mass - in other words, it stops growing.
To look for cavity growth, the researchers first sorted the protostars by age by analyzing Herschel and Spitzer data of each star's light output. The protostars in the Hubble observations were also observed as part of the Herschel telescope's Herschel Orion Protostar Survey.
Then the astronomers observed the cavities in near-infrared light with Hubble's Near-infrared Camera and Multi-object Spectrometer and Wide Field Camera 3. The observations were taken between 2008 and 2017. Although the stars themselves are shrouded in dust, they emit powerful radiation which strikes the cavity walls and scatters off dust grains, illuminating the gaps in the gaseous envelopes in infrared light.
The Hubble images reveal the details of the cavities produced by protostars at various stages of evolution. Habel's team used the images to measure the structures' shapes and estimate the volumes of gas cleared out to form the cavities. From this analysis, they could estimate the amount of mass that had been cleared out by the stars' outbursts.
"We find that at the end of the protostellar phase, where most of the gas has fallen from the surrounding cloud onto the star, a number of young stars still have fairly narrow cavities," said team member Tom Megeath of the University of Toledo. "So, this picture that is still commonly held of what determines the mass of a star and what halts the infall of gas is that this growing outflow cavity scoops up all of the gas. This has been pretty fundamental to our idea of how star formation proceeds, but it just doesn't seem to fit the data here."
Future telescopes such as NASA's upcoming James Webb Space Telescope will probe deeper into a protostar's formation process. Webb spectroscopic observations will observe the inner regions of disks surrounding protostars in infrared light, looking for jets in the youngest sources. Webb also will help astronomers measure the accretion rate of material from the disk onto the star, and study how the inner disk is interacting with the outflow
Credits: NASA, ESA, and N. Habel and S. T. Megeath (University of Toledo)
An Unexpected Hubble Discovery Just Changed Our Understanding of Star Formation
What we thought may have been an off-switch for star formation doesn't appear to work that way after all.
© NASA/JPL-Caltech/T. Megeath, University of Toledo, Ohio Star formation in the Orion Complex.
New observations from the Hubble Space Telescope show the powerful astrophysical jets and stellar winds that flow from baby stars do not have the expected effect of quenching the stellar growth process. This poses quite a significant conundrum for our models of star formation.
The birth of a star is quite a long process on human timescales. It's not as if we can sit and watch a baby star form from go 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 commonly accepted model goes thus: Firstly, you need to start with a really dense clump of material in a cloud of cool, interstellar molecular gas.
With enough density, the clump collapses under its own gravity to form a protostar, which starts to spin. This spin causes the material in the cloud around it to form a disk, which spools into the growing star like water down a drain, inexorably drawn in by its strengthening gravitational pull.
But only 30 percent of the initial cloud's mass ends up in the star. Until now, we actually had a pretty good explanation as to why: As the star grows, it starts to produce a powerful stellar wind. In addition, material falling into the star starts to interact with the star's magnetic fields, flowing along magnetic field lines to the poles, where it is blasted into space in the form of powerful plasma jets.
The combined outward push of these two forces, known as stellar feedback, carves a larger and larger 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.
a star filled sky: baby stars(R. B. Andreo/DeepSkyColors.com, NASA, ESA, STScI, N. Habel and S. T. Megeath/University of Toledo)
In a study of 304 protostars in the Orion Complex star-forming region, highlighted in yellow in the image above, astronomers have found no evidence that the outflow cavities grow steadily as the star rapidly grows.
"In one stellar formation model, if you start out with a small cavity, as the protostar rapidly becomes more evolved, its outflow creates an ever-larger cavity until the surrounding gas is eventually blown away, leaving an isolated star," said astronomer Nolan Habel of the University of Toledo.
"Our observations indicate there is no progressive growth that we can find, so the cavities are not growing until they push out all of the mass in the cloud. So, there must be some other process going on that gets rid of the gas that doesn't end up in the star."
The study required data from a number of space telescopes. The Herschel Space Observatory and 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 surveys, 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. Although optical light can't penetrate a protostellar cloud, infrared wavelengths can, and infrared observations are an excellent tool for probing into densely clouded regions.
In this case, the light of the forming star reflects off the boundaries of the cavity, which allows astronomers to map its size.
a close up of a bright light: Hubble observations(NASA, ESA, STScI, N. Habel and S. T. Megeath/University of Toledo)
This painstaking work resulted in a catalog of protostars and their cavities, sorted by age… and older protostars did not seem to have larger cavities.
"We find that at the end of the protostellar phase, where most of the gas has fallen from the surrounding cloud onto the star, a number of young stars still have fairly narrow cavities," said astronomer Tom Megeath of the University of Toledo.
"So, this picture that is still commonly held of what determines the mass of a star and what halts the infall of gas is that this growing outflow cavity scoops up all of the gas. This has been pretty fundamental to our idea of how star formation proceeds, but it just doesn't seem to fit the data here."
Although it's still possible that the winds and jets play some role in star formation, that role doesn't seem 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 a longer time to clear the cavity - but without more detailed observations, it's impossible to tell.
So, that will be one of the next steps. No doubt astronomers will also be looking to model and simulate star formation - to try and identify other mechanisms that could call a halt to growth with a much smaller contribution from stellar feedback. Watch this space.
The team's research is due to appear in The Astrophysical Journal, and is available on arXiv
