Date: Thu, 18.03.21 22:30
Hubble shows torrential outflows from in
March 18, 2021
NASA/Goddard Space Flight Center
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
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.
Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style
1. Nolan M. Habel, S. Thomas Megeath, Joseph Jon Booker, William
Marina Kounkel, Charles Poteet, Elise Furlan, Amelia Stutz,
P. Manoj, John J. Tobin, Zsofia Nagy, Riwaj Pokhrel, Dan Watson. An
HST Survey of Protostellar Outflow Cavities: Does Feedback Clear
Envelopes? The Astrophysical Journal, 2021 [abstract]
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