Chaotically magnetized cloud is no place to build a star, or is it?

Artist impression of chaotic magnetic field lines very
near a newly emerging protostar. Credit: NRAO/AUI/NSF; D. Berry

For decades, scientists thought that the magnetic field lines
coursing around newly forming stars were both powerful and
unyielding, working like jail bars to corral star-forming
material. More recently, astronomers have found tantalizing
evidence that large-scale turbulence far from a nascent star
can drag magnetic fields around at will.

Now, a team of astronomers using the Atacama Large
Millimeter/submillimeter Array (ALMA) has discovered a
surprisingly weak and wildly disorganized very near a newly emerging
protostar. These observations suggest that the impact of
magnetic fields on star formation is more complex than
previously thought.

The researchers used ALMA to map the magnetic field surrounding
a young protostar dubbed Ser-emb 8, which resides about 1,400
light-years away in the Serpens star-forming region. These new
observations are the most sensitive ever made of the
small-scale magnetic field surrounding a young protostar. They
also provide important insights into the formation of low-mass
like our own sun.

Previous observations with other telescopes found that magnetic
fields surrounding some young protostars form a classic
“hourglass” shape – a hallmark of a strong magnetic field –
that starts near the protostar and extends many light-years
into the surrounding cloud of dust and gas.

“Before now, we didn’t know if all stars formed in regions that
were controlled by . Using ALMA, we
found our answer,” said Charles L. H. “Chat” Hull, an
astronomer and NRAO Jansky Fellow at the Harvard-Smithsonian
Center for Astrophysics (CfA) in Cambridge, Mass., and lead
author on a paper appearing in the Astrophysical Journal
Letters
. “We can now study magnetic fields in star-forming
clouds from the broadest of scales all the way down to the
forming star itself. This is exciting because it may mean stars
can emerge from a wider range of conditions than we once
thought.”

Texture represents the magnetic field orientation in the
region surrounding the Ser-emb 8 protostar, as measured by ALMA.
The gray region is the millimeter wavelength dust emission.
Credit: ALMA (ESO/NAOJ/NRAO); P. Mocz, C. Hull, CfA

ALMA is able to study magnetic fields at the small scales
inside star-forming clumps by mapping the polarization of light
emitted by dust grains that have aligned themselves with the
magnetic field.

By comparing the structure of the magnetic field in the
observations with cutting-edge supercomputer simulations on
multiple size scales, the astronomers gained important insights
into the earliest stages of magnetized star formation. The
simulations – which extend from a relatively nearby 140
astronomical units (an astronomical unit is the average
distance from the Earth to the sun) from the protostar to as
far out as 17 light-years – were performed by CfA astronomers
Philip Mocz and Blakesley Burkhart, who are co-authors on the
paper.

In the case of Ser-emb 8, the astronomers think they have
captured the original magnetic field around the protostar “red
handed,” before outflowing material from the star could erase
the pristine signature of the magnetic field in the surrounding
molecular cloud, noted Mocz.

“Our observations show that the importance of the magnetic
field in can vary widely from
star to star,” concluded Hull. “This protostar seems to have
formed in a weakly magnetized environment dominated by
turbulence, while previous observations show sources that
clearly formed in strongly magnetized environments. Future
studies will reveal how common each scenario is.”

Explore further:

Twisted magnetic fields give new insights on star formation

More information: Charles L. H. Hull et al. Unveiling
the Role of the Magnetic Field at the Smallest Scales of Star
Formation, The Astrophysical Journal (2017).
DOI: 10.3847/2041-8213/aa71b7