Monday, July 10, 2017

A Star is Born...in Surprising Circumstances

Stellar nurseries, the birthplace of new stars, are not as cozy and color-coordinated as Pinterest nurseries. Stellar nurseries feature dust and gas rather than lovable characters and perfect shades of blue or pink—cold expanses rather than cozy nooks.

As scientists have pieced together the story of how stars form, a model has emerged that highlights the role of a strong magnetic field. However, research recently published in The Astrophysical Journal Letters reveals that stellar nurseries may have environments that are much more varied and complex than previously thought. This information could help us better understand how stars like our sun form.

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

Before we get to these results, it helps to visualize where stellar nurseries form. It’s not just emptiness that fills the space between stars systems, this space is filled with a dilute mixture of gas and dust called the interstellar medium. Some areas are denser than others, forming giant interstellar clouds. (As a side note, NASA and the University of Colorado, Boulder launched a new rocket payload just last Tuesday that gathered data on the interstellar cloud between us and the star Beta Scorpii - details here.)

Under certain conditions, the gravity of an interstellar cloud can overcome the pressure of the gas within it and cause the cloud to collapse and break into pieces. These pieces become the birthplace of new stars as gravity pulls in more and more gas, creating increasingly dense, hot objects—protostars on the path to becoming full-fledged stars. During the protostar phase, a star-to-be is still pulling in mass from the surrounding interstellar cloud and hasn’t started fusion reactions yet.

This isn’t the whole story, however. Many pieces of interstellar clouds that should form stars according to gravity, do not. The process appears to be shaped by things like the turbulence and magnetic fields of the cloud. Although the details of their influence aren’t well understood, typical models suggest that magnetic fields play the dominant role in regulating star formation.
To better understand the impact of magnetic fields on the birth of stars, scientists in the global ALMA collaboration, the Atacama Large Millimeter/Submillimeter Array, turned their instruments on an object called Ser-emb 8. Ser-emb 8 is about 1,400 light years away. It’s a typical young protostar in a region where many stars are forming.

Located in the Atacama Desert of Chile, ALMA is a collection of ground-based telescopes (very precise antennas) that together study the universe in millimeter and submillimeter wavelengths—the range of light between infrared light and radio waves. This light contains valuable information on things like the origins of galaxies, stars, planets, and the molecules related to life.

ALMA can’t “see” magnetic fields directly. Instead it maps the polarization of the light given off by warm grains of dust that tend to align with the magnetic field of a region. Using this polarization map, scientists can infer the magnetic field in an area. Thanks to ALMA’s precise instruments, the Ser-emb 8 measurement is the most sensitive measurement scientists have ever made of a small-scale magnetic field around a young protostar.

The result was an unexpected surprise.

Previous research suggests that stars typically form in regions with strong magnetic fields. When observing a young protostar, this is evidenced by a magnetic field with an hourglass-shape, and astronomers have observed this before. However, the team, which was led by Charles Hull from the Harvard-Smithsonian Center for Astrophysics, found that Ser-emb 8 didn’t fit the model. Although it is clearly a young protostar, there is no hourglass in sight: The magnetic field of Ser-emb 8 is chaotic, randomly oriented, and doesn’t match up to the large-scale magnetic field of the region.

This 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.
Image Credit: ALMA (ESO/NAOJ/NRAO); P. Mocz, C. Hull, CfA.

To better understand this result and its implications, the team ran simulations of an interstellar cloud collapsing and forming a young protostar. Each simulation featured magnetic fields and turbulence of different strengths. From these simulations, the team created mock observations of the magnetic field.
By comparing the mock observations to the real ones, astronomers found that Ser-emb 8 is likely forming from the collapse of an interstellar cloud with a weak magnetic field. The star formation and the magnetic field of Ser-emb 8 appears to be controlled by turbulence within the cloud, not a strong magnetic field.

The implications of this result go beyond describing the environment in Ser-emb 8’s tiny piece of the sky. With this observation, astronomers have demonstrated that stars can form under a wider variety of conditions than previously thought. In other words, stellar nurseries may display more of nature’s creativity than we realized.

Kendra Redmond

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