Magnetic fields observed by SOFIA are shown as streamlines over an image from the Spitzer Space Telescope.

Magnetized gas flows feed a young star cluster

August 19, 2020 /

[Picture: Bildrechte: NASA/SOFIA/T. Pillai/J. Kauffmann; NASA/JPL-Caltech/L. Allen ]

Polarimetric Observations with SOFIA in interstellar clouds made of gas and dust, indicate that these clouds are strongly magnetized, and that magnetic fields influence the formation of stars within them. A key result is that the orientation of their internal structure closely correlates to that of the magnetic field.

To understand the role of magnetic fields when stars are formed, an international research team led by Thushara Pillai, Boston University & Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, observed the filamentary network of the dense gas surrounding a young star cluster in the solar neighborhood, with the HAWC+ polarimeter on the airborne observatory SOFIA at infrared wavelengths. Their research shows that not all dense filaments are created equal. In some of the filaments, the magnetic field succumbs to the flow of matter and is pulled into alignment with the filament. Gravitational force takes over in the denser parts of some filaments and the resulting weakly magnetized gas flow can feed the growth of young stellar clusters like a conveyor belt. The results are published in this week’s issue of “Nature Astronomy“.

The interstellar medium is composed of tenuous gas and dust that fills the vast amount of emptiness between stars. Stretching across the Galaxy, this rather diffuse material happens to be a significant mass reservoir in Galaxies.  An important component of this interstellar gas are the cold and dense molecular clouds which hold most of their mass in the form of molecular hydrogen. A major finding in the last decade has been that an extensive network of filaments permeates every molecular cloud. A picture has emerged that stars like our own sun form preferentially in dense clusters at the intersections of these filaments. 

In order to understand the role of magnetic fields, the researchers observed the filamentary network of dense gas around the Serpens South Cluster with HAWC+, a polarization-sensitive far-infrared detector onboard the airborne observatory SOFIA, that is unique in the world. Located about 1,400 light-years away from us, the Serpens South cluster is the youngest known cluster in the local neighborhood at the center of a network of dense filaments.

SOFIA 2017 auf dem Vorfeld des Internationalen Flughafens Christchurch, Neuseeland. Bildrechte: NASA/SOFIA/T. Pillai/J. Kauffmann; NASA/JPL-Caltech/L. Allen

Composite image of the Serpens South Cluster. Magnetic fields observed by SOFIA are shown as streamlines over an image from the Spitzer Space Telescope. This indicates that gravity can overcome the magnetic fields at some point, to deliver material for new stars. The magnetic fields have been dragged into alignment with the most powerful flows, as seen in the lower left, where the streamlines are following the direction of the narrow, dark filament. This is accelerating the flow of material from interstellar space into the cloud, and fueling the collapse needed to spark star formation.
Credit: NASA/SOFIA/T. Pillai/J. Kauffmann; NASA/JPL-Caltech/L. Allen

The observations show that low–density gaseous filaments are aligned parallel to the magnetic field orientation, and that their alignment changes to being perpendicular at higher gas densities. The high angular resolution of HAWC+ reveals a further, previously unseen twist to the story. “In some dense filaments the magnetic field succumbs to the flow of matter and and is pulled into alignment with the filament”, says Thushara Pillai (Boston University and MPIfR Bonn), the first author of the publication.  “Gravitational force takes over in the more opaque parts of certain filaments in the Serpens Star Cluster and the resulting weakly magnetized gas flow can feed the growth of young stellar clusters like a conveyor belt”, she adds.

It is understood from theoretical simulations and observations that the filamentary nature of molecular clouds actually plays a major role in channeling mass from the larger interstellar medium into young stellar clusters whose growth is fed from the gas. The formation and evolution process of stars is expected to be driven by a complex interplay of several fundamental forces — namely turbulence, gravity, and the magnetic field. In order to get an accurate description for how dense clusters of stars form, astronomers need to pin down the relative role of these three forces. Turbulent gas motions as well as the mass content of filaments (and therefore gravitation force) can be gauged with relative ease. “However, the signature of the interstellar magnetic field is weak, also because it is about 10,000–times weaker than even our own Earth’s magnetic field”, says Bernhard Schulz, SOFIA Science Mission Operations Deputy Director of the University of Stuttgart. “This has made measurements of magnetic fields in filaments through far-infrared polarization observations such a formidable ability of our observatory.”

Small dust grains that are mixed into the interstellar gas make up a small fraction of a molecular cloud’s mass. These interstellar dust grains tend to align perpendicular to the direction of the magnetic field. As a result, the infrared light emitted by the dust grains is polarized — and this polarization can be used to chart the magnetic field directions in molecular clouds. The magnetic field directions in the polarization map of Serpens South observed with HAWC+ onboard SOFIA align well with the direction of the gas flow along the narrow southern filament. Hence, these observations support the idea that filamentary accretion flows can help form a young star cluster.

Recently, the Planck space mission produced a highly sensitive all–sky map of the polarized dust emission at wavelengths smaller than 1 mm. This provided the first large–scale view of the magnetization in filamentary molecular clouds and their environments. Studies done with Planck data found that filaments are not only highly magnetized, but they are coupled to the magnetic field in a predictable way. The orientation of the magnetic fields is parallel to the filaments in low–density environments. The magnetic fields change their orientation to being perpendicular to filaments at high gas densities, implying that magnetic fields play a similarly important role in shaping filaments, compared to the influence of turbulence and gravity.

This observation pointed towards a problem. Matter doesn’t flow perpendicular to magnetic field lines, so how does the gas contract further within the filaments to form stars? “Since the HAWC+ instrument onboard of our airborne Observatory SOFIA provides an angular resolution which is an order of magnitude higher in comparison to Planck, it was now possible to resolve the smaller regions in the filaments, where the balance between magnetic fields and the other forces changes.”, Bernhard Schulz explains.

"The fact that we were able to capture a critical transition in star formation was somewhat unexpected. This just shows how little is known about cosmic magnetic fields and how much exciting science awaits us from SOFIA with the HAWC+ receiver”, concludes Thushara Pillai.

 

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The High-resolution Airborne Wideband Camera Plus (HAWC+), SOFIA’s newest instrument, uses far-infrared light to observe celestial dust grains, which align perpendicular to magnetic field lines. From these results, astronomers can infer the shape and direction of the otherwise invisible magnetic field. Far-infrared light provides key information about magnetic fields because the signal is not contaminated by emission from other mechanisms, such as scattered visible light and radiation from high-energy particles. The HAWC+ instrument was developed and delivered to NASA by a multi-institution team led by the Jet Propulsion Laboratory in Pasadena, California.

SOFIA, the "Stratospheric Observatory for Infrared Astronomy" is a joint project of the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR; German Aerospace Center, grant: 50OK0901, 50OK1301 and 50OK1701) and the National Aeronautics and Space Administration (NASA). It is funded on behalf of DLR by the Federal Ministry for Economic Affairs and Energy based on legislation by the German Parliament, the State of Baden-Württemberg and the University of Stuttgart. Scientific operation for Germany is coordinated by the German SOFIA-Institute (DSI) of the University of Stuttgart, in the USA by the Universities Space Research Association (USRA).

Contakt:
Dr. Thushara Pillai
Institute for Astrophysical Research
Boston University, Boston, USA.
E-mail: tpillai.astro@gmail.com

Prof. Dr. Karl Menten
Direktor und Leiter der Forschungsabteilung „Millimeter- und Submillimeterastronomie“
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-297
E-mail: kmenten@mpifr-bonn.mpg.de

Dr. Norbert Junkes
Presse- und Öffentlichkeitsarbeit
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-399
E-mail: njunkes@mpifr-bonn.mpg.de

Dr. Dörte Mehlert
Presse- und Öffentlichkeitsarbeit
Deutsches SOFIA Institut, Universität Stuttgart
Fon: +49 711 685-69632
E-mail: mehlert@dsi.uni-stuttgart.de

Contact Dörte Mehlert, mehlert@dsi.uni-stuttgart.de
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