FM04: Magnetic Fields Along the Star-Formation Sequence

Line segments indicate the orientations of magnetic fields in the Eagle Nebula’s “Pillars of Creation,” structures made famous by the underlying image from the Hubble Space Telescope. The pillars consist of cold, dense dust and gas and have stars forming at their tips. The magnetic fields that thread the pillars are oriented differently from the fields surrounding them, suggesting that the columns are shaped by magnetic fields and that stars form by the collapse of clumps of magnetically slowed gas (University of Central Lancashire).

A magnetic field map of the Vela C star-forming region made with the BLASTPol polarimeter aboard the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) at 500 microns. The background image shows the light emitted by the dust grains, revealing the structure of the cloud. The “streaks” overlaid on the image show the orientation of the magnetic field inferred from the BLASTPol data. (Northwestern University)

Magnetic fields are thought to play important roles in star formation, in particular in shaping structures in the interstellar medium (ISM), overcoming the angular momentum problem and regulating the evolution of protoplanetary disks and their young host stars.

Starting with a subtle interplay among gravity, turbulence and magnetic fields during the formation of molecular clouds, the complexity of the star-formation process is rooted in the conditions of the gas and the effectiveness of processes that remove angular momentum and magnetic flux — not just at the onset of gravitational instability, but all along the road to the zero-age main sequence (ZAMS). During the ~1 million years needed to form a star, gravity has to overcome two main barriers to transform a dense core into a star: the core’s angular momentum and its magnetic flux. The outcome of the collapse (manifested in the core mass function, stellar mass function, occurrence and properties of protoplanetary disks, stellar multiplicity etc.) depends critically on how and when these two barriers are surmounted. While the gas dynamics at most of the relevant scales (0.1 to 10 000 astronomical units) is now accessible through observations, a thorough understanding of star formation will not be achieved until we have characterized the role of the magnetic field across all of the relevant spatial scales and time scales.

Polarimetry from optical to centimeter wavelengths has been the most powerful observing technique to study magnetic fields. High-sensitivity, high-resolution polarization observations by a variety of facilities (ALMA; Planck; BLASTPol and BLAST-TNG; HAWC+ on SOFIA; POL-2 on the JCMT; NIKA 2 on the IRAM 30 m antenna; PolKa at APEX; TolTEC at the LMT; CanariCam at the GTC; SPIRou on the CFHT; CRIRES+ on the VLT; PEPSI at the LBT; Mimir and others) are ushering in a new era in the study of polarized light, allowing us to uncover the properties of magnetic fields over a broad range of wavelengths and physical scales. In FM4 we are bringing together communities working with polarimetric observations of the various stages and objects along the star-formation sequence, from the structure of star-forming molecular clouds to the arrival of young stars on the ZAMS.

Our goal in FM4 is to discuss how to compare observations of magnetic fields at different evolutionary stages and physical scales such that we can establish a coherent view of their key roles in the multi-scale process of star formation. Combining results from different fields is complex, and thus we wish to emphasize how different measurements of magnetic fields can be compared and combined despite widely varying observing techniques applied to different objects and physical scales. FM4 features three main sessions: Observations and models of (i) magnetic fields in star-forming clouds, (ii) magnetic fields during the main accretion phase, and (iii) magnetic fields in protoplanetary disks and pre-main-sequence (PMS) stars. We will dedicate the last session to a general discussion on how to combine diagnostics to make robust estimates of magnetic fields both in a given object and across scales, as well as how to improve cross-pollination among communities studying cosmic magnetic fields at different scales and using different techniques.

We expect our Focus Meeting to be lively because of the imminent development of polarimetric capabilities at a wide range of observational facilities and the need to establish cross-disciplinary collaborations transcending historical barriers that have arisen due to different observational techniques.

We invite all IAU GA30 attendees to join us and participate in building new perspectives on magnetic field research from the scales of the ISM to the stars. We look forward to building bridges between polarised communities!

ANAËLLE MAURY works in the Astrophysics Department at the Alternative Energies and Atomic Energy Commission (CEA) in France, mostly investigating the small-scale properties of the youngest solar-type protostars in the Milky Way.

SWETLANA HUBRIG was a European Southern Observatory (ESO) staff astronomer on Cerro Paranal in Chile and is now at the Leibniz-Institute for Astrophysics Potsdam (AIP), Germany. Her research interests include stellar magnetism and high-resolution spectroscopy.

CHAT HULL is a Fellow of the National Astronomical Observatory of Japan (NAOJ) and is based at the Joint ALMA Observatory in Santiago, Chile. His main scientific interests are star formation, polarisation, magnetic fields and radio instrumentation and calibration.