title: Spatially resolved magnetic field structure in the disk of a T Tauri star.
abstract: Magnetic fields in accretion disks play a dominant part during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars and possibly in the innermost part of their accretion disks, but the strength and morphology of the field in the bulk of a disk have not been observed. Spatially unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicularly to the field) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal or a mixture of the two, which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau (ref. 11). Here we report resolved measurements of the polarized 1.25-millimetre continuum emission from the disk of HL Tau. The magnetic field on a scale of 80 astronomical units is coincident with the major axis (about 210 astronomical units long) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field also does not fit the data well. The unexpected morphology suggests that the role of the magnetic field in the accretion of a T Tauri star is more complex than our current theoretical understanding.
authors.affiliation: 1] Institute for Astrophysical Research, Boston University, Boston, Massachusetts 02215, USA [2] Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Ian W
authors.initials: IW
authors.lastname: Stephens
authors.affiliation: Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Leslie W
authors.initials: LW
authors.lastname: Looney
authors.affiliation: SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands.
authors.forename: Woojin
authors.initials: W
authors.lastname: Kwon
authors.affiliation: 1] Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA [2] Instituto Argentino de Radioastronomía, CCT-La Plata (CONICET), CC5, 1894 Villa Elisa, Argentina.
authors.forename: Manuel
authors.initials: M
authors.lastname: Fernández-López
authors.affiliation: Van Vleck Observatory, Astronomy Department, Wesleyan University, Middletown, Connecticut 06459, USA.
authors.forename: A Meredith
authors.initials: AM
authors.lastname: Hughes
authors.affiliation: Astronomy Department and Laboratory for Millimeter-wave Astronomy, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Lee G
authors.initials: LG
authors.lastname: Mundy
authors.affiliation: Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Richard M
authors.initials: RM
authors.lastname: Crutcher
authors.affiliation: Astronomy Department, University of Virginia, Charlottesville, Virginia 22904, USA.
authors.forename: Zhi-Yun
authors.initials: ZY
authors.lastname: Li
authors.affiliation: Institute of Astronomy and Astrophysics, Academia Sinica, Hilo, Hawaii 96720, USA.
authors.forename: Ramprasad
authors.initials: R
authors.lastname: Rao