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.

The Circumestellar Disk of the B0 Protostar Powering the HH 80-81 Radio Jet

“…The MM1 upper limit is significantly lower than the typical values found in millimeter interferometric observations of dense cores around low and high mass star forming regions (∼2-10%, e.g. Girart et al 1999;Lai et al 2003;Girart et al 2006Girart et al , 2009Hull et al 2014;Zhang et al 2014), but it is not far from the values found in disks around lowmass young stellar objects (Hughes et al 2013;Rao et al 2014;Stephens et al 2014;Kataoka et al 2016). …”

Scattering-produced (sub)millimetre polarization in inclined discs: optical depth effects, near–far side asymmetry and dust settling

“…For example, there is no clear indication of a near-far side asymmetry in the polarized intensity of the HL Tau (Class I/II) disk from the CARMA observations at 1.3 mm (Stephens et al 2014), although the spatial resolution is rather limited. The lack of asymmetry is to be expected if the large grains are indeed settled to the mid-plane, as inferred from the shape of the gaps on the disk (Pinte et al 2016).…”

“…The polarization fraction for emission from aligned grains in the optically thin limit, p0, depends on many factors, including grain shape, composition and alignment efficiency. It can be quite high on parsec scales or larger (up to ∼ 20%; Planck Collaboration et al 2015), although the (sub)millimeter polarization fraction detected on the scale of protoplanetary disks is typically on the order of a few percent or less (e.g., Stephens et al 2014). For illustration purposes, we adopt p0 = 3% in Fig.…”

“…Initial calculations of disk polarization from aligned grains assume a purely toroidal magnetic field (Cho & Lazarian 2007;Bertrang & Wolf 2017), as expected in a weakly magnetized disk (Fromang 2013). Such a config-uration would produce a polarization pattern that appears inconsistent with the pattern observed in HL Tau (Stephens et al 2014). The apparent inconsistency led Yang et al (2016a) to propose that the disk polarization in HL Tau comes from dust scattering, based on Kataoka et al (2015) theory of scattering-induced millimeter polarization (see also Kataoka et al 2016a).…”

“…The initial searches for disk polarization yielded only upper limits (Hughes et al 2009(Hughes et al , 2013. These were soon followed by detections in IRAS 16293B (Rao et al 2014) using the Submillimeter Array (SMA), in HL Tau (Stephens et al 2014), L1527 (Segura- Cox et al 2015), and Cepheus A HW2 (Fernández-López et al 2016) using the Combined Array for Research in Millimeter-wave Astronomy (CARMA), and in NGC 1333 IRAS 4A at 8 mm and 1 cm Liu et al 2016) using the Karl G. Jansky Very Large Array (VLA). Most excitingly, there are a large number of approved disk polarization programs using Atacama Large Millimeter/submillimeter Array (ALMA), with some results already published (Kataoka et al 2016b) and several more in preparation (e.g., J. M. Girart, in prep).…”