Understanding the Origin of the Magnetic Field Morphology in the Wide-binary Protostellar System BHR 71

Charles L H Hull,Tyler L Bourke,John J Tobin,Valentin J M Le Gouellec,Josep M Girart

doi:10.3847/1538-4357/ab5809

Abstract

Abstract We present 1.3 mm Atacama Large Millimeter/submillimeter Array observations of polarized dust emission toward the wide-binary protostellar system BHR 71 IRS1 and IRS2. IRS1 features what appears to be a natal, hourglass-shaped magnetic field. In contrast, IRS2 exhibits a magnetic field that has been affected by its bipolar outflow. Toward IRS2, the polarization is confined mainly to the outflow cavity walls. Along the northern edge of the redshifted outflow cavity of IRS2, the polarized emission is sandwiched between the outflow and a filament of cold, dense gas traced by N2D+, toward which no dust polarization is detected. This suggests that the origin of the enhanced polarization in IRS2 is the irradiation of the outflow cavity walls, which enables the alignment of dust grains with respect to the magnetic field—but only to a depth of ∼300 au, beyond which the dust is cold and unpolarized. However, in order to align grains deep enough in the cavity walls, and to produce the high polarization fraction seen in IRS2, the aligning photons are likely to be in the mid- to far-infrared range, which suggests a degree of grain growth beyond what is typically expected in very young, Class 0 sources. Finally, toward IRS1 we see a narrow, linear feature with a high (10%–20%) polarization fraction and a well-ordered magnetic field that is not associated with the bipolar outflow cavity. We speculate that this feature may be a magnetized accretion streamer; however, this has yet to be confirmed by kinematic observations of dense-gas tracers.

Highlights

Theories of magnetized star formation suggested that the formation of stars within molecular clouds should be regulated by a strong magnetic field (Mestel & Spitzer 1956; Shu et al 1987; McKee et al 1993; McKee & Ostriker 2007)
Jansky Very Large Array (VLA) observations of Class 0 and I protostellar cores by Segura-Cox et al (2018), where sources with larger disks tend to have misaligned magnetic fields and outflows. These results hint at a relationship between multiplicity and the magnetic field that can be further revealed by targeted Atacama Large Millimeter/submillimeter Array (ALMA) studies such as the one we present here, as well as future surveys of dust polarization toward sources whose multiplicity has already been determined, such as those observed as part of the VLA Nascent Disk and Multiplicity (VANDAM) survey (Tobin et al 2016b) using the VLA
By comparing by eye with the synthetic models of Q and U for an hourglass-shaped magnetic field morphology shown in Figure 4 of Frau et al (2011), we can constrain the inclination of the magnetic field in BHR 71 IRS1 with respect to the the plane of the sky

Summary

Introduction

Theories of magnetized star formation suggested that the formation of stars within molecular clouds should be regulated by a strong magnetic field (Mestel & Spitzer 1956; Shu et al 1987; McKee et al 1993; McKee & Ostriker 2007). The Combined Array for Research in Millimeter-wave Astronomy (CARMA), including the bright, deeply embedded Class 0 protostellar sources NGC 1333-IRAS 4A (Girart et al 1999, 2006), IRAS 16293A (Rao et al 2009), and L1157 (Stephens et al 2013) These sources exhibit other hallmarks of strong-field star formation, including powerful outflows (whose generation is intimately connected to the magnetic field; Frank et al 2014) and high (inferred) magnetic field strengths, on the order of a few milli-Gauss

Results

Discussion

Conclusion