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The magnetic field structure of a star-forming Bok globule BHR 71 was determined based on near-infrared polarimetric observations of background stars. The magnetic field in BHR 71 was mapped from 25 stars. By using a simple 2D parabolic function, the plane-of-sky magnetic axis of the core was found to be $theta_{rm mag} = 125^{circ} pm 11^{circ}$. The plane-of-sky mean magnetic field strength of BHR 71 was found to be $B_{rm pos} = 8.8 - 15.0$ $mu$G, indicating that the BHR 71 core is magnetically supercritical with $lambda = 1.44 - 2.43$. Taking into account the effect of thermal/turbulent pressure and the plane-of-sky magnetic field component, the critical mass of BHR 71 was $M_{rm cr} = 14.5-18.7$ M$_{odot}$, which is consistent with the observed core mass of $M_{rm core} approx 14.7$ M$_{odot}$ (Yang et al. 2017). We conclude that BHR 71 is in a condition close to a kinematically critical state, and the magnetic field direction lies close to the plane of sky. Since BHR 71 is a star-forming core, a significantly subcritical condition (i.e., the magnetic field direction deviating from the plane of sky) is unlikely, and collapsed from a condition close to a kinematically critical state. There are two possible scenarios to explain the curved magnetic fields of BHR 71, one is an hourglass-like field structure due to mass accumulation and the other is the Inoue & Fukui (2013) mechanism, which proposes the interaction of the core with a shock wave to create curved magnetic fields wrapping around the core.
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BHR 71 is a well isolated Bok globule located at ~200 pc, which harbours a highly collimated bipolar outflow. The outflow is driven by a very young Class 0 protostar with a luminosity of ~9 L_sun. It is one of a very small number that show enhanced abundances of a number of molecular species, notably SiO and CH3OH, due to shock processing of the ambient medium. In this paper the properties of the globule and outflow are discussed.
The magnetic field structure, kinematical stability, and evolutionary status of the starless dense core Barnard 68 (B68) are revealed based on the near-infrared polarimetric observations of background stars, measuring the dichroically polarized light produced by aligned dust grains in the core. After subtracting unrelated ambient polarization components, the magnetic fields pervading B68 are mapped using 38 stars and axisymmetrically distorted hourglass-like magnetic fields are obtained, although the evidence for the hourglass field is not very strong. On the basis of simple 2D and 3D magnetic field modeling, the magnetic inclination angles on the plane-of-sky and in the line-of-sight direction are determined to be $47^{circ} pm 5^{circ}$ and $20^{circ} pm 10^{circ}$, respectively. The total magnetic field strength of B68 is obtained to be $26.1 pm 8.7$ $mu {rm G}$. The critical mass of B68, evaluated using both magnetic and thermal/turbulent support, is $M_{rm cr} = 2.30 pm 0.20$ ${rm M}_{odot}$, which is consistent with the observed core mass of $M_{rm core}=2.1$ M$_{odot}$, suggesting nearly critical state. We found a relatively linear relationship between polarization and extinction up to $A_V sim 30$ mag toward the stars with deepest obscuration. Further theoretical and observational studies are required to explain the dust alignment in cold and dense regions in the core.
In this study, the detailed magnetic field structure of the dense protostellar core Barnard 335 (B335) was revealed based on near-infrared polarimetric observations of background stars to measure dichroically polarized light produced by magnetically aligned dust grains in the core. Magnetic fields pervading B335 were mapped using 24 stars after subtracting unrelated ambient polarization components, for the first time revealing that they have an axisymmetrically distorted hourglass-shaped structure toward the protostellar core. On the basis of simple two- and three-dimensional magnetic field modeling, magnetic inclination angles in the plane-of-sky and line-of-sight directions were determined to be $90^{circ} pm 7^{circ}$ and $50^{circ} pm 10^{circ}$, respectively. The total magnetic field strength of B335 was determined to be $30.2 pm 17.7$ $mu {rm G}$. The critical mass of B335, evaluated using both magnetic and thermal/turbulent support against collapse, was determined to be $M_{rm cr} = 3.37 pm 0.94$ ${rm M}_{odot}$, which is identical to the observed core mass of $M_{rm core}=3.67$ M$_{odot}$. We thus concluded that B335 started its contraction from a condition near equilibrium. We found a linear relationship in the polarization versus extinction diagram, up to $A_V sim 15$ mag toward the stars with the greatest obscuration, which verified that our observations and analysis provide an accurate depiction of the core.
We present a characterization of the binary protostar system that is forming within a dense core in the isolated dark cloud BHR71. The pair of protostars, IRS1 and IRS2, are both in the Class 0 phase, determined from observations that resolve the sources from 1 um out to 250 um and from 1.3 mm to 1.3cm. The resolved observations enable the luminosities of IRS1 and IRS2 to be independently measured (14.7 and 1.7L_sun, respectively), in addition to the bolometric temperatures 68~K, and 38~K, respectively. The surrounding core was mapped in NH3 (1,1) with the Parkes radio telescope, and followed with higher-resolution observations from ATCA in NH3 (1,1) and 1.3cm continuum. The protostars were then further characterized with ALMA observations in the 1.3~mm continuum along with N2D+ (J=3-2), 12CO, 13CO, and C18O (J=2-1) molecular lines. The Parkes observations find evidence for a velocity gradient across the core surrounding the two protostars, while ATCA reveals more complex velocity structure toward the protostars within the large-scale gradient. The ALMA observations then reveal that the two protostars are at the same velocity in C18O, and N2H+ exhibits a similar velocity structure as NH3. However, the C18O kinematics reveal that the rotation on scales $<$1000~AU around IRS1 and IRS2 are in opposite directions. Taken with the lack of a systematic velocity difference between the pair, it is unlikely that their formation resulted from rotational fragmentation. We instead conclude that the binary system most likely formed via turbulent fragmentation of the core.
We present 1.3 mm ALMA observations of polarized dust emission toward the wide-binary protostellar system BHR 71 IRS1 and IRS2. IRS1 features what appears to be a natal, hourglass-shaped magnetic field. In contrast, IRS2 exhibits a magnetic field that has been affected by its bipolar outflow. Toward IRS2, the polarization is confined mainly to the outflow cavity walls. Along the northern edge of the redshifted outflow cavity of IRS2, the polarized emission is sandwiched between the outflow and a filament of cold, dense gas traced by N$_2$D$^+$, 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.
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