We report on an Atacama Large Millimeter/submillimeter Array study of the Class I or II intermediate-mass protostar DK Cha in the Chamaeleon II region. The 12CO(J = 2–1) images have an angular resolution of ∼1″ (∼250 au) and show high-velocity blueshifted (≳70 km s−1) and redshifted (≳50 km s−1) emissions, which have 3000 au scale crescent-shaped structures around the protostellar disk traced in the 1.3 mm continuum. Because the high-velocity components of the CO emission are associated with the protostar, we concluded that the emission traces the pole-on outflow. The blueshifted outflow lobe has a clear layered velocity gradient with a higher-velocity component located on the inner side of the crescent shape, which can be explained by a model of an outflow with a higher velocity in the inner radii. Based on the directly driven outflow scenario, we estimated the driving radii from the observed outflow velocities and found that the driving region extends over 2 orders of magnitude. The 13CO emission traces a complex envelope structure with arc-like substructures with lengths of ∼1000 au. We identified the arc-like structures as streamers because they appear to be connected to a rotating infalling envelope. DK Cha is useful for understanding characteristics that are visible by looking at nearly face-on configurations of young protostellar systems, providing an alternative perspective for studying the star formation process.
Combining numerical simulations and analytical modeling, we investigate whether close binary systems form by the effect of magnetic braking. Using magnetohydrodynamics simulations, we calculate the cloud evolution with a sink, for which we do not resolve the binary system or binary orbital motion to realize long-term time integration. Then, we analytically estimate the binary separation using the accreted mass and angular momentum obtained from the simulation. In unmagnetized clouds, wide binary systems with separations of >100 au form, in which the binary separation continues to increase during the main accretion phase. In contrast, close binary systems with separations of <100 au can form in magnetized clouds. Since the efficiency of magnetic braking strongly depends on both the strength and configuration of the magnetic field, they also affect the formation conditions of a close binary. In addition, the protostellar outflow has a negative impact on close binary formation, especially when the rotation axis of the prestellar cloud is aligned with the global magnetic field. The outflow interrupts the accretion of gas with small angular momentum, which is expelled from the cloud, while gas with large angular momentum preferentially falls from the side of the outflow on to the binary system and widens the binary separation. This study shows that a cloud with a magnetic field that is not parallel to the rotation axis is a favorable environment for the formation of close binary systems.
Protostellar outflows are one of the most outstanding features of star formation. Observational studies over the last several decades have successfully demonstrated that outflows are ubiquitously associated with low- and high-mass protostars in solar-metallicity Galactic conditions. However, the environmental dependence of protostellar outflow properties is still poorly understood, particularly in the low-metallicity regime. Here we report the first detection of a molecular outflow in the Small Magellanic Cloud with 0.2 Z ⊙, using Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of 0.1 pc toward the massive protostar Y246. The bipolar outflow is nicely illustrated by high-velocity wings of CO(3–2) emission at ≳15 km s−1. The evaluated properties of the outflow (momentum, mechanical force, etc.) are consistent with those of the Galactic counterparts. Our results suggest that the molecular outflows, i.e., the guidepost of the disk accretion at the small scale, might be universally associated with protostars across the metallicity range of ∼0.2–1 Z ⊙.
Understanding the initial conditions of star formation requires both observational studies and theoretical works taking into account the magnetic field, which plays an important role in star formation processes. Herein, we study the young nearby dense cloud core L1521 F [n(H2) ∼104−6 cm−3] in the Taurus Molecular Cloud. This dense core hosts a 0.2 M⊙ protostar, categorized as a very low luminosity object with complex velocity structures, particularly in the vicinity of the protostar. To trace the magnetic field within the dense core, we conducted high-sensitivity submillimeter polarimetry of the dust continuum at λ = 850 μm and 450 μm using the POL-2 polarimeter situated in front of the SCUBA-2 submillimeter bolometer camera on the James Clerk Maxwell Telescope. This was compared with millimeter polarimetry taken at λ = 3.3 mm with ALMA. The magnetic field was detected at λ = 850 μm in the peripheral region, which is threaded in a north–south direction, while the central region traced at λ = 450 μm shows a magnetic field with an east–west direction, i.e., orthogonal to that of the peripheral region. Magnetic field strengths are estimated to be ∼70 μG and 200 μG in the peripheral and central regions, respectively, using the Davis–Chandrasekhar–Fermi method. The resulting mass-to-flux ratio of three times larger than that of magnetically critical state for both regions indicates that L 1521 F is magnetically supercritical, i.e., gravitational forces dominate over magnetic turbulence forces. Combining observational data with magnetohydrodynamic simulations, detailed parameters of the morphological properties of this puzzling object are derived for the first time.
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