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Register a new userJCMT POL-2 and ALMA polarimetric observations of 6000-100 au scales in the protostar B335: linking magnetic field and gas kinematics in observations and MHD simulations
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We present our analysis of the magnetic field structures from 6000 au to 100 au scales in the Class 0 protostar B335 inferred from our JCMT POL-2 observations and the ALMA archival polarimetric data. To interpret the observational results, we perform a series of (non-)ideal MHD simulations of the collapse of a rotating non-turbulent dense core, whose initial conditions are adopted to be the same as observed in B335, and generate synthetic polarization maps. The comparison of our JCMT and simulation results suggests that the magnetic field on a 6000 au scale in B335 is pinched and well aligned with the bipolar outflow along the east-west direction. Among all our simulations, the ALMA polarimetric results are best explained with weak magnetic field models having an initial mass-to-flux ratio of 9.6. However, we find that with the weak magnetic field, the rotational velocity on a 100 au scale and the disk size in our simulations are larger than the observational estimates by a factor of several. An independent comparison of our simulations and the gas kinematics in B335 observed with the SMA and ALMA favors strong magnetic field models with an initial mass-to-flux ratio smaller than 4.8. We discuss two possibilities resulting in the different magnetic field strengths inferred from the polarimetric and molecular-line observations, (1) overestimated rotational-to-gravitational energy in B335 and (2) additional contributions in the polarized intensity due to scattering on a 100 au scale.
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Context. The relationship between outflow launching and formation of accretion disks around young stellar objects is still not entirely understood, which is why spectrally and spatially resolved observations are needed. Recently, the Atacama Large Millimetre/sub-millimetre Array (ALMA) has carried out long-baseline observations towards a handful of sources, revealing connections between outflows and the inner regions of disks. Aims. Here we aim to determine the small-scale kinematic and morphological properties of the outflow from the isolated protostar B335 for which no Keplerian disk has, so far, been observed on scales down to 10 au. Methods. We use ALMA in its longest-baseline configuration to observe emission from CO isotopologs, SiO, SO$_2$ and CH$_3$OH. The proximity of B335 provides a resolution of ~3 au (0.03). We also combine our long-baseline data with archival data to produce a high-fidelity image covering scales up to 700 au (7). Results. $^{12}$CO has a X-shaped morphology with arms ~50 au in width that we associate with the walls of an outflow cavity, similar to what is observed on larger scales. Long-baseline continuum emission is confined to <7 au of the protostar, while short-baseline continuum emission follows the $^{12}$CO outflow and cavity walls. Methanol is detected within ~30 au of the protostar. SiO is also detected in the vicinity of the protostar, but extended along the outflow. Conclusions. The $^{12}$CO outflow shows no clear signs of rotation at distances $gtrsim$30 au from the protostar. SiO traces the protostellar jet on small scales, but without obvious rotation. CH$_3$OH and SO$_2$ trace a region <16 au in diameter, centred on the continuum peak, which is clearly rotating. Using episodic, high-velocity, $^{12}$CO features, we estimate the launching radius of the outflow to be <0.1 au and dynamical timescales on the order of a few years.
Using ALMA observations of the C$^{18}$O(2-1) line emission of the gas envelope of protostar L1527, we have reconstructed its morphology and kinematics under the assumption of axisymmetry about the west-east axis. The main original contribution to our understanding of the formation process of L1527 is the presentation of a simple 3D parameterisation based solely on regions that are not dominated by absorption. In the explored range ($sim$0.7 to 5 arcsec from the star) the model reproduces observations better than earlier attempts. The main results include: a measurement of the rotation velocity that confirms its evolution to Keplerian toward short distances; a measurement of the mean in-fall velocity, 0.43$pm$0.10 kms$^{-1}$, lower than free fall velocity, with no evidence for the significant $r$-dependence suggested by an earlier analysis; a measurement of the central mass, 0.23$pm$0.06 M$_{odot}$ within a distance of 1.5 arcsec from the star, in agreement with earlier estimates obtained from a different range of distances; evidence for a strong disc plane depression of the in-falling flux resulting in an $X$ shaped flow possibly caused by the freeze-out of CO molecules on dust grains; a measurement of the accretion rate, 3.5$pm$1.0 10$^{-7}$ M$_{odot}$yr$^{-1}$ at a distance of 1 arcsec (140 au) from the star; evidence for a 10$^circ$ tilt of the symmetry plane of the envelope about the line of sight, cancelling below $sim$3 arcsec from the star, but matching infrared observations and being also apparent on the sky map of the mean Doppler velocity.
We present 850$mu$m polarization observations of the L1689 molecular cloud, part of the nearby Ophiuchus molecular cloud complex, taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We observe three regions of L1689: the clump L1689N which houses the IRAS 16293-2422 protostellar system, the starless clump SMM-16, and the starless core L1689B. We use the Davis-Chandrasekhar-Fermi method to estimate plane-of-sky field strengths of $366pm 55$ $mu$G in L1689N, $284pm 34$ $mu$G in SMM-16, and $72pm 33$ $mu$G in L1689B, for our fiducial value of dust opacity. These values indicate that all three regions are likely to be magnetically trans-critical with sub-Alfv{e}nic turbulence. In all three regions, the inferred mean magnetic field direction is approximately perpendicular to the local filament direction identified in $Herschel$ Space Telescope observations. The core-scale field morphologies for L1689N and L1689B are consistent with the cloud-scale field morphology measured by the $Planck$ Space Observatory, suggesting that material can flow freely from large to small scales for these sources. Based on these magnetic field measurements, we posit that accretion from the cloud onto L1689N and L1689B may be magnetically regulated. However, in SMM-16, the clump-scale field is nearly perpendicular to the field seen on cloud scales by $Planck$, suggesting that it may be unable to efficiently accrete further material from its surroundings.
In the disk-mediated accretion scenario for the formation of the most massive stars, gravitational instabilities in the disk can force it to fragment. We investigate the effects of inclination and spatial resolution on observable kinematics and stability of disks in high-mass star formation. We study a high-resolution 3D radiation-hydrodynamic simulation that leads to the fragmentation of a massive disk. Using RADMC-3D we produce 1.3 mm continuum and CH3CN line cubes at different inclinations. The model is set to different distances and synthetic observations are created for ALMA at ~80 mas resolution and NOEMA at ~0.3. The synthetic ALMA observations resolve all fragments and their kinematics well. The synthetic NOEMA observations at 800 pc (~300 au resolution) are able to resolve the fragments, while at 2000 pc (~800 au resolution) only a single slightly elongated structure is observed. The position-velocity (PV) plots show the differential rotation of material best in the edge-on views. As the observations become less resolved, the inner high-velocity components of the disk become blended with the envelope and the PV plots resemble rigid-body-like rotation. Protostellar mass estimates from PV plots of poorly resolved observations are therefore overestimated. We fit the emission of CH3CN lines and produce maps of gas temperature with values in the range of 100-300 K. Studying the Toomre stability of the disks in the resolved observations, we find Q values below the critical value for stability against gravitational collapse at the positions of the fragments and the arms connecting the fragments. For the poorly resolved observations we find low Q values in the outskirts of the disk. Therefore we are able to predict that the disk is unstable and fragmenting even in poorly resolved observations. This conclusion is true regardless of knowledge about the inclination of the disk.
Observations of the isolated globule B335 with ALMA have yielded absorption features against the continuum that are redshifted from the systemic velocity in both HCN and HCO$^+$ lines. These features provide unambiguous evidence for infall toward a central luminosity source. Previously developed models of inside-out collapse can match the observed line profiles of HCN and HCO$^+$ averaged over the central 50 AU. At the new distance of 100 pc, the inferred infall radius is 0.012 pc, the mass infall rate is $3 times 10^{-6}$ solar masses per year, the age is 50,000 years, and the accumulated mass in the central zone is 0.15 solar masses, most of which must be in the star or in parts of a disk that are opaque at 0.8 mm. The continuum detection indicates an optically thin mass (gas and dust) of only $7.5times 10^{-4}$ solar masses in the central region, consistent with only a very small disk mass.
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