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Register a new userDisk Formation in Magnetized Dense Cores with Turbulence and Ambipolar Diffusion
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Ka Ho Lam
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Disks are essential to the formation of both stars and planets, but how they form in magnetized molecular cloud cores remains debated. This work focuses on how the disk formation is affected by turbulence and ambipolar diffusion (AD), both separately and in combination, with an emphasis on the protostellar mass accretion phase of star formation. We find that a relatively strong, sonic turbulence on the core scale strongly warps but does not completely disrupt the well-known magnetically-induced flattened pseudodisk that dominates the inner protostellar accretion flow in the laminar case, in agreement with previous work. The turbulence enables the formation of a relatively large disk at early times with or without ambipolar diffusion, but such a disk remains strongly magnetized and does not persist to the end of our simulation unless a relatively strong ambipolar diffusion is also present. The AD-enabled disks in laminar simulations tend to fragment gravitationally. The disk fragmentation is suppressed by initial turbulence. The ambipolar diffusion facilitates the disk formation and survival by reducing the field strength in the circumstellar region through magnetic flux redistribution and by making the field lines there less pinched azimuthally, especially at late times. We conclude that turbulence and ambipolar diffusion complement each other in promoting disk formation. The disks formed in our simulations inherit a rather strong magnetic field from its parental core, with a typical plasma-$beta$ of order a few tens or smaller, which is 2-3 orders of magnitude lower than the values commonly adopted in MHD simulations of protoplanetary disks. To resolve this potential tension, longer-term simulations of disk formation and evolution with increasingly more realistic physics are needed.
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We investigate the formation and evolution of a first core, protostar, and circumstellar disc with a three-dimensional non-ideal (including both Ohmic and ambipolar diffusion) radiation magnetohydrodynamics simulation. We found that the magnetic flux is largely removed by magnetic diffusion in the first core phase and that the plasma $beta$ of the centre of the first core becomes large, $beta>10^4$. Thus, proper treatment of first core phase is crucial in investigating the formation of protostar and disc. On the other hand, in an ideal simulation, $betasim 10$ at the centre of the first core. The simulations with magnetic diffusion show that the circumstellar disc forms at almost the same time of protostar formation even with a relatively strong initial magnetic field (the value for the initial mass-to-flux ratio of the cloud core relative to the critical value is $mu=4$). The disc has a radius of $r sim 1$ AU at the protostar formation epoch. We confirm that the disc is rotationally supported. We also show that the disc is massive ($Qsim 1$) and that gravitational instability may play an important role in the subsequent disc evolution.
We discuss the effects of the magnetic field observed in molecular clouds on the process of star formation, concentrating on the phase of gravitational collapse of low-mass dense cores, cradles of sunlike stars. We summarize recent analytic work and numerical simulations showing that a substantial level of magnetic field diffusion at high densities has to occur in order to form rotationally supported disks. Furthermore, newly formed accretion disks are threaded by the magnetic field dragged from the parent core during the gravitational collapse. These disks are expected to rotate with a sub-Keplerian speed because they are partially supported by magnetic tension against the gravity of the central star. We discuss how sub-Keplerian rotation makes it difficult to eject disk winds and accelerates the process of planet migration. Moreover, magnetic fields modify the Toomre criterion for gravitational instability via two opposing effects: magnetic tension and pressure increase the disk local stability, but sub-Keplerian rotation makes the disk more unstable. In general, magnetized disks are more stable than their nonmagnetic counterparts; thus, they can be more massive and less prone to the formation of giant planets by gravitational instability.
We investigate the formation and early evolution and fragmentation of an accretion disk around a forming massive protostar. We use a grid-based self-gravity-radiation-hydrodynamics code including a sub-grid module for stellar and dust evolution. On purpose, we do not use sink particles to allow for all paths of fragment formation and destruction, but instead keeping the spatial grid resolution high enough to properly resolve the physical length scales of the problem. We use a 3D grid in spherical coordinates with a logarithmic scaling in the radial direction and cosine scaling in the polar direction. Because of that, roughly 25% of the total number of grid cells, corresponding to $sim$ 26 million grid cells, are used to model the disk physics. They constitute the highest resolution simulations performed up to now on disk fragmentation around a forming massive star with the physics considered here. We study the convergence of our results by performing the same simulation for 5 different resolutions. We start from the collapse of a molecular cloud; a massive (proto)star is formed in its center, surrounded by a fragmenting Keplerian-like accretion disk with spiral arms. The fragments have masses of $sim 1 M_odot$, and their continuous interactions with the disk, spiral arms and other fragments results in eccentric orbits. Fragments form hydrostatic cores, surrounded by secondary disks with spiral arms that also produce new fragments. We identified several mechanisms of fragment formation, interaction and destruction. Central temperatures of the fragments can reach the hydrogen dissociation limit, form second Larson cores and evolve into companion stars. Based on this, we study the multiplicity predicted by the simulations and find $sim 6$ companions at different distances from the primary: from possible spectroscopic multiples, to companions at distances between 1000 and 2000 au.
The first long-baseline ALMA campaign resolved the disk around the young star HL Tau into a number of axisymmetric bright and dark rings. Despite the very young age of HL Tau these structures have been interpreted as signatures for the presence of (proto)planets. The ALMA images triggered numerous theoretical studies based on disk-planet interactions, magnetically driven disk structures, and grain evolution. Of special interest are the inner parts of disks, where terrestrial planets are expected to form. However, the emission from these regions in HL Tau turned out to be optically thick at all ALMA wavelengths, preventing the derivation of surface density profiles and grain size distributions. Here, we present the most sensitive images of HL Tau obtained to date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a spatial resolution comparable to the ALMA images. At this long wavelength the dust emission from HL Tau is optically thin, allowing a comprehensive study of the inner disk. We obtain a total disk dust mass of 0.001 - 0.003 Msun, depending on the assumed opacity and disk temperature. Our optically thin data also indicate fast grain growth, fragmentation, and formation of dense clumps in the inner densest parts of the disk. Our results suggest that the HL Tau disk may be actually in a very early stage of planetary formation, with planets not already formed in the gaps but in the process of future formation in the bright rings.
(Abridged) Context. Massive stars form in magnetized and turbulent environments, and are often located in stellar clusters. Their accretion mechanism, as well as the origin of their systems stellar multiplicity are poorly understood. Aims. We study the influence of both magnetic fields and turbulence on the accretion mechanism of massive protostars and their multiplicity. Methods. We present a series of four Radiation-MHD simulations of the collapse of a massive magnetized, turbulent core of 100 $M_odot$ with the AMR code Ramses, including a hybrid radiative transfer method for stellar irradiation and ambipolar diffusion. We vary the Mach and Alfvenic Mach numbers to probe sub- and superalfvenic turbulence as well as sub- and supersonic turbulence regimes. Results. Subalfvenic turbulence leads to single stellar systems while superalfvenic turbulence leads to binary formation from disk fragmentation following spiral arm collision, with mass ratios of 1.1-1.6. In those runs, infalling gas reaches the individual disks via a transient circumbinary structure. Magnetically-regulated, thermally-dominated (plasma beta $beta>1$), Keplerian disks form in all runs, with sizes 100-200 AU and masses 1-8 $M_odot$. The disks around primary and secondary sink particles share similar properties. We observe higher accretion rates onto the secondary stars than onto their primary star companion. The primary disk orientation is found to be set by the initial angular momentum carried by turbulence. Conclusions. Small (300 AU) massive protostellar disks as those frequently observed nowadays can only be reproduced so far in the presence of (moderate) magnetic fields with ambipolar diffusion, even in a turbulent medium. The interplay between magnetic fields and turbulence sets the multiplicity of stellar clusters. A plasma beta $beta>1$ is a good indicator of streamers and disks.
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