In the context of star and planet formation, understanding the formation of disks is of fundamental importance. Previous studies found that the magnetic field has a very strong impact on the collapse of a prestellar cloud, particularly in possibly suppressing the formation of a disk even for relatively modest values of the magnetic intensity. Since observations infer that cores have a substantial level of magnetization, this raises the question of how disks form. However, most studies have been restricted to the case in which the initial angle, $α$, between the magnetic field and the rotation axis equals 0$^\circ$. We explore and analyse the influence of non aligned configurations on disk formation. We perform 3D ideal MHD, AMR numerical simulations for various values of $μ$, the ratio of the mass-to-flux to the critical mass-to-flux, and various values of $α$. We find that disks form more easily as $α$ increases from 0 to 90$^\circ$. We propose that as the magnetized pseudo-disks become thicker with increasing $α$, the magnetic braking efficiency is lowered. We also find that even small values of $α$ ($\simeq$ 10-20$^\circ$) show significant differences with the alligned case. Within the framework of ideal MHD and for our choice of initial conditions, centrifugally supported disks cannot form for values of $μ$ smaller than $\simeq$3, if the magnetic field and the rotation axis are perpendicular, and smaller than about $\simeq$5-10 when they are perfectly aligned.
B-field Orion Protostellar Survey (BOPS). IV: The Relative Orientation Between Magnetic Fields and Density Structures in Young Protostellar Envelopes
Guangxi University（广西大学）；Institut de Ciències de l’Espai (ICE-CSIC)（空间科学研究所）；Korea Astronomy and Space Science Institute（韩国天体空间科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究学院）；Worcester State University（伍斯特州立大学）
The evolution of protostellar outflows is investigated under different mass accretion rates in the range $\sim10^{-5}-10^{-2} {\rm M}_\odot$ yr$^{-1}$ with three-dimensional magnetohydrodynamic simulations. A powerful outflow always appears in strongly magnetized clouds with $B_0 \gtrsim B_{\rm 0, cr}$ $=10^{-4} (M_{\rm cl}/100 {\rm M}_\odot){\rm G}$, where $M_{\rm cl}$ is the cloud mass. When a cloud has a weaker magnetic field, the outflow does not evolve promptly with a high mass accretion rate. In some cases with moderate magnetic fields $B_0$ slightly smaller than $B_{\rm 0,cr}$, the outf…
We investigate collapse of magnetic protostellar clouds of mass $10$ and $1 M_{\odot}$. The collapse is simulated numerically using the two-dimensional magnetohydrodynamic (MHD) code `Enlil'. The simulations show that protostellar clouds acquire a hierarchical structure by the end of the isothermal stage of collapse. Under the action of the electromagnetic force, the protostellar cloud takes the form of an oblate envelope with the half-thickness to radius ratio $Z/R \sim (0.20-0.95)$. A geometrically and optically thin primary disk with radius $0.2-0.7 R_0$ and $Z/R \sim (10^{-2}-10^{-1})$ for…
The degree of coupling between the gas and the magnetic field during the collapse of a core and the subsequent formation of a disk depends on the assumed dust size distribution. We study the impact of grain-grain coagulation on the evolution of magnetohydrodynamic (MHD) resistivities during the collapse of a prestellar core. We use a 1-D model to follow the evolution of the dust size distribution, out-of-equilibrium ionization state and gas chemistry during the collapse of a prestellar core. To compute the grain-grain collisional rate, we consider models for both random and systematic, size-de…
Protostellar disks in their natural habitat -- the formation of protostars and their accretion disks in the turbulent and magnetized interstellar medium
Using three-dimensional magnetohydrodynamics simulations, the driving of protostellar jets is investigated in different star-forming cores with the parameters of magnetic field strength and mass accretion rate. Powerful high-velocity jets appear in strongly magnetized clouds when the mass accretion rate onto the protostellar system is lower than $\dot{M} \lesssim 10^{-3}\,{\rm M}_\odot$ yr$^{-1}$. On the other hand, even at this mass accretion rate range, no jets appear for magnetic fields of prestellar clouds as weak as $μ_0 \gtrsim 5$--$10$, where $μ_0$ is the mass-to-flux ratio normalized b…
Through the magnetic braking and the launching of protostellar outflows, magnetic fields play a major role in the regulation of angular momentum in star formation, which directly impacts the formation and evolution of protoplanetary disks and binary systems. The aim of this paper is to quantify those phenomena in the presence of non-ideal magnetohydrodynamics effects, namely the Ohmic and ambipola r diffusion. We perform three-dimensional simulations of protostellar collapses varying the mass of the prestellar dense core, the thermal support (the $α$ ratio) and the dust grain size-distribu tio…
We model the coagulation and fragmentation of dust grains during the protostellar collapse with our newly developed shark code. It solves the gas-dust hydrodynamics in a spherical geometry and the coagulation/fragmentation equation. It also computes the ionization state of the cloud and the Ohmic, ambipolar and Hall resistivities. We find that the dust size distribution evolves significantly during the collapse, large grain formation being controlled by the turbulent differential velocity. When turbulence is included, only ambipolar diffusion remains efficient at removing the small grains from…
Dust grains influence many aspects of star formation, including planet formation, opacities for radiative transfer, chemistry, and the magnetic field via Ohmic, Hall, and ambipolar diffusion. The size distribution of the dust grains is the primary characteristic influencing all these aspects. Grain size increases by coagulation throughout the star formation process. We describe here numerical simulations of protostellar collapse using methods described in earlier papers of this series. We compute the evolution of the grain size distribution from coagulation and the non-ideal magnetohydrodynami…
The role of the magnetic field during protostellar collapse is poorly constrained from an observational point of view, although it could be significant if we believe state-of-the-art models of protostellar formation. We present polarimetric observations of the 233 GHz thermal dust continuum emission obtained with ALMA in the B335 Class 0 protostar. Linearly polarized dust emission arising from the circumstellar material in the envelope of B335 is detected at all scales probed by our observations, from radii of 50 to 1000 au. The magnetic field structure producing the dust polarization has a ve…
We present the results of a suite of numerical simulations designed to explore the origin of the angular momenta of protostellar cores. Using the hydrodynamic grid code \emph{Athena} with a sink implementation, we follow the formation of protostellar cores and protostars (sinks) from the subvirial collapse of molecular clouds on larger scales to investigate the range and relative distribution of core properties. We find that the core angular momenta are relatively unaffected by large-scale rotation of the parent cloud; instead, we infer that angular momenta are mainly imparted by torques betwe…
Dust plays a key role during star, disk and planet formation. Yet, its dynamics during the protostellar collapse remains a poorly investigated field. Recent studies seem to indicate that dust may decouple efficiently from the gas during these early stages. We aim to understand how much and in which regions dust grains concentrate during the early phases of the protostellar collapse, and see how it depends on the properties of the initial cloud and of the solid particles. We use the multiple species dust dynamics solver of the grid-based code RAMSES to perform various simulations of dusty colla…
Understanding the formation mechanisms of protoplanetary disks and multiple systems, and their pristine properties, is a key question for modern astrophysics. The properties of the youngest disks, embedded in rotating infalling protostellar envelopes, have largely remained unconstrained up to now. In the framework of the IRAM-PdBI CALYPSO survey, we have obtained sub-arcsecond observations of the dust continuum emission at 231 GHz and 94 GHz, for a sample of 16 solar-type Class 0 protostars. In an attempt to identify disk-like structures embedded at small scales in the protostellar envelopes, …
We numerically model the collapse of magnetic rotating protostellar clouds with mass of 10 $M_{sun}$. The simulations are carried out with the help of 2D MHD code Enlil. The structure of the cloud at the isothermal stage of the collapse is investigated for the cases of weak, moderate, and strong initial magnetic field. Simulations reveal the universal hierarchical structure of collapsing protostellar clouds, consisting of flattened envelope with the qausi-magnetostatc disk inside and the first core in its center. The size of the primary disk increases with the initial magnetic energy of the cl…
3D radiative hydrodynamic simulations of protostellar collapse with H-C-O dynamical chemistry
将协同演化化学、流体动力学和辐射转移结合起来是恒星形成研究的重要一步。它既可以更好地与观测联系起来，又可以对坍缩核心中的磁耗散进行自洽监测。我们的目标是遵循一个减少气体颗粒化学网络的坍塌致密核心的化学动力学演化。本文给出了1 M $_\odot$孤立致密核坍缩的辐射流体动力学（RHD）模拟结果.物理设置包括RHD和化学网络的动力学演化。为了进行这些模拟，我们合并了多维自适应网格细化代码RAMSES和热化学巴黎达勒姆冲击代码。我们模拟了第一个流体静力核心（FHSC）的形成和56个物种的共同进化，主要描述H-C-O化学。进行准确的基准测试，测试减少化学网络对一个完善的复杂网络。我们表明，通过使用一组紧凑的反应，可以密切配合CO丰度与一个更复杂的网络的结果。我们的主要结果是：（a）我们发现，气体-颗粒化学后处理可以导致一个数量级的CO气相丰度比动力学化学低，最强的效果在等温阶段的崩溃。(b)对于我们选择的参数，自由落体时间对化学丰度的影响很小。(c)动态化学演化需要描述的CO气相丰度，以及CO冰的形成大于1 $μ$ m的平均晶粒尺寸。(d)此外，尘埃的平均粒径和粒径分布有很强的影响化学丰度，从而对电离度和磁耗散。我们的结论是，尘埃颗粒的增长崩溃模拟耦合崩溃与化学一样重要。
Planet-forming disks are fundamental objects thought to be inherited from large scale rotation, through the conservation of angular momentum during the collapse of a prestellar dense core. We investigate the possibility for a protostellar disk to be formed from a motionless dense core which contains non-axisymmetric density fluctuations. The rotation is thus generated locally by the asymmetry of the collapse. We study the evolution of the angular momentum in a non-axisymmetric collapse of a dense core from an analytical point of view. To test the theory, we perform three-dimensional simulation…
The collapse of the magnetic rotating protostellar cloud with mass of $10\,M_{\odot}$ is numerically studied. The initial ratios of the thermal, magnetic, and rotational energies of the cloud to the modulus of its gravitational energy are 0.3, 0.2 and 0.01, respectively. The emphasis is on the evolution and properties of the quasi-magnetostatic primary disk formed at the isothermal stage of the collapse. Simulations show that the primary disk size and mass increase during evolution from $1500$ au to $7400$ au and from $0.3\,M_{\odot}$ to $5.2\,M_{\odot}$, respectively. Magnetic field is quasi-…
Non-ideal MHD effects have been shown recently as a robust mechanism of averting the magnetic braking "catastrophe" and promoting protostellar disc formation. However, the magnetic diffusivities that determine the efficiency of non-ideal MHD effects are highly sensitive to microphysics. We carry out non-ideal MHD simulations to explore the role of microphysics on disc formation and the interplay between ambipolar diffusion (AD) and Hall effect during the protostellar collapse. We find that removing the smallest grain population ($\lesssim$10 nm) from the standard MRN size distribution is suffi…
Disks are essential to the formation of both stars and planets, but how they form in magnetized molecular cloud cores remains debated. This work focuses on how the disk formation is affected by turbulence and ambipolar diffusion (AD), both separately and in combination, with an emphasis on the protostellar mass accretion phase of star formation. We find that a relatively strong, sonic turbulence on the core scale strongly warps but does not completely disrupt the well-known magnetically-induced flattened pseudodisk that dominates the inner protostellar accretion flow in the laminar case, in ag…
On the 3D time evolution of the dust size distribution in protostellar envelopes
Université Paris-Saclay（巴黎萨克雷大学）；Université Paris Cité（巴黎西岱大学）；CEA（法国原子能和替代能源委员会）；CNRS（法国国家科学研究中心）；AIM（天体物理研究所）；Institute of Space Sciences (ICE), CSIC（空间科学研究所（CSIC））；ICREA（加泰罗尼亚高级研究学院）
We use numerical simulations of molecular cloud formation in the colliding flow scenario to investigate the reliability of observational estimates of the angular momenta of early-state, low-mass protostellar cores. We show that, with suitable corrections for projection factors, molecular line observations of velocity gradients in NH$_3$ can be used to provide reasonable estimates of core angular momenta within a factor of two to three, with a few large underestimates due to unfavorable viewing angles. Our results differ from previous investigations which suggested that observations might overe…
We present a large suite of simulations of the formation of low-mass star clusters. Our simulations include an extensive set of physical processes -- magnetohydrodynamics, radiative transfer, and protostellar outflows -- and span a wide range of virial parameters and magnetic field strengths. Comparing the outcomes of our simulations to observations, we find that simulations remaining close to virial balance throughout their history produce star formation efficiencies and initial mass function (IMF) peaks that are stable in time and in reasonable agreement with observations. Our results indica…
The first stars are known to form in primordial gas, either in minihalos with about $10^6$~M$_\odot$ or so-called atomic cooling halos of about $10^8$~M$_\odot$. Simulations have shown that gravitational collapse and disk formation in primordial gas yield dense stellar clusters. In this paper, we focus particularly on the formation of protostellar binary systems, and aim to quantify their properties during the early stage of their evolution. For this purpose, we combine the smoothed particle hydrodynamics code GRADSPH with the astrochemistry package KROME. The GRADSPH-KROME framework is employ…
Truncated abstract: The formation of a protostellar disc is a natural outcome during the star formation process. As gas in a molecular cloud core collapses under self-gravity, the angular momentum of the gas will slow its collapse on small scales and promote the formation of a protostellar disc. Although the angular momenta of dense star-forming cores remain to be fully characterized observationally, existing data indicates that typical cores have enough angular momenta to form relatively large, rotationally supported discs. However, molecular clouds are observed to be permeated by magnetic fi…
