Kohji Tomisaka

Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows

The collapse of rotating magnetized molecular cloud cores is studied with axisymmetric magnetohydrodynamic (MHD) simulations. Because of the change of the equation of state of the interstellar gas, molecular cloud cores experience several phases during the collapse. In the earliest isothermal runaway collapse (n 1010 H2 cm-3), a pseudodisk is formed, and it continues to contract until an opaque core is formed at the center. In this disk, a number of MHD fast and slow shock pairs appear whose wave fronts are parallel to the disk. We assume that the interstellar gas obeys a polytropic equation of state with the exponent of Γ > 1 above the critical density at which the core becomes optically thick against the thermal radiation from dusts ncr ~ 1010 cm-3. After the equation of state becomes hard, an adiabatic quasi-static core forms at the center (the first core), which is separated from the isothermal contracting pseudodisk by the accretion shock front facing radially outward. By the effect of the magnetic tension, the angular momentum is transferred from the disk midplane to the surface. The gas with an excess angular momentum near the surface is finally ejected, which explains the molecular bipolar outflow. Two types of outflows are found. When the poloidal magnetic field is strong (its energy is comparable to the thermal one), a U-shaped outflow is formed, in which gas is mainly outflowing through a region whose shape looks like a capital letter U at a finite distance from the rotation axis. The gas is accelerated by the centrifugal force and the magnetic pressure gradient of the toroidal component. The other is a turbulent outflow in which magnetic field lines and velocity fields seem to be randomly oriented. In this case, globally the gas moves out almost perpendicularly from the disk, and the outflow looks like a capital letter I. In this case, although the gas is launched by the centrifugal force, the magnetic force working along the poloidal field lines plays an important role in expanding the outflow. The continuous mass accretion leads to a quasi-static contraction of the first core. A second collapse due to the dissociation of H2 occurs in it. Finally, another less massive quasi-static core is formed by atomic hydrogen (the second core). At the same time, it is found that another outflow is ejected around the second atomic core, which seems to correspond to the optical jets or the fast neutral winds.

Equilibria and evolutions of magnetized, rotating, isothermal clouds. II: The extreme case: nonrotating clouds

Equilibrium solutions of magnetized nonrotating clouds in a static external medium were obtained for a wide ranges of parameters. Two different equilibrium states with the same mass but different central densities were obtained, and it is shown that the solution with lower central density is stable, while that with higher central density is unstable. Two possibilities for the initiation of star formation are suggested: (1) the mass of cloud exceeds M(cr) and (2) the cloud is compressed beyond the unstable equilibrium solution with higher central density. 23 references.

Collapse-Driven Outflow in Star-Forming Molecular Cores

Dynamical collapses of magnetized molecular cloud cores are studied with magnetohydrodynamic simulations from the runaway collapse phase to the accretion phase. In the runaway collapse phase, a disk threaded by magnetic field lines is contracting owing to its self-gravity, and its evolution is well expressed by a self-similar solution. The central density increases greatly in a finite timescale and reaches a density at which an opaque core is formed at the center. After that, matter accretes to the newly formed core (accretion phase). In this stage, a rotationally supported disk is formed in a cloud core without magnetic fields. In contrast, the disk continues to contract in the magnetized cloud core, since the magnetic fields transfer angular momentum from the disk. Its rotation motion winds up the threading magnetic field lines. Eventually, strong toroidal magnetic fields are formed and begin to drive the outflow, even if there is no toroidal field component initially. Bipolar molecular outflows observed in protostar candidates are naturally explained by this model.

Collapse and fragmentation of rotating magnetized clouds — II. Binary formation and fragmentation of first cores

Subsequent to Paper I, the evolution and fragmentation of a rotating magnetized cloud are studied with use of three-dimensional magnetohydrodynamic nested grid simulations. After the isothermal runaway collapse, an adiabatic gas forms a protostellar first core at the centre of the cloud. When the isothermal gas is stable for fragmentation in a contracting disc, the adiabatic core often breaks into several fragments. Conditions for fragmentation and binary formation are studied. All the cores which show fragmentation are geometrically thin, as the diameter-to-thickness ratio is larger than 3. Two patterns of fragmentation are found. (1) When a thin disc is supported by centrifugal force, the disc fragments into a ring configuration (ring fragmentation). This is realized in a rapidly rotating adiabatic core as �> 0.2τ −1 , where � and τ ff represent the angular rotation speed and the free-fall time of the core, respectively. (2) On the other hand, the disc is deformed to an elongated bar in the isothermal stage for a strongly magnetized or rapidly rotating cloud. The bar breaks into 2‐4 fragments (bar fragmentation). Even if a disc is thin, the disc dominated by the magnetic force or thermal pressure is stable and forms a single compact body. In either ring or bar fragmentation mode, the fragments contract and a pair of outflows is ejected from the vicinities of the compact cores. The orbital angular momentum is larger than the spin angular momentum in the ring fragmentation. On the other hand, fragments often quickly merge in the bar fragmentation, since the orbital angular momentum is smaller than the spin angular momentum in this case. Comparison with observations is also shown.

Formation Scenario for Wide and Close Binary Systems

Fragmentation and binary formation processes are studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core ( -->n = 104 cm ?3) to the stellar core ( -->n 1022 cm ?3), where n denotes the central density. We calculated 147 models with different initial magnetic, rotational, and thermal energies and the amplitudes of the nonaxisymmetric perturbation. In a collapsing cloud, fragmentation is mainly controlled by the initial ratio of the rotational to the magnetic energy, regardless of the initial thermal energy and amplitude of the nonaxisymmetric perturbation. The cloud rotation promotes fragmentation, while the magnetic field delays or in some cases suppresses fragmentation through all phases of cloud evolution. The results are categorized into three types. When the clouds have larger rotational energies in relation to magnetic energies, fragmentation occurs in the low-density phase ( -->1012 cm ?3 n 1015 cm ?3) with separations of 3-300 AU. Fragments that appeared in this phase are expected to evolve into wide binary systems. On the other hand, when initial clouds have larger magnetic energies in relation to the rotational energies, fragmentation occurs only in the high-density phase ( -->n 1017 cm ?3) after the clouds experience a significant reduction of the magnetic field owing to the ohmic dissipation. Fragments appearing in this phase have mutual separations of 0.3 AU and are expected to evolve into close binary systems. No fragmentation occurs in the case of sufficiently strong magnetic field, in which single stars are expected to be born. Two types of fragmentation epoch reflect wide and close separations. We might be able to observe a bimodal distribution for the radial separation of the protostar in extremely young stellar groups.

Directions of Outflows, Disks, Magnetic Fields, and Rotation of Young Stellar Objects in Collapsing Molecular Cloud Cores

The collapse of slowly rotating molecular cloud cores threaded by magnetic fields is investigated by high-resolution numerical simulation. Outflow formation in the collapsing cloud cores is also followed. In the models examined, the cloud core and parent cloud rotate rigidly and are initially threaded by a uniform magnetic field. The simulations show that the cloud core collapses along the magnetic field lines. The magnetic field in the dense region of the cloud core rotates faster than that of the parent cloud as a consequence of spin-up of the central region during the collapse. The cloud core exhibits significant precession of the rotation axis, magnetic field, and disk orientation, with precession highest in the models with low initial field strength (20 μG). Precession in models with initial fields of ~40 μG is suppressed by strong magnetic braking. Magnetic braking transfers angular momentum form the central region and acts more strongly on the component of angular momentum oriented perpendicular to the magnetic field. After the formation of an adiabatic core, outflow is ejected along the local magnetic field lines. Strong magnetic braking associated with the outflow causes the direction of angular momentum to converge with that of the local magnetic field, resulting in the convergence of the local magnetic field, angular momentum, outflow, and disk orientation by the outflow formation phase. The magnetic field of a young star is inclined at an angle of no more than 30° from that of the parent cloud at initial field strengths of ~20 μG, while at an initial field strength of ~40 μG, the magnetic field of the young star is well aligned with that of the parent cloud.

Collapse and fragmentation of rotating magnetized clouds — I. Magnetic flux-spin relation

We discuss evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of > 5 × 10 10 cm 3 . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towa rds a single relationship between the two quantities. If the core is magnetically dominated it s magnetic flux density approaches 1.5(n/5 × 10 10 cm 3 ) 1/2 mG, while if the core is rotationally dominated the angular velocity approaches 2.57 × 10 3 (n/5 × 10 10 cm 3 ) 1/2 yr 1 , where n is the density of the gas. We also find that the ratio of the angular velocity to the magne tic flux density remains nearly constant until the density exceeds 5 × 10 10 cm 3 . Fragmentation of the very dense core and emergence of outflows from fragments are shown in the subsequ ent paper.

Formation of a keplerian disk in the infalling envelope around l1527 IRS: Transformation from infalling motions to kepler motions

We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0 observations of C

First MHD simulation of collapse and fragmentation of magnetized molecular cloud cores

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Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field

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