Protostellar Jet and Outflow in the Collapsing Cloud Core (original) (raw)
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The Astrophysical Journal, 2008
The driving mechanisms of low-and high-velocity outflows in star formation processes are studied using threedimensional resistive MHD simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate cloud evolution from the molecular cloud core (n c ¼ 10 4 cm À3 ) to the stellar core (n c ¼ 10 22 cm À3 ), where n c denotes the central density. In the collapsing cloud core, we found two distinct flows: low-velocity flows ($5 km s À1 ) with a wide opening angle, driven from the adiabatic core when the central density exceeds n c k 10 12 cm À3 ; and high-velocity flows ($30 km s À1 ) with good collimation, driven from the protostar when the central density exceeds n c k 10 21 cm À3 . High-velocity flows are enclosed by low-velocity flows after protostar formation. The difference in the degree of collimation between the two flows is caused by the strength of the magnetic field and configuration of the magnetic field lines. The magnetic field around an adiabatic core is strong and has an hourglass configuration; therefore, flows from the adiabatic core are driven mainly by the magnetocentrifugal mechanism and guided by the hourglass-like field lines. In contrast, the magnetic field around the protostar is weak and has a straight configuration owing to ohmic dissipation in the high-density gas region. Therefore, flows from the protostar are driven mainly by the magnetic pressure gradient force and guided by straight field lines. Differing depth of the gravitational potential between the adiabatic core and the protostar causes the difference of flow speed. Low-velocity flows may correspond to the observed molecular outflows, while high-velocity flows may correspond to the observed optical jets. We suggest that the protostellar outflow and the jet are driven by different cores, rather than the outflow being entrained by the jet.
Driving Mechanism of Jets and Outflows in Star Formation Process
2007
The driving mechanism of jets and outflows in star formation process is studied using resistive MHD nested grid simulations. We calculated cloud evolution from the molecular cloud core to the stellar core. In the collapsing cloud core, we found two distinct flows: Low-velocity flows (sim 5 km/s) with a wide opening angle, driven from the adiabatic core, and high-velocity flows (sim 30 km/s) with good collimation, driven from the protostar. High-velocity flows are enclosed by low-velocity flows after protostar formation. The difference in the degree of collimation between the two flows is caused by the strength of the magnetic field and configuration of the magnetic field lines. The magnetic field around an adiabatic core is strong and has an hourglass configuration; therefore, flows from the adiabatic core are driven mainly by the magnetocentrifugal mechanism and guided by the hourglass-like field lines. In contrast, the magnetic field around the protostar is weak and has a straight configuration owing to Ohmic dissipation in the high-density gas region. Therefore, flows from the protostar are driven mainly by the magnetic pressure gradient force and guided by straight field lines. Differing depth of the gravitational potential between the adiabatic core and the protostar cause the difference of the flow speed. Low-velocity flows correspond to the observed molecular outflows, while high-velocity flows correspond to the observed optical jets. We suggest that the outflow and the jet are driven by different cores, rather than that the outflow being entrained by the jet.
A Unified Model for Bipolar Outflows from Young Stars: Apparent Magnetic Jet Acceleration
The astrophysical journal, 2023
We develop a unified model for molecular outflows in star formation. The model incorporates essential features expected of the primary wind, which is thought to be driven magnetocentrifugally from close to the central stellar object, and the ambient core material shaped by anisotropic magnetic support. The primary wind is modeled as a toroidally magnetized fast outflow moving radially away from the origin, with an angle-dependent density distribution: a dense axial jet surrounded by a more tenuous wide-angle wind, as expected in the X-wind model. If dynamically significant magnetic fields are present, the star-forming core will settle faster along the field lines than across, forming a toroid-like structure. We approximate the structure with a singular isothermal toroid whose density distribution can be obtained analytically. The interaction of the laterally stratified wind and the ambient toroid is followed using the Zeus2D magnetohydrodynamics (MHD) code. We find that the lobes produced by the interaction resemble many systematics observed in molecular outflows from very young stars, ranging from Class 0 to I sources. In particular, both the dense axial jet and the wide-angle wind participate in the wind-ambient interaction. In our model, the jet-and wind-driven pictures of molecular outflows are unified. We discuss the observational implications of the unified picture, including the possibility of detecting the primary jet /wind directly.
Jets and bipolar outflows from young stars: Theory and observational tests
Protostars and Planets V, 2007
Jets and outflows from young stars are an integral part of the star formation process. A particular framework for explaining these phenomena is the X-wind theory. Since PPIV, we have made good progress in modeling the jet phenomena and their associated fundamental physical processes, in both deeply embedded Class I objects and more revealed classical T Tauri stars. In particular, we have improved the treatment of the atomic physics and chemistry for modeling jet emission, including reaction rates and interaction cross-sections, as well as ambipolar diffusion between ions and neutrals. We have broadened the original X-wind picture to include the winds driven magnetocentrifugally from the innermost disk regions. We have carried numerical simulations that follow the wind evolution from the launching surface to large, observable distances. The interaction between the magnetocentrifugal wind and a realistic ambient medium was also investigated. It allows us to generalize the shell model of Shu et al. (1991) to unify the the jet-driven and wind-driven scenarios for molecular outflow production. In addition, we review related theoretical works on jets and outflows from young stars, and make connection between theory and recent observations, particularly those from HST/STIS, VLA and SMA.
The Astrophysical Journal, 2014
We investigate protostellar outflow evolution, gas entrainment, and star formation efficiency using radiation-hydrodynamic simulations of isolated, turbulent low-mass cores. We adopt an X-wind launching model, in which the outflow rate is coupled to the instantaneous protostellar accretion rate and evolution. We vary the outflow collimation angle from θ = 0.01 − 0.1 and find that even well collimated outflows effectively sweep up and entrain significant core mass. The Stage 0 lifetime ranges from 0.14-0.19 Myr, which is similar to the observed Class 0 lifetime. The star formation efficiency of the cores spans 0.41-0.51. In all cases, the outflows drive strong turbulence in the surrounding material. Although the initial core turbulence is purely solenoidal by construction, the simulations converge to approximate equipartition between solenoidal and compressive motions due to a combination of outflow driving and collapse. When compared to simulation of a cluster of protostars, which is not gravitationally centrally condensed, we find that the outflows drive motions that are mainly solenoidal. The final turbulent velocity dispersion is about twice the initial value of the cores, indicating that an individual outflow is easily able to replenish turbulent motions on sub-parsec scales. We post-process the simulations to produce synthetic molecular line emission maps of 12 CO, 13 CO, and C 18 O and evaluate how well these tracers reproduce the underlying mass and velocity structure.
A global jet/circulation model for young stars
Astronomy & Astrophysics, 2002
Powerful, highly collimated jets, surrounded by bipolar molecular outflows, are commonly observed near Young Stellar Objects (YSOs). In the usual theoretical picture of star formation, a jet is ejected from a magnetized accretion disk, with a molecular outflow being driven either by the jet or by a wider wind coming from the disk. Here, we propose an alternative global model for the flows surrounding YSOs. In addition to a central accretion-ejection engine driving the jet, the molecular outflow is powered by the infalling matter and follows a circulation pattern around the central object without necessarily being entrained by a jet. It is shown that the model produces a heated pressure-driven outflow with magneto-centrifugal acceleration and collimation. We report solutions for the three different parts of this self-similar model, i.e. the jet, the infalling envelope and the circulating matter that eventually forms the molecular outflow. This new picture of the accretion/outflow phase provides a possible explanation for several observed properties of YSO outflows. The most relevant ones are the presence of high mass molecular outflows around massive protostars, and a realistic fraction (typically 0.1) of the accretion flow that goes into the jet.
The First Jets in the Universe: Protostellar Jets from the First Stars
The Astrophysical Journal, 2006
The protostellar jets driven by the formation of the first stars are studied by using three-dimensional magnetohydrodynamical (MHD) nested grid simulations. Starting from a slowly rotating spherical cloud of 5.1 × 10 4 M ⊙ permeated by a uniform magnetic field, we follow the evolution from the central number density n c = 10 3 cm −3 (where the radius of the object r = 6.6 pc) to n c ≃ 10 23 cm −3 ( r ≃ 1 R ⊙ ). We resolve the cloud structure more than 8 orders of magnitude in spatial extent and 20 orders in density contrast. We calculate four models that differ in initial magnetic field strengths and angular velocities. In all models, protostars of ≃ 10 −3 M ⊙ are formed at n c ≃ 10 22 cm −3 in accordance with one-dimensional calculations. By this epoch, the magnetic flux density is amplified by 10 orders of magnitude from the initial value. Consequently, the formed protostar possesses the magnetic field of ∼ 10 6 G that is much larger than the flux density of the present counterparts, reflecting the fact that the dissipation of a magnetic field is ineffective in primordial gas clouds. If the initial magnetic field B > 10 −9 (n c /10 3 cm −3 ) 2/3 G, the protostellar jet is launched and its velocities reaches ∼ 70 km s −1 by the time the protostellar mass becomes (4 − 6) × 10 −3 M ⊙ , and a fraction (3 − 10%) of the accreting matter is blown off from the central region. Owing to the interaction of these ejecta with surrounding matter, expanding bow shocks are created at both heads of the jet. If this jet continues to sweep out the surrounding gas that otherwise accretes onto the central star or circumstellar disk, the final mass of the first star can be substantially reduced. In addition, dense post-shock regions behind the bow shocks are expected to promote the chemical reactions (formation of H 2 and HD), and this provides possible environments for subsequent low-mass star formation in the early universe.
The role of initial magnetic field structure in the launching of protostellar jets
Monthly Notices of the Royal Astronomical Society
Magnetic fields are known to play a crucial role in the star formation process, particularly in the formation of jets and outflows from protostellar discs. The magnetic field structure in star forming regions is not always uniform and ordered, often containing regions of magnetic turbulence. We present grid-based, magneto-hydrodynamical simulations of the collapse of a 1 M cloud core, to investigate the influence of complex magnetic field structures on outflow formation, morphology and efficiency. We compare three cases: a uniform field, a partially turbulent field and a fully turbulent field, with the same magnetic energy in all three cases. We find that collimated jets are produced in the uniform-field case, driven by a magneto-centrifugal mechanism. Outflows also form in the partially turbulent case, although weaker and less collimated, with an asymmetric morphology. The outflows launched from the partially turbulent case carry the same amount of mass as the uniform-field case but at lower speeds, having only have 71% of the momentum of the uniform-field case. In the case of a fully turbulent field, we find no significant outflows at all. Moreover, the turbulent magnetic field initially reduces the accretion rate and later induces fragmentation of the disc, forming multiple protostars. We conclude that a uniform poloidal component of the magnetic field is necessary for the driving of jets.
The First Jet in the Universe: Protostellar Jets from the First Stars
2006
The protostellar jets driven by the formation of the first stars are studied by using MHD nested grid simulations. Starting from a slowly rotating spherical cloud of 5.1 times 10^4 Msun permeated by a uniform magnetic field, we follow the evolution from the central number density n = 10^3 cm^-3 to n simeq 10^23 cm^-3. We calculate four models that differ in initial magnetic field strengths and angular velocities. In all models, protostars of simeq 10^-3 Msun are formed at n simeq 10^22 cm^-3 in accordance with one-dimensional calculations. By this epoch, the magnetic flux density is amplified by 10 orders of magnitude from the initial value. Consequently, the formed protostar possesses the magnetic field of \sim 10^6 G that is much larger than the flux density of the present counterparts, reflecting the fact that the dissipation of a magnetic field is ineffective in primordial gas clouds. If the initial magnetic field B > 10^-9 (n/10^3 cm^-3)^2/3 G, the protostellar jet is launch...
On the Influence of Magnetic Fields on the Structure of Protostellar Jets
The Astrophysical Journal, 2001
We here present the first results of fully three-dimensional (3-D) MHD simulations of radiative cooling pulsed (time-variable) jets for a set of parameters which are suitable for protostellar outflows. Considering different initial magnetic field topologies in approximate equipartition with the thermal gas, i.e., (i) a longitudinal, and (ii) a helical field, both of which permeating the jet and the ambient medium; and (iii) a purely toroidal field permeating only the jet, we find that the overall morphology of the pulsed jet is not very much affected by the presence of the different magnetic field geometries in comparison to a nonmagnetic calculation. Instead, the magnetic fields tend to affect essentially the detailed structure and emission properties behind the shocks at the head and at the pulse-induced internal knots, particularly for the helical and toroidal geometries. In these cases, we find, for example, that the H α emissivity behind the internal knots can be about three to four times larger than that of the purely hydrodynamical jet. We also find that some features, like the nose cones that often develop at the jet head in 2-D calculations involving toroidal magnetic fields, are smoothed out or absent in the 3-D calculations.