Transition in the Outflow Evoution of the Massive Star-forming Region W75N (original) (raw)
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Two-component magnetohydrodynamical outflows around young stellar objects
Astronomy and Astrophysics, 2006
Context. We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar magnetospheric winds coupled with discdriven jets where the resistive and viscous accretion disc is self-consistently described. Aims. These innovative MHD simulations are devoted to the study of the interplay between a stellar wind (having different ejection mass rates) and an MHD disc-driven jet embedding the stellar wind. Methods. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. We first investigate the various angular momentum transports occurring in the magneto-viscous accretion disc. We then analyze the modifications induced by the interaction between the two components of the outflow. Results. Our simulations show that the inner outflow is accelerated from the central object's hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force. Conclusions. The simulations show that the MHD disc-driven outflow extracts angular momentum more efficiently than do viscous effects in near-equipartition, thin-magnetized discs where turbulence is fully developed. We also show that, when a dense inner stellar wind occurs, the resulting disc-driven jet has a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass-ejection rate in the disc-driven jet and a larger radial extension that is in better agreement with the observations, besides being more consistent.
Outflows from Young Stars: Theory and Observation
Symposium - International Astronomical Union, 2004
Recent observations have revealed that young stellar objects are associated with jet-like structures and Herbig-Haro objects emitting at wavelengths ranging from optical lines to radio continua. These phenomena are similar in morphologies, and have mostly comparable energetics, dynamics, and kinematics. Probing such phenomena observed at various wavelengths with self-consistent physical and radiative processes arising within an inner disk-wind driven magnetocentrifugally from the circumstellar accretion disk is a challenge for confronting theory and observation of outflows. How such early outflow phase may play a role in forming planetary materials may help solve puzzles posed by meteorites. We will discuss the relevant observations, theoretical foundations for modelling approaches, magnetic structures and dynamical effects, and the connection to the early solar system.
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.
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.
Collimation of astrophysical MHD outflows
Astrophysics and Space Science, 2000
We explain in simple terms why a rotating and magnetized outflow forms a core with a jet and show numerical simulations which substantiate this argument. The outflow from a solar-type inefficient magnetic rotator is found to be very weakly collimated while the outflow from a ten times faster rotating YSO is shown to produce a tightly collimated jet. This gives rise to an evolutionary scenario for stellar outflows. We also propose a two-component model consisting of a wind outflow from a central object and a faster rotating outflow launched from a surrounding accretion disk which plays the role of the flow collimator.
Astrophysical Journal, 2006
We present adaptive optics (AO)-assisted near-infrared Fabry-Perot observations of both the H 2 v ¼ 1 0 S(1) line in the area surrounding the shell-like ultracompact H ii region (UCH ii) G5.89À0.39 and the Br emission in the region of ionized gas. This work aims at investigating the near-IR counterpart to the widely debated massive outflow detected toward this source. We also study the connection of the outflow(s) with the possible driving source(s) to better constrain the stellar content within this UCH ii region. Our data show evidence of a total of three outflows in this region, with distinct orientations and different driving sources. Two prominent bow-shock structures are identified in our H 2 data in a north-south orientation. The molecular jet, likely associated with these features, is not compatible with the orientation of the outflow previously detected at high spatial resolution in SiO emission. Moreover, we propose the driving source of this jetlike structure as the O5 V star recently detected by Feldt and coworkers. However, we report the detection of a bipolar structure, separated by a dark lane, at the location of the 1.3 mm continuum source (i.e., the candidate source to power the SiO outflow). Finally, a third bipolar outflow is traced through the Br emission. The confirmation through CO interferometric observations of this outflow activity would therefore favor an accretion scenario for high-mass star formation.
The Role of Magnetic Fields in Protostellar Outflows and Star Formation
Frontiers in Astronomy and Space Sciences
The role of outflows in the formation of stars and the protostellar disks that generate them is a central question in astrophysics. Outflows are associated with star formation across the entire stellar mass spectrum. In this review, we describe the observational, theoretical, and computational advances on magnetized outflows, and their role in the formation of disks and stars of all masses in turbulent, magnetized clouds. The ability of torques exerted on disks by magnetized winds to efficiently extract and transport disk angular momentum was developed in early theoretical models and confirmed by a variety of numerical simulations. The recent high resolution Atacama Large Millimeter Array (ALMA) observations of disks and outflows now confirm several key aspects of these ideas, e.g., that jets rotate and originate from large regions of their underlying disks. New insights on accretion disk physics show that magneto-rotational instability (MRI) turbulence is strongly damped, leaving magnetized disk winds as the dominant mechanism for transporting disk angular momentum. This has major consequences for star formation, as well as planet formation. Outflows also play an important role in feedback processes particularly in the birth of low mass stars and cluster formation. Despite being almost certainly fundamental to their production and focusing, magnetic fields in outflows in protostellar systems, and even in the disks, are notoriously difficult to measure. Most methods are indirect and lack precision, as for example, when using optical/near-infrared line ratios. Moreover, in those rare cases where direct measurements are possible-where synchrotron radiation is observed, one has to be very careful in interpreting derived values. Here we also explore what is known about magnetic fields from observations, and take a forward look to the time when facilities such as SPIRou and the SKA are in routine operation.
H2 observations of outflows from young stars
Analizamos observaciones del IR cercano recientes de jets Herbig-Haro (HH) y de flujos moleculares de protoestrellas muy jóvenes profundamente embebidas. Mediciones de movimiento propio y estudios espectroscópicos de baja y de alta resolución muestran la excitación y la cinemática de objetos individuales, que podrían interpretarse en términos de choques de proa de alta velocidad que barren y incorporan material del ambiente para formar flujos moleculares de "CO". Las propiedades observadas de muchos objetos puede explicarse razonablemente bien con modelos de choques de proa tipo "C" magnetizados, aunque choques del tipo "J" no puede excluirse del todo. Analizamos también nuevas observaciones echelle de las fuentes mismas de los flujos. Estos datos de H 2 muestran emisión de línea de velocidades intermedias y altas en la base del flujo (de menos de unas pocos cientos de unidades astronómicas de la fuente que lo impulsa) en la mayoría de las fuentes observadas. Las propiedades de estas regiones de líneas de emisión de hidrógeno molecular-o MHEL, por sus siglas en inglés-son similares a las regiones de líneas prohibidas (FEL, por sus siglas en inglés) hacia estrellas T Tauri.
A Unified Model for Bipolar Outflows from Young Stars
The Astrophysical Journal, 2006
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.
Magnetically Driven Outflows in a Starburst Environment
The Astrophysical Journal, 1999
We here investigate the possibility that the observed collimated outflows in luminous infrared galaxies (LIGs) and some Seyfert galaxies can be produced in a starburst (SB) environment. In the former source class, in particular, there seems to be some observational evidence for the presence of nuclear SBs in some objects (e.g. Scoville, Lonsdale, & Lonsdale 1998).