Life Cycles of Magnetic Fields in Stellar Evolution

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.

Science, 2009

2001

The generation of magnetic field in astrophysical bodies, e.g., galaxies, stars, planets, is one of the outstanding theoretical problems of physics and astrophysics. The initial magnetic fields of galaxies and stars are weak, and are amplified by the turbulent motion of the plasma. The generated field gets saturated due to nonlinear interactions. The above process is called "dynamo" action. Qualitatively, the magnetic field is amplified by the stretching of the field lines due to turbulent plasma motion. A fraction of kinetic energy of the plasma is spent in increasing the tension of the magnetic field lines, which effectively enhances the magnetic field strength. Current dynamo theories of are of two types, kinematic and dynamic. In the kinematic theories, one studies the evolution of magnetic field under a prescribed velocity field. In kinematic α-dynamo, the averaged nonlinear term u × b (u, b are velocity and magnetic field fluctuations respectively) is replaced by a constant α times mean magnetic field B 0 . This process, which is valid for small magnetic field fluctuations, yields linear equations that can be solved for a given boundary condition and external forcing fields 1−3 . In dynamic theories 4−6 , the modification of velocity field by the magnetic field (back reaction) is taken into account. Using a different approach, here we compute energy transfer rates from velocity field to magnetic field using field-theoretic method. The striking result of our field theoretic calculation is that there is a large energy transfer rate from the large-scale velocity field to the large-scale magnetic field. We claim that the growth of large-scale magnetic energy is primarily due to this transfer. We reached the above conclusion without any linear approximation like that in α-dynamo.

Proceedings of the International Astronomical Union, 2005

Magnetic fields may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio M/Φ and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Examples of massive star formation regions for which detailed observations have been made of magnetic field strengths and morphologies include DR21OH, OMC1, and S106; observational results for these regions and relevant results for the diffuse ISM and masers will be reviewed. Results are that the strength of interstellar magnetic fields remains invariant at B ∼ 6µG between 0.1 cm −3 < n(H) < 10 3 cm −3 , but increases as B ∝ ρ 0. 4−0. 5 for 10 3 cm −3 < n(H2) < 10 8 cm −3. Moreover, M/Φ is significantly subcritical (strong B with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend critically on the M/Φ ratio in each clump.

2001

High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the first year of the Chandra mission are providing important clues about the mechanisms which produce X-rays on very young stars. For zeta Puppis (O4 If) and zeta Ori (O9.5 I), the broad, blue-shifted line profiles, line ratios, and derived temperature distribution suggest that the X-rays are produced

AIP Conference Proceedings, 2005

Highly collimated supersonic jets and less collimated outflows are observed to emerge from a wide variety of astrophysical objects. They are seen in young stellar objects (YSOs), proto-planetary nebulae, compact objects (such as galactic black holes or microquasars, and X-ray binary stars), and in the nuclei of active galaxies (AGNs). Despite their different physical scales (in size, velocity, and amount of energy transported), they have strong morphological similarities. What is the universal mechanism that can explain their origin? In this lecture, I briefly review the role that magnetic fields seem to play on the formation, structure, and propagation of these jets.

Magnetic fields of stars and planets., 2020

A new version of the appearance of a magnetic field in stars and planets is presented. From the point of view of the theory of an elastic universe, elementary particles are axially symmetric, but spherically not symmetric. They take the form of electric and magnetic dipoles. At high pressures inside the stars and planets elementary particles gradually become the same orientation. In this case, a magnetic field arises around the stars and planets. I core of a star or planet is approaching the structure of a neutron star.

Monthly Notices of the Royal Astronomical Society, 2022

Beginning with cosmological initial conditions at z = 100, we simulate the effects of magnetic fields on the formation of Population III stars and compare our results with the predictions of Paper I. We use gadget-2 to follow the evolution of the system while the field is weak. We introduce a new method for treating kinematic fields by tracking the evolution of the deformation tensor. The growth rate in this stage of the simulation is lower than expected for diffuse astrophysical plasmas, which have a very low resistivity (high magnetic Prandtl number); we attribute this to the large numerical resistivity in simulations, corresponding to a magnetic Prandtl number of order unity. When the magnetic field begins to be dynamically significant in the core of the minihalo at z = 27, we map it onto a uniform grid and follow the evolution in an adaptive mesh refinement, MHD simulation in orion2. The nonlinear evolution of the field in the orion2 simulation violates flux-freezing and is consistent with the theory proposed by Xu & Lazarian. The fields approach equipartition with kinetic energy at densities ∼ 10 10 -10 12 cm -3 . When the same calculation is carried out in orion2 with no magnetic fields, several protostars form, ranging in mass from ∼ 1 to 30 M ⊙ ; with magnetic fields, only a single ∼ 30 M ⊙ protostar forms by the end of the simulation. Magnetic fields thus suppress the formation of low-mass Pop III stars, yielding a top-heavy Pop III IMF and contributing to the absence of observed Pop III stars.

Magnetic field is playing an important role at all stages of star evolution from star formation to the endpoints. The main effects are briefly reviewed. We also show that O-type stars have large convective envelopes, where convective dynamo could work. There, fields in magnetostatic balance have intensities of the order of 100 G.

2019

Magnetic fields are involved in every astrophysical process on every scale:from planetary and stellar interiors to neutron stars, stellar wind bubbles and supernova remnants; from the interstellar medium in galactic disks, nuclei, spiral arms and halos to the intracluster and intergalactic media.They are involved in essentially every particle acceleration process and are thus fundamental to non-thermal physics in the Universe.Key questions include the origin of magnetic fields, their evolution over cosmic time, the amplification and decay processes that modify their strength, and their impact on other processes such as star formation and galaxy evolution.Astrophysical plasmas provide a unique laboratory for testing magnetic dynamo theory. The study of magnetic fields requires observations that span the wavelength range from radio through infrared, optical, UV, X-ray, and gamma-ray.&lt;br&gt; &lt;br&gt; Canada has an extremely strong record of research in cosmic magnetism, and has a ...