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Register a new userGiant protostellar flares: accretion-driven accumulation and reconnection-driven ejection of magnetic flux in protostars
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Protostellar flares are rapid magnetic energy release events associated with formation of hot plasma in protostars. In the previous models of protostellar flares, the interaction between a protostellar magnetosphere with the surrounding disk plays crucial roles in building-up and releasing the magnetic energy. However, it remains unclear if protostars indeed have magnetospheres because vigorous disk accretion and strong disk magnetic fields in the protostellar phase may destroy the magnetosphere. Considering this possibility, we investigate the energy accumulation and release processes in the absence of a magnetosphere using a three-dimensional magnetohydrodynamic simulation. Our simulation reveals that protostellar flares are repeatedly produced even in such a case. Unlike in the magnetospheric models, the protostar accumulates magnetic energy by acquiring large-scale magnetic fields from the disk by accretion. Protostellar flares occur when a portion of the large-scale magnetic fields are removed from the protostar as a result of magnetic reconnection. Protostellar flares in the simulation are consistent with observations; the released magnetic energy (up to $sim 3times 10^{38}$ erg) is large enough to drive observed flares, and the flares produce hot ejecta. The expelled magnetic fields enhance accretion, and the energy build-up and release processes are repeated as a result. The magnetic flux removal via reconnection leads to redistribution of magnetic fields in the inner disk. We therefore consider that protostellar flares will play an important role in the evolution of the disk magnetic fields in the vicinity of protostars.
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The interaction between a YSO stellar magnetic field and its protostellar disc can result in stellar accretional flows and outflows from the inner disc rim. Gas flows with a velocity component perpendicular to disc midplane subject particles to centrifugal acceleration away from the protostar, resulting in particles being catapulted across the face of the disc. The ejected material can produce a dust fan, which may be dense enough to mimic the appearance of a puffed-up inner disc rim. We derive analytic equations for the time dependent disc toroidal field, the disc magnetic twist, the size of the stable toroidal disc region, the jet speed and the disc region of maximal jet flow speed. We show how the observed infrared variability of the pre-transition disc system LRLL~31 can be modelled by a dust ejecta fan from the inner-most regions of the disc whose height is partially dependent on the jet flow speed. The greater the jet flow speed, the higher is the potential dust fan scale height. An increase in mass accretion onto the star tends to increase the height and optical depth of the dust ejection fan, increasing the amount of 1--8~$mu$m radiation. The subsequent shadow reduces the amount of light falling on the outer disc and decreases the 8-- 40~$mu$m radiation. A decrease in the accretion rate reverses this scenario, thereby producing the observed see-saw infrared variability.
The early evolution of protostellar disks with metallicities in the $Z=1.0-0.01~Z_odot$ range was studied with a particular emphasis on the strength of gravitational instability and the nature of protostellar accretion in low-metallicity systems. Numerical hydrodynamics simulations in the thin-disk limit were employed that feature separate gas and dust temperatures, and disk mass-loading from the infalling parental cloud cores. Models with cloud cores of similar initial mass and rotation pattern, but distinct metallicity were considered to distinguish the effect of metallicity from that of initial conditions. The early stages of disk evolution in low-metallicity models are characterized by vigorous gravitational instability and fragmentation. Disk instability is sustained by continual mass-loading from the collapsing core. The time period that is covered by this unstable stage is much shorter in the $Z=0.01~Z_odot$ models as compared to their higher metallicity counterparts thanks to the higher mass infall rates caused by higher gas temperatures (that decouple from lower dust temperatures) in the inner parts of collapsing cores. Protostellar accretion rates are highly variable in the low-metallicity models reflecting a highly dynamical nature of the corresponding protostellar disks. The low-metallicity systems feature short, but energetic episodes of mass accretion caused by infall of inward-migrating gaseous clumps that form via gravitational fragmentation of protostellar disks. These bursts seem to be more numerous and last longer in the $Z=0.1~Z_odot$ models in comparison to the $Z=0.01~Z_odot$ case. Variable protostellar accretion with episodic bursts is not a particular feature of solar metallicity disks. It is also inherent to gravitationally unstable disks with metallicities up to 100 times lower than solar.
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Eduard I. Vorobyov The Institute for Computational Astrophysics
, n Saint Marys University
, Halifax
2010
We perform a comparative numerical hydrodynamics study of embedded protostellar disks formed as a result of the gravitational collapse of cloud cores of distinct mass (M_cl=0.2--1.7 M_sun) and ratio of rotational to gravitational energy (beta=0.0028--0.023). An increase in M_cl and/or beta leads to the formation of protostellar disks that are more susceptible to gravitational instability. Disk fragmentation occurs in most models but its effect is often limited to the very early stage, with the fragments being either dispersed or driven onto the forming star during tens of orbital periods. Only cloud cores with high enough M_cl or beta may eventually form wide-separation binary/multiple systems with low mass ratios and brown dwarf or sub-solar mass companions. It is feasible that such systems may eventually break up, giving birth to rogue brown dwarfs. Protostellar disks of {it equal} age formed from cloud cores of greater mass (but equal beta) are generally denser, hotter, larger, and more massive. On the other hand, protostellar disks formed from cloud cores of higher beta (but equal M_cl) are generally thinner and colder but larger and more massive. In all models, the difference between the irradiation temperature and midplane temperature triangle T is small, except for the innermost regions of young disks, dense fragments, and disks outer edge where triangle T is negative and may reach a factor of two or even more. Gravitationally unstable, embedded disks show radial pulsations, the amplitude of which increases along the line of increasing M_cl and beta but tends to diminish as the envelope clears. We find that single stars with a disk-to-star mass ratio of order unity can be formed only from high-beta cloud cores, but such massive disks are unstable and quickly fragment into binary/multiple systems.
The kinetic features of secondary magnetic reconnection in a single flux rope undergoing internal kink instability are studied by means of three-dimensional Particle-in-Cell simulations. Several signatures of secondary magnetic reconnection are identified in the plane perpendicular to the flux rope: a quadrupolar electron and ion density structure and a bipolar Hall magnetic field develop in proximity of the reconnection region. The most intense electric fields form perpendicularly to the local magnetic field, and a reconnection electric field is identified in the plane perpendicular to the flux rope. An electron current develops along the reconnection line in the opposite direction of the electron current supporting the flux rope magnetic field structure. Along the reconnection line, several bipolar structures of the electric field parallel to the magnetic field occur making the magnetic reconnection region turbulent. The reported signatures of secondary magnetic reconnection can help to localize magnetic reconnection events in space, astrophysical and fusion plasmas.
Coronal mass ejections (CMEs) are the primary drivers of severe space weather disturbances in the heliosphere. Models of CME dynamics have been proposed that do not fully include the effects of magnetic reconnection on the forces driving the ejection. Both observations and numerical modeling, however, suggest that reconnection likely plays a major role in most, if not all, fast CMEs. Here, we theoretically investigate the accretion of magnetic flux onto a rising ejection by reconnection involving the ejections background field. This reconnection alters the magnetic structure of the ejection and its environment, thereby modifying the forces acting upon the ejection, generically increasing its upward acceleration. The modified forces, in turn, can more strongly drive the reconnection. This feedback process acts, effectively, as an instability, which we refer to as a reconnective instability. Our analysis implies that CME models that neglect the effects of reconnection cannot accurately describe observed CME dynamics. Our ultimate aim is to understand changes in CME acceleration in terms of observable properties of magnetic reconnection, such as the amount of reconnected flux. This flux can be estimated from observations of flare ribbons and photospheric magnetic fields.
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