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Register a new userThe Hall effect in accretion flows
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Magnetic diffusion in accretion flows changes the structure and angular momentum of the accreting material. We present two power law similarity solutions for flattened accretion flows in the presence of magnetic diffusion: a secularly-evolving Keplerian disc and a magnetically-diluted free fall onto the central object. The influence of Hall diffusion on the solutions is evident even when this is small compared to ambipolar and Ohmic diffusion, as the surface density, accretion rate and angular momentum in the flow all depend upon the product eta_H(B.Omega), and the inclusion of Hall diffusion may be the solution to the magnetic braking catastrophe of star formation simulations.
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We study the effects of accretion environment (gas density, temperature and angular momentum) at large radii ($sim 10$pc) on luminosity of hot accretion flows. The radiative feedback effects from the accretion flow on the accretion environment are also self-consistently taken into account. We find that the slowly rotating flows at large radii can significantly deviate from Bondi accretion when radiation heating and cooling are considered. We further find that when the temperature of environment gas is low (e.g. $T=2times 10^7$K), the luminosity of hot accretion flows is high. When the temperature of gas is high (e.g. $Tgeq4times 10^7$K), the luminosity of hot accretion flow significantly deceases. The environment gas density can also significantly influence the luminosity of accretion flows. When density is higher than $sim 4times 10^{-22}text{g} text{cm}^{-3}$ and temperature is lower than $2times 10^7$K, hot accretion flow with luminosity lower than $2%L_{text{Edd}}$ is not present. Therefore, the pc-scale environment density and temperature are two important parameters to determine the luminosity. The results are also useful for the sub-grid models adopted by the cosmological simulations.
Magnetic fields play an important role in star formation by regulating the removal of angular momentum from collapsing molecular cloud cores. Hall diffusion is known to be important to the magnetic field behaviour at many of the intermediate densities and field strengths encountered during the gravitational collapse of molecular cloud cores into protostars, and yet its role in the star formation process is not well-studied. We present a semianalytic self-similar model of the collapse of rotating isothermal molecular cloud cores with both Hall and ambipolar diffusion, and similarity solutions that demonstrate the profound influence of the Hall effect on the dynamics of collapse. The solutions show that the size and sign of the Hall parameter can change the size of the protostellar disc by up to an order of magnitude and the protostellar accretion rate by fifty per cent when the ratio of the Hall to ambipolar diffusivities is varied between -0.5 <= eta_H / eta_A <= 0.2. These changes depend upon the orientation of the magnetic field with respect to the axis of rotation and create a preferred handedness to the solutions that could be observed in protostellar cores using next-generation instruments such as ALMA. Hall diffusion also determines the strength and position of the shocks that bound the pseudo and rotationally-supported discs, and can introduce subshocks that further slow accretion onto the protostar. In cores that are not initially rotating Hall diffusion can even induce rotation, which could give rise to disc formation and resolve the magnetic braking catastrophe. The Hall effect clearly influences the dynamics of gravitational collapse and its role in controlling the magnetic braking and radial diffusion of the field merits further exploration in numerical simulations of star formation.
Although the magnetospheric accretion model has been extensively applied to T Tauri Stars with typical mass accretion rates, the very low accretion regime is still not fully explored. Here we report multi-epoch observations and modeling of CVSO 1335, a 5 Myr old solar mass star which is accreting mass from the disk, as evidenced by redshifted absorption in the H$alpha$ profile, but with very uncertain estimates of mass accretion rate using traditional calibrators. We use the accretion shock model to constrain the mass accretion rate from the Balmer jump excess measured with respect to a non-accreting template, and we model the H$alpha$ profile, observed simultaneously, using magnetospheric accretion models. Using data taken on consecutive nights, we found that the accretion rate of the star is low, $4-9 times 10^{-10} ,$ M$_{odot},$ yr$^{-1}$, suggesting a variability on a timescale of days. The observed H$alpha$ profiles point to two geometrically isolated accretion flows, suggesting a complex infall geometry. The systems of redshifted absorptions observed are consistent with the star being a dipper, although multi-band photometric monitoring is needed to confirm this hypothesis.
We obtained photometric observations of the nova-like cataclysmic variable RW Tri and gathered all available AAVSO and other data from the literature. We determined the system parameters and found their uncertainties using the code developed by us to model the light curves of binary systems. New time-resolved optical spectroscopic observations of RW Tri were also obtained to study the properties of emission features produced by the system. The usual interpretation of the single-peaked emission lines in nova-like systems is related to the bi-conical wind from the accretion discs inner part. However, we found that the Halpha emission profile is comprised of two components with different widths. We argue that the narrow component originates from the irradiated surface of the secondary, while the broader components source is an extended, low-velocity region in the outskirts of the accretion disc, located opposite to the collision point of the accretion stream and the disc. It appears to be a common feature for long-period nova-like systems -- a point we discuss.
Magnetic fields play an important role in star formation by regulating the removal of angular momentum from collapsing molecular cloud cores. Hall diffusion is known to be important to the magnetic field behaviour at many of the intermediate densities and field strengths encountered during the gravitational collapse of molecular cloud cores into protostars, and yet its role in the star formation process is not well-studied. This thesis describes a semianalytic self-similar model of the collapse of rotating isothermal molecular cloud cores with both Hall and ambipolar diffusion, presenting similarity solutions that demonstrate that the Hall effect has a profound influence on the dynamics of collapse. ... Hall diffusion also determines the strength of the magnetic diffusion and centrifugal shocks that bound the pseudo and rotationally-supported discs, and can introduce subshocks that further slow accretion onto the protostar. In cores that are not initially rotating Hall diffusion can even induce rotation, which could give rise to disc formation and resolve the magnetic braking catastrophe. The Hall effect clearly influences the dynamics of gravitational collapse and its role in controlling the magnetic braking and radial diffusion of the field would be worth exploring in future numerical simulations of star formation.
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