辐射效率高的磁化邦迪吸积
Radiatively Efficient Magnetized Bondi Accretion
摘要：
我们用数值方法研究了大尺度磁场对均匀等温气体到电阻性静止质点吸积速率的影响。只有质量，而不是磁通量，吸积到质点上。这项研究的模拟避免了复杂的边界条件所产生的保持边界远离吸积物体。我们的模拟利用自适应细化方法，以达到接近吸积物体的高空间保真度。我们的结果是特别相关的问题，恒星形成的磁化分子云，其中热能辐射的时间尺度远短于动力学的时间尺度。相反的绝热的情况下，我们的模拟表明收敛到一个有限的吸积率的限制，其中吸积对象的半径为零，无论磁场强度。对于非常弱的磁场，吸积率首先接近Bondi值，然后随着磁通量在质点附近积累而下降约2倍。对于强磁场，稳态吸积率与邦迪值相比降低了约0.2 β^{1/2}倍，其中β是气体压力与磁压力的比值。我们给出了吸积率随磁场强度变化的简单表达式。文中给出了弱磁场极限下含时吸积和强磁场下稳态吸积的近似解析结果。
英文摘要：
We have carried out a numerical study of the effect of large scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on time scales much shorter than the dynamical time scale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor ~ 2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor ~ 0.2 β^{1/2} compared to the Bondi value, where βis the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendixes for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.
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