Twisted accretion discs - IV. Alignment in polytropic discs and low limit

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Abstract
Twisted thin accretion discs are of interest in explaining long-term periodicities in X-ray binaries, the eclipse in Epsilon Aurigae, and perhaps precessing radio jets and possible warped molecular outflows in star-forming regions. Earlier results used isothermal discs to determine the alignment radius, Ra. Now we see how poly tropic discs affect earlier results for discs around compact objects and around close binaries. We find that Ra for polytropic discs can be up to an order of magnitude larger, depending on the polytropic index, the viscosity parameter and the precession mechanism. There is little change in the earlier conclusion that the case for alignment in Her X-1 is marginal, while there is substantial alignment in ε Aur. A lower limit is put on α for isothermal discs, which is expected to hold for polytropic discs as well. A bound is put on the energy of the perturbed flows.

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