Michel accretion of a polytropic fluid with adiabatic index γ>5/3\gamma \gt 5/3: global flows versus homoclinic orbits

Nov 24, 2015
17 pages
Published in:
  • Class.Quant.Grav. 33 (2016) 10, 105016
  • Published: Apr 27, 2016
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Abstract: (IOP)
We analyze the properties of a polytropic fluid that is radially accreted into a Schwarzschild black hole. The case where the adiabatic index γ lies in the range of 1<γ5/31\lt \gamma \leqslant 5/3 has been treated in previous work. In this article, we analyze the complementary range of 5/3<γ25/3\lt \gamma \leqslant 2. To this purpose, the problem is cast into an appropriate Hamiltonian dynamical system, whose phase flow is analyzed. While, for 1<γ5/31\lt \gamma \leqslant 5/3, the solutions are always characterized by the presence of a unique critical saddle point, we show that, when 5/3<γ25/3\lt \gamma \leqslant 2, an additional critical point might appear, which is a center point. For the parametrization used in this paper, we prove that, whenever this additional critical point appears, there is a homoclinic orbit. Solutions corresponding to homoclinic orbits differ from standard transonic solutions with vanishing asymptotic velocities in two aspects: they are local (i.e., they cannot be continued to arbitrarily large radii), the dependence of the density or the value of the velocity on the radius is not monotonic.
Note:
  • 13 pages, 3 figures
  • 04.20.-q
  • 04.70.-s
  • 98.62.Mw
  • accretion flows
  • black holes
  • Hamiltonian dynamical systems