Marginally Stable Circular Orbits in Schwarzschild Black Hole Surrounded   by Quintessence Matter
  Marginally stable circular orbits (MSCOs) of a massive test particle are investigated in the spacetime geometry of Schwarzschild black hole surrounded by quintessence. For that matter we consider three important scenarios where the equation of state parameter $\omega_{q}$, has one of the following forms (i) $\omega_q=-1$ (ii) $\omega_q=-2/3$ and (iii) $\omega_q= -1/3$. The existence of such marginally stable circular orbits in these scenarios depend on the range of normalization factor $\alpha$. Briefly, we show that in the first case such orbits exist only if $0<\alpha<4/16875$. Moreover in the second case which is a special Kiselev black hole it is found that MSCOs exist when the value of the normalization factor satisfy $0<\alpha\leq 0.00536165238$. In the last case the MSCOs are also shown to exist. 
Ibrar Hussain, Sajid Ali
gr-qc